At the moment of impact of the injury, the brain is subject to sudden onset of both acceleration-deceleration and rotational forces, which cause characteristic types of structural brain damage (contusions and diffuse axonal injury, respectively) and set in train a series of neurochemical reactions (for further detail on the medical aspects of the acute period, which are varied and complex, see Refs. [20, 24]). Other neurological events that can occur after TBI (e.g. skull fractures, hypoxic damage, intracranial haemorrhage, brain infection) can exacerbate brain dysfunction.

Areas of the brain that are the most vulnerable to structural damage from severe contusions are the frontal and temporal lobes (because of contact between the brain and the sharp sphenoidal ridges of the interior surface of the skull); the brain area most vulnerable to diffuse axonal injury is the subcortical white matter as a consequence of rotational forces at the moment of impact causing shearing and tearing of nerve fibres.

With the rare exception of some penetrating brain injuries, TBI initially results in coma. Following emergence from coma, most patients have a variable period of post-traumatic amnesia (ranging from mere minutes to many months), its duration being a function of the severity of the initial injury and any subsequent medical complications (e.g. brain infection). Post-traumatic amnesia is primarily characterised by confusion, disorientation and inability to lay down new memories. By convention, if a patient has not emerged from post-traumatic amnesia by 6 months post-trauma, it is likely that he or she is experiencing global and profound cognitive impairment, rather than the transient state of post-traumatic amnesia. In cases of the most extreme severity, a patient may transit from coma to vegetative/minimally conscious states before entering the phase of post-traumatic amnesia. A very small number of patients remain in the vegetative/minimally conscious state in the longer term [25].

Following emergence from post-traumatic amnesia, a period of active recovery of brain function ensues. For severe TBI, spontaneous recovery plateaus around 6–12 months post-trauma [26]. During this time, many patients undergo inpatient and/or community-based neurorehabilitation from a multidisciplinary team including the following disciplines: rehabilitation medicine, nursing, physiotherapy, occupational therapy, speech and language therapy, clinical and neuropsychology and social work. Continued recovery, along with functional adaptation, may continue to occur over the next year or so. The recovery trajectory for milder degrees of TBI is less protracted. Thus, unlike many other health conditions, the process of recovery from severe degrees of TBI is measured in terms of months or years, rather than weeks or days.

The clinical profiles of mild versus the more severe degrees of TBI differ significantly. Post-concussion symptoms are common after mild TBI, comprising a mixture of somatic (most commonly headaches and dizziness), cognitive (forgetfulness and poor concentration) and behavioural (fatigue and irritability) symptoms. These symptoms can be debilitating but resolve within weeks or the first few months in the majority of cases [27].

With severe degrees of injury, the contusions and diffuse axonal injury in the brain, which were described earlier, bear a direct relationship to the patterns of impairments characterising TBI. Contusions in the frontal lobes have an impact on executive functions and the regulation of behaviour, contusions in the temporal lobes are responsible for impairments in memory and learning functions, and diffuse axonal injury in the subcortical white matter disrupts speed of information processing. These three areas of impairments fall within the neuropsychological domain, and indeed cognitive and behavioural impairments are more common than physical impairments after TBI. A state-wide multicentre cohort study of severe TBI [28] found that by 18 months post-trauma, a smaller proportion (15 % and 20 %) continued to have clinically significant problems with mobility and hand function, respectively, in comparison with the key cognitive domains of executive functioning (54 %) and memory (62 %). Consequently, while acknowledging that a small proportion of people certainly experience physical disability after TBI, the fact is that many people with clinically significant impairments do not present as typically disabled, and for this reason TBI has been described as the ‘hidden disability’.

The introduction of ICF Core Sets both for TBI [29] and vocational rehabilitation [30, 31] are welcome developments. One advantage of core sets is the streamlining of the assessment process to ensure a comprehensive yet efficient evaluation of function. The Comprehensive Core Set for TBI contains 139 categories (37 for body functions, 2 for body structure, 61 for activities/participation and 39 for environmental factors), and the Brief Core Set contains 23 codes (see Table 12.2). These categories are the best ones to target in evaluating TBI.

There is a large, but not complete, overlap between the ICF Core Set for TBI and the ICF Core Set for vocational rehabilitation (see Table 12.2), the latter containing 90 categories in the Comprehensive Core Set and 13 in the Brief Core Set. Only 4 of the 17 body function categories from the Vocational Rehabilitation Comprehensive Core Set are not also represented in the TBI Comprehensive Core Set, 2 of the 40 activities/participation categories and 8 of the 33 environmental categories. This close correspondence between the TBI and Vocational Rehabilitation Core Sets is advantageous.

