Joe Mayhew

Large Animal Neurology


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of pupillary responses to the swinging light test are useful here as in the evaluation of vision. Consider a bright and alert patient with rather complex responses to visual and light pathway testing. The patient has no menace response but has a normal palpebral response to touch, and is blind with a widely dilated pupil, all in the left eye. There is a normal menace response in the right eye, and light shone in the right eye results in the right pupil constricting and also in a positive dazzle (blink) response. When the light is then swung briskly to enter the left eye, the left pupil is and remains dilated and there is no dazzle response in the left eye. When the light is then quickly retransferred to enter the right eye, the right pupil constricts strongly again. The lesion must involve both the left retina or optic nerve and the left oculomotor parasympathetic fibers. Retrobulbar empyema, cellulitis, or neoplasm can readily explain these findings. With such lesions behind the globe, the sympathetic fibers to the dilator muscles of the pupil can also be involved, confounding this clinical picture further.

      CASE STUDY

      Picture a patient being examined for visual and light acuity, but outside in daylight with both pupils somewhat constricted. The right menace response appears to be less than that for the left eye with no convincing anisocoria detectable in shaded but bright daylight. The left pupil responds directly to light shone in the left eye. The right pupil does respond to light shone in the right eye, although being in daylight it is not possible to be convinced of any asymmetry in the rate or degree of pupillary responsiveness. Where is the lesion? With such information available, a partial lesion should be in the left central visual pathways, i.e., postchiasmal. However, now note the responses to the swinging light test using a very bright light source. Light shone in the left eye results in pupillary constriction in that eye. The light is quickly swung to be redirected into the right eye, avoiding a dazzle response. Although the right pupil is initially constricted it dilates back to its resting size as light reaches that eye! When the light is swung quickly to be redirected into the left eye again the left pupil, that may or may not initially appear somewhat dilated, responds by constricting very well. This can be repeated as the light is quickly redirected into each eye in turn, pausing long enough to observe each pupillary size and response. Also, being outside, when the left eye is covered for 10 s with a hand the right pupil dilates to a resting state. When the right eye is covered the left pupil remains constricted as appropriate for the degree of bright ambient light. Indeed, such maneuvers may allow convincing anisocoria to become more apparent with the right pupil becoming less constricted than the left pupil. At least one lesion is in the right eye or right optic nerve. When a darkened examination space becomes available, anisocoria with right relative mydriasis should then be noted. Two points are of note in this case. First, we are often not discerning enough to visually detect minor degrees of anisocoria; and second, outside, even in shaded daylight, there is enough ambient light entering the normal eye to maintain considerable pupillary constrictor tone in the blind eye.

      With anisocoria, particularly when due to partial lesions, it can sometimes be difficult to determine which pupil is abnormal. As a rule, an abnormally small pupil (i.e., sympathetic denervation) in one eye will not dilate fully in darkness, but it will respond to light directed into that eye and into the other, normal eye. In comparison, a unilateral abnormally dilated pupil (i.e., parasympathetic denervation) will be most evident in bright light and will not constrict fully in response to light shone into either eye. With reference to bright light, it should be remembered that daylight, and especially direct sunlight, is so much more powerful than any portable light source. It is thus best to perform light pathway tests both in ambient and in quite dim lighting.

Schematic illustration of bilateral pupillary dilation found in normal ambient room light (yellow bar), the possibility that it is due to a frightened or painful patient must be considered.