A Novel Electrophysiological Instrument for Rapid and Objective Assessment of Glaucomatous Damage

Abstract

Abstract: : Purpose: To introduce a noninvasive device that can objectively and efficiently measure responses from select visual pathways known to be affected in the early stages of glaucoma. Methods: Based on previous work that demonstrated differential glaucomatous effects on ON and OFF divisions of the magnocellular pathway (1,2) an instrument that employs a visual evoked potential (VEP) technique designed to detect dysfunction in each of these subsystems was developed. To assess the integrity of each subsystem for each eye tested, only eight 1–s epochs of data were required. Critical stimulus conditions included arrays of 24x24 bright or dark checks set at peak contrasts of 10% or 15% and modulated sinusoidally at 10 Hz (appearance–disappearance mode) against a static background field (60 cd/m²). The stimulus field (11x11 deg) was viewed monocularly at a distance of 115 cm. Fourier analysis was used to extract the response at the stimulus frequency for each 1–s epoch of EEG. Each stimulation/recording period was immediately preceded by an identical 1–s stimulus presented for adaptation. A signal–to–noise ratio was derived jointly from the amplitude and phase data by means of a multivariate statistic. Automatic artifact rejection of noisy signals associated with electrical drift, 60 Hz power lines, or outliers improved detection of responses. Pilot data were collected from 23 participants (12 patients with open–angle glaucoma, 3 glaucoma suspects, 2 ocular hypertensives, and 6 age–similar controls); participants had visual acuities of 20/30 or higher. Results: Reliable responses were obtained within the brief period of data collection and the integrity of each subsystem under investigation was determined. Even in glaucoma cases with 20/20 visual acuity, 10% contrast stimulation conditions revealed neural deficits in 11 of 14 eyes tested. Results are consistent with those reported previously (1,2) that used a more laborious technique which produced entire contrast response functions. Preliminary receiver operating characteristic (ROC) curve analysis of the present data yielded accuracy estimates that ranged from 79% to 96% with the highest accuracy obtained under the 10% dark condition. Conclusions: A unique functional measure offers the possibility of quick and early detection of central visual deficits in patients with glaucoma. 1. Greenstein et al., 1998, Vis Res 38, 1901–1911; 2. Badr et al., 2003, ARVO.

Keywords: neuro-ophthalmology: diagnosis • electrophysiology: clinical • clinical research methodology