Electrophysiological Investigation of the Effects of Systemic Oxygen Inhalation on the Oscillatory Potentials and Scotopic B–Wave in Diabetes Mellitus.

Abstract

Abstract: : Purpose: The oscillatory potentials (OPs) are known to reflect inner retinal function and have been found to be reduced in subjects with diabetes mellitus (DM). The scotopic b–wave is thought to reflect primarily the activity of the rod bipolars and has also been found to be reduced in DM. Since retinal hypoxia has been implicated in abnormalities of visual function we investigated the effect of 5 minutes of systemic oxygen (O₂) inhalation on the OPs and scotopic b–wave in subjects with DM. Methods: Fifteen subjects with Type II DM, mean disease duration 8yrs, with no diabetic retinopathy (NDR), mean age 62yrs (SD 6.0), were compared to a group of 15 age–matched controls, mean age 62yrs (SD 10.6). OPs were recorded monocularly after 20mins dark adaptation to 6 white flashes, intensity 3cdsm^–2 (ISCEV standard), 3ms duration at 15s intervals (first two flashes treated as conditioning flashes). This was then repeated: a) after 2mins of 100% O₂ inhalation through a 60% Ventimask, b) 2mins, c) 7 mins and d) 12mins after removal of the mask. The scotopic b–wave was recorded monocularly (n =17 NDR, n=18 controls), after 20mins dark adaptation at the same time points to a 5ms green flash (peak 515nm), intensity 0.0012cdsm^–2 (ISCEV standard), at 0.5Hz. Results: O₂ inhalation increased the summed OP amplitude with a significant increase of 12% and 19% from baseline at 7 and 12mins after removal of the mask respectively using Bonferroni pairwise comparisons (p<0.001 RM ANOVA). No significant change in summed amplitude was found in the controls (p=0.154). An increase in amplitude was also observed for the scotopic b–wave, of 10% and 8% in the NDR group at these respective time points, though this did not reach significance using the RM ANOVA. Conclusions: O₂ inhalation significantly increased the summed amplitude of the OPs in NDR subjects while control subject amplitudes remained stable. The scotopic b–wave amplitude increased for both groups though not significantly in this experiment. The increase in summed OP amplitude with O₂ inhalation supports the suggestion of impaired retinal autoregulation in subjects with DM even when no retinopathy is apparent, and suggests that tissue hypoxia may be present in the surface layers of the retina in these subjects.

Keywords: electrophysiology: clinical • diabetes