Hereditary Congenital Nystagmus

Three members of the same family with hereditary congenital nystagmus (eN) were studied. Nystagmus amplitude, fre­ quency, and Intensity functions deter­ mined the gaze angle with least nys­ tagmus. Visual acuity was increased in all cases by the use of version or composite prisms. In both pendular and jerk forms of eN, the fovea oscillated on alternate sides of the fixation spot and exhibited simultaneous bilateral shifts in the result­ Ing fixation bias. The nystagmus wave­ forms were often complex and required velocity information for distinction of type and direction. The "attempt" to fixate was a driving stimulus for eN and ambient il­ lumination or eyelid position were unre­ lated to its genesis. Pendular and jerk nystagmus are different manifestations of the same ocular motor instability and simple classification on the basis of waveform is erroneous when used to infer etiology.