Optometry Semester 2

OPTOMETRY · SEMESTER 2 Schematic Eye Visual Optics and Assessment START READING NOTES Contents of This Topic APPLYING KNOWLWDGE OF SCHEMATIC EYE IN CALCULATING REFRACTIVE POWER OF THE EYE RELATED TASKS BASIC OPTICAL PRINCIPLES OF HUMAN EYE Introduction to schematic eye Different types of schematic eyes are designed taking into account various parameters from a simple one to a complex one. Gullstrand schematic eye 1 Key features of gullstrand exact schematic eye Gullstrand schematic eye #2 Reduced schematic eye Numerical values of various parameter as follows; Clinical application of reduced eye APPLYING KNOWLWDGE OF SCHEMATIC EYE IN CALCULATING REFRACTIVE POWER OF THE EYE CHAPTER TWO RELATED TASKS Define schematic eye Describe types of schematic eye Describe Gullstrand schematic eye number one Describe Gullstrand schematic eye number two Describe reduced eye Demonstrate skills of calculating refractive power. BASIC OPTICAL PRINCIPLES OF HUMAN EYE The optics of the eye consist of fluid optical mediums and solid mediums. Aqueous humour and vitreous humour constitute the fluid mediums, where as the cornea and crystalline lens form the solid mediums. Practically the refraction of light in the eye takes place at the anterior cornea and the two surfaces of the crystalline lens. Introduction to schematic eye Schematic eye is a simplified eye model that is designed to replace the complex optics of the human eye and to facilitate the understanding of the optics of the eye. The schematic eye assumes that; The eye is homocentric( has a common optical axis) The refracting surfaces are spherical The cornea and lens form the optical refracting elements. Different types of schematic eyes are designed taking into account various parameters from a simple one to a complex one. The schematic eye include; Gullstrand schematic eye 1 Gullstrand Emsley schematic eye 2 Reduced eye Gullstrand schematic eye 1 This eye provides us with numerical value for the radii of curvature, indices of refraction , distance btn refracting surfaces and location of principal points, nodal points and focal points. Also known as Gullstrand exact schematic eye , is a hyperopic eye ( about +1.00d) and consist of six refractive surfaces , four which associated with equivalent core lens. The primary advantage is that all of the optical constants for the eye provide a very good approximation of the dimensions of the “average eye”. Diagram gullstrand schematic eye 1 Key features of gullstrand exact schematic eye Has six refractive surfaces Equivalent core lens Hyperopic ( +1.00) Accommodated and unaccommodated version Good approximation of the dimension of the average eye The power of cornea is +43.00D and lens +19.11D. Gullstrand schematic eye #2 Also known as the Simplified schematic eye, Several schematic models have been developed that consist of 3 refracting surfaces, the Gullstrand is one of them. Here , the corneal is considered thin and it represent a single spherical refracting surface. The crystalline lens is assumed to have a homogenous index of refraction. The simplified schematic eye is emmetropic. Reduced schematic eye Even the simplified eye is too cumbersome for the majority of clinical applications , so Listing and a number of other investigators reduced the optics of the eye to a single spherical refracting surface. Since there is only one refracting surface, the first and second principal planes, points and nodal points merge to form only one principal plane , principal point and nodal point. Numerical values of various parameter as follows; Refractive index= 1.333 Dioptric strength= +60.00 First focal point(f1)= -16.67 infront of the cornea Second focal point (f2)= +22.22mm behind the cornea Axial length of reduced eye =+22.22mm Radius of curvature of cornea = 5.55mm diagram Clinical application of reduced eye Calculation of retinal image size Designing of ophthalmic instruments Calculation of intraocular lens (IOL) power. ← PREVIOUS TOPICNEXT TOPIC →VIEW MODULE NOTESVIEW SEMESTER NOTESALL OPTOMETRY NOTES Need These Notes as PDF? Request a formatted copy for offline study, printing or revision. GET PDF NOTES ON WHATSAPP

