Visual Optics and Assessment

OPTOMETRY · SEMESTER 2 Clinical Optometric Procedures: Subjective Refraction Visual Optics and Assessment START READING NOTES Contents of This Topic Clinical Optometric Procedures: Subjective Refraction INTRODUCTION CONSIDERATIONS BEFORE SUBJECTIVE REFRACTION INSTRUMENTS FOR SUBJECTIVE REFRACTION STARTING POINT OF THE SUBJECTIVE REFRACTION BEST VISION SPHERE DETERMINATION VISION MONOCULAR BALANCING TECHNIQUES ASTIGMATIC ERROR DETERMINATION Feb 2010, Ve Jackson Cross Figure 2.11 J B2 – 11 Slit Method Figure 2 B2 – 15 BINOCULAR BALANCING TECHNIQUES  Phoropter/Trial Frame SECTION B – VISUAL FUNCTION ASSESSMENT Clinical Optometric Procedures: Subjective Refraction CHAPTER 2 – SUBJECTIVE REFRACTION This chapter will include a review of: Considerations before subjective refraction Instruments for subjective refraction Starting point of the subjective refraction Best vision sphere determination Monocular balancing techniques Astigmatic error determination Binocular balancing techniques INTRODUCTION Subjective refraction determines the refractive status of the eye using the patient' input. Subjective refraction is to determine by subjective means, the spherical and cylindrical lenses that are necessary to provide the patient with the best VA with accommodation relaxed. This is termed as subjective refraction. This is determined by responses/ input from the patient. This examination relies completely on the patient’s perceived differences between letters on a VA chart being viewed through each variation in refractive power. It should be noted that since the technique is subjective in nature, it does not always represent the full refractive status of the patient being tested. CONSIDERATIONS BEFORE SUBJECTIVE REFRACTION The discrimination between dioptric changes differs from individual to individual. Some individuals are very sensitive to even the smallest changes in dioptric change. Others are less sensitive and require significant changes in dioptric power for differences in VA to be perceived. According to Benjamin in Borish’s Clinical Refraction (2006) intelligence, past experience, accustomed visual imagery and uncertainty in discriminating between small differences may prevent perfect correlations between the subjective findings and that of the true refractive status of the eye. Some patients may perform poorly on subjective refractions because of the forced/paired choice presentations of lenses being presented. Malingers and tend to mislead the examiner, due to a preference for spectacles as a fashion item, seeking attention, etc. Other factors that should be considered when performing a subjective refraction which could influence the patient’s performance on the subjective refraction include: Health status of the eye Systemic health Use of medications or drugs that may have ocular and visual effects Age of the patient Extended depth of focus of a small pupil International Centre for Eyecare Education Choice of target, its distance and composition Room illumination Physiological pupil size and retinal adaptation Time allowed for discrimination between lens changes INSTRUMENTS FOR SUBJECTIVE REFRACTION Trial Case, Trial Frame (TF) and Phoropter (Fig. 2.1 a, b) and (Fig. 2.2) (It is important to acknowledge that phoropters are expensive and that with the context of the developing world, trial frames are more readily available, however, they have an equal degree of importance especially when conducting binocular vision testing). Figure 2.1 (a) Trial frame Figure 2.1 (b) Trial case The use of the TF and trial case versus the use of a phoropter for the subjective refraction can be based on availability and various other factors including age of patient, refractive status of the patient, compactness of instrumentation, etc. In many cases where a patient presents with high amounts of refractive error, the variation of the phoropter in terms of face form, vertex distance (in some instances) and pantoscopic angle from that of spectacles makes the trial frame the preferred instrument in the subjective refraction. Figure 2.2 Phoropter used in refraction International Centre for Eyecare Education The subjective refraction is a step-by-step procedure involving the patient’s ability to evaluate the clarity of a distance target as a series of paired comparisons of lenses are presented by the examiner. The patient fixates throughout the procedure on a distance target which consists of symbols or letters on a chart or projected chart. The examiner must ensure that this chart is of optimal quality in terms of contrast (should be 100%). Room lighting must allow normal pupil size and retinal adaptation and the chart to be visible. Subjective refractions can be of 2 types, viz. monocular or binocular. The binocular subjective is sometimes preferred because it keeps the eyes in its normal state of binocularity, i.e. both eyes are open during the course of the refraction and the accommodative state is more stable and relaxed for distance viewing. Monocular refractions involve occlusion of the non-tested eye. In monocular refraction, the final end point of the refraction requires accommodative balance and binocular determinations. In cases where a patient is monocular in nature, i.e. strabismus, blind in one eye, uni-ocular, a monocular refraction is only performed without binocular balance. Subjective refraction comprises a host of various techniques. They include: 1. Determination of the best vision spherical Rx and visual acuity 2. Duochrome (monocular balancing of spherical power) 3. Astigmatism determination a. Jackson crossed cylinder technique or b. Fan-and-block technique or c. Humphriss dots d. Stenopaic slit technique 4. Binocular balancing techniques a. Equalization by alternate occlusion or b. Prism dissociation method or c. Fogging method (Humphriss) or d. A method using a Septum (Turville) or e. Polarization method (Vectographic) or f. Dissociated duochrome method 5. Recording of final refractive error with best visual acuity in each eye and both eyes together STARTING POINT OF THE SUBJECTIVE REFRACTION The starting point of the monocular subjective refraction is determined by various objective techniques, including retinoscopy or autorefraction. These findings (lenses) are placed within the trial frame or phoropter. Alternatively, the refraction can begin from the patient’s previous spectacle correction in the trial frame (TF). The starting lenses must be placed in the TF such that the optical centers of the lenses are coincident with the geometric centers of the TF aperture and the entrance pupil of the eyes (Fig. 2.3). This is ensured by adjusting the TF to the patient’s correct interpupillary distance measurement. (It must be noted whether IPD setting should be in the binocular form

