Time-varying, slow-phase component interaction in congenital nystagmus

We investigated the nystagmus of a 12-year-old boy with suspected X-linked congenital nystagmus (CN) and exophoria to determine the underlying mechanisms and component signals in the 'dual-velocity' and other slow phases of his Asymmetric (a)Periodic Alternating Nystagmus (APAN). Fast Fourier transforms (FFT) were performed on the waveforms and residual data after subtracting a sawtooth waveform whose amplitude and frequency matched those of the jerk nystagmus. The FFT analyses identified two frequency components (jerk--4 Hz and pendular--4 and 8 Hz, variable) that varied differently in intensity and frequency/phase over the time-course of the APAN. We synthesized each of the patient's slow phases using summation of sawtooth and sinusoidal waveforms. The resulting waveforms included jerk (with different slow-phase appearances), dual jerk, and pendular. We demonstrated that the pendular nystagmus seen during the neutral phase of APAN and the appearance of either decelerating (mimicking latent nystagmus), dual-velocity, or dual-jerk slow phases can be explained and produced by the summation of linear and pendular components of variable amplitudes and frequencies/phases. Thus, one mechanism may be responsible for all the variation seen in this patient's slow phases, rather than the less parsimonious hypothesis of a switched-tonic-imbalance mechanism that we had originally suggested to simulate the dual-velocity waveform.