As aforementioned, it is well established that TBI alters cognitive function and can produce lasting deficits at the moderate to severe end of the injury spectrum (Christensen et al., 2008; Ponsford, Draper & Schonberger, 2008; Schretlen & Shapiro, 2003; Sigurdardottir,
94 Andelic, Roe & Schanke, 2009). The mTBI area is less researched and the findings more mixed. The area is worthy of consideration forthwith, not only because it is a controversial one, but also as the bulk of the current investigations population of interest suffer mild injuries. It is often argued that litigants are motivated to ‘fake bad’ or approach testing with sub-optimal effort due to the potential financial gains of demonstrating injury related deficits (Larrabee, 2005). As such, some researchers control for, or examine, the impact of litigation status. Several key papers from the literature regarding cognitive function post-mTBI are covered forthwith.
Ponsford et al., (2000) compared an mTBI group (n = 83, M age = 26.4 years) with injury controls recruited from the same medical facility, tested at one week and three months post injury. The test battery included memory measures while being biased towards
processing speed. One week post injury the mTBI group performed more poorly on processing speed measures than control subjects. At three month post-injury there were no differences in neuropsychological test performance between mTBI sufferers and controls although 24% of the head inured sample continued to self-report difficulties. In exploring the findings of the subset with persisting difficulties, the authors explain that it is unclear whether MVA resulted in greater brain injury, added more complicating physical injuries, or if MVA was simply more psychologically traumatising.
Mathias et al., (2004) were interested in the sensitivity of measures of processing speed to mTBI. A sample of 40 mTBI sufferers (M age = 32.4 years) were prospectively recruited and compared with well matched controls. Only 28% of the clinical sample were involved in litigation at the time of testing. Testing occurred at around one month post injury. Information processing was measured using choice reaction time tasks while measures from the Test of Everyday Attention (TEA; Robertson et al., 1994) were used to test selective and divided attention. An executive domain was comprised of Phonemic
Fluency and the Ruff Figural Fluency Test (RFFT; Ruff, Evans & Marshall, 1986). Memory performance was captured using the RAVLT. Results showed mTBI sufferers to be slower than controls on the TEA Telephone Search Task control condition, but not on the Telephone Search while Counting (TSC) condition (the dual-task divided attention trial). The mTBI sufferers performed worse than controls on the selective attention task (Visual Elevator from the TEA), on the RAVLT and on the RFFT, but not on the Phonemic Fluency task. There was also evidence of slowed information processing for mTBI sufferers. The authors suggest that the effects size for the speed measures were at least medium, and of a similitude to that of the other domains.
Binder et al., (1997) conducted a meta-analysis of studies of mTBI sufferers, 3 months post injury and beyond. The rationale for doing so was to investigate the critical issue, the chronicity of deficits. Executive function as a domain in its own right was not studied although TMT parts A and B, and the Stroop test were included among the measures of attention, and the WCST and Phonemic Fluency were assigned the domain ‘mental flexibility.’ Severity of injury accounted for a greater proportion of the variance than any particular cognitive domain. Attentional measures were most sensitive to mTBI. A small percentage of mild TBI cases continue to exhibit small deficits 3 months post injury, and Binder et al. concluded by calling for more research into the nature of this phenomenon.
Belanger et al., (2005) also conducted meta-analysis into outcome post mTBI, investigating cognitive domain and litigation status as potential moderating factors. Thirty- nine studies met entry criteria. Cognitive domains included general cognition, attention, memory acquisition, delayed memory, language, visuospatial ability and motor ability. Fluency was categorised as a domain in its own right, additional to executive function. Time since injury was demarcated into greater or less than 90 days. The largest effects were evident in the fluency and delayed memory domains, and the smallest were for motor
96 functions and executive function. Unfortunately, the authors do not discuss the relationship (or lack thereof) between fluency and executive function. Belanger et al. suggest that by three months post injury, that the cognitive sequelae of mTBI are resolved. Effect sizes were similar irrespective of litigation status for those examined within 90 days post injury. At the more distal time-point however, mTBI non-litigants and controls were not differentiated on the basis of performance, whereas deficits experienced by mTBI litigants remained or
worsened. In exploring this finding Belanger et al. do not comment on the potential for those with persisting deficits to be potentially more motivated to participate in research as
suggested by Binder et al., (1997) and Ponsford et al., (2000). A weakness of Belanger et al.’s (2005) design is the inclusion of studies using both prospective and clinical recruitment.
Studies of mTBI are not confined to this section of the literature review. Cases among older adult TBI populations, and studies including data from mTBI sufferers that explore executive function in greater detail, are examined within subsequent Sections. Attention is now turned to the literature that focuses on the cognitive impact of TBI for the older patient.
8.10 Review of the Literature concerning TBI, Age and Cognition