Periodic alternating nystagmus (PAN) is a remarkable horizontal jerking that periodically (every 90 s, or so) changes direction, interposed with a brief neutral period during which the eyes show no nystagmus, or jerk downward. PAN is seen with lesions in the lower brainstem but has also been reported with Creutzfeldt-Jakob disease, hepatic encephalopathy, lesions of the cerebellar nodulus, carcinomatous meningitis, anti-GAD antibodies, and varied other processes. A congenital form is associated with albinism. It differs from ping-pong gaze, which is a saccadic variant with a more rapid alternating of gaze from side to side and usually the result of severe bilateral hemispheric disease.
So-called oculopalatal tremor is caused by a lesion of the central tegmental tract and may be accompanied by a pendular nystagmus that has the same beat as the palatal and pharyngeal muscles, as discussed in Chap. 4.
Roving conjugate eye movements are characteristic of light coma. Horizontal ocular deviations that shift every few seconds from side to side (ping-pong gaze) is a form of roving eye movement that occurs with bihemispheric infarctions or sometimes with posterior fossa lesions. Fisher has noted a similar slower, side-to-side pendular oscillation of the eyes (“windshield-wiper eyes”). This phenomenon has been associated with bilateral hemispheric lesions that have presumably released a brainstem oscillatory pacemaker.
Ocular bobbing is a term coined by Fisher to describe a distinctive spontaneous fast downward jerk of the eyes followed by a slow upward drift to the midposition. It is observed in comatose patients in whom horizontal eye movements have been obliterated by large destructive lesions of the pons, less often of the cerebellum. The movements may be disconjugate in the vertical plane, especially if there is an associated third-nerve palsy on one side.
Other spontaneous vertical eye movements have been given a variety of confusing names: atypical bobbing, inverse bobbing, reverse bobbing, and ocular dipping. For the most part, they are observed in coma of metabolic or anoxic origin in which reflexive horizontal eye movements may be preserved (in distinction to ocular bobbing). Ocular dipping describes an arrhythmic slow conjugate downward movement followed in several seconds by a more rapid upward movement; it occurs spontaneously but may at times be elicited by moving the limbs or neck. Anoxic encephalopathy has been the most common cause, but a few cases have followed drug overdose (Ropper, 1981).
Oculogyric crisis, formerly associated with postencephalitic parkinsonism, is now most often caused by phenothiazine drugs, as discussed earlier.
Saccadic Intrusions (Opsoclonus, Ocular Flutter, and Square Wave Jerks)