Clinical Optometric Procedures: Accommodation And Presbyopia

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.

• Expected findings
• 18 – 1/3 age (± 2.00DS) (Scheiman and Wick, 2008)
• Push-out-to-Clear Method

Procedure

This method is similar to the push-in-to-blur method except that the target is placed inside of the near point and is gradually moved away from the patient until the print is no longer blurred and the patient can read the letters. The measurement is simply read off the RAF-rule scale.

• Minus Lens to Blur Method
• Procedure
• 1. Distance prescription should be in place

2. The test is performed first monocularly and then binocularly 3. The near card is held at 40cm and the patient is asked to look at a target one line above the best near VA 4. The target is illuminated by overhead lamp or lighting International Centre for Eyecare Education 5. Minus lenses are added in 0.50-1.00D steps until the patient reports first sustained blur (patient blinks and can no longer clear the target).

6. The amplitude is determined by adding the absolute value of the minus lenses cleared to the dioptric value of the working distance, i.e. 2.50DS for a 40 cm working distance. For example, a patient presents with a minus-lens-to-blur measurement of -4.00DS at a working distance of 40cm (-2.50DS). The total amplitude of accommodation for this patient would be -6.50DS.

• Expected findings
• 2.00DS < push-up test (Scheiman and Wick, 2008)

Dynamic / Monocular Estimated Method (MEM) Retinoscopy Dynamic retinoscopy provides an objective evaluation of the accommodative response. Dynamic retinoscopy is used when the patient’s accommodation is kept in an active state by encouraging the patient to fixate on an accommodative target at approximately 50-66 cm on the retinoscope.

Procedure

1. The test is conducted under conditions of normal room illumination. This is especially important since accommodation is affected by illumination 2. Distance prescription on.

3. The patient is asked to fixate on an appropriate target on the MEM card attached to the retinoscope.

4. The examiner then moves the retinoscope and target closer to the patient while observing the retinoscopic reflex.

5. The initial reflex at the patient’s Harmon’s distance will be a bright, broad and fast “with” movement (indicating a normal accommodative lag).

6. A change of this reflex to a narrower, dimmer and slower reflex indicates the focusing is lost and that the amplitude of accommodation has been exceeded.

7. The distance at which the movement changes is noted and converted to a dioptric equivalent from the spectacle plane to provide the amplitude of accommodation.

8. If the initial reflex was against, the amplitude is reached when there is a change in the reflex to a “with” movement.

• (Bennet and Rabbets, 1984; Eskridge, 1991)
• Expected findings

+0.25DS to +0.50DS ± 0.25DS (Scheiman and Wick, 2008) It is suggested that a value below +0.25DS or over +0.75DS should raise suspicion and therefore be investigated further. International Centre for Eyecare Education Estimation of Amplitude of Accommodation There are several methods than one may utilize to estimate the amplitude of accommodation in order to ascertain the strength of the patient’s ability to stimulate accommodation. These include:

1. Hofstetter’s rule

Hoffstetter’s rule provides an estimation of the average amplitude of accommodation, in diopters, for a patient of a given age. A measurement lower than the expected is regarded as a deficiency in accommodation.

2. Duane-Hofstetter formula

The Duane-Hoffstetter formula for probable amplitude of accommodation should assist one in determining whether the amplitude for the patient is adequate or not. It considers not only the average amplitude but the range of minimum and maximum values when considering the age of the patient.

• Maximum amplitude = 25.0 – 0.40 x age
• Average amplitude = 18.5 – 0.30 x age
• Minimum amplitude = 15.0 – 0.25 x age

Points to remember:

• For a presbyopic patient you may have to add +1.00DS before each eye and carry out the procedure as the

Px may not be able to see the target. Sometimes, if the reading addition has already been determined, measure the amp of acc with the 'add on'. However, the lens used must be subtracted from your findings e.g. if the reading on the RAF-rule is +3.00D and you used +1.00DS lens, the amp of acc will be: amp of acc

= +3.00-(+1.00) =+2.00D

• Similarly if you had a young patient or a highly myopic patient, you may have to put up minus lenses before

taking the readings. Remember to add that power on. E.g. Reading on RAF-rule is +10.00D, and you place a -2.00 DS lens:

• Amp of Acc = +10.00 – (-2.00D)

= +12.00D

• The above four methods will produce different values for the amplitude of accommodation in the same

patient. In the push-in method, the patient will tend to report blur when experiencing difficulty to read. In the push-out method, the patient will wait until the print becomes clear. Therefore instruct your patient clearly or ask the patient to read letters/words out loud. One will also experience variability of the results if the slide on Average amp = 18.5 – 1/3rd Age

Clinical pearls

• The above methods can produce different values for the amp because of the varied techniques.
• Subjective measures of amp give larger values than objective measures because pupil constriction

during subjective measures keeps the image from blurring. Thus objective and subjective measures can differ by up to 2D.

