Traumatic brain injury (TBI) is a global health problem and one of the most common causes of loss of function and disability. Approximately 69 million individuals are estimated to suffer TBI each year [1]. The incidence of TBI in northern Sweden has been reported as 354 per 100,000 inhabitants and the incidence of hospitalization because of TBI in the whole country is 190 per 100,000 inhabitants [2]. The most common injury events that cause TBI in Sweden are vehicle-related events and falls, as is the case in the rest of the Western world [3].

TBI is defined as a head trauma that leads to a disruption in the brain’s normal function. The head trauma can be due to a penetrating injury, a blow or jolt to the head [4]. In the acute setting, the severity of the TBI is usually classified with the Glasgow Coma Scale (GCS) score based on eye opening, verbal and motor response. The injury is defined as mild (GCS 13–15), moderate (GCS 9–12) or severe (GCS 3–8), depending on the patient responses during the acute physical examination [5]. In Sweden, a computed tomography (CT) is typically performed after a head trauma with a GCS score of less than 15 points and/or anamnestic or clinical risk factors, such as focal neurological deficits and anticoagulation therapy [6]. More than 90% of TBI patients treated in the emergency room (ER) have mild TBI (mTBI) where the majority recover within a few months but identifying the minority at risk of persistent symptoms is of utmost importance.

The ER at the Umeå University Hospital in Northern Sweden was a recruiting institute of data from TBI patients for the Collaborative European Neurotrauma Effectiveness Research in Traumatic Brain Injury study (CENTER-TBI). The aim of the CENTER-TBI project was to better characterize TBI as a disease and improve the care of the patients [7, 8]. In the same manner, the goal of physical and rehabilitation medicine is to assess functioning in relation to disorder or disease, personal and environmental factors, such as evaluation of disability after brain injury [9]. The Glasgow Outcome Scale Extended (GOSE) is widely used to to measure disability after TBI, assess independence and return of lifestyle [10]. The Rivermead Post- Concussion Symptoms Questionnaire (RPQ) is likewise a well-established way of measuring common symptoms after TBI [11]. Although TBI often causes disability, the outcome and the need for rehabilitation varies. Previous studies have shown that many TBI patients, especially those with mild TBI, have long-term residual symptoms and are not offered rehabilitation despite their need [12, 13]. The overall aim of this study was to evaluate symptoms and disability five years after mild TBI in a defined population in the Umeå area. The specific aim was to investigate how characteristics such as age, gender, pre-injury systemic disease, CT-result and additional TBI affect the outcome.

In this follow-up study on TBI patients, the majority (93%) of the participants were classified as mild, therefore only this group was studied further. Out of this population, as many as three of four patients still reported post-TBI symptoms five years after the initial injury. The number of symptoms was significantly correlated to the level of disability on the GOSE. No significant difference was found between women and men regarding the number and severity score of symptoms. Both repeated TBI and pre-injury systemic disease were significantly related to the level of disability.

In the current study, the proportion of mild TBI is in line with previous research that has estimated that 80–90% of all cases of TBI are mild [15]. Likewise, previous studies have shown that fatigue is the most common symptom in all participants [16]. Interestingly, there was a difference between women and men, the most common symptoms in women being frustration and poor concentration. In addition, two of the symptoms were significantly more frequent in women than in men. There are mixed findings regarding gender differences, where some studies have found an association between women and post-concussion symptoms [13, 17], while other studies report no significant difference between gender [18]. The reasons for possible gender differences are unclear.

Surprisingly, 6% of the mTBI patients reported a severe disability, meaning they could not live entirely independently. GOSE is considered as the “golden standard” for assessing outcomes after TBI [19]. In the current study, post-concussion symptoms correlated significantly with disability. This finding has also been shown in previous studies that have used the RPQ questionnaire, a specific instrument for the assessment of disability after mild TBI [20]. Furthermore, there was good correlation between GOSE and both repeated TBI and pre-injury systemic disease. Multiple head injuries are described as having a cumulative effect on function, which may be the reason for the higher level of disability in this group. Within the TBI research area, there is more evidence of a prevailing association between TBI and neurodegeneration [21] which can be influenced by repeated trauma.

CT scans of the brain remain a standard diagnostic tool for assessing TBI patients and are used to predict outcome. No significant correlation was seen between GOSE and CT results which is in line with previous studies by Smith et al. [22] that showed limitations of CT results as a prognostic factor of long-time consequences. Similar findings were reported in a study on severe TBI from our hospital, namely that CT results were significantly related to GOSE three months after injury, but not after one year [23].

Only a minority of the participants had received some form of rehabilitation. Since mild TBI is considered to be a “mild injury”, this could be a reason why this category of patient was not offered treatment and rehabilitation. However, in a recent meta-analysis by Möller et al. [12] , it was concluded that “persons with mild traumatic brain injury who are at risk of, or who experience, prolonged symptoms should be considered for specialist treatment”.

One strength of this study is that it is based in part on the neurotrauma CENTER-TBI project with defined inclusion and exclusion criterion. Therefore, we gathered a large amount of randomised patient material, with high generalizability, including representatives from a wide range of ages. Another strength is that validated questionnaires with instruments designed for TBI were used and patients were recruited from the same hospital. This study suffers from a few limitations. Firstly, there was no healthy control group, which makes it difficult to draw conclusions based on the overall percentage of reported symptoms. It should be noted that post-concussion symptoms do not occur exclusively in patients with mTBI but are also frequently reported in the general population and for patients with migraine, chronic pain or those who have suffered whiplash injuries [24]. Another limitation is the relatively long follow-up period which may lead to recall bias. Therefore, we cannot rule out systematic distortions as other factors might have influenced the results. Only a few studies have investigated long-term follow-up after TBI and therefore studying the long-term consequences of mild TBI was an interesting aspect of this study.

This study shows that five years after sustaining mild TBI, patients reported high frequencies of symptoms and disabilities. These findings indicate that it might be clinically meaningful to quantify symptoms earlier after the injury in order to identify patients in need of rehabilitation and to tailor specific treatment and rehabilitation interventions. The relationship between multiple trauma and disability highlights the importance of informing patients to be more careful in risky situations. In a future perspective, further research within this area may result in a way to identify subgroups of patients and study gender differences.

The data are not publicly available due to ethical restrictions.

BMS, EA and BMM designed the research study. EA performed the research. EA analyzed the data. EA, BMM and BMS wrote the manuscript. BMS and BMM provided help and advice on writing and editing the manuscript. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

The study was approved by the Swedish Ethical Review Authority (Dnr 2019-05337). All participants gave informed consent prior to their enrolment. The study was performed in accordance with the principles of the Declaration of Helsinki.

We sincerely thank Maud Stenberg, M.D, PhD for her help in writing the manuscript.

This research received no external funding.

The authors declare no conflict of interest.