OPTOMETRY · SEMESTER 2 Optical Aberration Visual Optics and Assessment START READING NOTES Contents of This Topic APPLYING KNOWLEDGE OF OPTICAL ABERRATIONS IN ENHANCING PATIENT’S VISUAL CLARITY RELATED TASKS INTRODUCTION TO ABERRATION There are two general categories of optical aberrations ; MONOCHROMATIC ABERRATIONS Spherical aberration The extent of aberration can be expressed in one or two ways ; 2. Coma aberration 3. Oblique or radial astigmatism it is produced by spherical lens not astigmatic lens the shells are arranged according to their meridians which are tangential and sagittal 4. Curvature of field 5. Distortion aberration Types of distortion Chromatic aberration Types of chromatic aberration Way of over coming C.A Assignment APPLYING KNOWLEDGE OF OPTICAL ABERRATIONS IN ENHANCING PATIENT’S VISUAL CLARITY CHAPTER THREE RELATED TASKS Describe aberration , monochromatic and chromatic List types of aberration List monochromatic aberration Describe spherical aberration Describe coma aberration Describe astigmatic aberration Describe curvature of field aberration Describe distortion aberration Describe transverse chromatic aberration Describe tangential chromatic aberration Explain how to overcome chromatic aberration Demonstrate how to overcome aberration. INTRODUCTION TO ABERRATION Aberration, may be referred to a defect in an optical system that causes light rays to fail to converge at a single point, leading to a blurred or distorted image. OR Is the inability of a lens to bring all incident rays to a desired image point. Optical aberrations are defects in an optical system that degrade the quality of the dioptric image, even when the image is in focus. It is important to have knowledge of the aberrations pf the eye because they impair the quality of the retinal image. There are two general categories of optical aberrations ; Monochromatic aberration that occurs when a single wavelength of light is employed. Chromatic aberration that result from the use of mixed wavelength of light eg white light. MONOCHROMATIC ABERRATIONS Also known as Von Seidel aberration. Are a characteristic of the shape or configuration of an optical system. There are five monochromatic aberrations that can affect the uality of the retinal image. These are; Spherical aberration Comma aberration Oblique or radial astigmatism Curvature of field Distortion Spherical aberration Spherical aberration exists when the marginal rays ( rays refracted through the peripheral parts) and paraxial rays fail to come to focus in the same plane. Occurs when rays from axial object are refracted by the periphery of the lens. Pencil of light refracted on the large aperture of refracting system, where rays tend to refracted on diff zones which have different prismatic effects. Maybe positive spherical aberration or negative spherical aberration ; where in positive the marginal rays comes to focus infront of paraxial rays. ( more refractive power at the peripheral) and vice versa. The extent of aberration can be expressed in one or two ways ; Longitudinal aberration; refers to dioptric distance between points where the marginal and paraxial rays come to a focus ( dioptric difference btn the two regions.) Lateral aberration ; denotes the radius of the patch of light produced by the marginal rays in the plane of focus. 2. Coma aberration Coma affects the clarity of images formed for “off axis” objects. Coma occurs when rays from an off axis point source come to focus in the same image plane, but they fail to focus at the same point in that plane.( refracted on the periphery) Variation in linear magnification cause coma that lead to asymmetry in the image. Forms a comet shape like image of a point source. When the rays through the paraxial region of the optical system comes to focus closer to the optical axis within the image plane is referred to as positive coma and vice versa. 3. Oblique or radial astigmatism occurs when off-axis points are refracted near the optic axis and the image is blurred due to the resulting astigmatic effect. also known as radial or maginal astigmatism caused by incidenting ray at an oblique angle leading the pencil of light to form two image lines at two different focal locations separated by an interval of sturm involves the part of the lens which is in use doesnot affected by the pupil it is produced by spherical lens not astigmatic lens due to prismatic effect image produced by oblique astigmatism is known as image shells this is from the set of lines corresponding to each focus so as to look like a shell structure the shells are arranged according to their meridians which are tangential and sagittal tangential shells are horizontal arranged looks like a cup upper part saggital shells are verticlly arranged so it form an image like a saucer they are perpendicular to each other by combination of these image lead to