Hypoglycaemia in Clinical Diabetes

38 SYMPTOMS OF HYPOGLYCAEMIAThe hyperinsulinaemic glucose clamp technique allows more controlled experiments ofacute hypo- and hyperglycaemia. However, although this technique is used in most studiesof cognitive function in hypoglycaemia, it does not mimic the physiological or temporalcharacteristics of ‘natural’ or intercurrent episodes of hypoglycaemia experienced by peoplewith type 1 diabetes. From laboratory experiments using the glucose clamp technique it wasfound that blood glucose concentrations between 3.1 and 3.4 mmol/l caused the followingeffects (Holmes, 1987; Deary, 1993):• Slowed reaction times (this experiment involves making a fast response when a lightappears on a computer screen. Hypoglycaemia had more effect on reaction times whenthe reaction involved making a decision).• Slowed mental arithmetic.• Impaired verbal fluency (in this test one has to think of words beginning with a givenletter, probably involving the frontal lobes of the brain).• Impaired performance in parts of the Stroop test (in this test one has to read aloud a seriesof ink colours when words are printed in a different colour from that of the name, e.g. theword RED printed in green ink).Some mental functions were spared during hypoglycaemia, for example:• simple motor (like the speed of tapping) and sensory skills;• the speed of reading words aloud.By 1993 over 16 studies had investigated cognitive functions during acute and mild–moderate hypoglycaemia (Deary, 1993). The levels of blood glucose ranged from 2.0 to3.7 mmol/l. The way that hypoglycaemia was induced varied among studies, as did themethods of blood sampling (e.g. arterialised or venous blood). Moreover, the ability levelsof the people in different studies varied, and there was much heterogeneity in the testbatteries used to assess mental performance. An authoritative statement as to the mentalfunctions disrupted during hypoglycaemia is still not possible. However, in at least one ormore of the studies a number of tests were significantly impaired during hypoglycaemia(Box 2.2).Few areas of mental function are preserved at normal levels during acute hypoglycaemia.There is a general dampening of many abilities that involve conscious mental effort. In theface of so many deleterious effects, what mental functions remain intact during acute hypoglycaemia?At blood glucose concentrations similar to those indicated above, the followingmental tests are not significantly impaired:• finger tapping;• forward digit span (repeating back a list of numbers in the same order);• simple reaction time;• elementary sensory processing.