Visual evoked potentials to multiple temporal frequencies. Use in the differential diagnosis of optic neuropathy
P. Bobak, R. Friedman, M. Brigell, J. Goodwin and R. Anderson
Department of Ophthalmology, University of Illinois, Chicago.
The usefulness of the visual evoked potential (VEP) in differential
diagnosis increases when stimulus parameters such as check size and grating
orientation are varied. In this study we varied the stimulation frequency.
Temporal frequency-specific abnormalities were compared in three patient
categories, including retrobulbar optic neuritis (eight patients),
pseudotumor cerebri (11 patients), and thyroid eye disease (seven
patients). All patients had minimal clinical evidence of optic nerve damage
when tested. A 2.3 cycle-per-degree sinusoidal grating of 55% contrast was
phase reversed at either 1 or 4 Hz. The P1 latency of the 1-Hz data and the
phase at 8 Hz, the second harmonic of the 4-Hz input frequency, were
measured. In retrobulbar neuritis, latency (phase) was severely abnormal at
both temporal frequencies. In thyroid eye disease, VEP phase was abnormal
at 8 Hz while the P1 latency was normal at 1 Hz. The P1 latency and phase
were normal in most cases of pseudotumor cerebri. The results suggest
differing mechanisms for damage in compressive vs primary demyelinating
neuropathies.