teacup and saucer oblique astigmatism can not be overcomed by any form of the lens instead is overcomed by tilting the front part of the frame here have to deal with pantascopic tilt 4. Curvature of field it is a curved image surface from the flat object surface occurred when the object surface doesnot match with the image surface always the lens tends to affect the vergence of object rays which follows the curvature of the lens which is not equal to that of the cornea and retina image become out of focus and therefore blurred image surface is known as petzval's surface can be overcomed by making the lens back curvature which resemble with retina, for cameras image surface should be curved 5. Distortion aberration Is an aberration occurred btn the ray which passes at the centre of the lens and the actual principal ray. Is produced by unequal linear mx across the image plane. depending on the refractive nature and position of the aperture. Happens when magnification change through lens. This is regarded as a spherical aberration of the principal ray Image produced is sharply defined But the marginal points are the one affected by either highly magnified or minified so as to displace them far or closer than the respected points of object Types of distortion

OPTOMETRY · SEMESTER 2 Clinical Optometric Procedures: Ocular Health Examination Visual Optics and Assessment START READING NOTES Contents of This Topic Clinical Optometric Procedures: Ocular Health Examination INTRODUCTION TESTS USED FOR ASSESSMENT OF OCULAR HEALTH Grade 4 Grade 3 DIRECT OPHTHALMOSCOPY  Portable/Handheld NORMAL FUNDUS Retinal Vessels DIAGRAMMATIC REPRESENTATION OF THE VARIOUS Figure 10 C – 13 Figure 21 Diag C – 19 Central R Retina Arte Figure 25 P C – 21 SOAP FORMAT OF RECORD KEEPING A = Assessment Diagnosis Consultation/Referral OD-11.00 DS 6/6 Assessment / Plan: Clinical Optometric Procedures: Ocular Health Examination SECTION C – OCULAR HEALTH EXAMINATION This section includes a review of: Tests included in assessment of ocular health Direct ophthalmoscopy Normal fundus Diagrammatic representation of the various structures of the fundus INTRODUCTION Anectodal reports reveal that most practitioners assume that an assessment of ocular health involves a posterior segment evaluation of the fundus. The ocular health examination reveals information on the current status of ocular health, conditions that have the potential to result in ocular damage and to monitor the effect of systemic disease on the ocular system. The effects of systemic disease is not only limited to an assessment of the posterior segment of the eye, but involves assessment of pupils, visual fields, binocularity, etc. TESTS USED FOR ASSESSMENT OF OCULAR HEALTH Assessment of ocular health should include tests such as: 1. Pupillary responses (which can also be performed as part of the preliminary examination) 2. Evaluation of the anterior segment and adnexae (may be achieved by direct observation and slit lamp biomicroscopy 3. Evaluation of ocular media (achieved by slit lamp biomicroscopy, direct ophthalmoscopy, visual acuity) 4. Evaluation of the posterior segment. An assessment of the posterior segment may be achieved by several methods of examination, namely: a. Direct fundus examination (direct ophthalmoscopy) b. Dilated fundus examination Stereoscopic fundus biomicroscopy Binocular indirect ophthalmoscopy 5. Visual field screening (confrontation) 6. Measurement of intraocular pressure (this may be achieved by using non-contact and contact methods of measurement) 7. Systemic health screening tests We shall describe now the techniques used for the assessment of ocular health in more detail: 1. Pupillary responses This has been discussed in Section A Chapter 7 of the preliminary eye examination. 2. Evaluation of the anterior segment and adnexae 1. This may be achieved by gross direct observation or with the slit lamp biomicroscopy 2. Gross observation of the patient’s body, face and head must take into consideration. These observations provide clues regarding the visual, ocular and general health issues involved o Body positioning: this could provide information about the presence or evidence of a stroke o Head turn, tilt or elevation could indicate extraocular muscle difficulties, visual field problems and possible ocular pathology (like ptosis or macular degeneration) leading to compensatory head position changes o An assessment of the external ocular structures, i.e. the eyelids, eyelashes, puncta, conjunctiva, cornea and iris. Some of these structures can be observed with the naked eye, with extra light and slight magnification (+10.00DS lens or Burton lamp) or with a slit lamp biomicroscope. 