• Target size, target illumination, and speed of target approach will affect the measurement of the
• amplitude of accommodation. International Centre for Eyecare Education

the RAF-rule is moved too fast towards or away from the patient

• The minus-lens-to-blur method generally produces a lower amp of acc because of the unnatural manner of

the procedure. Due to the minification of the target through a minus lenses  brain perceiving it further away and is difficult to see

• Implications of the Amplitude of Accommodation

Accommodative insufficiency

• This is a situation in which accommodation is persistently lower than expected for the patient’s age
• A reduction of the amp of acc by 2.00D or more is a hallmark
• Accommodative insufficiency would present as general asthenopia
• There may be subcategories of accommodative insufficiency, viz

o Ill-sustained accommodation: which is accommodation which is initially sustained, however, over time, it cannot be maintained. It is sometimes referred to as accommodative fatigue o Paralysis of `accommodation: is a condition in which the amp of acc is markedly reduced or totally absent. It is usually the result of an organic condition or head trauma o Unequal accommodation: is an amp that is a difference of at least 0.50D or more between the 2 eyes.

• It can be due to an organic condition, head trauma or functional amblyopia

Accommodative excess

• This refers to a condition in which accommodation is persistently higher than that expected for the patient’s

age

• More recent definitions regard this condition as the inability to relax accommodation readily, leading to a
• spasm of accommodation

Accommodative infacility

• This condition is characterized by a slowing of the dynamics of accommodation, viz. latency, time constant
• and peak velocity.
• Change in accommodation only occurs with effort and difficulty.
• The most common symptom experienced by patients is the difficulty to change focus from distance to near
• and vice versa.
• Factors Affecting the Amplitude of Accommodation

1. The original refractive state: A myope may show a greater amp of acc than a hyperope.

2. General health of the patient: The amp may be reduced and poorly sustained if the patient is ill. Systemic conditions that can affect the amplitude of accommodation include anaemia, influenza, mumps, measles, whooping cough, tonsillitis, multiple sclerosis, mysasthenia gravis, myotonic dystrophy, diabetes, whiplash injuries, secondary glaucoma, Parinaud’s syndrome. Sinusitus can also affect accommodation without affecting pupil reflexes.

3. Vascular and Glandular defects: Compromise of the vascular supply to the ciliary body may affect the accommodative effort.

4. Presbyopia: Loss of elasticity of the crystalline lens 5. Drugs: Certain drugs that affect the autonomic nervous system will have an effect on the accommodation e.g. Antihistamines, tranquilizers, drugs used for Parkinson’s disease.

6. Convergence problems: Any problems with convergence can affect the binocular accommodative effort.

7. Method of measurement: Different techniques may produce different amps of acc.

PRESBYOPIA

“Presbyopia refers to the gradual, naturally occurring, age-related, irreversible reduction in maximal accommodative amplitude resulting in symptoms of blur and ocular discomfort or asthenopia at the customary near working distance”.

In healthy individuals, the amplitude of accommodation diminishes gradually from the first few years of life until 55 years at which time it is considered to be zero. When this amp of acc is insufficient for comfortable, clear vision at his or her customary working distance, the individual suffers from presbyopia.

It has been documented that most people become presbyopic around the age of 40-45 years. Presbyopia occurs earlier in some ethnic groups, people with short arms or working distances, and hyperopes. The reason this occurs is due to a loss of accommodative effort. These patients therefore require additional plus power than expected.

