Causes of death

All-cause mortality statistics give no clue as to why mortality might be raised. As well as the acute complications of diabetes, such as hypoglycaemia and ketoacidosis, a number of chronic complications are well recognised. Almost all of these relate in some way to micro- or macrovascular disease, and include CVD, nephropathy, neuropathy and retinopathy. Some may feature largely in studies of morbidity but not be a major cause of mortality, for example peripheral arterial disease is a common condition among diabetic patients but is rarely the primary cause of death (Chapter 8). Studies of patients with type 2 diabetes, although not usually subdivided by age, indicate that CVD is the major cause of death in these patients, accounting for as much as 80% of the excess deaths (Blendea etal., 2003). In younger patients the chronic complications of diabetes develop some time after the initial diagnosis.

Data from the Diabetes UK Cohort Study illustrates how the predominant cause of death in people with type 1 diabetes changes with age (Table 1.1). Under the age of 20 the greatest single cause of death was acute complications of diabetes, which accounted for 38% of the deaths in men and 54% of the deaths in women. In males, between the ages of 20 and 39 years, acute complications remained the greatest single cause of death but in females CVD was the cause of the greatest number of deaths even at this young age. By the 40-59 age groups CVD accounted for at least half of all the deaths in patients with type 1 diabetes. This same pattern has been seen in other studies of young people with diabetes (Lounamaa etal., 1991; Moss etal., 1991), acute complications initially being responsible for the greatest number of deaths but CVD complications becoming the predominant cause of death at fairly young ages.

Table 1.1 Cause of death, expressed as a percentage of the total, by age group. Data from the Diabetes UK Cohort Study.

Males

Females

1-19

20-39

40-59

1-19

20-39

40-59

years

years

years

years

years

years

Diabetes

38

26

7

54

17

10

Renal disease

0

8

9

0

16

11

Cardiovascular disease

6

17

61

10

26

50

Other

56

49

23

36

41

29

Renal disease has previously been identified as a major cause of death in patients with type 1 diabetes. From Pittsburgh (Dorman etal., 1984) it was reported that renal disease was responsible for the majority of deaths in the 20-29 age group of the Pittsburgh morbidity and mortality study, and studies from Denmark suggested that the high relative mortality after 20-30 years' duration of diabetes was due to the development of proteinuria (Borch-Johnsen etal., 1986). In the Diabetes UK Cohort Study the proportion of deaths due to renal disease was lower than the proportion due to CVD at all ages.

Clearly if mortality and morbidity are to be reduced then the prevention and treatment of CVD must be addressed. Cardiovascular disease itself is a generic term, encompassing many specific components, and can be further divided into peripheral arterial disease, cerebrovascular disease and heart disease as well as other types of vascular disease such as venous disease and aneurysms. Even within these groups further divisions can be made, for example the term 'heart disease' includes not only ischaemic heart disease, but also valve disorders, hypertensive heart disease, cardiomyopathy, dysrrhythmias and heart failure. Large cohort studies are necessary if separate statistics are to be calculated for the individual components of CVD. A number of studies of patients with type 2 diabetes have calculated these separate statistics, although the results are rarely reported by specific age group, but among the studies of patients with type 1 diabetes the Diabetes UK Cohort Study is alone in being of sufficient size and having sufficient follow-up to examine some of the CVD outcomes separately by age (Laing etal., 2003a, 2003b).

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