Head oscillations in infantile nystagmus syndrome

To quantitatively characterize eye and head oscillations in patients with infantile nystagmus syndrome (INS).

Vertical and horizontal eye and head position in INS patients were measured simultaneously at a sampling frequency of 500 Hz. Eye and head movements were measured continuously for 180 seconds. The data was calibrated and converted to angular vectors, which were further analyzed with custom software.

A total of 10 patients with INS were included: 3 with pseudo-jerk, 3 with pure-jerk, 2 with pseudo-pendular with foveating saccade form of jerk, 1 with bidirectional jerk, and 1 with asymmetric pendular nystagmus waveforms. None of the patients had periodic, aperiodic, or a superimposed latent nystagmus component. Two types of head oscillations were observed: one with a frequency of 1-3 Hz, present in all patients; and another with a frequency range of 5-8 Hz, present in only 7 patients. High-frequency oscillations were episodic, whereas low-frequency oscillations were constantly present. Peak velocity of the high-frequency head oscillations and eye velocity of nystagmus were not correlated, suggesting that these oscillations did not influence foveation.

Two types of head oscillations were found in INS patients: a constant, low-frequency and an episodic, high-frequency. Lack of correlation between the foveation period of nystagmus and peak head velocity during high-frequency oscillations suggests a coexisting pathological phenomenon rather than a compensatory mechanism used to improve the visual acuity.