Clinical electrophysiology of two rod pathways: normative values and clinical application

Background: The scotopic 15-Hz flicker electroretinogram (ERG) has two limb s (slow and fast ERG rod signals), and these have been attributed to two re tinal rod pathways (the ON rod bipolar and AII amacrine pathway and the rod -cone gap-junction pathway). The aim of this study was to provide normative values of the scotopic 15-Hz flicker ERG, to estimate the inter-individual variability, and to apply this method to a clinical setting. Methods: Twenty-two normal subjects, one patient with retinitis pigmentosa (RP), and two patients with Stargardt's macular dystrophy (SMD) participate d in the study. The SMD patients were screened for mutations in the 50 exon s of the ABCA4 (for merry ABCR) gene. We measured ERG response amplitudes a nd phases to flicker intensities ranging from -3.37 to -0.57 log scotopic t rolands s at a flicker frequency of 15 Hz. Results: The normal scotopic 15-Hz flicker ERG showed a biphasic amplitude pattern with a minimum at about -1.57 log scotopic trolands s, where there was an abrupt phase shift of about 180 deg. The inter-individual variabilit y in ERG amplitude ranged from 47% to 67% for the slow and from 41% to 64% for the fast rod signal. Both the RP patient and the SMD patients (who were compound heterozygotes for mutations in the ABCA4 gene) showed reduced amp litudes for the two rod ERG pathways. Conclusion: The inter-individual vari ability might be explained by anatomical differences between individual ret inae. In the RP patient, the amplitude reductions corresponded well with th e standard rod ERG. In the SMD patients, however, the scotopic 15-Hz flicke r ERG revealed rod dysfunction, whereas the standard rod ERG was within nor mal limits. The scotopic 15-Hz flicker method may be more sensitive than th e standard rod ERG.