Impaired Fixation Suppression
The diagnosis of impaired fixation suppression is made by observing the effect of asking the subject to fix his eyes upon a clearly visible target, upon any ongoing nystagmus. The fixation index is the ratio of nystagmus peak slow-phase velocity with fixation to nystagmus intensity with fixation removed (e.g. complete darkness or eyes closed).
For nystagmus induced by caloric input, Takemori and Cohen (Takemori and Cohen, 1974) found the normal mean fixation index to be 48 + 10 percent. Similarly, Alpert reported the normal humans had a fixation index between 60 and 70 (Alpert, 1974). However, it is questionable whether these value are appropriate normal values for several reasons. It seems likely that fixation index should increase with age, as the pursuit declines with age, but fixation data according to age are not available. Similarly, fixation suppression is probably affected by the many other variables which affect pursuit. It also seems likely that the efficiency of fixation is related to the intensity of nystagmus that one is attempting to suppress.
Peripheral vestibular nystagmus is usually well suppressed by fixation. Congenital nystagmus and many varieties of central nystagmus are unaffected by or even increased by fixation. Nystagmus which is increased by fixation is called "fixation nystagmus".
Maire and Duvoisin (1999) reported that visual suppression of static positional nystagmus is poor in central vestibular lesions and good in peripheral lesions. This seems reasonable, although it would seem unrealistic to rely on fixation suppression alone to differentiate between these two entities.
© 1996-2002, Timothy C. Hain
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