Abnormal flash but normal pattern VEP in a cavernous sinus meningioma.

Abstract

The visual evoked cortical potential (VEP) to pattern reversal stimulation is a sensitive indicator of optic nerve function, and has the advantage that there is a highly reproducible waveform across subjects. VEPs can also be recorded to diffuse flash stimulation, and although flash VEP waveforms display considerable inter-subject variability, there is low interocular asymmetry enabling interocular comparison [1,2]. Usually, the pattern reversal VEP is much more sensitive than the flash VEP in revealing optic neuropathy. A 52 year old white female presented with nonspecific visual symptoms (intermittent blurring of vision) in her left eye. On examination of the left eye there was a stye on the lower lid and questionable proptosis; examination of the right eye was normal. Visual acuity, visual fields, colour vision (Ishihara) and fundus examination were unremarkable in both eyes. Electrophysiology was performed in an attempt to exclude significant underlying pathology and provide reassurance. Initial findings are shown in Figure 1a. The pattern reversal VEPs from both right and left eyes are within normal limits, although there is slight interocular waveform asymmetry. Flash VEP however, is grossly abnormal from the left eye with amplitude reduction and simplification of the waveform relative to that from the right eye. Pattern and full-field ERGs were normal (not shown). The findings therefore suggested possible left optic nerve dysfunction. MRI scan demonstrated a cavernous sinus meningioma extending into the left orbit. No surgical intervention was undertaken. At 14 month follow-up left visual acuity had fallen to 6/9; the repeat electrophysiological data are shown in Figure 1b. Left eye pattern reversal VEP now shows delay and degradation of the waveform. Flash VEPs continue to be abnormal. Pattern ERG shows N95 component loss in keeping with presumed retrograde degeneration to the retinal ganglion cells. The combination of an abnormal flash VEP with a normal pattern VEP is very unusual. We have been unable to identify any previous reports detailing such a combination, but a similar pattern of abnormality can occur in nerve sheath meningioma, retinal disease, or very rarely in optic nerve demyelination (unpublished observations). This case demonstrates the complimentary nature of pattern and flash stimulation in anterior visual pathway diagnosis.