Long-lasting mental fatigue is a pathological state with an extreme mental energy deficit and reduced ability to regain this energy in response to activity, while “normal” fatigue is time-limited and alleviated by rest (Rönnbäck and Johansson, 2022). Pathological mental fatigue results in reduced quality of life and impaired ability to maintain employment or educational status after an acquired brain injury, endocrine or burn-out syndrome (Cantor et al., 2008; Glise et al., 2010; Johansson and Rönnbäck, 2014b; Palm et al., 2017; Leso et al., 2020). Work ability defined by Tengland (2011) is having the health status to perform work with tasks that are reasonable in an acceptable work environment. The so-called invisible mental energy deficit is difficult to conceptualize particularly in relation to work ability. From clinical experience, patients usually have a strong wish to return to ordinary life, including work, but they may not always understand or report their reduced capacity to work. If adequate support is missing, some will return to work and increase their working hours too quickly, with an increased risk of becoming more fatigued followed by a reduced work ability. Others may seem to lack the motivation or appear to be lazy. As many as 30% of family members and friends interpret fatigue after traumatic brain injury (TBI) as laziness (Norrie et al., 2010). This shows how difficult it can be to both understand mental fatigue and assess the patient’s work capacity, even for health professionals, if there is no team trained in assessing work ability in relation to mental fatigue.

Patients included in this study, commonly affected by long-term mental fatigue are those who have suffered an acquired brain injury (ABI, here TBI or stroke), been diagnosed with burn-out syndrome (BO) or hypothyroidism (HYT). Studies reporting work ability for these patient groups in relation to fatigue show that work ability can be reduced during many years and return to work can be difficult.

Twenty-three health care centers specializing in primary care in Gothenburg were contacted, and 15 of these consented to participate. Included patients have had contact with their health center during the past 5 years for acquired brain injury (ABI, TBI and stroke, diagnostic codes ICD-10: S06, I60, I63.0-9), hypothyroidism (HYT, deficiency of thyroid hormone, E03.9), and burn-out syndrome (BO, Swedish definition, Exhaustion disorder, F43.8A) (Åsberg et al., 2003). A randomized selection of participants was made by the Department of Data Management and Analysis, Region Västra Götaland, with 200 participants/group. Only one diagnosis was used for the selection of patients. The ABI group was prioritized for the selection, as this was the group with the least number of patients, second priority BO and lastly HYT as being the largest group. The ambition was to capture those who had recovered and those who were still struggling with mental fatigue in relation to work ability. A mail was sent to all the participants providing information concerning the study, the questionnaires, copy of a letter of approval from the head of the health center and a stamped addressed envelope. The study was approved by the Swedish Ethical Review Authority (2019-05177) and the Department of Data Management and Analysis, Region Västra Götaland (202-03789).

A total of 154 people (26%) responded and 2% of the letters were returned without having been responded to out of a total of 600. The groups were similar in number of respondents, age and education. The ABI group had an even gender distribution, but significantly more women responded in the HYT and BO groups. Work status did not differ between groups, but the self-perceived work status differed significantly between groups (Table 1).

A majority of the respondents had a reduced work ability as indicated with WAI scores and problems with mental fatigue as indicated with MFS. The BO group reported the lowest/worst WAI score and the highest MFS rating; the HYT group, the highest/best WAI score and least problems with mental fatigue and the ABI group reported scores in-between these (Table 2). The gender distribution differed between groups. The within-group comparison (t-test/group) did not find any difference between men and women of their MFS rating (ABI p = 0.594, HYT p = 0.683. BO p = 0.691) or WAI rating (ABI p = 0.715, HYT p = 0.767. BO p = 0.603).

The purpose of the study was to evaluate the relationship between work ability and mental fatigue using a linear regression analysis. For this, the goal to include people suffering from mental fatigue (cutoff MFS over 10) and those not, concerning work ability was achieved, even if the group without problems was smaller (Table 2). The numbers of men and women differed between groups and gender was included in the analysis. The dependent variable was work status in the workplace and the independent variables (predictors) were diagnostic group, age, gender, WAI and MFS. The result showed that the model was significant, F_(5,138) = 32.711, p < 0.001, and explained 53% of the variance (Adjusted R2 = 0.526). WAI and MFS were significant predictors for work status in the workplace, but neither diagnostic group, gender nor age were (Table 3).

Work Ability Index and MFS showed a high significant correlation and both scales correlated significantly with work status in the workplace and self-perceived own estimated work status (groups merged). Work status in the workplace correlated significantly with age and WAI and decreased with increasing age. Age did not correlate with MFS and self-perceived work status (Table 4). The relationship between WAI and MFS for each patient group is shown in Figure 1.

