typically pendular, slow in frequency, and similar in amplitude between eyes. See-saw nystagmus may be congenital, but it is most commonly observed in patients with large tumors of the parasellar region that impinge on the third ventricle. Craniopharyngioma is a frequent cause. Other parasellar– diencephalic tumors and trauma may also produce see-saw nystagmus; congenital achiasma is a rare cause. Retinitis pigmentosa, albinism, and optic nerve hypoplasia also have been observed with seesaw nystagmus. There may be associated vision loss, often bitemporal hemianopia. Asymmetric vision loss may influence the amplitude of the eye movements (ie, the amplitude may be larger in the poorer-seeing eye).
Dissociated Nystagmus
Dissociated nystagmus is characterized by a difference between the 2 eyes in the amplitude of the ocular oscillation. Perhaps the most common form of this disorder is one that is associated with lesions of the medial longitudinal fasciculus (MLF), which produce an internuclear ophthalmoplegia (INO; see Chapter 8). Isolated slowing of adduction of the eye ipsilateral to an MLF lesion is the primary feature required to establish a diagnosis of INO. In addition, nystagmus of the abducting eye often occurs when gaze is directed to the side opposite the lesion. One explanation for this pattern of dissociated nystagmus is the development of increased neural pulsing in an attempt to overcome the adduction weakness. According to Hering’s law, the increased neural signaling would also be delivered to the contralateral yoke muscle, which would create excessive saccadic movements in the contralateral lateral rectus muscle.
Saccadic Intrusions
Several forms of saccadic intrusions have been identified that are based on eye movement recordings (Fig 9-3). Two classes are distinguished by the presence or absence of an intersaccadic interval—the temporal separation between sequential saccades lasting 180–200 milliseconds.
Figure 9-3 Schematic representation of saccadic intrusions and oscillations. The baseline of each graph represents ontarget fixation. A, Square-wave jerks: small, uncalled-for saccades away from and back to the target position. B, Macrosquare-wave jerks. C, Macrosaccadic oscillations: hypermetric saccades to either side of the target position. D, Ocular flutter: back-to-back, to-and-fro saccades without an intersaccadic interval. (Modified with permission from Leigh RJ, Zee
DS. The Neurology of Eye Movements. 3rd ed. Contemporary Neurology Series. New York: Oxford University Press; 1999.)
Saccadic Intrusions With Normal Intersaccadic Intervals
The most common saccadic intrusions are square-wave jerks, which have a normal intersaccadic interval (amplitude typically <2°; latency to refixation: 200 ms). Macrosquare-wave jerks are much less common but also include an intersaccadic interval (amplitude 5°–15°; latency to refixation: 70– 150 ms). The smaller-amplitude square-wave jerks may occur normally in low frequencies in older people. The larger-amplitude macrosquare-wave jerks may occur normally in low frequencies in older people. The larger-amplitude macrosquare-wave jerks tend to have a slightly higher frequency and are always pathologic; they are observed most frequently in patients with cerebellar disease or multiple sclerosis. Lesions that disrupt the fastigial nucleus of the cerebellum or the superior colliculus, or their interconnecting fibers, may give rise to these saccadic intrusions. The abnormal eye movements may be caused by an alteration of the omnipause neurons of the pons, which would lower the typically high threshold for initiation of saccadic eye movements during attempted fixation.
Macrosaccadic oscillation is another saccadic intrusion that breaks fixation. It differs from square-wave jerks in that the oscillations extend equally to one side and then the other side of the