3. In addition to the gross observation of the external ocular structures, the practitioner can also make an assessment of the anterior chamber angle width in the absence of other instrumentation such as a slit-lamp or gonioscope or when these techniques cannot be performed like in the paediatric population. This method of examination is also referred to as the Shadow method. Procedure o Direct the patient to a distance target in primary gaze o The practitioner holds a penlight temporal to the eye at about 15cm from the eye so that the penlight lies in line with the pupil (Fig. 1a). To achieve an accurate positioning of the penlight, the practitioner can begin by placing the penlight behind the eye/head position and then move it anteriorly until the practitioner is able to observe the illumination of the iris surface (Fig 1b). Figure 1 Positioning of the penlight for the shadow method of anterior chamber angle width estimation o The tangential illumination source (Fig. 2) will allow the practitioner to observe a crescent shaped shadow on the iris area opposite to the position of the penlight (Fig. 3) when the patient’s eye is in primary gaze International Centre for Eyecare Education Figure 2 Illumination of the iris surface with the temporal tangentially placed penlight o The crescent-like shadow observed is produced by the elevation of the iris on the side of the penlight illumination, thus preventing the light from passing across the entire iris surface unobstructed (Fig. 3a). The width of the crescent-like shadow will vary depending on the width of the anterior chamber angle. If the practitioner notices no shadow created (Fig. 3b), then it would indicate that there is a deep anterior chamber depth and therefore a lesser chance of the presence of angle closure glaucoma or a lesser chance of precipitating angle closure glaucoma with dilated fundus examinations. Figure 3 (a) Grade 2 anterior chamber depth (risk of angle closure); (b) Grade 4 anterior chamber depth (open angle) o The grading of the anterior chamber angle depth and its likelihood to precipitate an angle closure is illustrated in figure 4. These guidelines are used to complement practitioner sketches in an effort to allow the practitioner to grade angle depth. Grade 4 Grade 3 Angle closure is unlikely – Dilation possible Grade 2 Grade 1 Dilation maybe risky Dilation is contraindicated Figure 4 Grading scale used for the classification of the anterior chamber angle depth based on LOCS III photographs International Centre for Eyecare Education 3. Evaluation of ocular media The ocular media comprises the cornea, anterior chamber, crystalline lens and vitreous body. The main feature of the ocular media is that they should be transparent. The purpose of the ocular health examination to assess the transparency of these structures. The practitioner shines the direct ophthalmoscope light into the eye and observes the light reflected from the retina. This is observed as a red glow within the pupil

OPTOMETRY · SEMESTER 2 Clinical Optometric Procedures: Muscle Balance Assessment Visual Optics and Assessment START READING NOTES Contents of This Topic Clinical Optometric Procedures: Muscle Balance Assessment INTRODUCTION FUSIONAL VERGENCES Horizontal Smooth Fusional Vergences Break 19 ±8 Break 21 ±6 Vertical Fusional Vergences Break: 3 STEREOACUITY Test Correct Answer Stereo Angle Clinical Optometric Procedures: Muscle Balance Assessment CHAPTER 4 – MUSCLE BALANCE ASSESSMENT This chapter will include a review of: Fusional vergences Vergence facilities Stereoacuity INTRODUCTION The end point of refraction is to enable the patient to have clear, comfortable binocular vision for all of his visual tasks. To ensure this, the practitioner needs to investigate various visual skills. The patient must be able to align his 2 eyes and maintain alignment for sustained periods of time. In addition, the patient must have sufficient accommodation to enable him to focus on a task and sustain his accommodation comfortably. Muscle balance techniques are therefore performed to ensure that the patient’s accommodation and convergence interact appropriately. The practitioner by conducting the techniques will be able to determine if the patient’s can be corrected with lenses. If there needs to be modifications to the final lens prescription to ensure that