Understanding the phenotype of genetically associated electronegative ERG retinopathies: comparing the full-field ERG b:a ratio.

PURPOSE: The electronegative electroretinogram (ERG) is a specific clinical finding usually indicating inner retinal dysfunction occurring post-phototransduction. X-linked retinoschisis (XLRS) and complete and incomplete congenital stationary night blindness (cCSNB, iCSNB) are inherited retinal dystrophies classically associated with electronegative ERGs. Comparing the full-field ERG b:a ratio expands current ERG diagnostic criteria and aids in localising physiological sites and pathological mechanisms.

METHODS

A retrospective review of patients with a clinical diagnosis of iCSNB, cCSNB and XLRS was conducted. ERG and genetic results were analysed. Average b:a ratios between groups were compared, and prevalence of electropositivity was assessed using thresholds of b:a > 1.0 and b:a > 1.50.

RESULTS

53 patients were included, and genetic confirmation was available in 7/24 iCSNB, 3/14 cCSNB and 11/15 XLRS patients respectively. In genetically proven cases, mean b:a ratio in XLRS patients (b:a = 1.04) was significantly higher than cCSNB (b:a = 0.60, p < 0.001) and iCSNB (b:a = 0.60, p < 0.001). An electropositive ERG was significantly more likely to be associated with RS1 than iCSNB (p < 0.001) or cCSNB (p = 0.001) at b:a > 1.0 threshold, and more likely RS1 than iCSNB (p = 0.040) at b:a > 1.5 threshold.

CONCLUSION

Our study highlights the distinct ERG findings between these typically electronegative inner retinal dystrophies. In a clinical setting, the traditional electronegative definition of b:a < 1.0 appears very insensitive to detect XLRS patients. Our data suggests clinical suspicion should remain even in patients with a b:a ratio > 1.50, and highlights the importance of genetic testing in these cases.