Mental illness as an independent risk factor for unintentional injury and injury recidivism

Twelve percent of Americans are diagnosed and treated for mental illness annually. The relationship between mental illness and intentional injuries such as assault and suicide has previously been described. However, unintentional injury among mentally ill adults has not been characterized. The purpose of this study was to identify relationships between mental illness diagnosis and unintentional injury. We hypothesized that diagnosed mental illness is an independent risk factor for unintentional injury and it increases the risk of recidivism.

In this retrospective cohort study, trauma registry data, medical records, and outpatient mental health care data from the San Francisco Department of Public Health Billing Information System (BIS) were used to identify patients admitted with unintentional injury at a Level I urban trauma center in 2003 and 2004. Data collected included mechanism of injury, patient outcome and disposition, mental health diagnoses, substance abuse history, presence of homelessness, number of repeat injury events, and outpatient mental health treatment history. The incidence of unintentional injury requiring admission to a trauma center and the risk of intentional injury recidivism in subjects with a mental illness diagnosis were compared with those in subjects without a mental illness diagnosis. The risk of recidivism in those who had received publicly funded outpatient treatment before their injury was also evaluated.

Of the 1,709 patients admitted for unintentional injury, 20% had a diagnosis of mental illness. Individuals with mental illness had twice the rate of unintentional injury requiring admission (2.2-2.4 people in 1,000 vs. 1.0-1.1 in 1,000) and 4.5 times the odds of injury recidivism (odds ratio [OR] = 4.5, 95% confidence interval [CI] 3.3-6.1) as those who did not have a mental illness diagnosis. Mental illness was a more robust predictor of injury recidivism than substance abuse (OR = 3.2, 95% CI 2.3-4.3) or homelessness (OR = 2.3, 95% CI 1.5-3.4). Compared with the nonmentally ill group, subjects with mental illness had a longer hospital stay and were less likely to be discharged home. Also, their injuries were more likely the result of falling or being hit by cars, and less likely the result of motor vehicle collisions than subjects without mental illness.

Mental illness is an independent risk factor for unintentional injury and injury recidivism. Individuals with mental illness also have a different pattern of injury and hospitalization. They tended to suffer from different mechanisms of injury, stayed in the hospital longer, and were more likely to be discharged to a skilled nursing facility. Recognition of mental illness as a risk factor for injury may prompt re-examination of resource allocation for mental health and injury prevention and highlights the mentally ill as a prime target population for unintentional injury prevention efforts.