OPTOMETRY · SEMESTER 2 Clinical Optometric Procedures: Objective Refraction Visual Optics and Assessment START READING NOTES Contents of This Topic Clinical Optometric Procedures: Objective Refraction RETINOSCOPES Spot Retinoscopes Streak Retinoscopes RETINOSCOPY Mohindra Retinoscopy Retinoscope Reflex Movements Performing Static Retinoscopy SECTION B – VISUAL FUNCTION ASSESSMENT Clinical Optometric Procedures: Objective Refraction CHAPTER 1 – OBJECTIVE REFRACTION This chapter includes a review of: Retinoscopes Retinoscopy INTRODUCTION Objective refraction comprises a host of various techniques that may be employed to determine the patient’s refractive correction. It is usually performed for the determination of the starting point of a subjective refraction. More importantly, it is a technique which the practitioner has to rely on entirely when a subjective refraction cannot be ascertained. This would be in cases of the patient malingering (i.e. feigning poor/better acuity than they actually have), uncooperative patients like children, patients who are unable to communicate subjective responses to you and in patients who are unreliable in terms of subjective responses. It is a procedure that becomes more accurate with a greater amount of practice. Both retinoscopy and autorefraction are methods of objective refraction. In many cases, autorefraction is used to replace retinoscopy, however, issues of instrument reliability, problems inherent due to the nearness of the target in the instrument, and poor cooperation on the part of patients makes retinoscopy a far more superior and more adaptable technique to perform than autorefraction. Retinoscopy is a technique that is used to objectively determine the refractive error of a patient. It does not require the patient’s responses and therefore can be performed on children and non-verbal individuals. There are two kinds of retinoscopes: 1. Spot retinoscopes – contain an ordinary light source which projects a “patch” or “spot” of light 2. Streak retinoscopes – contains a special source with a linear filament which produces a “line” or “streak” of light RETINOSCOPES The first retinoscopes used a light source placed just behind the patient’s shoulder while modern retinoscopes have the light source built into them (i.e. they are self-luminous retinoscopes). Retinoscopes allow us to shine a light into a person’s eye and look at the light reflected back from their retina. This reflected light as seen in the person’s pupil is called the retinoscopic reflex or “ret reflex”. Advantages of Retinoscopes Spot Retinoscopes 1. Astigmatism can be detected with spot retinoscopy by noting the shape of the ’ret reflex’. In an uncooperative patient, it may not be possible to achieve prolonged viewing if the patient’s fixation is not steady, and the examiner has to depend on occasional glimpses for assessment of the reflex. In streak retinoscopy the reflex is always a slit, regardless of the presence of astigmatism, whereas in spot retinoscopy significant astigmatism will make the reflex appear elliptical. The correct axis of the astigmatic correction is more rapidly determined than with spot retinoscopy. International Centre for Eyecare Education 2. The time it takes for the examiner to rotate the streak from one position to another, an uncooperative patient may change their fixation position to one requiring more or less accommodation. This might be interpreted as astigmatism. With the spot retinoscope, the eye with no astigmatism (or corrected astigmatism) will always return a circular reflex, even though the speed and/or direction may change. Streak Retinoscopes 1. The end point of retinoscopy is sometimes easier to observe with streaks than with spot retinoscopes. 2. All modern streak retinoscopes have an adjustment for changing the retinoscope beam vergence from being divergent to convergent. This is not always available on spot retinoscopes. Apart from the problems with uncooperative patients noted above, the choice of “spot” or “streak” is a matter of individual preference. Optics of the Retinoscope The retinoscope consists of 2 major systems that have various parts. The systems of the retinoscope are the projection system and the observation system (Fig. 1.1). Projection system This part of the sytem illuminates the retina and comprises the following parts: Light source, i.e. a small bulb that projects light onto the retina (RPE and choroid) Condensing lens which lies in the path of the light projected from the bulb to focus the light onto the mirror Mirror which is placed within the head of the instrument. It bends the light at right angles to the axis of the handle so that the light is projected from the head of the instrument Focusing sleeve which varies the distance between the bulb and the lens so that the light projected from the retinoscope either diverges (plane mirror effect) or converges (concave mirror effect) Current source or rechargeable/replaceable battery in the retinoscope handle Observation system This part of the retinoscopic optical system allows the practitioner to view the retinal reflex. The light reflected off the retina passes through an aperture in the mirror and out through the sight hole at the rear of the head. The light that is reflected from the retina are acted upon by the optical components of the eye and therefore the observation of this reflected light provides the practitioner information about the optics of the patient’s eye International Centre for Eyecare Education Figure 1.1 The retinoscope Beam Settings of the Retinoscope The sleeve not only rotates the streak orientation but also changes the beam from divergent to convergent (Fig. 1.2). When the beam is divergent (sleeve down) add plus for a “with” motion and minus for an “against” motion. This is also referred to as plane mirror retinoscopy ( “with” motion = same direction of reflex movement as the motion from the restinoscope’s streak/ “against” motion = opposite direction of reflex movement as the motion from the retinoscope’s streak) When the beam is convergent (sleeve up) add plus for an “against” movement and minus for a “with” movement. This is also referred to as concave mirror retinoscopy (see the below diagrams) Convergent and divergent beam settings a: The divergent beam b: The convergent beam Figure 1.2 (a) Retinoscope in divergent beam position; (b) Retinoscope in convergent beam position International Centre for Eyecare Education In most

OPTOMETRY · SEMESTER 2 Schematic eyes Visual Optics and Assessment START READING NOTES Contents of This Topic Schematic eyes OCULAR PARAMETERS COMMONLY USED IN SCHEMATIC EYES LENS POWER GULLSTRAND’S EXACT SCHEMATIC EYES Key Features SIMPLIFIED SCHEMATIC EYES F = 1.376−1.0 F = 1.386−1.0 BIBLIOGRAPHY Schematic eyes SCHEMATIC EYES INTRODUCTION AND OVERVIEW This chapter includes a review of: Ocular Parameters Commonly Used in Schematic Eyes Gullstrand’s Exact Schematic Eye Effects of changing optical constantsPhysiological Optics OCULAR PARAMETERS COMMONLY USED IN SCHEMATIC EYES ANTERIOR CHAMBER DEPTH The Anterior Chamber Depth (ACD) is the distance between posterior corneal surface and apex of the anterior crystalline lens surface. The range of ACD extends from 2.7 to 4.5 mm with a mean value of about 3.45 mm. REFRACTIVE INDEX OF THE CRYSTALLINE LENS The parameters of a typical crystalline lens are shown in Figure 3.1. The lens typically has an anterior surface with a radius of curvature of approximately 6mm and a steeper posterior radius of curvature of approximately 10mm. Measurement of the refractive index of the cornea (1.376), the aqueous humor (1.336), and the vitreous humor (1.336) are straightforward because these media are relatively uniform. However, the crystalline lens is not homogeneous, instead it consists of a series of laminae in which the refractive index gradually increases from the surface toward the centre of the lens. In young, adolescent individuals, the changes in the refractive index are continuous throughout the lens. With age, as the lens matures, the refractive indices of portions of the lens become sharply separated from the surrounding lens forming iso-index surfaces that may be observed with the slit lamp. In particular, the central nuclear portion or core of the lens becomes sharply demarcated from the surrounding cortical portion of the lens. The discontinuity in refractive index between the nucleus and cortex is sufficient to form catoptric images at the interface between the core and cortex. Figure 3.1: Ocular parameters of a typical crystalline lens The indices of refraction of the ocular media cannot be measured accurately in the living eye. The values for index of refraction that have been used to develop schematic eye models are based on measurements obtained from postmortem studies. The instrument usually used to measure the indices of refraction is the