Section 9: Metabolic Diseases

Predictors of Hypoglycemia

The major morbidity associated with hypoglycemia is temporary neurologic deficit and coma, seizures with central nervous system injury, and permanent neurologic impairment if treatment is absent or delayed. Given the potential seriousness of hypoglycemic reactions for individuals with diabetes, it is not surprising that a number of studies have attempted to find reliable predictors of hypoglycemia. Table 26 presents a summary of studies that have examined potential risk factors for hypoglycemia. In general, the majority of studies have examined the relationships between demographic and medical factors using severe hypoglycemia as the dependent or criterion variable. Most, if not all of the studies, have relied on self-report for documentation of hypoglycemic reactions. Despite the obvious limitations in the methodology, it is noteworthy that research suggests that most diabetic individuals are able to recall and retrospectively report severe episodes with considerable accuracy (Pramming et al., 1991).

Generally, the associations between age or insulin dosage and severe hypoglycemic reactions have been insignificant. Higher levels of insulin have been significantly associated with an increased risk of severe episodes in some studies (Casparie and Elving, 1985; The DCCT Research Group, 1991) but not in others (Goldgewicht et al., 1983; MacLeod et al., 1993; Nilsson et al., 1988; Ward et al., 1990). Significant differences have been found in mean hemoglobin A1c (HbA_1c ) levels, with lower recent levels reported in the hypoglycemic groups (Casparie and Elving, 1983; The DCCT Research Group, 1991), although Nilsson et al. (1988) failed to find significant differences.

A previous history of severe hypoglycemic episodes is a consistent and significant predictor of future episodes. Results from Nilsson et al. (1988) revealed that 78 percent of individuals in the severe hypoglycemia group (n = 46) had experienced episodes of severe hypoglycemia prior to the study compared with 22 percent in the control group (n = 22 for those individuals not experiencing a severe hypoglycemic reaction during study). A history of severe hypoglycemia was a significant predictor of severe hypoglycemia in intensively treated subjects in the DCCT Research Group study (1991), with almost a three-fold increase in relative risk (RR = 2.54, 95 percent CI = 1.67-3.88). Significant risk factors for severe hypoglycemia in the MacLeod et al. (1993) investigation included a history of severe hypoglycemia, a history of hypoglycemia related injury, or hypoglycemia related convulsion. A retrospective review of driving records of insulin-dependent diabetic and non-diabetic drivers revealed that the number of hypoglycemic episodes while driving during the past year was significantly related to total number of crashes during a five-year period for 354 diabetic drivers. Individuals reporting two or more hypoglycemic episodes were twice as likely to be involved in one or more crashes compared to those diabetic drivers reporting no hypoglycemic episodes.

In summary, a previous history of severe hypoglycemic reactions is significantly associated with future episodes of severe hypoglycemia. Thus, a history of severe hypoglycemia could serve as a medical red flag for licensing decisions. A review of the driver licensing regulations in the United States reveals, however, that few of the states currently take history of hypoglycemic reactions into consideration when making licensing decisions. For example, summary guidelines of licensing regulations for diabetic drivers are available for 19 of the states in the United States (U.S. Department of Transportation, 1992). For 11 of the 19 states, there is no specific mention of diabetes in the summary guidelines. For 8 of the 19 states, the specific guidelines for diabetes range from Medical Report requirements (District of Columbia, Florida, West Virginia) to functional ability profiles (Maine, Utah). For seven of the states, there is no specific mention of diabetes mellitus in the summary guidelines. Presumably, acute complications of diabetes mellitus (e.g., hypoglycemic episodes leading to temporary neurological impairment, including seizures and/or coma) would be covered in guidelines for altered states of consciousness. Finally, for those states that have specific licensing guidelines for diabetes mellitus, a review of those guidelines reveals considerable variability between states. Utah provides the most comprehensive guidelines for the licensure of diabetic drivers and the reader is encouraged to review those guidelines for more details.

In addition to a history of severe hypoglycemia, recent research suggests that hypoglycemia unawareness may be an important risk factor for severe hypoglycemic episodes. The following section reviews the literature on hypoglycemic unawareness and its relevance for driving.

Table 26  Summary of Studies Examining Predictors of Severe Hypoglycemic Reactions in Individuals with Diabetes Mellitus