including the lack of representativeness, low participation rates, and self-reporting of TBI exposure and outcomes. Some studies have inappropriate control groups or no control groups, and the definition of mild, moderate, and severe TBI differs from study to study, particularly in the moderate category.
It is clear that sustaining TBI can have detrimental effects on a person, whether the injury is mild, moderate, or severe. The committee found many instances of long-term outcomes that are associated with TBI; some acute outcomes resolved or lessened over time (such as some neurocognitive and psychosocial findings), and other sequelae became more apparent several years after injury (such as psychiatric conditions). Many studies found a dose–response relationship with regard to TBI severity and outcome: generally, the more severe the TBI, the more severe the outcome. For example, with regard to neurocognitive outcomes, the committee found sufficient evidence of an association between penetrating TBI and decline in neurocognitive function associated with the region of the brain affected and the volume of brain tissue lost. The evidence was consistent in veterans of World War II and Vietnam. With regard to closed head injuries, the committee found sufficient evidence of an association between severe TBI and neurocognitive deficits, limited but suggestive evidence of an association between moderate TBI and neurocognitive deficits, and inadequate and insufficient evidence of an association between mild TBI and neurocognitive deficits.
With regard to neurologic effects, the studies reviewed had numerous findings, including a strong association between TBI and unprovoked seizures. For example, there is a causal association between penetrating TBI or severe closed TBI and unprovoked seizures, whereas the evidence of risk of unprovoked seizures after mild TBI is limited and suggestive of an association. In general, the risk of seizure after all levels of TBI severity appears to be highest in the first year after trauma and to decline thereafter. Some of the literature reviewed supports an association between TBI and neurodegenerative diseases, for example, studies that yielded sufficient evidence of an association between moderate or severe TBI and dementia of the Alzheimer type or parkinsonism, although an association with dementia pugilistica could be supported only in professional boxers. Other studies reviewed did not support a relationship between TBI and multiple sclerosis or amyotrophic lateral sclerosis and were categorized as inadequate and insufficient to determine whether an association exists. There were endocrine outcomes, such as sufficient evidence of an association between moderate to severe TBI and growth hormone insufficiency and hypopituitarism, however, the studies only supported a finding of limited and suggestive evidence of an association between moderate to severe TBI and diabetes insipidus.
Psychiatric outcomes have been discussed by the committee, and there is some uncertainty regarding the mechanisms linking TBI and psychiatric diagnoses. For example, it is not clear whether psychopathologic conditions after TBI are biologic consequences of the injury, a reaction to the person’s cognitive and social dysfunction after TBI, or a continuation of pre-existing conditions. The committee has chosen to use the terminology of primary psychiatric disorders, as has been the custom in the TBI literature. The committee notes that the predominance of studies indicated that groups with TBI (mild, moderate, or severe) had higher rates of major depression 6 months or more after TBI than did appropriate comparison groups.