Hypoglycaemia in Clinical Diabetes

CONSEQUENCES OF NOCTURNAL HYPOGLYCAEMIA 89reported by patients because they remain asleep, but may contribute to the development ofimpaired awareness of hypoglycaemia.The ‘Dead in Bed’ SyndromeThe possible contribution of hypoglycaemia to cardiac arrhythmias and sudden death isdiscussed in detail in Chapter 12. However, since it has been proposed that nocturnalhypoglycaemia may be a specific precipitant, it merits a brief mention here. A detailed surveyof unexpected deaths in young people with type 1 diabetes in the early 1990s highlighted arare but distinct mode of death in which young people were found lying in an undisturbed bed.Autopsy failed to reveal a structural cause but circumstantial evidence implicated nocturnalhypoglycaemia as a precipitant. Not all affected individuals had strict glycaemic control, butmany were known to have been susceptible to developing nocturnal hypoglycaemia. Thistype of sudden and unexpected death has since been confirmed in epidemiological surveysin other countries, possibly accounting for between 5–10% of all deaths under the age of40 in people with diabetes. Hypoglycaemia causes an increase in the QT interval and onepossible explanation is that an episode of nocturnal hypoglycaemia triggers a ventriculararrhythmia in a susceptible individual. However, further work is required to establish thisplausible hypothesis as the cause of these deaths.Neurological Consequences of Nocturnal Hypoglycaemia on CerebralFunctionThe possibility that recurrent exposure to hypoglycaemia, particularly occurring during sleep,might insidiously damage cerebral function and cause permanent cognitive impairment wasraised by the finding that children who had developed type 1 diabetes before the age offive years, exhibited cognitive impairment when compared with non-diabetic controls (Ryanet al., 1985). This observation was replicated in different studies using a range of cognitivetests (Bjorgaas et al., 1997; Rovet and Ehrlich, 1999).However, the relationship to hypoglycaemia, particularly during the night, was unclear(Golden et al., 1989); hypoglycaemia-induced convulsions have also been implicated, and itis possible that other factors associated with early-onset diabetes may contribute to cognitiveimpairment.Several studies have explored the extent to which nocturnal episodes may affect performanceon the following day. Bendtson et al. (1992) found no difference in cognitive performanceamong adults with type 1 diabetes when tested on the morning after an episode ofnocturnal hypoglycaemia in comparison with a night with no hypoglycaemia. Similar findingshave been reported after the induction of experimental hypoglycaemia during the night (Kinget al., 1998) although the subjects were more fatigued on the following morning. It seemsthat even children are relatively unaffected by a single episode of nocturnal hypoglycaemia.Matyka et al. (1999b) tested pre-pubertal children after episodes of prolonged, spontaneouslyoccurring, hypoglycaemia that occurred during sleep, many of which lasted several hours,and found no deleterious effect on cognitive function on the following morning, althoughmood was adversely affected.In summary, although it seems plausible that recurrent nocturnal hypoglycaemia mightcontribute to cognitive decline, on the available evidence the verdict remains unproven.