the patient reaches the goal of clear, comfortable binocular vision, a more comprehensive binocular work-up or vision therapy is required. It is important for the practitioner to note that a patient’s prescription will have influenced the status of the patient’s accommodative and vergence systems. The muscle balance techniques are conducted through the patient’s habitual prescription. If the practitioner determines changes, then a new Rx would be considered. Distance and Near Lateral and Vertical Phorias The assessment of the distance and near lateral and vertical phorias are determine as outlined in section A, Chapter 4, except that this time, these tests are carried out through the patient’s Rx. This gives the practitioner an idea of how he has changed the patient’s muscle balance with the introduction of a prescription. It also assesses the vergence and accommodative system. FUSIONAL VERGENCES Fusional vergences are generally performed to determine through the application of prisms, the patient’s ability to use their vergence system to maintain binocularity. In this way, prisms are used to induce retinal disparity. The prism is gradually increased in magnitude, forcing the patient’s vergence system to compensate for the disparity that has been created. Fusional vergences may be horizontal and vertical. It is more common to find someone conducting a horizontal fusional vergence as opposed to both horizontal and vertical. This is due to the fact that a normal vertical phoria can be up to ½ ∆ and the amplitude of vertical vergences to compensate is about 1∆. It has been recommended that free space methods (using a prism bar) are preferred since they mimic natural viewing conditions as opposed to using the phoropter method. You will learn more about this in the binocular vision module of the course. Horizontal Smooth Fusional Vergences When testing horizontal fusional vergences the following aspects are recorded: 1. Blur point: this is the point when the patient can no longer compensate the prism induced retinal disparity while maintaining stable accommodation. In other words, it’s the point at which the accommodative system can no longer assist in holding the eyes together. Only the vergence system is involved in keeping the image single. This point is not usually found in patients at distance divergence since the accommodative effort of the patient is completely relaxed with the refractive correction. If a blur value is obtained, then the practitioner should consider re-refraction. 2. Break point: this is the point at which the patient’s vergence system can no longer compensate the further increase in prism and cannot maintain the target single anymore. Fusion breaks and produces a double image. 3. Recovery point: this is point at which the patient’s vergence system can recover its fusional ability and regain single vision as the induced retinal disparity decreases. Equipment Phoropter A distance VA chart in which one can isolate single letters Figure 4.1 Setup of Risley prism for positive fusional reserves Figure 4.2 Setup of Risley prism for negative fusional reserves International Centre for Eyecare Education Horizontal Smooth Fusional Vergences at Distance and Near Procedure 1. The patient must be wearing their best corrected distance or near prescription. The distance or near Pd must be set depending on the distance at which the vergences (reserves) are being measured. 2. The target is an isolated letter one line larger than the patient’s best corrected VA in the poorer eye. 3. The Risley prism is set at zero before both eyes. 4. Direct the patient to the target and instruct him to keep both eyes open. The patient should be able to see one clear image. If the patient sees 2 targets (i.e. reports diplopia), either BI or BO prism must be added before the patient to achieve fusion and this point becomes the starting point of the test (Scheiman and Wick, 2008). Carlson and Kurtz (2004) on the other hand suggest that diplopia must be recorded if the patient reports seeing two targets and the test should be abandoned. 5. The patient is instructed to look at the target and to keep it clear and single. The patient is then asked to report if the target blurs (blur point), becomes double (break point) and when the target is single once again (recovery point). 6. BI or positive fusional reserves (Fig. 4.1) are always measured before BO or negative fusional reserves (Fig. 4.2) since BO testing affects accommodation and convergence which may affect the results obtained during the determination of BI reserves. 7. Prism is gradually increased from a zero point either on the phoropter or a prism bar until the patient reports the first perceptible blur. Prism is added at a rate of 2 per second. The patient should be encouraged to determine the point of sustained blur (i.e. when the patient can no longer clear the target at the point of blur). This point is

