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Head shaking and vestibulo-ocular reflex in congenital nystagmus.

The authors investigated the mechanisms underlying the head shaking shown by some patients with congenital nystagmus (CN). In order to improve visual function by head shaking, a patient with CN must have some visual acuity loss due to retinal image motion created by the nystagmus; an abnormal vestibulo-ocular reflex (VOR); and the head shaking must be correlated with the nystagmus. The authors measured the VOR gain (eye velocity/head velocity) and examined eye-head coordination in five patients with CN with various combinations of these three factors. One patient met all three criteria and was able to increase his acuity by shaking his head. Other patients who shook their heads either had no loss of visual acuity due to the nystagmus or had a normal VOR. In either case, head shaking was of no apparent visual benefit and may represent, instead, an associated pathologic tremor in the cephalomotor control system.