Abbe Refractometer. The refractometer consists of an observation telescope and a prism with a known refractive index. The telescope is used to find the critical angle of incidence for the interface between the prism and the substance in question. When a ray strikes the interface at the critical angle, the angle of refraction will be 90°, i.e. the refracted ray will coincide with the surface of the interface. Therefore, the sine of the critical angle for a given media varies with the ratio between the refractive index of the substance in question and the refractive index of the prism (nunknown/nprism).Physiological Optics The crystalline lens does not have a uniform refractive index. Even in a normal you eye, concentric zones of discontinuity, probably associated with different periods of growth can be noted. The discontinuity zones may be particularly apparent when an opacity is observed in one layer. Although the lens is difficult to deal with in an optical sense, there are two important advantages to having a lens that does not have a uniform refractive index, but instead demonstrates a gradual increase in refractive index as you go from the surface toward the centre. First, the arrangement results in greater total refractive power. Even if the lens were homogenous with a refractive index equal to that of the lens centre, the equivalent core lens (i.e. a lens with a gradually increasing index of refraction) would have a greater refracting power. Second, the equivalent core arrangement reduces the amount of positive spherical aberration in the eye. Since it is very difficult from an optical point of view to deal with structures that have a continuously changing refractive index, the lens is generally considered to consist of two discrete parts, a central biconvex core or nucleus surrounded by a larger biconvex lens called the cortex. The refractive indices of the cortex and core of the lens are usually considered to be 1.386 and 1.406, respectively. Schematic lens models that consist of two discrete areas, one inside the other, that have different but uniform refractive indices are called equivalent core lenses (see Figure 3.3). Figure 3.3: Schematic showing the uniform refractive index needed to represent an equivalent core lens for the eye LENS POWER In the unaccommodated state, the average lens power is generally taken to be about +20.8 D. However, the available data are highly variable and largely indirect estimates of power.Physiological Optics AXIAL LENGTH Frequency distributions for individual ocular components, with the exception of axial length, conform to a normal, Gaussian distribution. Sorsby (1981) reported that axial length was randomly distributed in the general population. Although randomly selected, the population was small. Stenstrom (1948) reported that distribution is more peaked than “normal” (leptokurtotic) and asymmetric, including a larger number of longer eyes (see Figure 3.8). Since the distribution of refractive errors is leptokurtic, there cannot be free association between individual components. Highest correlation is typically found between refractive error and axial length. GULLSTRAND’S EXACT SCHEMATIC EYES Schematic eyes are models of the optical system of the eye. There are as many different schematic eyes as there are people studying the eye as an optical instrument. However, there are three basic types of schematic eyes that differ primarily in terms of their complexity. One of the most complete descriptions of the optical system of the eye is provided by Gullstrand's Exact Schematic Eye (also referred to as Gullstrand's # 1 eye; see Figure 3.4 and the accompanying Table 3.1). Gullstrand's exact schematic eye is a hyperopic eye (about +1.00 D) and consists of six refracting surfaces, four of which are associated with an equivalent core lens. The primary advantage of Gullstrand's exact eye is that all of the optical constants for the eye provide a very good

OPTOMETRY · SEMESTER 2 Clinical Optometric Procedures: Interpupillary Distance Measurement (Ipd) Visual Optics and Assessment START READING NOTES Contents of This Topic Clinical Optometric Procedures: Interpupillary Distance Measurement (Ipd) INTRODUCTION DISTANCE PD MEASUREMENTS NEAR PD MEASUREMENTS MONOCULAR PD MEASUREMENTS Clinical Optometric Procedures: Interpupillary Distance Measurement (Ipd) CHAPTER 9 – INTERPUPILLARY DISTANCE MEASUREMENT (IPD) This chapter will include a review of: Distance PD measurements Near PD measurements Monocular PD measurements INTRODUCTION The Interpupillary distance (IPD) measure is also referred to as the pupillary distance (PD). The determination of this measurement is an important part of the ophthalmic prescription and the refraction routine. The use of the PD measurement begins when refracting the patient with the trial frame or phoropter. Thereafter, it is indicated on a prescription form for the ordering of spectacle lenses. The specification of this measurement is essential in order to place the optical centers of lenses in their required position, which in most cases lies in front of the patient’s line of sight or pupil centers. The entrance pupil of the eye determines the size and location of the bundle of light that enters the eye to stimulate the retina. By definition, the PD is the horizontal distance in millimeters between the entrance pupils of the 2 eyes for a given viewing distance (Fig. 9.1). Practically, it is the distance between the centre of the pupil of one eye and the centre of the pupil of the other eye. The PD will not be the same for all people and is measured for distance and near fixation. Figure 9.1 Interpupillary distance of a patient’s eyes. The distance PD is placed at the optical centers of the spectacle lenses before the entrance pupils of the eyes when patients are looking in primary gaze. The distance PD is related to the amount of binocular convergence required by a patient for bi-foveal fixation. Patients with larger distance PDs have greater demands for convergence to a near target than patients with smaller PDs. There are several factors one must take into consideration when measuring the PD of a patient. They include: Binocularity of the patient (is strabismus/tropia present?) Is there any facial asymmetry Are the irises too dark to determine the pupil center International Centre for Eyecare Education These factors would contribute to the method that is used to determine the PD measurement. For example, if the pupils are too dark, and there is no facial asymmetry and the patient is binocular, one may use other reference points as opposed to using the center of the pupil. One may in this case, use the limbal edges as the reference points to take the measurements. If there is facial asymmetry or a strabismus present, the lenses will be placed such that the optical center of the lens is coincident with the entrance pupil of the eye. In this case, the practitioner would need to take a monocular PD measurement. DISTANCE PD MEASUREMENTS Instrumentation Millimeter ruler (accurate) Procedure 1. The practitioner and the patient must be positioned directly in front of each other (at eye level) at an arm’s length away (± 40cm). 2. The patient is directed to fixate the practitioner’s left open eye, while the examiners right eye is closed. 3. Align the zero mark on the millimeter rule with the temporal limbus of the patient’s right eye (Fig. 9.2a) (The limbal reference point is used when it is difficult to precisely locate the center of the entrance pupil in a clinical situation). Some refraction manuals suggest that the practitioner use the pupillary margins as the reference points, however, this is provided that the pupils are symmetric (Fig. 9.2b). The practitioner can also align the PD rule with the pupil center of the patient’s right eye to pupil center of the patient’s left eye (Fig. 9.2c). Figure 9.2 (a) IPD temporal limbal alignment; (b) IPD temporal pupil margin alignment; IPD pupil center alignment 4. The practitioner then closes his left eye and opens his right eye and directs the patient to bi-foveally fixate the practitioner’s open right eye. 5. The examiner then notes the position on the ruler that corresponds to the nasal limbus of the patient’s left eye (or pupil center of the patient’s left eye). 