Three members of the same family with hereditary congenital nystagmus (CN) were studied. Nystagmus amplitude, frequency, and intensity functions determined the gaze angle with least nystagmus. Visual acuity was increased in all cases by the use of version or composite prisms. In both pendular and jerk forms of CN, the fovea oscillated on alternate sides of the fixation spot and exhibited simultaneous bilateral shifts in the resulting fixation bias. The nystagmus waveforms were often complex and required velocity information for distinction of type and direction. The "attempt" to fixate was a driving stimulus for CN and ambient illumination or eyelid position were unrelated 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.
Dell'Osso LF, Flynn JT, Daroff RB. Hereditary Congenital NystagmusAn Intrafamilial Study. Arch Ophthalmol. 1974;92(5):366-374. doi:10.1001/archopht.1974.01010010378002