Atypical voluntary nystagmus.

Department of Otology and Laryngology, Massachusetts Eye and Ear Infirmary, Boston, MA 02114, USA.
Neurology (Impact Factor: 8.3). 03/2009; 72(5):467-9. DOI: 10.1212/01.wnl.0000341876.76523.99
Source: PubMed
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    ABSTRACT: The proposal that there is an inherent capability in humans to produce bursts of fluttering saccades was tested by comparing Purkinje image eye movement recordings in subjects with voluntary nystagmus and control subjects. Voluntary nystagmus is composed of recurrent saccades without an intersaccade interval and has been proposed to be an inherited event. No difference in saccade peak velocity-amplitude curves or microsaccades during visual fixation was found between the two groups. With training control subjects learned to produce runs of saccadic flutter identical to voluntary nystagmus. This learned flutter was composed of recurrent complete saccades rather than saccades interrupted in midflight. Voluntary flutter is thus not a genetic trait but a learned event that is usually undeveloped in man. These observations can be explained by the Robinson model of saccade generation and indicate that similar models must have an inherent ability to produce saccadic flutter.
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    ABSTRACT: The ability to hold an eccentric position of gaze has been attributed to a brainstem network termed the ocular motor neural integrator. For this integrator to function properly, an intact cerebellum is necessary. This report describes a patient with cerebellar dysfunction who showed an unusual form of nystagmus: each slow phase had a waveform of increasing velocity. This contrasts with gaze paretic nystagmus, the more typical manifestation of cerebellar disorder, in which each slow phase has a waveform of decreasing velocity. Based on these observations and results from basic research, we propose that (1) the cerebellum controls neural integration in the brainstem by a positive feedback loop, and (2) pathological alterations in the strength of transmission (or gain) through the feedback loop cause the eyes to drift off target, with either an exponentially increasing (gain too high) or decreasing (gain too low) velocity.
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