6. This measurement is noted as the distance PD (Fig. 9.3). International Centre for Eyecare Education Figure 9.3 IPD measurement using pupil center alignment. 7. The procedure may be repeated to ensure alignment of the ruler and accuracy of the measurement. 8. While the patient is not actually directed to a distance target, it should be noted that the patient’s fixation of the practitioner’s eye is not enough to deviate the patient’s eyes from the straight ahead position by an amount that would significantly alter the measurement. 9. In cases where a patient has an alternating strabismus, the measurement is taken in a similar manner, except that the patient’s non-fixating eye is occluded during the alignment process. In this way, the practitioner is confident that the patient will be viewing through the optical center of the lens if that particular eye is fixating a distance target (Fig. 9.4). Figure 9.4 PD measurement in a case of an alternating strabismus, each eye fixating at a time with the non-fixating eye being occluded. NEAR PD MEASUREMENTS Procedure 1. The position of the patient and the examiner is as per the distance measurement. 2. The patient is directed to view the tip of the examiner’s nose (or some other near target), thereby causing the patient to converge slightly. 3. Align the zero mark on the millimeter rule with the temporal limbus of the patient’s right eye (or pupil center of the patient’s right eye) while the examiner’s right eye is closed. 4. The examiner closes his left eye and opens his right eye and notes the position on the ruler that corresponds to the nasal limbus of the patient’s left eye (or pupil center of the patient’s left eye). 5. This measurement is noted as the near

OPTOMETRY · SEMESTER 2 Clinical Optometric Procedures: Visual Fields And Visual Field Screening Visual Optics and Assessment START READING NOTES Contents of This Topic Clinical Optometric Procedures: Visual Fields And Visual Field Screening VISUAL FIELD – DEFINITION PERIMETRY Static Perimetry TERMINOLOGY Scotoma Type Lesion Margins Scotoma Size FIELD DEFECTS FEATURES USED IN DESCRIBING A VISUAL FIELD DEFECT Bilateral Shapes TESTING THE VISUAL FIELD USING NON-AUTOMATED Patient X: Chart #1 Chart #2 Chart #3 Chart #4 Chart #6 Modified Procedures: Clinical Optometric Procedures: Visual Fields And Visual Field Screening CHAPTER 8 – VISUAL FIELDS AND VISUAL FIELD SCREENING This chapter includes a review of: Visual field – Definition Perimetry Terminology Visual pathway, lesions and accompanying visual field defects Features used in describing a visual field defect Testing the visual field using non-automated instruments and techniques There are some basic definitions and terms that one should be familiar with before knowing how to conduct a visual field screening. VISUAL FIELD – DEFINITION According to Benjamin in Borish’s clinical refraction, the visual field (VF) is “that area of space that a person can see at one time”. Even though we function binocularly in most circumstances, the clinical testing of the visual field is rarely conducted binocularly. The monocular visual field is 3-dimensional known as the “Hill of Vision”. The outer edges of which represent the outermost limits of the area in space termed the visual field that can be seen at any one time. Any target, irrespective of its size or intensity cannot be seen beyond this area. The outer or absolute limits of the monocular visual field are: superior: 55-60 degrees, inferior 70 degrees, temporal 100 degrees, & nasal 60 degrees (Fig. 8.1). The shape of the VF is therefore that of a horizontal oval. Figure 8.1 Schematic representation of the extent of the normal visual field of the RE International Centre for Eyecare Education The nasal limit of the VF in the primary gaze position is limited by the bridge of the nose while superior it may be limited by anatomical variations including deep-set eyes, ptosis, blepharochalasis or a prominent brow. The practitioner can verify if the restriction is due to an anatomic restriction by asking the patient to turn their head toward the area of the suspected anatomic restriction while still maintaining fixation in the straight ahead position. If the restriction was initially due to an anatomic variation, then the VF will expand outward however, if it does not, then the practitioner may suspect organic VF defects. When considering the VF of both eyes in combination, the lateral extend of the VF is about 200 degrees extending from the right temporal to the left temporal edge. There is an area of overlap between the VF of each eye in the central 120 degrees (nasal edge to nasal edge). This central 120 degrees is referred to as the binocular VF where each eye is able to detect a stimulus within this region. The ability to detect a stimulus within the boundaries of the VF depends on the sensitivity which varies with eccentricity, testing stimulus and the state of retinal adaptation. The Hill of Vision is a three dimensional representation that represents the sensitivity of eye, with the higher Hill of Vision indicating that there is a greater sensitivity at that location of the hill. The central part of the eye (i.e. foveal area) contains the highest resolution and is represented by the peak of the Hill of vision. The peak sensitivity of the Hill of vision exists under photopic conditions. Lowering of the lighting conditions to scotopic conditions depresses the Hill of Vision. It is therefore important for the practitioner to take cognisance of this since testing should be conducting under photopic conditions. These conditions must remain constant for successive VF testing in order to make comparisons between successive VF plots. The blindspot which represents the optic nerve head lays 15.5 temporal to fixation and 1.5 below the horizontal meridian. The physiological blindspot is 5.5° wide by 7.5 high in diameter. The peripheral parts of the retina display a decrease in resolution, especially as the distance gets further from the macula. PERIMETRY The science of measuring the visual field is referred to as perimetry. There are generally 2 testing strategies to perform perimetry. Kinetic Perimetry Kinetic perimetry involves the placement of a selected test stimulus of known size and intensity outside the borders of the visual field or within the blindspot and then moving it until a point at which it is detected. The boundary at which the target is first seen have equal sensitivity and if connected, they will form a ring-shaped locus of points referred to as the isopter. When the size and intensity of the stimulus is changed, another boundary is mapped. These isopters therefore provide the practitioner with the overall extent of the visual field and gives a measure of the sensitivity of the visual field. The isopter may be plotted by target stimuli of various sizes and intensities. This method of visual field measurement was the traditional approach and remains useful in determining the borders of larger or deeper visual field defects. It is also generally more useful in peripheral field defects (greater than 30 degree from fixation) and not so useful in central fields. This has been attributed to the fact that the sensitivity within the central field slopes off at a slow rate resulting in a zone of greater variability of patient responses. It has also been noted that small isolated reductions in sensitivity referred to as scotomas are easily missed with kinetic perimetry. Static Perimetry Static perimetry involves the presentation of a stimulus to a specific location on the retina. The intensity of the stimulus is presented in increasing levels until it is detected by the retina is that specific location. The intensity of the stimulus at the point of detection is referred to as the threshold. Static perimetry is a good method to detect the sensitivity of the visual field in

