Figure 7-8 A, The major amblyoscope. Targets can be placed in each arm of the device to be presented separately to each eye. The arms can then be moved to compensate for ocular misalignment. B, Testing with a major amblyoscope for retinal correspondence in a patient with 20Δ of esotropia. NRC = (harmonious) normal retinal correspondence; ARC = anomalous retinal correspondence; UHARC = unharmonious anomalous retinal correspondence. (Part A courtesy of Steven Archer, MD; part B

modified with permission from von Noorden GK, Campos EC. Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 6th ed. St Louis: Mosby; 2002:229.)

Assessment of Eye Movements

Ocular Rotations

Generally, when eye movements are assessed, versions are tested first. The examiner should pay particular attention to the movements of both eyes into the 9 diagnostic positions of gaze. Limitations in movement into these positions and asymmetric excursion of the 2 eyes (such as “overaction”)

should be noted. Spinning the child, or provoking the doll’s head maneuver, may be helpful in eliciting vestibular-stimulated eye movements. If versions are not full, duction movements should be tested for each eye separately. BCSC Section 5, Neuro-Ophthalmology, also discusses testing of the ocular motility system.

Convergence

Alignment at near fixation is usually measured at 33 cm directly in front of the patient in the horizontal plane. Comparing the alignment in primary position at both distance and near fixation helps the examiner assess the accommodative convergence (synkinetic near) reflex. The near point of convergence is determined by placing a fixation object at 40 cm in the midsagittal plane of the patient’s head. As the patient fixates on the object, it is moved toward the patient until 1 eye loses fixation and turns out. The point at which this occurs is the near point of convergence. The eye that maintains fixation is considered the dominant eye. The normal near point of convergence is 8–10 cm or less. This determination does not distinguish between fusional convergence and accommodative convergence.

Accommodative convergence/accommodation ratio

The accommodative convergence/accommodation (AC/A) ratio is defined as the amount of convergence (in prism diopters) per unit change in accommodation (in diopters). There are 2 methods of clinical measurement (see also BCSC Section 3, Clinical Optics): the gradient method and the heterophoria method.

1.The gradient method arrives at the AC/A ratio by dividing the change in deviation in prism diopters by the change in lens power. An accommodative target must be used, and the working distance (typically .33 m or 6 m) is held constant. Plus or minus lenses (eg, +1.00, +2.00, +3.00, – 1.00, –2.00, –3.00) are used to vary the accommodative requirement. This method measures the stimulus AC/A ratio, which is not necessarily identical to the response AC/A ratio. The latter can be determined only with the use of an optometer that records the change in accommodation actually produced.

2.In the heterophoria method, the distance and near deviations are used, along with the interpupillary distance, to calculate the AC/A ratio. A rough clinical estimate can be made by comparing distance and near fixation alignment. If the patient is more exotropic or less esotropic at near fixation, too little convergence, or a low AC/A ratio, is present; if the patient is more esotropic or less exotropic at near, a high AC/A ratio is present. In accommodative esotropia, a difference in esotropia of 10Δ or more between distance and near fixation is considered to represent a high AC/A ratio.

Fusional Vergence

Vergences are movements of the 2 eyes in opposite directions; they are also discussed in Chapter 5. Fusional vergences are motor responses that serve to eliminate horizontal, vertical, or, to a limited degree, torsional image disparity. They can be grouped by the following functions:

Fusional convergence eliminates bitemporal retinal disparity and controls an exophoria. Fusional divergence eliminates binasal retinal disparity and controls an esophoria. Vertical fusional vergence controls a hyperphoria or hypophoria.

Torsional fusional vergence controls an incyclophoria or excyclophoria.

Fusional vergence can be measured by using a major amblyoscope, a rotary prism, or a bar prism