Section 9: Metabolic Diseases

9. 1 Diabetes Mellitus

9. 2 Thyroid Disease

9.2.a. Hyperthyroidism
9.2.b. Hypothyroidism

A summary of the current fitness-to-drive guidelines (Metabolic Diseases) for medical practitioners from Australia (1998) and Canada (2000) is presented in Table 29.

9.1 Diabetes Mellitus

Prevalence

Diabetes mellitus, one of the most common endocrine diseases, affects approximately 16 million Americans(Centers for Disease Control [CDC], 1998). Prevalence rates in the United States range from two percent to six percent (CDC, 1998; National Institutes of Health [NIH], 1995). The number of people diagnosed with diabetes increased five-fold between 1958 and 1993 (NIH, 1995). Statistics reveal that the prevalence of diabetes increases with age, and recent estimates are that 18 percent to 20 percent of those 65 and over in the United States have diabetes (CDC, 1998).

Typically, the disease is categorized into two forms: insulin-dependent diabetes mellitus (IDDM or Type I diabetes) and non-insulin-dependent diabetes mellitus (NIDDM or Type II diabetes). IDDM may occur at any age, but it primarily appears before age 30. NIDDM, on the other hand, usually occurs in individuals over the age of 40. The diseases also differ in severity, underlying deficit, and type of therapeutic control. IDDM usually is more severe and is characterized by impairment in the ability to produce insulin. Daily insulin injections are required to manage the disease. NIDDM, on the other hand, typically is less severe, and is marked by an impaired ability to recognize and utilize insulin. Therapeutic control often is achieved by diet alone or in combination with oral hypoglycemic agents. Some individuals with NIDDM, are, however, treated with insulin. Of the two, NIDDM is the most common, with IDDM comprising only five percent to 10 percent of the total diabetic population (Canadian Diabetic Association, 2000; NIH, 1995).

The problems associated with diabetes which may affect driving competency can be classified as either acute or chronic. Chronic effects of diabetes include cardiovascular disease (coronary artery disease, hypertension, cerebrovascular accidents, microangiopathy), neuropathy, and diabetic retinopathy. The effects of the chronic complications of diabetes mellitus on driving are discussed under their respective headings (e.g., cardiovascular disease, peripheral neuropathy, etc.). Hypoglycemic reactions among diabetic drivers represent the most acute risk and are a primary factor of concern for traffic safety. Importantly, hypoglycemia does not occur in NIDDM treated only with diet and is unlikely to occur in those individuals with NIDDM treated with oral hypoglycemic. For example, six percent of individuals treated with sulfonylurea-derivatives in a study by Jennings, Wilson, and Ward (1989) experienced hypoglycemic symptoms monthly, and only 14 percent experienced hypoglycemic symptoms less frequently. Hypoglycemic reactions are most likely to occur in insulin treated individuals with IDDM, particularly those who are under tight glycemic control (The DCCT Research Group, 1987). Because of the importance for traffic safety, a more detailed discussion of hypoglycemic reactions in the diabetic driver is provided following the general review of research on diabetes and driving given below.