OPTOMETRY · SEMESTER 2 Clinical Optometric Procedures: Pupillary Testing Visual Optics and Assessment START READING NOTES Contents of This Topic Clinical Optometric Procedures: Pupillary Testing ANATOMY OF THE PUPIL AND IRIS Ocular Voluntary Nervous System EVALUATION OF PUPILS  Dim =  Bright EVALUATION OF PUPIL REFLEXES  Evaluate & Note: P Pupils OD 4 3+ E R R A (-)RAPD Pupil Test Summary Clinical Optometric Procedures: Pupillary Testing CHAPTER 7 – PUPILLARY TESTING This chapter includes a review of the: Anatomy of the pupil and iris Pupillary pathways Evaluation of pupil size and symmetry Evaluation of pupil reflexes Background information Pupillary testing provides the practitioner information regarding the integrity and function of the iris, the optic nerve, anterior visual pathways, parasympathetic and sympathetic nervous systems and general systemic health. In order to fully understand pupillary testing, one must be familiar with the iris anatomy and neural pathways which regulate pupil size. Terminology Miosis = pupil constriction Mydriasis = pupil dilation Anisocoria = unequal pupil size ANATOMY OF THE PUPIL AND IRIS The pupil is bounded by the iris, which determines its shape and size. The iris is made up of 2 groups of muscle, viz. the circular muscle fibres at the pupil margin = sphincter pupillae. The other group consists of the radial fibres that extend from the iris root to the border of the iris = dilator pupillae. PUPILLARY PATHWAYS There are several parts that make up the pupil reflex pathways. It involves transmission of signals from the outside world to the brain via the afferent papillary pathway. The control of the pupil size is controlled by the Parasympathetic Nervous System (PNS) and Sympathetic Nervous System (SNS) which makes up the efferent pathways involved in transmission of signals from the brain to the effector organs. A simple way to remember this is: Afferent = At brain Efferent = Exit or Effector International Centre for Eyecare Education Afferent pupil pathway The afferent pupil pathway begins with the primary sensor for light, i.e. the photopic system which comprises primarily the cones of the retina. The light reaching the retina is the primary determinant of the pupillary light reflex. Once the fovea is stimulated by light entering the eye, it sets ups signals that are transmitted by retinal nerve fibers. This then relays information for pupil control through the optic nerve to the optic chiasm, at which point half the fibers decussate to the contralateral (opposite) optic tract which the remaining fibers continue on the ipsilateral (same) optic tract to the pupil light control centers in the midbrain. This entire section of the light reflex pathway is the afferent system. The system returning from the midbrain is the parasympathetic pathway (discussed below). Parasympathetic pathway This is a 3 neuron pathway (Fig. 6.1). The pathway begins at the Pretectal nucleus of the midbrain and ends at the iris. The fibres from the pretectal nucleus semidecussate to the Edinger Westphal nuclei (i.e. one fibre from each pretectal nucleus to both Edinger Westphal nuclei) at which point they exit the midbrain as the 3rd nerve and synapse in the ciliary ganglion, and finally reach the sphincter pupillae of the iris via the short ciliary nerves. The majority of the fibres of the 3rd nerve, innervate the ciliary muscle which controls accommodation while only about 3% of fibers innervate the iris sphincter muscle (Benjamin: Borish’s Clincial refraction). Flow chart of the parasympathetic pathway Figure 6.1 The parasympathetic pathway International Centre for Eyecare Education Sympathetic pathway This is a 3 neuron pathway that begins in the posterior hypothalamus (Fig. 6.2). The efferent fibers travel to the brain stem and synapse in ciliospinal center of Budge (intermediolateral gray matter of the spinal cord) at the level of T2 along the spinal cord. The fibers from this point on, are the preganglionic fibers (second order neurons). They exit the spine at the level of the thorax (T1) and travel to the synapse in the superior cervical ganglion which is located at the level of the angle of the jaw. From here the post ganglionic fibers (third order neurons) follow the internal carotid artery, through the cavernous sinus and reach the orbit through the superior orbital fissures. These nerves join the ophthalmic division of the 5th cranial nerve to reach the iris dilator pupillae via the long ciliary nerves. The nerves split at the carotid bifurcation, to supply various organs including the sweat glands, iris dilator fibers and Muller’s muscle in the upper lid. Flow chart of the sympathetic pathway Figure 6.2 The sympathetic pathway Ocular Voluntary Nervous System The ocular voluntary nervous system provides the output for the Near Synkinesis (also called Near Triad). The Near Synkinesis is a combination of 3 functions (accommodation + convergence + pupil constriction) that normally function in a connected way. Under normal conditions, they occur simultaneously and are inseparable from one another. When one accommodates, both convergence and pupil constriction automatically occur. This function is technically not controllable but is categorized as a voluntary function because it requires conscious voluntary effort. It can occur even in the blind. The pathway for the Near Synkinesis is not fully known but it appears to begin in the frontal visual association areas (# 19): Anterior visual association areas CN III nuclei (more ventral than pre-tectal input) Follows p route Accommodation Convergence Pupillary ConstrictionInternational Centre for Eyecare Education EVALUATION OF PUPILS When evaluating a pupil we have to consider several criteria, viz. Size and symmetry/shape of the pupil Pupil reaction to light and accommodation (briskness) Speed of the reflex Analysis of pupil size Ruler or HAAB scale method The assessment of pupil size requires that the patient be adapted to the level of light within the environment. This is typically done under normal room illumination. The patient fixates a distance target. The diameter of the pupil is measured using a millimeter rule (PD rule) or a HAAB scale which comprises series of coloured circles of known diameters. The ruler is held against the patient’s cheekbones

