Hypoglycaemia in Clinical Diabetes

146 IMPAIRED AWARENESS OF HYPOGLYCAEMIA(Heller et al., 1995; Jorgensen et al., 2003). This will provide a witnessed descriptionof how hypoglycaemia develops in a patient, with information on its true frequency andseverity. Patients often underestimate the frequency of severe hypoglycaemia, partly becauseof post-hypoglycaemia amnesia.ClassificationIn one study, Hepburn et al. (1990) subdivided hypoglycaemia awareness into three categories:normal, partial and absent awareness. These were defined as follows:• Normal awareness: the individual is always aware of the onset of hypoglycaemia.• Partial awareness: the symptom profile has changed with a reduction either in the intensityor in the number of symptoms and, in addition, the individual may be aware of someepisodes of some episodes of hypoglycaemia but not of others.• Absent awareness: the individual is no longer aware of any episode of hypoglycaemia.Although the subdivision into partial and absent awareness is artificial, it reflects the naturalhistory of this clinical problem, illustrating the gradual progression of this disability, andemphasising that in some patients the abnormality is severe (absent awareness) althoughtotal absence of clinical manifestations of hypoglycaemia (particularly the neuroglycopenicfeatures) is exceptionally rare (Gold et al., 1994, Clarke et al., 1995). The problem maynot be simply an absence of symptoms, but rather that the time during which warningsymptoms can be detected is extremely short, allowing the affected individual a very limitedopportunity to take avoiding action. Some patients describe how the onset of hypoglycaemiaappears to have become much more rapid compared with their previous experience andprogresses quickly to severe neuroglycopenia. However, impaired awareness may not necessarilyevolve into total unawareness of hypoglycaemia, and may vary over time, presumablybecause of major influences of environmental factors on the generation and perception ofsymptoms.The above classification of awareness of hypoglycaemia is far from comprehensive. Inaddition, the state of hypoglycaemia awareness can be ascertained only when the individualis in a physical state in which recognition of the onset of hypoglycaemia is possible.Therefore, if the person is asleep, intoxicated, inebriated, anaesthetised or sedated, so thattheir conscious level is reduced, they are not able to perceive (as subjective symptoms)the normal physiological manifestations of hypoglycaemia. An individual’s awareness ofhypoglycaemia can be evaluated only if hypoglycaemia occurs while the individual is awake.A further prerequisite is that the person must have had previous experience of hypoglycaemiaat some time during treatment with insulin. In assessing the present state ofhypoglycaemia awareness, it is desirable that the patient should have experienced one ormore episodes of hypoglycaemia (confirmed biochemically) within a recent time intervalsuch as the preceding year, so that a comparison of the symptoms can be made with earlierepisodes of hypoglycaemia. A diagnosis of impaired hypoglycaemia awareness cannot beentertained or surmised if a patient has either never been exposed previously to acute hypoglycaemiaor has only started to experience hypoglycaemic events very recently. Becausehypoglycaemia awareness and its impairment is a continuum ranging from normality to