The results showed that there was a clear relationship between work ability (WAI) and mental fatigue (MFS) and both scales correlated with work status in the workplace. WAI and MFS were also significant predictors for work status in the workplace. Neither diagnostic group, age or gender, turned out to be significant predictors for work status. The connection between fatigue and work ability found here is in agreement with what is reported from other ABI, HYT and BO studies (Glise et al., 2010; Andersen et al., 2012; Waljas et al., 2014; Palm et al., 2017; Leso et al., 2020; Rutkowski et al., 2021; Western et al., 2021). This shows that mental fatigue is important to routinely screen for when work ability is assessed, specifically in patient groups where fatigue commonly occurs. This also shows the need to inform employers and the patient about the impact mental fatigue may have on return to work and to adapt working hours and working environment to what is sustainable for the patient.

It is here suggested that WAI and MFS can be used as a screening tool and this can indicate when an in-depth investigation is required. If a patient reports mental fatigue, the underlying causes should always be identified before initiation of treatment, rehabilitation and adaptation of the working environment. The intention is to avoid or minimize long-term sick leave and to improve well-being in the workplace. The patient may have several other medical disorders and/or cognitive, psychological and social problems not included in this study, which need to be evaluated in relation to mental fatigue. It is also necessary to evaluate and understand the answers from the items included in WAI and MFS. Some individuals may over-estimate their problems while others may under-estimate them due to a lack of insight, knowledge or memory problems. However, both the WAI and MFS scales have pre-defined alternatives (no likert scale), are highly consistent between and within patients and this, in turn facilitates patient follow-up.

The screening method suggested here with WAI and MFS can help health care professionals to identify people who are mentally fatigued with a reduced ability to work or return to work after illness. It has previously been shown that WAI is a sensitive screening tool to identify people on sick leave with a probable need for rehabilitation (Bethge et al., 2015) and that MFS is related to work ability (Palm et al., 2017). In this study, it was also indicated that some of the participants probably worked more than they could manage. Their self-perceived percentage of full-time working hours was lower than the level corresponding to their work status in the workplace and of those working full-time, several reported a poor to moderate work ability according to WAI. This indicates that some worked more than what they could manage. From clinical experience, several patients have a strong wish to return to ordinary life including work but they have not understood or adequately reported their reduced capacity to work. They have returned to work too early and have increased working hours too fast and worked above their limits. This resulted in total exhaustion and a need to reduce the working hours for a long time. To avoid this, early identification of patients and adaption of the working hours and working environment is important.

Among the respondents, 19 reported a second diagnosis from the background questionnaire whether they have an additional diagnosis of the three included in the study. Only one diagnosis was used for the selection of participants. The ABI was prioritized in the selection process due to the lower numbers of patients having an ABI diagnosis reported in the primary care. An additional diagnosis of those included here, as well as other diagnoses not included, could increase the burden to the perceived mental fatigue as well as working ability. As this was a survey and no additional in-depth analysis of participants was done, it was not possible to evaluate which diagnosis was the most prominent for the 19 participants with a self-reported second diagnosis, nor whether more than one diagnosis would exacerbate the mental fatigue and cause a decline in work ability. Several factors can worsen the state of people’s work ability. This highlights the need of a thorough assessment for the patient when planning for treatment and rehabilitation. In this study, the respondents only reported their total burden on mental fatigue in relation to work ability.

The design of the study was not intended to compare the diagnostic groups in terms of mental fatigue and work ability, although the results may, to some extent reflect the reality. BO was the group that reported the highest MFS and the lowest WAI ratings. BO have high impact on work ability and can cause long-term sick leave and it may take several years to return to work (Beno et al., 2021). BO is a diagnosis related to work ability and feelings of energy depletion or exhaustion while the others are medical diagnoses and are not diagnosed based on work disability and mental fatigue. The ABI group rated MFS and WAI in-between the ratings reported for BO and HYT. After an ABI, many patients are initially affected by mental fatigue to varying degrees (Andelic et al., 2020; Rutkowski et al., 2021) and recover to some extent over time (Olver, 1996). A majority with mild TBI will recover within months (Carroll et al., 2004). However, several with acquired mild injury may experience long-term problems with fatigue (Johansson et al., 2009; Winward et al., 2009). HYT was the group that reported the least problems with mental fatigue and diminished work ability. HYT can be treated with hormone replacement drugs that can have a beneficial effect on MFS and WAI, although not always (Louwerens et al., 2012).

Twenty-six percent of the people addressed responded. The number of respondents was similar between the diagnostic groups with no difference in age and education level. There was a predominance of university graduates (50–62%). By comparison, about 50% of the population in this age group in Gothenburg have a university education. There was an even gender distribution in ABI. The women were in the majority in the HTY (79%) and BO (78%) groups, and this corresponds to the gender distribution for HYT and BO in Sweden (Socialstyrelsen, 2018). The rating of MFS and WAI did not differ between men and women within the respective diagnostic groups.

The raw data supporting the conclusions of this article will be made available by the author, without undue reservation.

The studies involving human participants were reviewed and approved by Etikprovningsmyndigheten. The patients/participants provided their written informed consent to participate in this study.

BJ designed the study, collected the data, analyzed the data, wrote the manuscript, and approved the submitted version.

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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