and gradually increasing the prism power until diplopia occurs. Accommodation must be controlled during fusional vergence testing. Normal fusional vergence amplitudes are listed in Table 7-1. Fusional vergence can be altered by the following:

Table 7-1

Compensatory mechanisms: As a tendency to deviate evolves, a larger-than-normal fusional vergence develops for that deviation. Very large fusional vergences are common in compensated, long-standing vertical deviations and in exodeviations.

Change in visual acuity: An improvement in acuity may improve the fusional vergence mechanism and change a symptomatic intermittent deviation to an asymptomatic heterophoria. State of awareness: Fatigue, illness, or drug or alcohol ingestion may decrease the fusional vergence mechanism, converting a heterophoria to a heterotropia.

Orthoptics: Orthoptic exercises may increase the magnitude of the fusional vergence mechanism (mainly fusional convergence). This treatment works best for near fusional convergence, particularly convergence insufficiency.

Optical stimulation of fusional vergence: In controlled accommodative esotropia, reducing the strength of the hyperopic or bifocal correction induces an esophoria that stimulates fusional divergence. In other cases, the power of prisms used to control diplopia may be gradually reduced to stimulate compensatory fusional vergence.

Special Tests

Motor Tests

Special motor tests include the forced-duction test, active force generation, and saccadic velocity measurement. See also BCSC Section 5, Neuro-Ophthalmology.

In the forced-duction test, forceps are used to move the eye into various positions and thereby determine resistance to passive movement. This test is usually performed at the time of surgery but can sometimes be performed in the clinic with topical anesthesia in cooperative patients.

Active force generation assesses the relative strength of a muscle. The patient is asked to move the eye in a given direction while the observer grasps the eye with an instrument. If the muscle tested is paretic, the examiner feels less-than-normal tension.

Saccadic velocity measurement can be performed using a special instrument that graphically records the speed and direction of eye movement. This test is useful for distinguishing paralysis from restriction. If the muscle is paralyzed, the saccadic velocity remains low throughout the movement of the involved eye, whereas if the muscle is restricted, the velocity is initially normal but rapidly decelerates when the eye reaches the limit of its movement. Clinical observation can allow qualitative assessment of saccadic velocity; with this method, floating saccades indicate muscle paresis.

Assessment of the Field of Single Binocular Vision

The field of single binocular vision may be tested on a Goldmann perimeter. This is useful for following the recovery of a paretic muscle or for measuring the outcome of surgery to alleviate

diplopia. A small white test object is followed by both eyes in the cardinal positions throughout the visual field. When the patient indicates that the test object is seen double, the point is plotted. The field of binocular vision normally measures about 45°–50° from the fixation point except where it is blocked by the nose (Fig 7-9). Weighted templates that reflect the greater importance of single binocular vision in primary and reading positions can be used to quantify the diplopia field.

Figure 7-9 The normal field of single binocular vision.

Sullivan TJ, Kraft SP, Burack C, O’Reilly C. A functional scoring method for the field of binocular single vision. Ophthalmology. 1992;99(4):575–581.

3-Step Test

There are 8 cyclovertical muscles (4 in each eye): the 2 depressors of each eye are the inferior rectus (IR) and superior oblique (SO) muscles; the 2 elevators of each eye are the superior rectus (SR) and inferior oblique (IO) muscles. Weakness of the cyclovertical muscles, especially the superior oblique muscles, is often responsible for hyperdeviations. The 3-step test is an algorithm that can be used to help identify the weak cyclovertical muscle. As helpful as the test is, however, it is not always diagnostic and can be misleading, especially in patients in whom more than 1 muscle is paralyzed, in patients who have undergone strabismus surgery, in the presence of a skew deviation, and in the

presence of restrictions or dissociated vertical deviation (see Chapter 11). The 3-step test is performed as outlined in the following subsections (Fig 7-10; also see Chapter 11, Fig 11-4).