OPTOMETRY · SEMESTER 2 Clinical Optometric Procedures: Accommodation And Presbyopia Visual Optics and Assessment START READING NOTES Contents of This Topic Clinical Optometric Procedures: Accommodation And Presbyopia ACCOMMODATION – DEFINITION & MECHANISM TYPES OF ACCOMMODATION Convergence Accommodation / Vergence Accommodation Tonic Accommodation AMPLITUDE OF ACCOMMODATION = +3.00-(+1.00) =+2.00D PRESBYOPIA RELATIVE ACCOMMODATION NRA/PRA: +2.00/-2.25 DETERMINATION OF AC/A RATIO SECTION B – VISUAL FUNCTION ASSESSMENT Clinical Optometric Procedures: Accommodation And Presbyopia CHAPTER 3 – ACCOMMODATION AND PRESBYOPIA This section on accommodation and presbyopia will include a review of: Accommodation – Definition and mechanism Types of accommodation Amplitude of accommodation Presbyopia Relative accommodation Determination of AC/A ratio ACCOMMODATION – DEFINITION & MECHANISM Definition “Accommodation refers to the process whereby changes in the dioptric power of the crystalline lens maintains a clear focus of an object as it draws closer to the eye”. Mechanism If an eye is emmetropic, an object viewed at a distance, will form a clear image on the retina (Fig. 3.1a). When the eye shifts its fixation to a near object, the retina senses a blurred image (Fig. 3.1b). The sphincter muscles of the ciliary body constrict thereby decreasing the diameter of the circular support for the (zonule of Zinn) zonular fibres. These fibers relax allowing the fibrous contents of the lens to increase pressure on the capsule. The capsule then bulges at the centre and flattens at the periphery thereby taking a more convex form (Fig. 3.2a & 3.2b). In the fully accommodated state the retina will be conjugated with the near point of accommodation (punctum proximum) (Fig. 3.1c). Figure 3.1 Focus changes with distance viewing and accommodation Figure 3.2 Cross-section of lens showing an increase in convexity with accommodation Accommodation reaction time Because accommodation (acc) is under the control of the Autonomic Nervous System and the ciliary body comprises smooth muscles, the system is relatively slow in reacting as compared to the extra-ocular muscles which are striated. Accommodative-reaction time is approximately 0.36secs + 0.09secs, and is further reduced by low levels of illumination (Campell and Westheimer, 1960) Amplitude of accommodation The closest distance a target can be seen clearly is referred to as the near point, while the farthest point that a static eye can see clearly is referred to as the far point. The dioptric equivalent of this near point is the amplitude of accommodation. E.g. if the near point is 8cm, then the amplitude of accommodation is 100cm/8cm = 12.50D. TYPES OF ACCOMMODATION There are various types of accommodation, namely: Reflex Accommodation This is an autonomic reaction or adjustment of refractive state to obtain and maintain a sharply defined retinal image in response to a blur input. This occurs for relatively small amounts of blur, around 2.00D, however, beyond this magnitude, voluntary accommodation is required. This is the largest and most important component of accommodation under both monocular and binocular viewing conditions. Convergence Accommodation / Vergence Accommodation This accommodation is induced by the innate neurological linking and action of disparity (fusional) vergences. It gives rise to the convergence accommodation/convergence ratio (i.e. CA/C ratio). This is the second major component of accommodation. Proximal Accommodation This is accommodation due to the influence of knowledge of the apparent nearness of an object. It is initiated by objects within 3 meters of the individual. Tonic Accommodation This type of accommodation is found in the absence of blur, disparity, proximal and voluntary inputs. There is no stimulus for tonic accommodation. It is the baseline neural innervation from the midbrain. It represents the normal tonus of the ciliary body at rest. The mean tonic accommodation in young adults is 1.00D. Tonic accommodation decreases with age because of the biomechanical limits of the crystalline lens. Accommodation Associated with the Dark (Dark Focus of Acc) The dark focus of accommodation is the accommodative posture that the eye assumes in the absence of visual stimuli. It lies approximately 1.00D inside the far point. This phenomenon helps to explain “night myopia” or “empty field myopia”. AMPLITUDE OF ACCOMMODATION Measurement of the Amplitude of Accommodation There are several methods of measuring of the amplitude of accommodation. Any of these measurements can only be made after distance correction. Push-in-to-Blur Method (Push-up Test) This method utilizes the RAF (Royal Air Force) – rule. Procedure 1. The patient’s distance Rx should be in place. 