Outcome studies have repeatedly documented the wide variety of impairments that occur after TBI. For example, in a sample of 55 people with TBI of all severity levels drawn from a rehabilitation hospital, Koskinen et al. [32] found that 100 of the 123 ICF Checklist categories were problem areas, with 48 of the categories present in 30 % or more of the sample. These findings vindicate the large number of categories contained in the TBI Comprehensive Core Set. They also indicate that at the clinical level, there is wide individual variability – even though some areas of function are almost always compromised (e.g. neurocognitive) – other types of dysfunction (e.g. motor-sensory) may also occur.

The evaluation of function and disability after TBI is the vital first step to good clinical management. At the body function level, a thorough evaluation of both motor-sensory and neuropsychological performance is necessary to evaluate functions that may impact the client’s work capacity. The TBI and Vocational Rehabilitation ICF Core Sets provide helpful guidance in this respect. The components of a comprehensive vocational rehabilitation evaluation for TBI are described in detail in Sect. 12.6.1.

Neuropsychological assessment is necessary in order to understand the cognitive strengths and weaknesses of the individual client with TBI (which are not always apparent with formal interview), so that the goals and strategies used in the rehabilitation programme can be tailored appropriately. In this context, assessment takes several hours for the administration of performance-based cognitive tests conducted by a certified neuropsychologist. The reader is referred to Lezak et al. [33] which is one of the most commonly used texts in the field, and the systematic review of Tate et al. [34] details all instruments currently used in the TBI field (n = 728) which are linked to ICF categories. Because of the complexity of neuropsychological impairments, we elaborate below on the characteristic neurocognitive impairments after TBI (but keeping in mind that key motor-sensory impairments may also profoundly affect work performance, e.g. balance problems for a roof tiler).

As noted, the typical neurocognitive impairments after TBI are in the following body function (designated as code b in the ICF) areas, which will affect day-to-day activities (ICF code d) within those respective areas:
Neurocognitive impairments may also have an impact more broadly across multiple activity/participation domains, for example, work and employment (cf. d840-d859 – work and employment).

It is emphasised that these neurocognitive labels are umbrella terms covering a multitude of different types of component processes, which need to be evaluated. The ‘memory’ domain, for example, includes the following functional systems: autobiographical, episodic, implicit, procedural, prospective and semantic memory; working memory predominantly involves executive abilities (see Ref. [35]).¹ Episodic (i.e. memory for events) and prospective (i.e. remembering to remember) memory functions are commonly impaired after TBI. A distinction needs to be made between memories that were established prior to the injury (which will be largely intact) versus the acquisition of new information to be consolidated in memory and retrieved when required (which is the typical problem area after TBI). Knowing the stage at which the memory process is disrupted is also important – problems may occur at any of the initial encoding/acquisition (cf. ICF b1440 – short-term memory), consolidation (cf. ICF b1441 – long-term memory) and/or retrieval (cf. ICF b1442 – retrieval of memory) stages.

Similarly, ‘executive function’ is an umbrella term referring to ‘specific mental functions especially dependent on the frontal lobes of the brain, including complex goal-directed behaviours such as decision-making, abstract thinking, planning and carrying out plans, mental flexibility, and deciding which behaviours are appropriate under what circumstances’ [6, p. 57]. The factor that unites these superordinate and integrative functions is the capacity to utilise intellectual skills in a flexible and adaptive way to regulate behaviour. Although processing speed is a more unidimensional construct than memory or executive functions, a particular challenge in this area is to confirm that the underlying deficit is indeed processing speed, as opposed to other dysfunction which may masquerade and affect processing speed yet is not the underlying problem (e.g. inertia, depression and attentional problems can also affect processing speed).

Thus, it is not sufficient to say a client has a ‘memory’, ‘executive’ or ‘processing speed’ problem – rather, understanding the nature, stage of processing and severity of the impairment is crucial, along with the combined impact of other impairments and knowing about the way in which the client’s ‘memory’ (or other) impairment will interfere with work tasks. Such information will also form the basis for providing the client with specific and targeted compensatory strategies to address his or her problem areas.

Outcomes in relation to return to work vary widely after TBI, particularly as a function of injury severity. Although a proportion of people are able to resume work at a similar level to their pre-injury employment, many people have reduced work hours compared to pre-injury, particularly after severe TBI [36]. Some also report that their work skills and performance are adversely affected [37]. For others with major disability resulting from the TBI, return to the workforce is not an option, and other avocational alternatives need to be considered. As a result of such changes, significant numbers experience decreased earnings from pre- to post-injury [38].

