individual’s initial response to trauma and subsequent response to treatment. Gaps in knowledge exist regarding the effects of preexisting conditions on outcome following TBI, and it is often difficult to differentiate the effects of preinjury factors from those related to the injury itself or the postinjury environment. Preinjury conditions, such as attention deficit hyperactivity disorder (ADHD), learning disabilities, or mild forms of syndromes on the autism spectrum (e.g., Asperger’s), may also affect an individual’s cognitive deficits after a TBI, as well an individual’s ability to acknowledge an injury, seek screening or treatment, understand a diagnosis and subsequent treatment plans, and set appropriate goals for treatment success.
Preinjury depression may affect the manifestation of various TBI-related effects. In a study of TBI by Bombardier et al. (2010), a prior history of depression among patients correlated with higher post-TBI rates of major depressive disorder. Although screening attempts to prevent individuals with most major affective disorders from military service, instances of bipolar disorder, schizophrenia, or substance use disorder (SUD), among others, may go undiagnosed. Corrigan et al. (2003) demonstrate that about half of the civilian subjects in TBI Model Systems, a national data repository of information about the acute and postacute care of individuals with TBI, had preinjury SUD. Emotional disturbance and ongoing substance abuse can also affect a survivor’s capacity to cognitively engage in and potentially benefit from even a well-designed cognitive rehabilitation program.
Other preexisting factors may contribute to poor outcomes following TBI, including a lack of social support systems and environmental factors. Socioeconomic status (SES) is an environmental factor that can affect cognitive, behavioral, and functional outcomes. Socioeconomic status is associated with low education status or low IQ. But the relationship between low SES and a worse outcome may be due to the limited resources available to the individual and the family, including access to high-quality rehabilitation and availability of family members to act as caregivers. If an individual from low SES suffers a TBI in the military, that person may be afforded the opportunity for continued treatment and care due to his service, which may otherwise be unavailable. However, due to work restrictions or other responsibilities, that person’s family or other caregivers may not be able to provide the support system and care the person needs after hospitalization and during a structured rehabilitation program.
Comorbidities are conditions that occur in addition to the primary insult, injury, or disease. Comorbidities can occur by chance (i.e., two or more conditions occurring simultaneously, with one condition not the direct origin of the other), or by causal association (Valderas et al. 2009). Causal