Pattern electroretinogram in glaucoma

Abstract

Purpose of review: Several studies have shown that the pattern electroretinogram, a direct, objective method of measuring retinal ganglion cell function, is altered early in ocular hypertension and glaucoma. Renewed interest in the pattern electroretinogram for early detection of pre-perimetric glaucoma has been sparked by noninvasive and reproducible methods of recording using skin electrodes.

Recent findings: With the noninvasive pattern electroretinogram, response abnormalities have been detected in up to 50% of glaucoma suspects with normal standard perimetry. In early glaucoma (with either normal or high intraocular pressure), a reduction of intraocular pressure has sometimes yielded improvement in pattern electroretinogram amplitude. A prolonged steady-state stimulus presentation reduces the pattern electroretinogram amplitude and increases optic nerve blood flow in normal subjects, suggesting that sustained activity of retinal ganglion cells is physiologically associated with autoregulatory changes of the neural-vascular system. It is unknown whether this autoregulation is altered in glaucoma. The multifocal pattern electroretinogram does not seem to have an advantage over the pattern electroretinogram in the early detection of glaucoma. The photopic negative response of the diffuse flash electroretinogram has shown changes in glaucoma, but may not be able to detect retinal dysfunction in normal tension glaucoma.

Summary: The pattern electroretinogram is a noninvasive, direct, objective method that may be useful to clinicians in detecting early retinal ganglion cell dysfunction in glaucoma suspects. The pattern electroretinogram may also optimize treatment strategies based on improvement of retinal ganglion cell function.

Figure 1. Setup for the non-invasive PERG

(a) Standard skin electrodes are taped on the lower eyelids (active), temples (reference), and forehead (common ground). (b) The subject looks for 3 minutes at the center of the stimulus placed inside a ganzfeld bowl. (c) Pattern stimulus consisting of black and white stripes alternating 16.6 times per second. (d) The stimulus covers a circular retinal area with a 12.5 degree radius centered on the fovea.

Figure 2. Example of normal PERG (right eye, upper left) and abnormal PERG (left eye, bottom left)

PERGs have an approximately sinusoidal waveform, whose frequency corresponds to the reversal rate. The analyzed period (122.8 ms) exactly corresponds to two contrast reversals. PERG waveforms are automatically analyzed by Digital Fourier Transform to isolate the PERG sinusoidal component (16.6 Hz) from the background noise, and measure its amplitude in microvolts and phase in radians (latency) (right-hand tables). Amplitude and phase are also expressed in standard deviations (SDs) from age-corrected normal averages (right-hand tables) and plotted one against the other in a polar diagram (center panels). The box in the polar diagram represents the 95% (±2 SD) tolerance interval for amplitude (vertical) and phase (horizontal). In the example, the right eye has an amplitude and phase close to the age-corrected normal average. The left eye has a reduced amplitude (about −6 SDs from normal) and a delayed phase (about −8 SDs from normal). The program also calculates an index of intrinsic variability (coefficient of variation (CV) between the amplitude and phase recorded during the first 1.5 minutes and the second 1.5 minutes of the acquisition) and an index of a noise response (amplitude and phase of a difference waveform between the first 1.5 minutes and the second 1.5 minutes of the acquisition).