Purpose: To quantify the spatial and temporal nature of congenital periodic alternating nystagmus (PAN) and to test the hypothesis that PAN results from a temporal shift in the null zone.
Methods: Twenty-five subjects with oculocutaneous albinism (16 tyrosinase negative and 9 tyrosinase positive) and 7 with ocular albinism (5 x-linked and 2 autosomal recessive) participated in the study. Using infrared oculography, five features of the nystagmus were examined: amplitude, frequency, waveform, beat direction, and temporal nature of the cycle.
Results: Twelve subjects (37.5%) exhibited a PAN. The nystagmus waveforms encountered during the PAN active phases were either jerk-with-extended-foveation or pseudocycloid, whereas a variety of oscillations (including triangular and bidirectional) were evident during the quiet phases. For most of the 12 subjects, there was an asymmetric variation in nystagmus intensity during each PAN cycle. None of the 12 demonstrated a convergence null or an abnormal head posture.
Conclusions: PAN is not an uncommon oscillation among humans with albinism. Changes in gaze position markedly influenced the periodicity of the ongoing nystagmus, thus supporting the hypothesis that PAN is the result of a temporal shift in the null zone.