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Visual problems
Approximately 80 per cent of flight information is accrued visually and thus any pathological process
which interferes with visual function may result in human error and may contribute to an accident.
Diabetes mellitus is known to affect all parts of the eye, e.g. cataract, retinal vein occlusion, ischaemic
optic neuritis and cranial nerve palsies resulting in diplopia. Diabetic retinopathy, however, is a highly
specific vascular complication of diabetes mellitus and is estimated to be the most frequent cause of new
blindness among adults between 20 and 74 years of age. Twenty years after the onset of the disease,
almost all insulin dependent patients, and more than 60 per cent of those who are non-insulin dependent,
have some degree of retinopathy (Klein et al, 1984). More than four fifths of cases of blindness among
Type 1 patients, and one third of cases among Type 2 patients, are caused by diabetic retinopathy. Many
forget that Type 2 diabetes is not a benign disease, which has caused it to be called a wolf in sheep’s
clothing. The major determinants for the development of retinopathy are the quality of diabetic control
and the duration of the diabetes.
ICAO Preliminary Unedited Version — March 2010 III-4-19
HYPOGLYCAEMIA
R D Lawrence was a unique physician. He became a prominent specialist in a disorder from which he
himself suffered most of his career. He was a meticulous physician and researcher and, in 1923,
documented his first hypoglycaemic attack. He observed he felt just a little shaky some hours after
injecting insulin and the next day was slightly faint, dizzy, weak and tremulous. He later wrote I felt weak,
sweaty, with an intense hunger which led me to the biscuit box and slow restoration. Obviously my first
hypoglycaemic attack (Lawrence, 1961). This description by Lawrence illustrates the dual
symptomatology of this un-physiological state: a combination of neuroglycopenia and autonomic neural
stimulation. Either of these symptom complexes may degrade pilot performance. A study carried out
(Holmes, 1986) in Type 1 patients subjected to modest hypoglycaemia of 3.1 mmol/L showed a
decrement in performance which increased with the complexity of the task performed. In this and other
studies researchers have shown that reaction times do not return to normal until some 20-30 minutes after
euglycaemia has been restored. The implications in the aviation environment are self-evident.
As hypoglycaemia is a significant concern in the aviation environment, accurate risk assessment is vitally
important. This requires good data on the incidence of hypoglycaemia in both Type 1 and Type 2
patients. Such data, however, have proven difficult to obtain.
Type 1 Diabetes
It is very difficult to assess the frequency of hypoglycaemia in insulin-treated diabetic populations,
because of the wide variation in severity and outcome. Other problems include the common occurrence of
asymptomatic biochemical hypoglycaemia that is only evident if blood glucose is measured frequently
and the failure to recognise or record many mild episodes, including those during sleep. The development
of diminished symptomatic awareness of hypoglycaemia also reduces the identification of episodes by the
affected patient and sometimes symptoms are attributed to hypoglycaemia when the blood sugar is not in
fact low. The true prevalence of unawareness has been estimated at between three and 22 per cent (Heller
et al, 1995).
Severe hypoglycaemia, defined by the need for external assistance to resuscitate the patient, is a more
robust and consistent measure for estimating frequency and is reliable even in retrospective reporting.
Where a similar definition for severe hypoglycaemia has been applied, the lowest annual prevalence is
nine per cent, but the average is approximately 20-30 per cent. The higher figures come from studies in
which the patients’ relatives or other observers were asked about the symptoms, rather than the patients
themselves. Despite the difficulties in assessment, the frequency of mild hypoglycaemia in one good
study was 1.6 episodes per patient per week, approximately 83.6 episodes per patient per year (Praming et
al, 1991). This is an alarmingly high figure.
Strict glycaemic control, usually resulting from intensive insulin therapy, is recognised to be a risk factor
for severe hypoglycaemia. In the Diabetes Control and Complications Trial (1993), strict glycaemic
control was associated with a threefold increase in severe hypoglycaemia. The risk of severe
hypoglycaemia increased continuously with lower monthly glycosylated haemoglobin values.
Unfortunately, analysis of the glycosylated haemoglobin data did not support the prediction of a specific
target value at which the benefits of intensive therapy were maximised and the risks minimised. Other risk
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Manual of Civil Aviation Medicine 1(132)