The Dikmen et al. [22] study referred to earlier in this chapter provided detailed examination of one of the largest, most representative and controlled outcome studies of TBI in civilians. The samples comprised a consecutive series of 466 patients with TBI admitted to a level 1 trauma hospital in Seattle, USA, along with two control groups (one with non-neurological trauma, n = 124, and the other consisted of 88 non-injured friends of the TBI patients, hence providing control for important demographic features). The TBI sample spanned the entire injury severity spectrum and was followed up at 12 months post-trauma.

A range of outcomes was reported, but in terms of work status, a significantly smaller percentage of the TBI group (49 %) was working at 12 months post-trauma compared to the trauma controls (63 %) or the ‘friend controls’ (82 %). Six TBI severity subgroups were compared, using a proxy measure of coma duration (time to follow commands), and a dose-response relationship was apparent within the TBI group. The least injury severity subgroup (time to follow commands less than 1 h post-injury) contained the largest proportion of those employed at 12 months (64 %), followed by 50 % and 51 % working for the next two severity subgroups (1–24 h and 1–6 days, respectively, post-injury), then 36 % working in the 1–2-week subgroup, 18 % in the 2–4-week subgroup and 6 % in the most severe subgroup (more than 1 month to follow commands).

The pattern of these results has been confirmed in other studies from different countries, using the duration of post-traumatic amnesia as the injury severity marker; return to work rates for mild TBI usually fall in the 75–80 % range (e.g. Refs. [37, 39]). Levels of returning to work after moderate injury may be lower, with van der Naalt and colleagues [39] reporting a 61 % return to work. In contrast, return to work rates for people with severe TBI usually fall within the 20–40 % range [40–42].

People who sustain a mild TBI and were employed at the time of their injury usually resume work rapidly, returning after an average of 9 working days, with 95 % back at work by 3 months post-injury [37]. By contrast, people with severe TBI generally take much longer and may not commence a vocational rehabilitation programme until 3–6 months post-injury. As a result of such differences, longitudinal studies documenting the trajectory of return to employment post-injury have found that the rates of people resuming mainstream competitive employment peak around the first or second year post-injury. The rate then remains stable or slowly declines over time [42–44].

Given that a large number of people, particularly those with severe TBI, are unable to return to mainstream competitive employment, other reported outcomes are relevant, including home duties, volunteer work and ‘sheltered employment’ (now referred to as disability segregated employment). Rates of people taking up these options range from 7 % to 18 % [37, 41]. Although these activities are important options in terms of meaningful occupation after TBI, they do not constitute mainstream competitive employment.

Belonging to an increasingly recognised subgroup are those adults who sustained their TBI as children; their vocational outcomes are also adversely impacted [45, 46]. Anderson et al. [47] studied outcomes for 124 young adults who sustained a mild, moderate or severe TBI between the ages of 0–16 years and were followed up at 13.7 years post-injury. Overall, young adults with a history of paediatric TBI were 1.7 times more likely to be unemployed or working in an unskilled capacity compared with the general population, 2.1 times less likely to be working in a skilled occupation and 1.6 times less likely to be working in a professional capacity.

Identifying valid prognostic factors associated with return to work can assist in clinical decision making and setting rehabilitation goals with clients. In their systematic review, Nightingale et al. [48] highlighted the importance of predictive models for return to work, sampling variables from pre-injury, injury and post-injury domains in order to adequately identify key prognostic factors. Furthermore, the authors highlighted the importance of identifying prognostic factors from the early phase post-injury as being of maximum clinical value in assessing return to work potential.

Nightingale et al. [48] found that although concerns about the employability of people with TBI who were older or more poorly educated were commonly highlighted in the literature, these factors were rarely associated with lower rates of return to work in the methodologically strongest studies. However, there was evidence to suggest that pre-injury employment was positively associated with post-injury return to work. The second category, injury severity, can only really be tested in studies that include participants with more than one category of injury severity (e.g. mild vs. severe). In the only high-quality study in the review that investigated a broad range of injury severity among a large number of variables, Temkin et al. [49] found that injury severity was in fact the only predictor of return to work status. In terms of early post-injury predictors, Nightingale and colleagues [48] found limited evidence among good quality studies for any of the key domains of factors including cognitive (i.e. attention and processing speed, memory, executive functioning) or neurophysical impairment, functional status and community participation, highlighting the need for future research.

Van Velzen and colleagues [50] reviewed the evidence for return to work after TBI among prognostic variables grouped according to the ICF components (body function, activity, participation) and contextual (i.e. environmental, personal). The authors found limited evidence supporting any prognostic factors in any of the ICF domains and made an observation similar to Nightingale et al. [48] about the fragmented nature of the evidence base due to the large diversity of variables tested across studies. Further research is clearly required into prognostic factors.