Electroretinograms and Normative Data

Electroretinography (ERG) is an important clinical tool that provides an objective quantitative measure of retinal function. Decreased a and b wave amplitudes and prolonged latencies correlate to reductions in retinal function that may be the result of toxicity, ischemic damage, or retinal dystrophy (Fishman et al. 2001, Ophthalmology monographs). Furthermore, since the different components of the ERG waveform correspond to the different layers of the retina, one is able to attribute changes in the ERG to damage to specific retinal layers. This data can be a useful surrogate for retinal health, for example establishing safety profiles for drugs under clinical development. Since 1989 the International Society for Clinical Electrophysiology of Vision (ISCEV) has provided standards for the recording of ERGs. These documents provide a framework for the clinical electrophyisologist to obtain “standard” ERG recordings (Marmor 1989). The variety of permissible ERG instruments and their individual calibration requirements contributes to significant inter-laboratory variability. This variability is recognized in the ISCEV standards and partly addressed by stating “it is incumbent on the manufactures and users to verify that full-field stimulation meets the requirements of this standard.” Placing the onus for compliance on the manufactures but leaving the clinical electrophyisologist to determine if the recording standards are indeed met. ERG standards have extended beyond the aand b-wave of the full field flash ERG. The pattern ERG (PERG) is the electroretinal response to a pattern reversing stimulus such as bar gratings or checkerboard pattern. The PERG primarily reflects ganglion cell function and since it is viewed on display monitors it largely represents ganglion cell function within the macula. The peak and trough components of the PERG have been formally defined as the N35, P50 and N95 which represent the polarity (Negativity or Positivity) and the mean latency of occurrence. The ISCEV has produced standards for the recording and reporting of the PERG (Holder et al. 2007). While the PERG provides a single waveform which represents the electroretinal response of the entire macular region, the clinical multifocal electroretinogram (mERG) provides information of local retinal function. The mERG is recorded typically displaying the local retinal response of 61 or 103 local regions within the central 45 ̊ of the posterior pole. The responses represent localized cone-driven ERGs obtained in the light adapted state. While the waveform morphology of the mERG is similar to the fullfield ERG the electroretinal