Introduction: A negative electroretinogram (ERG) is one in which there is a selective reduction in amplitude of the b-wave, such that it does not exceed that of the a-wave. The purpose of this study was to determine the incidence and clinical causes of negative ERGs at a tertiary referral centre. In addition, interesting and previously unreported aetiologies are described.
Patients and methods: Retrospective review of all ERGs done at Moorfields Eye Hospital from November 1995 to December 1998 under ISCEV standard conditions. Many patients had photopic ON- and OFF-response recording in addition to conventional ISCEV Standard ganzfeld ERG.
Results: A total of 2,640 ERGs were performed during the study period. 128 cases (4.8%) showed a negative ERG. The causes, where a firm clinical diagnosis was possible, include X-linked juvenile retinoschisis, congenital stationary night blindness, central retinal artery occlusion, birdshot chorioretinopathy and melanoma-associated retinopathy (MAR). Unilateral negative ERG waveforms with normal fundal appearances were seen in 7 patients. Photopic ON- responses could be selectively affected.
Conclusions: The incidence of negative ERGs over a 34-month period presenting to a large tertiary centre was almost 5%. The presence of a negative ERG may be instrumental in demonstrating the site of visual dysfunction, with many cases showing minimal or no fundus abnormality. ON- and OFF-response recording yielded additional information regarding photopic post-receptoral/phototransduction function.