In some cases, the hypertropic eye has been known to alternate with the direction of gaze (“alternating skew”), with the right eye higher in right gaze and the left eye higher in left gaze. A cerebellar or other posterior fossa lesion is the usual cause. A mechanism for this sign has been proposed based on otolithic influences on cerebellar centers. Ford and coworkers have described a rare form of skew deviation caused by a monocular palsy of elevation stemming from a lesion immediately rostral to the ipsilateral oculomotor nucleus; a lesion of upgaze efferents from the ipsilateral riMLF was postulated but an abnormality of the vertical gaze holding mechanism related to the function of the INC is an alternative explanation.
Among the most unusual of the complex ocular disturbances is a subjective tilting of the entire visual field that may produce any angle of divergence but most often creates an illusion of environmental tilting of 45 to 90 degrees (tortopia) or of 180-degree vision (upside-down vision). Objects normally on the floor, such as chairs and tables, are perceived to be on the wall or ceiling. Although this symptom may arise as a result of a lesion of the parietal lobe or in the otolithic (utricular) apparatus, it has most often been associated in our experience with an internuclear ophthalmoplegia and slight skew deviation. Presumably the vestibular-otolithic nucleus or its connections in the MLF that maintain the vertical position of the ipsilateral eye are impaired. Lateral medullary infarction has been a common cause; other cases may be migrainous (Ropper, 1983). Ocular lateropulsion, in which the eyes are driven to one side and the patient feels pushed or pulled in the same direction, is another component in some cases of lateral medullary infarction as discussed in Chap. 33.
Nystagmus refers to involuntary rhythmic movements of the eyes and is of two general types. In the more common jerk nystagmus, the movements alternate between a slow component and a fast corrective component, or jerk, in the opposite direction. In pendular nystagmus, the oscillations are roughly equal in rate in both directions, although on lateral gaze the pendular type may be converted to the jerk type with the fast component to the side of gaze. Nystagmus reflects an imbalance in one or more of the systems that maintain stability of gaze. The causes may therefore be viewed as originating in (1) structures that maintain steadiness of gaze in the primary position; (2) the system for holding eccentric gaze—the so-called neural integrator; or (3) the VOR system, which maintains foveal fixation of images as the head moves. For the purposes of clinical work, however, certain types of nystagmus are identified as corresponding to lesions in specific structures within each of these systems, and it is this approach that we take in the following pages. One classification considers nystagmus as the result of a disturbance in the vestibular apparatus or its brainstem nuclei, the cerebellum, or a number of specific regions of the brainstem such as the MLF.
In testing for nystagmus, the eyes should be examined first in the central position and then during upward, downward, and lateral movements. Jerk nystagmus is the more common type. It may be horizontal or vertical and is elicited particularly on ocular movement in these planes, or it may be rotatory and, rarely, retractory or vergent. By custom the direction of the nystagmus is designated according to the direction of the fast component (referred to as “beating” to that side). There are several varieties of jerk nystagmus. Some occur spontaneously; others are readily induced in normal persons by drugs or by labyrinthine or visual stimulation.