Research has found that the time of appearance and progression of presbyopia depends on various factors including:

• Patient’s habitual working distance
• Distance refractive error
• Visual needs of the individual
• Other factors such as race, gender, illumination conditions, ambient temperature and geographic factors

There are various methods that one may utilize to determine the presbyopic addition for a patient, namely:

• Tentative add determination
• Determining the patient’s exact requirements
• Fused cross cyl method

The determination of the final prescription that goes into the patient’s spectacles is given by the refinement of the add determined by whichever method

• Methods to Determine the Presbyopic Correction

Tentative add determination

The tentative add determination involves the use of a formula to determine the presbyopic addition that the patient would require. It is given by the formula: International Centre for Eyecare Education This formula has been suggested for patients less than 55 years of age and who have an average working distance of 40cm. For patients greater than 55 years of age, accommodation is virtually zero and therefore the formula above does not hold.

Amplitude of accommodation

This method assumes that the prescription of addition should not use more than ½ to 2/3 of the total amplitude of accommodation. The amplitude is determined as instructed in the section on accommodation. This value is used in a formula to determine the add that should be prescribed for a working distance of 40cm.

Age expected addition

Several authors have suggested a table of expected adds based on the age of the patient. The table provided below is from a recent article by Antona et al in Clinical and Experimental Optometry 2008; 91(3).

• Age in years Tentative add
• 40 – 42 +0.75 D
• 43 – 45 +1.00 D
• 46 – 47 +1.25 D
• 48 – 50 +1.50 D
• 51 – 52 +1.75 D
• 53 – 55 +2.00 D
• 56 – 57 +2.25 D
• 58 – 60 +2.50 D

Fused cross-cylinder method

The fused cross-cylinder test is a subjective evaluation of the accuracy of the accommodative response under binocular viewing conditions. It is a functional analysis that is performed when over- or under-accommodation is suspected, and it can be used to determine a tentative near addition for a presbyopic patient.

• Equipment
• Phoropter with ±0.50 D cylinder lenses
• Near point rod
• Cross cylinder grid target (which is a grid of vertical and horizontal sets of lines, Fig. 3.3).
• Adjustable illumination
• Add = 2.50D – 2/3 Amp of Acc
• Tentative add = Age/10 – 3.50 D International Centre for Eyecare Education
• Figure 3.3 Cross-cylinder grid target

Procedure

1. The patient is seated behind the phoropter with the best corrected distance Rx in place and near PD set.

2. The fused crossed cylinder is placed before each eye with the minus cylinder axis orientated vertically (red dots on 90 orientation).

3. The patient is directed to the grid target, which is placed at a distance of 40cm in a dimly lit room.

4. The test is performed binocularly.

5. With cylinders in place, the patient is asked to report which of the lines on the grid appear sharper and more distinct, the vertical (lines going up and down) or horizontal (lines going across).

6. If the minus cylinders are oriented to a vertical axis, then the horizontal lines are expected to appear sharper than the vertical lines.

7. If the patient reports that the vertical lines are sharper than the horizontal lines, then reduce the illumination in the room before proceeding to binocularly adding lenses before the eyes. If the patient continues to report that the vertical lines are still sharper than the horizontal lines, flip the cylinders before the eyes to an axis orientation of 180. If the patient still reports that the vertical lines are sharper, then terminate the test and record that the patient has a “vertical preference”. If with the new axis orientation, the patient reports that the horizontal lines are sharper than the vertical lines, then the practitioner should record the findings as “lead of accommodation”, “minus add indicated” or “minus projection”.

8. If upon starting with the minus cylinder axis oriented vertically, the patient reports that the horizontal lines are sharper than the vertical lines or that both sets of lines appear equally clear, then the practitioner may add +0.25DS lenses binocularly before the eyes until the patient reports that the vertical lines appear sharper than the horizontal lines.

9. At this point, the practitioner then decreases the amount of added plus lenses again in +0.25DS steps until both sets of lines appear equally clear.

10. If equality cannot be reached, then the practitioner must stop at the highest plus lens that result in the patient perceiving the horizontal lines sharper.

Recording findings

The final amount of added plus lenses are recorded, relative to the patient’s distance prescription. In some cases, the practitioner may have to record “minus add indicated” as explained above.

• Example
• FCC = +1.00DS International Centre for Eyecare Education
• Expected findings
• Non-presbyopic patients: +0.50DS ± 0.25DS (Scheiman and Wick, 2008)

Presbyopic patients: the lag of accommodation is expected to increase with age.

RELATIVE ACCOMMODATION

Relative accommodation is a measure of the patient’s ability to relax or stimulate accommodation to a fixed vergence target. These tests inform the practitioner of the interaction between the patient’s accommodativeconvergence system.

