Pattern Reversal Visual Evoked Potential and Cognitive Functions in Subclinical Hypothyroid Subjects

Background: Central nervous system (CNS) involvement is insidious and may occur early in subclinical hypothyroid (SCH) state which can be picked up by electrophysiological study. This study aims to record visual evoked potential (VEP), event-related latency and cognitive functions, and find their association with the levels of serum thyroid-stimulating hormone (TSH) in patients with SCH. Materials and Methods: In this cross-sectional study, 36 adult SCH patients and an equal number of age-and sex-matched euthyroid controls were included. Pattern reversal VEP, visual reaction time (RT), digit spanning test, and AB clock test (ABCT) were done in both SCH cases and euthyroid controls. The observed values were analyzed for comparison of mean values between the groups and correlation of recorded variables with the levels of serum TSH. Results: SCH cases showed a higher P100 (VEP) latency in both the right (103.2 +/- 12.3 vs. 102.7 +/- 6.8 ms) and left eye (101.1 +/- 9.1 vs. 96.2 +/- 10.7 ms) as compared to controls, but the difference was statistically insignificant. A significant delay in RT was observed on visible spectra of light in SCH cases (P < 0.001). Digit spanning score (forward and backward) in SCH cases was significantly lower than controls (P < 0.001), and a lower standardized score (< 124 or < 95th percentile) was significantly associated with SCH state (P = 0.027). No significant difference was observed in visuospatial domain by ABCT between both the groups although the median score was lower in SCH cases. Only digit spanning score showed a significant negative correlation with TSH levels (r = -0.4; P = 0.001). Conclusion: Decline in working memory and RT to visual stimuli is an evidence of the involvement of CNS in SCH. Prolonged latency in VEP may depend on the duration of SCH.