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May be increased

Hypometric, when self-generated and especially when instructed to generate successive saccades back and forth at a rapid pace; slow vertically in advanced casedes

Slow; hypometric; initiated with difficulty

Slow and hypometric in some patients

Hypometric and increased latency; deficit more marked in the presence of a visual background

Difficulty with initiation; may be slow and hypometic

Difficulty generating memorized sequences

Advanced cases make errors on antisaccade task

Increased errors on antisaccade task

Increased errors on antisaccade task

Markedly increased errors on antisaccade task; impaired predictive saccade tracking

Conditions that closely mimic PSP, causing slow vertical saccades, horizontal square-wave jerks, dysphagia, and frequent falls, include multiple infarcts affecting the basal ganglia, internal capsule, and midbrain (in the distribution of the perforating vessels arising from the proximal portions of the posterior cerebral artery) (65), infiltrative processes such as lymphoma, and paraneoplastic syndromes (66). Disorders causing the dorsal midbrain syndrome, such as tumor and hydrocephalus, can also produce a clinical picture that has some similarities to PSP with vertical-gaze palsy.

Whipple's disease can also closely mimic PSP, with vertical saccadic gaze palsy (67,68). In addition, there may be characteristic "oculomasticatory myorhythmia"— a pendular vergence oscillation with concurrent contractions of the masticatory muscles; occasionally the limb muscles also show rhythmic contractions (69). Whipple's disease can now be diagnosed using polymerase chain reaction (PCR) analysis of involved tissues (70), and can be treated with antibiotics (71).

Pure akinesia is characterized by profound disturbances of speech, handwriting, and gait, so that, for example, affected patients may suffer episodes during which they stand "frozen" for hours on end (72). Tremor, limb rigidity, akinesia, dementia, or responsiveness to levodopa are absent. Such patients may show slow and hypometric vertical saccades. The disorder may be a restricted form of PSP with a longer, more benign course.

Cortical-basal degeneration (CBD) may lead to a defect in range of vertical eye movements but it usually does not cause marked slowing of saccades; instead the defect is an increased saccadic reaction time (latency), which is evident at the bedside (21,73). Hypometria of upward saccades may occur early in the course, and should be differentiated from restricted upward range, which is present in elderly normal subjects. Other occasional findings in CBD include some decrease in horizontal saccade speed in the direction of the more affected limb, and increased distractibility during the antisaccade paradigm (looking at, instead of way from, the visual target). The other features of this degeneration—focal dystonia, ideomotor apraxia, alien hand syndrome, myoclonus, asymmetric akinetic-rigid syndrome with late onset of gait or balance disturbances—are more important in securing the diagnosis (74,75).

Multiple system atrophy (MSA) causes a parkinsonian syndrome with marked autonomic findings. Some patients show slowing of vertical saccades as well as hypometria (21,76), whereas other have cerebellar eye movement findings, including downbeat nystagmus during positional testing (77), impaired smooth ocular and eye-head pursuit.

Dementia with Lewy bodies, which causes parkinsonism and fluctuating dementia with florid visual hallucinations, may be associated with a vertical-gaze paralysis (78,79), but systematic measurements of vertical saccade are not yet available.

Other basal ganglia disorders that have been reported to show features similar to PSP include idiopathic striopallidodentate calcification (Fahr's disease) (80), and autosomal dominant parkinsonism and dementia withpallido-ponto-nigral degeneration (81). Some patients with Huntington's disease (HD) may present with vertical saccadic palsy and axial rigidity; this condition is discussed in a later section. Patients with the syndrome of amyotrophic lateral sclerosis (ALS), parkinsonism, and dementia (Lytico-Bodig), which is encountered in the inhabitants of the islands of the South Pacific Ocean, including Guam, may show more severe deficits than those with idiopathic PD, including limitation of vertical gaze (82). A variant of ALS has been described, in which slow vertical saccades, gaze-evoked nystagmus, and impaired pursuit were prominent (83). In the French West Indies, a PSP-like syndrome thought to result from neurotoxic alkaloids is associated with ingestion of herbal teas and fruits (84). Slow saccades, with a supranuclear gaze palsy, are also characteristic of Creutzfeldt-Jakob disease but usually in both the horizontal and vertical planes (85). Periodic alternating nystagmus, rebound nystagmus, and centripetal nystagmus (slow phases directed eccentrically) on lateral gaze are also characteristic of this condition owing to cerebellar involvement (85,86).

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