Negative Relative Accommodation (NRA) is an indication of the patient’s ability to relax accommodation for a fixed vergences demand.

Positive Relative Accommodation (PRA) is an indication of the patient’s ability to stimulate accommodation with a fixed vergence demand.

• Equipment
• Phoropter
• Near reading card
• Near point rod
• Illumination source

Procedure

1. The patient is seated usually behind a phoropter with the distance Rx in place and near PD set. In the case of presbyopic patients, their tentative near add is placed in the phoropter as well.

2. A near target is presented to the patient at 40cm.

3. The patient’s attention is directed to a row of letters that is 1 or 2 lines better than the near VA on the near card. One must ensure that these letters are clear before beginning the test since the end point of the test is to determine the lenses that make the letters blurry. If for some reason, the letters are not clear, then the practitioner must add lenses in +0.25DS steps until the letters become clear. This additional lens power now becomes the tentative near addition. In some cases, if clarity cannot be obtained even with the addition of the additional plus lenses, then the practitioner must terminate the test and record that the NRA/PRA cannot be performed.

4. The NRA is performed first. Lenses are added before the patient in increments of +0.25DS until the patient reports the first sustained blur.

5. Once the NRA is determined, proceed to determine the PRA. Lenses are added before the patient in increments of -0.25DS until the patient reports the first sustained blur.

Recording findings

The practitioner must take note of the added plus and minus lenses for the NRA and PRA respectively. This is determined relative to the starting point of either the distance refraction or the tentative near add that was placed in the phoropter. International Centre for Eyecare Education Examples of recording

NRA/PRA: +2.00/-2.25

• NRA/PRA: +2.00/-2.25 through a tentative add of +1.50DS
• Expected findings: Pre-presbyopic patients
• NRA: +2.00 ± 0.50D PRA: -2.37 ± 1.00D (Scheiman and Wick, 2008)

Expected findings: Presbyopic patients According to Carlson and Kurtz (1996), the NRA and PRA may vary widely in presbyopic patients. However, the sum of the add and NRA should not exceed +2.50DS.

Values of less than 1.50D for either the NRA or PRA, or a difference of 1.00D are indications of possible inadequate accommodation function.

DETERMINATION OF AC/A RATIO

Purpose

The determination of the AC/A ratio involves the determination of the change in accommodative convergence that occurs when a patient accommodates or relaxes accommodation by a given amount.

The AC/A ratio is a key element that is used in the determination of an appropriate management plan for a given patient condition. For example, a patient who presents with esophoria at near and a high AC/A ratio would benefit from the practitioner simply prescribing spectacle lenses as opposed to prisms. However, if the same patient had a low AC/A ratio, it would be advisable for the practitioner to prescribe prisms or vision therapy as opposed to lenses.

• The AC/A ratio may be calculated or performed practically:
• 1. Calculated AC/A ratio
• The calculated AC/A ratio is determined using the following formula:
• Where:
• IPD = Interpupillary distance (in centimeters)
• NFD = near fixation distance (in meters)
• Hn = near phoria (eso is plus and exo is minus)
• Hf = distance phoria (eso is plus and exo is minus)
• For example: IPD = 60mm, patient’s phoria is 4XOP at distance and 12XOP at near (40cm).
• Calculated AC/A = 6.0 + (0.4) (–12 + 4)  AC/A = 6.0 + (-3.2)  AC/A = 2.8 : 1
• AC/A = IPD (cm) + NFD(m) (Hn – Hf) International Centre for Eyecare Education

2. Gradient AC/A ratio

The gradient method of AC/A ratio determination involves measuring the phoria and then measuring it a second time using a lens of known power, either -1.00DS or -2.00Ds. The change in the phoria that is observed with the added minus lenses gives the AC/A ratio.

For example: If the near phoria was 2SOP with the patients distance Rx and with an added -1.00DS lens is 7SOP, then it implies that the change in the phoria is 5 and the AC/A ratio is 5:1.

The calculated AC/A ratio is usually larger than the gradient method because of the effect of proximal vergence with has an effect on the near phoria measurement.

Expected findings: The normal AC/A ratio is expected to be 4:1 (±2)International Centre for Eyecare Education

SECTION B – VISUAL FUNCTION ASSESSMENT