If the maximum vertical separation of images occurs on looking downward and to the left and the white image is projected farther down than the red, the paretic muscle is the left inferior rectus; if the red image (from the right eye) is lower than the white, the paretic muscle is the right superior oblique. As already mentioned, correction of vertical diplopia by a tilting of the head implicates the superior oblique muscle of the opposite side (or the ipsilateral trochlear nucleus). Separation of images on looking up and to the right or left will similarly distinguish paresis of the inferior oblique and superior rectus muscles. Most patients are attentive enough to open and close each eye and determine the source of the image thrown most outward in the field of maximal separation.
There are several alternative methods for studying the relative positions of the images of the two eyes. One, a refinement of the red-glass test, is the Maddox rod, in which the occluder consists of a transparent red lens with series of parallel cylindrical bars that transform a point source of light into a red line perpendicular to the cylinder axes. The position of the red line is easily compared by the patient with the position of a white point source of light seen with the other eye. Another technique, the alternate cover test, requires less cooperation than the red-glass test and is, therefore, a passive maneuver that is more useful in the examination of children and inattentive patients. It does, however, require sufficient visual function to permit central fixation with each eye. The test consists of rapidly alternating an occluder or the examiner’s hand from one eye to another and observing the deviations from and return to the point of fixation, as described earlier in the chapter in the discussion of tropias and phorias. Measuring the prismatic correction needed to neutralize the ocular misalignment in each field of gaze with a prism bar allows the quantification of deviation and provides a method to follow diplopia over time.
The more sophisticated Lancaster test uses red/green glasses and a red and green bar of laser light projected on a screen to accomplish essentially the same result but has the advantage of reflecting the actual position and torsion of each eye. Detailed descriptions of the Maddox rod and alternate cover tests, which are the ones favored by neuroophthalmologists, can be found in the monographs of Leigh and Zee and of Glaser. In all these tests, the examiner is aided by committing to memory the cardinal actions of the ocular muscles shown in Fig. 13-6 and Table 13-2.
The red-glass and other similar tests are most useful when a single muscle is responsible for the diplopia. If testing suggests that more than one muscle is involved, myasthenia gravis and thyroid ophthalmopathy are likely causes as they affect several muscles of ocular motility. Palsy of the oculomotor nerve causes a similar circumstance.
Monocular diplopia occurs most commonly in relation to diseases of the cornea and lens rather than the retina; usually the images are overlapping or superimposed rather than discrete. In most cases, monocular diplopia can be traced to a lenticular distortion or displacement but in some, no abnormality can be found and the symptom has a nonorganic basis. Monocular diplopia has been reported in association with cerebral disease (Safran et al), but this is a rare occurrence. Occasionally, patients with homonymous scotomas caused by a lesion of the occipital lobe will see multiple images (polyopia) in the defective field of vision, particularly when the target is moving.
Causes of Individual Third, Fourth, and Sixth Nerve Palsies (Table 13-3)