OPTOMETRY · SEMESTER 2 Clinical Optometric Procedures: Ocular Motilities Visual Optics and Assessment START READING NOTES Contents of This Topic Clinical Optometric Procedures: Ocular Motilities TYPES OF EYE MOVEMENTS Monocular Eye Movements RIGHT EYE LEFT EYE Pursuit Eye Movement Testing / Broad-H Test Saccadic Eye Movements Clinical Optometric Procedures: Ocular Motilities CHAPTER 6 – OCULAR MOTILITIES This chapter includes a review of: Types of eye movements Binocular eye movements INTRODUCTION Ocular motilities refer to eye movements. The purpose of these eye movements is to assess the patient’s ability on conjugate eye movements. Vertical and lateral gaze movements direct the lines of sight along the Y and X axis respectively. Rotations of the eyes along these axes result in the eyes moving toward targets within any of the four quadrants. In this process the lines of sight of the eyes are directed up, down, right or left, away from the primary position of gaze (straight ahead position of gaze). Reflexive and voluntary eye movements are controlled by a cortical network involving the frontal, parietal and occipital areas of the brain that send premotor signals to the nuclei of the 3rd, 4th and 6th cranial nerves. Voluntary movements originate in the frontal areas of the brain, while reflexive movements originate in the posterior portion of the brain. TYPES OF EYE MOVEMENTS Binocular Eye Movements Conjugate eye movements Versions are movements of both eyes in the same direction, i.e. the lines of sight are parallel to each other. They may be to the right (Fig. 6.1), left (Fig. 6.2), up or down. These movements are tested to determine if the neuromuscular systems controlling the movements are functioning and intact. Figure 6.1 Schematic of eye movement on dextroversion (to the right) International Centre for Eyecare Education Figure 6.2 Schematic of eye movement on levoversion (to the left) Disconjugate eye movements Vergences are movements of both eyes in opposite directions e.g. convergence (toward the nose) or divergence (away from the each other or the nose) where the lines of sight are not parallel to each other (Fig.6. 3). Figure 6.3 Schematic of eye movement in convergence and divergence Torsional eye movements These eye movements allow a rotation of the eyes clockwise and counterclockwise. They can be incyclotorsion refers to a movement when the eye rotates toward the nose and excyclotorsion is the movement of the eyes when the top of the eye rotates away from the nose. Control of conjugate eye movements The signal for eye movements originates in the cerebrum and is transmitted to the gaze centers in the midbrain and motor nuclei in the Pons. Signals from here are transmitted via the 3rd, 4th and 6th nerves to the EOMs. Supranuclear pathways conduct impulses to the gaze centers while internuclear pathways coordinate the gaze centers with the motor nuclei. Abnormalities detected with conjugate eye movements informs the practitioner of lesions which involve one or more of the 3 cranial nerves controlling eye movements or lesions located at the motor nuclei in the midbrain, Pons and gaze centers in the upper midbrain or the cerebral centers where the eye movements are initiated. Monocular Eye Movements Ductions refer to monocular eye movements. The muscle or muscles that contract to produce the movement of the eye are the agonist(s). When 2 muscles move the eye in the same direction to produce a movement, then the muscles are synergists. For each agonist that contracts to move the eye, an antagonist muscle which produces a movement in the direction opposite the agonist, relaxes. BINOCULAR EYE MOVEMENTS Bifoveal fixation is the goal of the coordinated action of the EOMs. Movement of the eyes to the right is referred to as dextroversion (Fig. 6.1) while movement to the left is levoversion (Fig. 6.2). An assessment of motilities allows us to evaluate the oculomotor system. Abnormal eye movements may be caused by fatigue, poor attention span, aging, drugs or medications, and neurological problems. Achievement of Binocular Eye Movements Conjugate eye movements are produced when the direction, speed and magnitude of rotation of the eyes are equal. This is enabled since the EOMs of both eyes are yoked together. There is identical excitatory or inhibitory innervation supplied to the corresponding yoked muscles. The equal, simultaneous innervation sent to the yoked EOMs to produce voluntary conjugate eye movements is governed by Hering’s Law of Equal Innervation. This law applied whether the eyes are fixating binocularly or monocularly. Table 6.1 Yoked Pairs of Ocular Muscles RIGHT EYE LEFT EYE Lateral rectus Medial rectus Medial rectus Lateral rectus Superior rectus Inferior oblique Inferior rectus Superior oblique Superior oblique Inferior rectus Inferior oblique Superior rectus Types of Conjugate Eye Movements There are 3 types of conjugate eye movements, viz. pursuits, saccades and vestibular eye movements. Pursuits and saccades are initiated in the cerebral hemispheres and are mediated by supranuclear pathways. Vestibular (reflex) eye movements are initiated in the ear canal and mediated by the cerebellum and brain stem. Pursuits Pursuits are slow, smooth tracking conjugate eye movements which are stimulated by target motion. They are elicited by asking a patient to follow a slow moving target. Saccades Saccades are rapid, voluntary or reflex fixational movements. It is elicited by having the patient fixate various targets. Vestibular eye movements Vestibular eye movements are a mix of smooth image stabilizing movements or rapid saccadic eye movements. They occur in response to continuously moving stimuli in the visual field or to stimulation of the semicircular canals. International Centre for Eyecare Education Pursuit Eye Movement Testing / Broad-H Test A pursuit or broad H test refers to a following eye movement when the eye moves to follow the visual clue of an object moving in the field of vision. The findings of the Broad H test consider the fields of action of the six extrinsic ocular muscles. The field of action of a muscle refers to the field in which a particular muscle has its greatest action, e.g. RLR has its field of action in the right hand field

OPTOMETRY · SEMESTER 2 Clinical Optometric Procedures: The Cover Test Visual Optics and Assessment START READING NOTES Contents of This Topic Clinical Optometric Procedures: The Cover Test FEATURES OF OCULAR DEVIATION Binocularity Implications COVER TEST ASSESSMENT OF OCULAR DEVIATION XOP ± 1 Subjective Cover Test OTHER METHODS TO DETERMINE THE PRESENCE OF A 12∆ BI 6∆ BU 12∆ BI 3∆ BU Clinical Optometric Procedures: The Cover Test CHAPTER 5 – THE COVER TEST This chapter will include a review of: Features of ocular deviation Cover test assessment of ocular deviation Von Graeffe assessment of ocular deviation Maddox rod assessment of ocular deviation FEATURES OF OCULAR DEVIATION Background Information The cover test is used to determine whether there is any tendency of the eyes to deviate from well coordinated behavior. If a deviation is detected, the cover test will show a deviation if it is latent (heterophoria – tendency to turn under certain conditions) or manifest (heterotropia – permanent turn). The cover test can also be used to estimate or measure the direction and size of the deviation, and give some indication whether it is compensated or not. Every patient’s deviation must be described in terms of its frequency, direction, magnitude, laterality and comitancy. Frequency: may be either constant or intermittent. Direction: may be horizontal, vertical or rotatory (cyclo). In addition, no deviation of the line of sight is termed othortropia/orthophoria. Horizontal deviations are either inward (nasal) deviation from the line of sight (known as an eso deviation) or outward (temporal) deviation of the line of sight (known as an exo deviation). A vertical upward deviation of the line of sight of one eye is referred to as a hyper deviation while a vertical downward deviation of the line of sight and is referred to as a hypo deviation of the eye (Fig. 5.1). When there is a deviation of the eye around the antero-posterior axis of the eye, then the eye is said to have a cyclo or torsional deviation. An excyclodeviation is characterized by a temporal rotation of the superior aspect of the globe while an incyclodeviation is characterized by a nasal rotation. Magnitude: According to Daum in Eskridge et al (1991), “the magnitude of the deviation is the angular measurement of the difference in direction of the lines of sight of the eyes for a specific fixation distance and direction of gaze”. The magnitude of a deviation is specified in prism dioptre () units. Laterality: is usually only specified when the deviation is constant. A constant strabismus may be either unilateral or alternating. If the deviation is unilateral, then it