Recurrent severe hypoglycaemia, intelligence and speed of information processing.

In a previous study, the cumulative effect of recurrent severe hypoglycemia on cognitive function was examined in 100 patients with insulin-treated diabetes and a significant correlation was observed between the frequency of severe hypoglycemic episodes and the difference between their current IQ and estimated premorbid IQ (Langan, Deary, Hepburn, & Frier, 1991). The study here extended this model for examination of the role of biological environmental influences on intelligence. Eighty-five of the original cohort of 100 patients were recalled and tested on Wechsler Performance IQ, a test of premorbid IQ, the Hick Reaction Time task, the Sternberg Memory Scan task and a test of Rapid Visual Information Processing (RVIP). The patients' frequency of severe hypoglycemia was assessed using a structured interview. Both IQ-decrement estimates and frequencies of severe hypoglycemia proved reliable after 18 months (correlations > .75) and the corrected correlation between these two variables was about .4. Therefore, repeated episodes of acute severe neuroglycopenia are associated with a lowering of IQ. Frequency of severe hypoglycemia was significantly related to Hick decision times and to response thresholds in the RVIP task. Factor analysis suggested that hypoglycemia affects decision or response-initiation processes rather than encoding, storage, comparison, or classification processes in short-term memory. Indices of these short-term memory processes were highly related to IQ-type test scores. This study also confirms, in a sample with a near-normal distribution of IQ, that measures of psychometric intelligence are significantly correlated with indices from elementary cognitive tests, especially the RVIP task, which is based on signal-detection theory.