1. Purpose Fixation disparity testing is designed to evaluate binocular vision under associated conditions. This is in contrast to cover testing, the von Graefe phoria test, and the modified Thorington techniques, which are done under conditions in which either one eye is covered or the eyes are ­dissociated.

(a) Fixation disparity testing is performed under binocular conditions The main deficiency of the typical phoria measurement is that the evaluation occurs under dissociated conditions. Wick (22) states that “the vergence error under binocular conditions is often not the same as it is under monocular conditions.” As a result, there are situations in which a patient may be symptomatic, but the conventional phoria/vergence analysis does not produce a clear understanding of the cause of the patient’s symptoms. Although some clinicians suggest the routine use of fixation disparity testing, we have found that in the majority of cases, phoria/vergence testing is sufficient to reach a tentative diagnosis and management plan. In those situations in which the diagnosis is unclear or a prism prescription is being considered, fixation disparity testing is a useful addition to the examination procedure.

(b) Associated phoria versus forced vergence fixation disparity assessment Various instruments are available for the evaluation of fixation disparity. Instruments, such as the Mallett unit, the American Optical vectographic slide, the Borish card, the Bernell lantern, the Wesson Card, the Sheedy Disparometer, and some computerized distance visual acuity charts (Chapter 15) can all be used to determine the associated phoria. The associated phoria is the amount of prism necessary to neutralize any perceived misalignment of the lines.

Studies suggest, however, that the use of forced vergence fixation disparity testing is more likely to yield data that are useful for determining those patients who are likely to have symptoms (23,24). The Wesson card is currently the only commercially available instrument for measuring the actual fixation disparity. Based on current information, forced vergence fixation disparity testing should be used when assessing a horizontal deviation. For a vertical deviation, associated phoria testing is sufficient.

(c) Determination of prism correction Fixation disparity is currently considered the method of choice for determining the amount of prism to prescribe for binocular disorders. Other methods tend to yield higher amounts of prism than fixation disparity analysis.

3. Expected values Refer to Chapter 15.

1. Purpose To determine the change in accommodative convergence that occurs when the patient accommodates or relaxes accommodation by a given amount.

(a) Significance in diagnosis and treatment Determination of the AC/A ratio is important in analysis of optometric data. The AC/A finding is a key characteristic in the final determination of the diagnosis. It is also one of the most important findings used to determine the appropriate management sequence for any given condition. For example, esophoria at near associated with a high AC/A ratio would generally respond well to plus lenses. If the same degree of esophoria were associated with a normal or low AC/A ratio, the recommended treatment approach would include prism correction or vision therapy or both.

(b) Calculated versus gradient AC/A ratio There are two methods for determining a patient’s AC/A ratio. The first, referred to as 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 = far phoria (eso is plus and exo is minus)

Example: IPD = 60 mm, the patient is 2 exophoric at distance and 10 exophoric at near (40 cm).

When using this formula, one should remember to use the correct signs for esophoria and exophoria. A rule of thumb is that a high AC/A ratio will result in more eso or less exo at near, and a low AC/A ratio will lead to less eso or more exo at near.

The second method, called the *gradient AC/A*, is determined by measuring the phoria a second time using $- 1.00$ or $- 2.00$ lenses. The change in the phoria, with the additional minus, is the AC/A ratio. For example, if the near phoria is 2 esophoria through the subjective finding and, with $- 1.00$, it is 7 esophoria, the AC/A ratio is 5:1.

There may be significant differences between the two methods of determining the AC/A ratio. For instance, divergence excess and convergence excess patients both have high calculated AC/A ratios, but many of these patients have approximately normal gradient AC/A ratios (20). The same phenomenon may occur with convergence insufficiency. The calculated AC/A ratio will be low, but the gradient AC/A may be normal (22). The reason for these differences is the effect of proximal convergence and the lag of accommodation. The calculated AC/A ratio is usually larger than the gradient because of the effect of proximal vergence, which affects the near phoria measurement. Because the gradient ratio is measured by testing the near phoria twice at a fixed distance, proximal vergence is held constant and theoretically does not alter the final result. The lag of accommodation also accounts for differences between the calculated and gradient AC/A ratio measurements. Although the stimulus to accommodation is 2.50 D at near, the accommodative response is typically less than the stimulus. This difference between the stimulus and response of the accommodative system is called the lag of accommodation. The lag of accommodation is generally +0.25 to +0.75 D. Because the patient will tend to underaccommodate for any given stimulus, the gradient AC/A tends to be lower than the calculated AC/A ratio.

(c) Controlling accommodation A source of measurement error in the AC/A evaluation is failure to control accommodation. The clinician should emphasize, in the instructional set, that clarity of the target is essential. It is easy to understand how variation in accommodative response from one measurement to another would adversely affect results. The gradient AC/A requires two measurements of the near phoria, first with only the subjective in place and then with −1.00 over the subjective. If a patient accurately accommodates for the first measurement, but underaccommodates for the second, the result will be an underestimation of the true AC/A ratio. It is, therefore, critical to ask the patient to maintain clarity, and it is advisable to ask the patient to read the letters periodically.

(d) Response versus stimulus AC/A ratio When evaluating the accommodative or binocular systems, we usually present the stimulus at 40 cm. This creates an accommodative demand of 2.50 D. This is referred to as the stimulus to accommodation. Although the stimulus to accommodation is 2.50 D, the accommodative response will generally be about 10% less than the stimulus (25). The expected finding for MEM retinoscopy, for example, which assesses the accommodative response, is a lag of accommodation of about +0.25 to +0.50 D. It is important to be aware of the difference between the response and stimulus to accommodation, realizing that most patients will underaccommodate by about 10%. An instance where this becomes important is when comparing the calculated AC/A ratio to the gradient AC/A ratio. The gradient AC/A ratio will tend to underestimate the AC/A ratio. For example, suppose the phoria is measured as 10 exophoria at near and, when repeated with −1.00 lenses, the phoria is 6 exophoria. Based on this information, the gradient AC/A ratio would be 4:1. However, if we assume that the patient underaccommodates by 10%, the phoria has changed by 4 Δ while accommodation has changed by 0.75 D. This would be an AC/A ratio of about 4.45:1.

3. Expected values The expected AC/A ratio is $4:1$, with a standard deviation of ± 2.