implies that the deviation is constantly in only one eye, for example right constant exotropia. If the patient uses either eye to fixate, then the deviation is referred to as an alternating deviation and the deviating eye cannot be specified due to its alternation. This type of deviation would simply be documented as an alternating tropia. Comitancy: refers to the magnitude of the deviation when the eye changes to various directions of gaze. If a deviation is comitant or concomitant then it implies that the magnitude of the deviation remains the same irrespective of the direction of gaze of the patient. If it is incomitant or inconcomitant then it implies that the magnitude of the deviation changes when the eye shifts its gaze from one direction to another. International Centre for Eyecare Education Figure 5.1 Diagrammatic representation of tropias Binocularity Implications The use of the information gathered from establishing the binocularity of the patient has several implications: The clinician may chose to perform further testing of the oculomotor system There may have to be modifications in the typical refraction routine. Additional specific tests may need to be performed. For example, a patient who has a tropia is considered monocular and therefore cannot undergo binocular balancing tests. The binocular status may suggest the presence of eyestrain, headaches, decreased performance, Amblyopia or reduced stereopsis. COVER TEST ASSESSMENT OF OCULAR DEVIATION The cover test can be divided into 2 categories, viz. objective and subjective. Objective Cover Test The objective cover test is sometimes termed the ’cover-uncover’ test. It is probably one of the most important of all of the tests to determine the oculo-motor balance of a patient. It may be carried out at distance and near. It can also be carried out on patients wearing no correction or wearing a habitual prescription. If the patient has a habitual prescription then the cover test should be conducted with the prescription on at all times when tested. According to Benjamin in Borish’s Clinical Refraction (2006), the unilateral cover test confirms the presence of a tropia or phoria and defines its directions. There are 2 types of cover tests: 1. Unilateral cover test (cover-uncover test) 2. Alternate cover test 1. Unilateral cover test (cover-uncover test) The unilateral cover test is performed by placing an occluder in front of one eye and then observing the movement, if any, in the fellow eye. This test is used to detect the presence of a tropia / heterotropia / strabismus. A tropia is a deviation of the eye that is visible by simply looking at the patient. In Figure 5.2, when observing the position of the patient’s eyes, once can see that the right eye is turned inward. This eye is the deviating eye while the other eye is the fixating (dominant) eye. One would therefore classify this deviation as an esotropia. Figure 5.2 Schematic of unilateral cover test 2. Alternate cover test The alternate cover test is performed by alternating the occluder from one eye to the other while the patient fixates a target and observing the movement, if any, in the eye that has just been uncovered (Fig.5.3). Figure 5.3 Schematic of unilateral cover test showing an esophoria. Performing the Cover Test Aim: The cover test is an objective method of evaluating the presence, direction and magnitude of a strabismus (tropia) or phoria www.icee.org 4 Equipment: VA chart Occluder Near point target Overhead lamp

OPTOMETRY · SEMESTER 2 Clinical Optometric Procedures: Convergence And The Near Point Of Convergence Visual Optics and Assessment START READING NOTES Contents of This Topic Clinical Optometric Procedures: Convergence And The Near Point Of Convergence Quantifying Convergence Prism Dioptre Convergence = 1/2  6/2 NEAR POINT OF CONVERGENCE (NPC) Red Lens Technique BIBLIOGRAPHY: Clinical Optometric Procedures: Convergence And The Near Point Of Convergence CHAPTER 4 – CONVERGENCE AND THE NEAR POINT OF CONVERGENCE This chapter will include a review of: Convergence Near Point of Convergence (NPC) CONVERGENCE Convergence is a fusional movement that is essentially reflex in nature; however, an individual can exert conscious control over the movement. Most people can be trained to exercise voluntary control. Convergence can thus be stimulated voluntarily, disparate stimulation [disparity or fusional vergence] and accommodation [accommodative-convergence synkinesis]. The voluntary centre is situated in the frontal oculogyric centre while the reflex action originates in the occipital lobe. Figure 4.1 Schematic showing movement of eyes when converging If an object is brought closer to the eye from a distant position, the light rays from that object are divergent and hence won’t form a clear, single image on the retina as the eye would be in a distance viewing position. The eye therefore has to make adjustments to clear the image and make it single. The change in the refractive power of the crystalline lens enables the Px to restore the clarity of the object, this is known as accommodation. Simultaneously, compound movements of the extra-ocular muscles, predominantly the medial rectii muscles position the eyes in such a way that the visual axes now intersect at the object of regard and it is seen singly, this is known as convergence (Fig. 4.1). The medial rectii like all other extraocular movements (EOM’s) is striate in nature and is therefore under control of the voluntary nervous system. Therefore, convergence is faster acting than accommodation. Types of Convergence Tonic convergence All muscles exist at a certain base line level of contraction, this inherent convergence due to muscle tonus is referred to as tonic convergence and the accompanying eye position is referred as the ’physiological position of rest’. When there is no tonus or innervation to any of the EOM’s then the position of the eye is referred to as the ’anatomical position of rest’. International Centre for Eyecare Education Proximal convergence Proximal convergence is also referred to as voluntary convergence and is induced by the awareness of the nearness of an object. Accommodative convergence Accommodative convergence occurs due to the link between accommodation and convergence and is described as a function of the accommodative effort. Fusional convergence This type of convergence is also referred to as reflex convergence and is characterized by the eye’s ability to move into a position to fuse 2 single images into a single concept. It compensates for any excess or deficiency in tonic convergence using retinal disparity as its stimulus. Quantifying Convergence Metre Angle While accommodation is measured in dioptres it was difficult to apply to convergence and therefore the concept of the METRE ANGLE was introduced by Nagel in 1880. METRE ANGLE is defined as that rotation (amount of convergence) of the eye to view an object on the midline at 1m distance (Fig.4.2). Figure 4.2 The metre angle The metre angle is also defined as “the reciprocal of the target distance in meters”, and is equal to the product of the vergence demand in prism dioptres and the interpupillary distance in centimeters. International Centre for Eyecare Education Therefore for both eyes to converge at a point 1m in distance, 2MA effort is required since each eye will rotate by 1MA. It was found that patients with wider interpupillary distances would need to make a greater rotational effort versus those that have a narrow PD. This presented a problem and thus the concept of the prism dioptre was introduced. Prism Dioptre A prism dioptre takes into account the distance by which an image appears to have been displaced. It expresses this displacement as a function of the distance of a prism from the object (Fig. 4.3). Figure 4.3 Prism diopter If a patient has with an