Control of Eye Movements—Pathology (Continued)

NEUROLOGIC DEFICITS

Eye movement disorders from brainstem involvement of the pathways subserving horizontal and vertical gaze are usually exquisitely localizing. For example, a lesion in the right abducens nucleus will cause a complete loss of gaze of either eye toward the right (usually with an associated ipsilateral lower motor neuron facial palsy because the fascicles of the facial nerve wrap around the abducens nucleus before exiting the brainstem), whereas a lesion of just to the right paramedian pontine reticular formation will cause an absence of voluntary and reflex saccades to the right, with relative preservation of the vestibulo-ocular reflex (VOR) and pursuit eye movements. A lesion of the right medial longitudinal fasciculus will disrupt only the abducens interneuron projections, and therefore the patient will have all eye movements intact except for poor adduction of the right eye (poor movement of the right eye toward the nose), a so-called internuclear ophthalmoplegia.

Vertical gaze may be selectively abnormal, with lesions in the midbrain and pretectal area, especially from compression from above, such as typically seen with pineal tumors. If the posterior commissure is primarily involved, these patients may have selective absence of upward eye movements with preservation of all other eye movements. Associated clinical abnormalities include upper lid retraction and nonreactive pupils to light with intact pupillary constriction when viewing a near target (all part of the so-called dorsal midbrain syndrome).

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