The incidence and waveform characteristics of periodic alternating nystagmus in congenital nystagmus.

PURPOSE: To investigate the incidence and waveform characteristics of periodic alternating nystagmus, (PAN) in congenital nystagmus (CN).
METHODS: In a prospective study, 18 patients with CN without associated sensory defects agreed to undergo eye movement documentation using binocular infrared oculography. Two of the 18 had a diagnosis of suspected PAN before entering the study. The patients sat in a dimly lit room and viewed an LED (4 min in diameter) located in the primary position, at a distance of 100 cm. During an 8-minute recording, patients were read a story of neutral interest to hold attention at a constant level. PAN was defined as a left-beating nystagmus, a transition phase, a right-beating nystagmus, and a final transition phase; the sequence was then repeated.
RESULTS: Seven of the 18 patients had PAN (median cycle: 223 seconds, range 180-307 seconds). The periodicity of the cycles for each adult patient was regular, although the phases within a cycle were often asymmetric. Six of the seven patients had an anomalous head posture (AHP), and in five the AHP was in only one direction. Except for one patient, the PAN waveforms had an increasing slow-phase velocity in at least one phase of the cycle; in the other phase they were linear.
CONCLUSIONS: The occurrence of PAN in CN is not as rare as previously thought and can be missed because of the long cycles and the use of only one AHP. The AHP was dependent on, and could be predicted from, the waveforms containing the longest foveation times. Although the waveforms and foveation times may differ among the phases of the PAN cycle, the periodicity of the cycle was usually regular and therefore predictable. Identification of PAN is essential in cases in which surgical treatment is considered for correction of AHPs.