The Effects of Optic Nerve Head Drusen on the Latency of the Multifocal Visual Evoked Potential

Abstract

Purpose:: To determine the effect of optic nerve head drusen (ONHD) on the latency of the multifocal visual evoked potential (mfVEP) and to better understand the pathophysiology of this condition.

Methods:: Monocular mfVEPs were recorded from a group of 10 patients (18 eyes) with ONHD, 18 patients (26 eyes) with high tension glaucoma (HTG), 19 patients (31 eyes) with low tension glaucoma (LTG) and a control group of 19 subjects (38 eyes). Mean age was 62.0 ± 11 (LTG); 62.0 ± 10.8 (HTG); 59.7 ± 9.3 (ONHD) and 53.0 ± 9.5 (controls) years. Average mean deviation (MD) was - 6.4 ± 5 (LTG); - 6.2 ± 4.3 (HTG) - 5.2 ± 7.0 (ONHD) and - 0.5 ± 0.9 (controls) dB. Mean recorded maximum intraocular pressure (IOP) was 17.2 ± 4.9 (LTG); 26.3 ± 5.5 (HTG); 15 ± 3.7 (ONHD) and 14 ± 3.5 (controls) mmHg. Glaucoma eyes were defined by the presence of a glaucomatous disc and an abnormal 24-2 visual field as described previously [1] and were characterized as LTG or HTG based on IOP. The mfVEPs were elicited by a 60-sector dartboard display. The mfVEPs monocular latencies were measured using custom software [2] and analyzed in two ways: 1) average relative latency (i.e. the latency relative to a normative group [3]) of all individual mfVEPs for an eye, and 2) percent of the 60-sectors with significantly delayed latencies.

Results:: For 50% (9/18) of the ONHD eyes, mfVEP latency values fell above the 95% limits for the controls, while for 39% (7/18) of the ONHD eyes, the percent of delayed sectors fell above the 95% limit. All patient groups showed an increase in mfVEP latency with average relative latencies, and percent of sectors delayed, of 4.5 ms & 15% (ONHD), 2.8 ms & 9.3% (LTG) and 5.5 ms & 20.9% (HTG) compared to 1 ms and 4.9% (controls). When one eye (the eye with the worse MD on perimetry) of each subject was analyzed, the difference between ONHD (4.5 ms; 15.6%), HTG (5 ms; 20.5%) and controls (1 ms; 4.5%) was significant (Wilcoxon test; p < 0.05).

Conclusions:: ONHD causes an optic neuropathy that produces significant latency delays on the mfVEP. These delays are greater than those seen in LTG and ischemic optic neuropathy [4], similar to those for HTG, where mechanical compression may play an important role, and below those seen in compressive optic neuropathy [5] were mechanical compression is involved. Mechanical compression may contribute to visual field loss in patients with ONHD. 1. Grippo et al IOVS (2006); 2. Hood et al Doc. Ophthal. (2004); 3. Fortune et al Doc. Ophthal. (2004); 4. Odel et al ARVO 2005; 5. Semela et al BJO (2006).

Keywords: electrophysiology: clinical • neuro-ophthalmology: optic nerve