interpupillary distance of 6cm converges to the midline to fixate a point at a distance of 1m, each eye will turn inward by 3 prism dioptres and there will be a total convergence for both eyes of 6 prism dioptres. ( = prism dioptre) For distance other than 1m, we used the following formula: (N.B.: The above formula applies to convergence of one eye only) Example: A patient having a Pd = 60mm fixates on an object along the midline 2m away. Calculate the total convergence in prism dioptre. Convergence = 1/2  6/2 = 1.5 for each eye Therefore total convergence = 3 In order for the above patient to rotate one eye to view an object on the midline 2m away, he will have to create the equivalent of 1.5 of deviational effort in order to make the same effort if a single binocular concept of a total of 3 is to be obtained. NEAR POINT OF CONVERGENCE (NPC) The NPC is the point of intersection of the lines of sight of the eyes when maximum convergence is utilized, while still preserving single binocular vision. The measurement obtained is more correctly called the ‘NPC distance’. However, in clinical practice it is referred to as the NPC. Patients who have reduced NPC distances may have visual and ocular discomfort when performing near point vision tasks. Instrumentation 1. RAF-rule (RAF= Royal Air Force) 2. Penlight (usually a preferred target) 3. Wolff ball, finger puppet (these are motility targets, but can be used for patients who have problems fixating on the penlight target). Measurement of the NPC There are several methods that may be utilised to measure the NPC. These include: RAF-Rule Technique Procedure 1. Patient is seated comfortably and is wearing the habitual Rx under full room illumination conditions. 2. The RAF-rule is held below the line

OPTOMETRY · SEMESTER 2 Clinical Optometric Procedures: Visual Acuity Visual Optics and Assessment START READING NOTES Contents of This Topic Clinical Optometric Procedures: Visual Acuity CLASSIFICATION OF VISUAL ACUITY (VA) NOTATIONS OF VISUAL ACUITY VISUAL ACUITY = 20/40 = 2MAR, 20/100 = 5MAR. CLINICAL TESTING OF VISUAL ACUITY Chart Formats DISTANCE VISUAL ACUITY MEASUREMENT A B C D E (6/9) = 6/9 ‐2 F G H I J OD: 6/6 OS: 6/9 OU: 6/7.5 RE: 6/6 LE: 6/9 BE: 6/7.5 Pinhole Visual Acuity Common Errors When Taking Visual Acuities B – E – S NEAR VISUAL ACUITY MEASUREMENT The Reduced Snellen / Equivalent Snellen Notation N 8 / 8 = 1.0 M BIBLIOGRAPHY Clinical Optometric Procedures: Visual Acuity CHAPTER 3 – VISUAL ACUITY This chapter includes a review of the following: Classification of visual acuity Notations of visual acuity Clinical testing of visual acuity Distance visual acuity measurement Near visual acuity measurement CLASSIFICATION OF VISUAL ACUITY (VA) Visual acuity is defined as the “spatial resolving capacity of the visual system” (Benjamin, 2006) and refers to the sharpness of vision or the patient’s ability to recognize a minimum size target. Visual acuity is customarily abbreviated as 'VA'. The measurement of VA should be performed at every patient’s (Px) visit on completion of a case history. It is one of the most informative tests conducted. Visual acuity provides information on; Refractive status of the eye Indication of macula function Indication of neural integrity One can compare visual status of an eye and/or between one eye and another to see if the visual acuity is comparable and similar or different. VA allows us to compare the sharpness of vision between the 2 eyes, the eyes of the same Px at different times, or between the eyes of different Px’s. It provides a scale by which we can communicate our assessment of a Px’s visual ability to other optometrists. Figure 3.1 A Visual acuity chart International Centre for Eyecare Education Tests of Visual Resolution According to Benjamin (2006), there are a variety of tests of visual performance that measure some aspect of the limits of the visual system’s ability to discern detail or to recognize detailed targets. These include: 1. Minimum detectable resolution Minimum detectable resolution refers to the threshold of an individual’s visual system to detect the presence of a spot or a line stimulus against its background. This assessment of visual performance does not require the discrimination of target detail but rather requires the individual to perceive the presence or absence of an aspect of the stimuli presented (Fig.3.2). Positive contrast is depicted by a bright stimulus (point or line) against a dark background (Fig. 3.2 a) while negative contrast is observed when a dark point or line is presented against a bright background (Fig. 3.2 b). Minimum detectable resolution is usually of little clinical application, except in nightdriving vision and is not considered to be a routine examination technique. Figure 3.2 (a) positive contrast; (b) negative contrast 2. Minimum resolvable Minimum resolvable visual performance involves the resolution of detail. Clinical evaluation of VA is based upon this type of visual performance. Most of our tests of VA are recognition tests that involve the measurement of the smallest symbols, shapes, letters that can be correctly identified/resolved by the Px. This type of acuity can be separated into 2 forms: a. Form sense: (Landolt rings, Tumbling E’s), in which a simple form is used and some task, such as orientation, is used to determine the acuity (Fig 3.3 a and b). Landolt C’s/Rings: These are circles with a gap (Fig. 3.3 a). The gap is orientated in four directions, viz. up, down, right or left. In some cases, charts also include oblique positions of the gaps in addition to cardinal positions. Tumbling E’s: This test involves the presentation of an “E” in different orientations (cardinal and/or oblique) at every acuity level (Fig. 3.3 b). The Px is instructed to identify the direction in which the limbs of the E are pointing. This test, like the Landolt C test is useful in measuring the VA in Pxs who are not familiar with letters of the alphabet such as toddlers. b. True minimum legible: In which complex patterns such as letters or numerals are used as the test stimulus (Fig. 3.3 c). These are referred to as letter optotypes (targets) (Fig. 3.3 c). These were originally designed by Snellen using sans-serif letters and later were changed to Sloan letters since the serifs did create some confusion. International Centre for Eyecare Education Figure 3.3 (a) Landolt C’s; (b) Tumbling E’s; (c) Letter optotypes 3. Minimum separable / Vernier acuity The assessment of vernier acuity involves the individual’s ability to detect that a group of points or lines are separate and distinct, e.g. detecting a break in a line. This type of acuity is usually used to measure the resolution capacity of the visual system. Targets are usually presented as either lines or gratings of equal separation from each other. The Px has the task of determining the minimum separation between line targets that allows them to distinguish the lines from each other. For grating targets, the Px has to determine the finest grating that can be distinguished from a uniform field of uniform luminance. 4. Stereoacuity Stereoacuity involves the use of both eyes. It represents the ability of the Px to resolve slight differences in distance of target objects when looking at special stereoscopes. Stereoacuity is measured in seconds of arc (using the disparometer). NOTATIONS OF VISUAL ACUITY VA expresses the angular size of the smallest target that can just be resolved by the Px. There are various different ways in which they can be specified, namely: 1. Snellen Fraction 2. Decimal Notation 3. Minimum angle of resolution 4. Logarithm of minimum angle of resolution 5. Visual acuity rating 6. Visual efficiency 1. Snellen Fraction Snellen principle The Snellen test is a form of minimum legible visual task (i.e. involving the resolution of detail). Helmholtz