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Editors Selection IGR 18-4

Clinical Examination Methods: Electroretinography

Comment by Brad Fortune on:

52562 Photopic negative response versus pattern electroretinogram in early glaucoma, Preiser D; Lagrèze WA; Bach M et al., Investigative Ophthalmology and Visual Science, 2013; 54: 1182-1191

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Electrophysiological measures of vision function have for decades generated interest among glaucoma researchers and clinicians alike because of their potential to help elucidate the pathophysiological process and sequence of glaucomatous damage as well as offer a potential complementary metric of function, perhaps one more sensitive than standard automated perimetry. In a study recently published by Preiser, Lagrèze, Bach and Poloschek, the investigators set out to compare the diagnostic utility of two promising electrophysiological measures of retinal ganglion cell function: the well-established pattern electroretinogram (PERG) and the full-field flash ERG component known as the photopic negative response (PhNR) discovered relatively recently by Viswanathan and colleagues.¹ The paper by Preiser et al. Provides a good review of the field (for a primary research paper), so it is recommended reading for that reason alone. The technical aspects of the study, including the ERG recording procedures and data analysis were all performed very well, being state-of-the-art yet also easily reproduced in other laboratories or clinical settings. Preiser et al. Found that the PERG and the PhNR performed about equally well, achieving relatively high diagnostic accuracy in patients with 'manifest glaucoma' (documented visual field defects) and above chance in patients with 'pre-perimetric glaucoma'. This latter result confirms earlier longitudinal PERG studies done at the same institution as well as research published by others showing that ERG measures often detect abnormal function prior to standard visual field testing in patients with glaucoma (and/ or in glaucoma suspects). One interesting aspect of their report was that there was relatively poor correlation between the PERG and PhNR amplitudes within eyes, a finding exploited by the investigators who then showed that the combined PERG+PhNR assessment achieved even higher diagnostic accuracy in both patient groups. The authors suggest this result could represent some degree of uniqueness (not just 'noise'), such that the PERG and PhNR might offer complementary, objective assessments of retinal function in glaucoma. Though tantalizing, it is also possible that these two different ERG signals represent currents with the same origin.² In the end, the authors conclude that practical aspects of recording might guide the choice between these two alternatives: 'The PhNR has the advantage of not requiring clear optics and refractive correction; the PERG has the advantage of being recorded with natural pupils.' One might also consider that the photopic full-field flash offers a clear and direct measure of function of distal retinal layers, which PERG does not, thus complicating its specificity for glaucoma diagnosis when applied in isolation.

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