2. The LE is occluded. 3. The RAF-rule is held firmly against the patient's cheeks and tilted inferiorly from the patient’s eye level at about 30°. The patient is directed to small reading print on the sliding target, which should be placed at the far end of the rule. 4. The target is moved slowly towards the patient. The movement toward the patient is stopped when the patient reports just blur. The patient is asked to blink a few times and try and clear the print. If the print clears the target is moved closer still till the patient reports that it is blurred again. 5. The end-point of this test is when the patient experiences sustained blur, i.e. the target blurs and remains blurred even after blinking and trying to clear it. The position of the target in centimeter is converted to dioptric value. 6. The procedure is repeated 3 times to obtain an average measurement of the amplitude. 7. The procedure is then repeated for the LE with the occluder over the RE. 8. The occluder is then removed and the measurement is taken for both eyes. This measurement should usually be greater than the individual 2 measurements provided that there are no binocular vision anomalies present. 9. The practitioner takes note of the distance at which sustained blur is obtained and converts this distance to a dioptric equivalent to reflect the amplitude of accommodation. NB: One of the disadvantages of this method of measurement is that it tends to over-estimate the amp. Due to linear magnification – as the target gets closer to the eye, it appears larger and hence blur is noticed later than it would be with a smaller target.

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 Clinical Optometric Procedures: Color Vision Visual Optics and Assessment START READING NOTES Contents of This Topic Clinical Optometric Procedures: Color Vision  Pseudoisochromatic Plates DEFECTIVE COLOR VISION  Deuteranomalous Trichromacy Protanope 1 (0.01) Normal 92 (910.6) Optimum COLOR VISION TESTING Pseudoisochromatic Plates Table 10.2 Ishihara Pseudoisochromatic Score Chart Protan Deutan OS: 1,2,4,6,8,10,12,14 Color Arrangement Tests Farnsworth D-15 Farnsworth-Munsell 100 Hue SECTION B – VISUAL FUNCTION ASSESSMENT Clinical Optometric Procedures: Color Vision CHAPTER 10 – COLOR VISION This chapter includes a review of: Normal color vision Defective color vision Color vision testing  Pseudoisochromatic Plates Color Arrangement Tests INTRODUCTION Color vision is the visual function that allows one to perceive variation among the physical wavelengths of light that compile the visible spectrum. NORMAL COLOR VISION Persons with normal color vision are termed trichromats. Trichromats have 3 different divisions of cone photoreceptors within the retina which contain photopigments to absorb the wavelengths of light within the visible spectrum. Erythrolabe refers to the photopigment that absorbs red wavelengths. Chlorolabe refers to the photopigment that absorbs green wavelengths. Cyanolabe refers to the photopigment that absorbs blue wavelengths. All the spectral hues can be matched by an additive mixture of the three primary colors taken from the long-wave (red), medium-wave (green), and short-wave (blue) parts of the spectrum (Fig.10.1). Figure 10.1 Colors of the rainbow viewed by an individual with normal color vision International Centre for Eyecare Education DEFECTIVE COLOR VISION Defective color vision is either congenital or acquired. Congenital Color Deficiencies Approximately 4% of the total population has congenital color deficiencies out of which about 95% are males. Deficiencies may present as a partial abnormality which is more common, or as a complete absence of specific cone photopigments which comprise the retinal pigment system. Certain abbreviations are designated to each portion of the retinal pigment system: Prot refers to the red or erythrolabe cone photopigment system Deut refers to the green or chlorolabe cone photopigment system Trit refers to the blue or cyanolabe cone photopigment system There are three different types of color deficiency that are based on how many colors of the cone pigment system it takes to make a color match on an anomaloscope, a device specifically used to distinguish the various color deficiencies. The patient must adjust the ratio of red and green lights to match a yellow light in terms of hue, brightness and saturation. Types of Color Deficiencies Monochromacy Monochromats have an absence of two or all three cone photopigments. There are two types, the rod or typical monochromat and the cone or atypical monochromat. The rod monochromat has no functioning photopic system. Persons with rod monochromatism generally have macular dysfunction, nystagmus and low vision. The cone monochromat has one functioning cone photopigment of the photopic system. Persons with cone monochromatism may be further characterized as red, green or blue monochromats. The red or green monochromat has reduced but adequate visual acuity while the blue monochromat has very poor visual acuity. All types of monochromatism which are extremely rare lead to no available color discrimination (Fig.10.2b). A B Figure 10.2 (a) Colors of the rainbow viewed by an individual with normal color vision; (b) Colors of the rainbow viewed by an individual with monochromatism Dichromats have an absolute defect or complete absence of one portion of the cone pigment system. Dichromatism is further separated into Protanopia, Deuteranopia and Tritanopia. In all dichromats there is a neutral point within the spectrum. Under photopic conditions, it is the point that appears achromatic. There is no hue present at the neutral point because it is the position where the remaining 2 photopigments are balanced. Protanopia is an abnormality of erythrolabe. There exists a major luminosity loss on the red end of the spectrum. These individuals experience color confusions or difficulty discriminating between green, yellow and red. For example, the color red is perceived as a darker color perhaps similar to brown (Fig. 10.3b). Finally, their spectral sensitivity shifts towards shorter wavelengths. International Centre for Eyecare Education A B Figure 10.3 (a) Colors of the rainbow viewed by an individual with normal color vision; (b) Colors of the rainbow viewed by an individual with protanopia Deuteranopia is an abnormality of chlorolabe. There is a luminosity loss in the green portion of the spectrum. These individuals experience color confusions or difficulty discriminating between green, yellow and red (Fig.10.4b). Their spectral sensitivity does not shift. A B Figure 10.4 (a) Colors of the rainbow viewed by an individual with normal color vision; (b) Colors of the rainbow viewed by an individual with deuteranopia Tritanopia is an abnormality of the cyanolabe. There exists a major luminosity loss on the blue end of the spectrum (Fig.10.5b). Their spectral sensitivity shifts towards longer wavelengths. A B Figure 10.5 (a) Colors of the rainbow viewed by an individual with normal color vision; (b) Colors of the rainbow viewed by an individual with tritanopia Anomalous trichromacy Anomalous Trichromats have a partial defect or alteration of one portion of the cone photopigment system. Anomalous Trichromats are further separated into: Protanomalous trichromacy  Deuteranomalous Trichromacy Tritanomalous Trichromacy The colors matches of anomalous trichromats appear a little different due to the weakness but there are no confusions in color. Persons with these types of defects generally disagree on exact shades of color. International Centre for Eyecare Education Table 10.1 Classification of Congenital Color Deficiency Designation (based on number of cone photopigments) Prevalence Male (female) Hue Discrimination 1 Monochromat Typical or Rod 0.003 Absent Atypical, Incomplete or Cone 0.000001 Absent 2 Dichromat Protanope 1 (0.01) Deuteranope 1 Severely impaired (0.01) Tritanope 0.002 3 Trichromat Anomalous (2 normal; 1 defective) Protanope 1 (0.01) Variable impairment Deuteranope 5 (0.25) Tritanope trace Normal 92 (910.6) Optimum Acquired Color Vision Deficiencies Acquired color deficiencies are usually associated with trauma, drug toxicity or disease. These all have the potential of affecting the retina or optic nerve. Persons with acquired color deficiencies most frequently present with a blue-yellow defect. The color deficiency may be present