Fatigue After Traumatic Brain Injury: A Systematic Review

"I moved from simply being to actively living." A qualitative study on peer mentorship camps for adults with acquired brain injury

PURPOSE

This study explored the experiences of an Active Rehabilitation peer mentorship camp on individuals with acquired brain injury (mentees) and its impact on their daily lives.

METHODS

The research involved field observations and longitudinal interviews with 18 mentees attending an Active Rehabilitation camp in Norway. Data were narratively analyzed using the Self-determination theory for insight and discussion.

RESULTS

Three themes were constructed from the analysis. The first theme, "Striking a balance between rest and activity," reflects how mentees managed post-injury fatigue and how camp experiences enhanced their understanding and coping strategies. The second theme, "Rebuilding who I am," addresses stigma, reduced self-esteem, and acceptance challenges faced by mentees, revealing positive effects from camp participation. The third theme, "Finding a new flock," describes the reduction in social circles after injury and how interactions at the camp expanded their peer networks during and after the camp.

CONCLUSIONS

Active Rehabilitation camps enhance community-based rehabilitation for individuals with acquired brain injury by improving knowledge, boosting self-esteem, and broadening social networks. It is recommended that camp providers assess the impact of room-sharing and devise strategies for personalizing camp activities and effectively matching mentees and peer mentors.

A virtual rehabilitation tool for cognitive rehabilitation after traumatic brain injury

PURPOSE

The aim of the current study was to test a virtual reality (VR) rehabilitation tool for traumatic brain injury (TBI) and determine if the system could be integrated into usual clinical practice.

MATERIALS AND METHODS

A mixed cohort study with 10 people with TBI, 4 Family/Whānau members and 13 clinicians from a single community centre specialising in TBI rehabilitation. TBI participants received two sessions of VR per week for six weeks integrated into usual rehabilitation. Fatigue measures, and system recorded therapist scores were collected from conversation, memory, and attention tasks embedded into each level of the VR. Participants also took part in interviews or focus groups.

RESULTS

The tool was successfully integrated into usual clinical care. TBI participants showed a trend of reduced fatigue and increased awareness of fatigue. The overarching theme identified from the qualitative data was 'Learn Reflect and Apply in Real Life'.

CONCLUSIONS

The VR tool can be integrated into clinical practice to provide a practical setting where clients with TBI can develop insight and awareness into the impact of their brain injury on their function, learn adaptive strategies and practice them repeatedly in a simulated, safe, realistic, and supported environment.

Clinician perspectives on cognitive behavioral therapy and health education for sleep disturbance and fatigue after acquired brain injury: A qualitative study

Sleep disturbance and fatigue are persistent, clinically significant problems for many with acquired brain injury (ABI). A recent randomized controlled trial (RCT) demonstrated that cognitive and behavioral therapy tailored for post-ABI sleep and fatigue (CBT-SF) produced larger and faster effects compared to an active health education (HE) control intervention. The current study aimed to qualitatively explore the perspectives of the six clinicians, all clinical neuropsychologists, who delivered the CBT-SF and HE interventions to 126 individuals with ABI in the RCT. One-on-one semi-structured interviews were conducted, and transcripts were analyzed using reflexive thematic analysis. Seven themes were generated, encompassing client-related factors (Client Awareness and Readiness as Precursors to Engagement and Navigating Client Differences Throughout Treatment) and clinician-related factors that influenced treatment delivery (Building on Pre-Existing Expertise and Strengths and Challenges of Delivering a Manualized Intervention), as well as other themes directly related to treatment delivery itself (Challenges and Successes Relating to Client Engagement, CBT Strategies That Work, and Navigating Practical Considerations in Treatment Delivery). The findings reinforce the value of psychoeducation and cognitive-behavioral strategies for managing sleep and fatigue problems after ABI. They also provide valuable insights to guide the translation of CBT-SF from an RCT to real-world clinical practice.

Goal management training for improving fatigue in children and adolescents with acquired brain injuries: A 2-year follow-up of a randomised controlled trial

To explore the long-term effectiveness of a paediatric adaptation of Goal Management Training (pGMT), relative to a psychoeducative program (pBHW), in reducing fatigue after pABI 2 years post-intervention. Thirty-eight youths and their parents completed the Paediatric Quality of Life - Multidimensional Fatigue Scale. Primary outcome measures were Total Fatigue Score, General fatigue, Cognitive fatigue, and Sleep/rest fatigue (parent-report). No significant differences in fatigue symptoms by the parental report was observed between the intervention groups at the 2-year follow-up (total score: F = .16, p = .69; general fatigue: F = .36, p = .55; sleep/rest: F = .48, p = .49; and cognitive fatigue: F = .09, p = .76), nor any time*group interactions (total score: F = .25, p = .86; general fatigue: F = .39, p = .76; sleep/rest: F = .20, p = .89; and cognitive fatigue: F = .08, p = .97). In total, 45% of the participants in the pGMT group and 25% in the pBHW group demonstrated a reliable positive clinical change. The significant improvements in fatigue symptoms that were demonstrated 6 months post-intervention could not be confirmed in this 2-year follow-up study. However, a continued positive tendency on most dimensions of fatigue for the participants in the pGMT group could be observed, suggesting that cognitive rehabilitation may help reduce fatigue.

Understanding participant experiences of cognitive behavioural therapy and health education for sleep disturbance and fatigue after acquired brain injury: a qualitative study

PURPOSE

A recent randomised controlled trial demonstrated the efficacy of a cognitive behavioural therapy intervention for sleep disturbance and fatigue (CBT-SF) after acquired brain injury (ABI), leading to more rapid improvement than an active health education (HE) control intervention. This study qualitatively examined participants' experiences of receiving the CBT-SF and HE interventions.

MATERIALS AND METHODS

20 individuals with ABI shared their experiences with the interventions through semi-structured interviews, with transcripts analysed using reflexive thematic analysis.

RESULTS

We identified eight themes, organised into a narrative spanning the start of treatment (Coming Into Therapy), through the process of change (Acceptance, Taking Control, and It's Hard Work) and helpful intervention elements (Building Knowledge and Skills and Facilitating Factors), to life post-treatment (Maintaining Improvements and Overall Intervention Outcomes). Treatment helped participants accept their sleep and fatigue problems as enduring consequences of a brain injury, while empowering them to regain control by learning strategies to manage their symptoms, which they continued to use post-treatment.

CONCLUSIONS

This qualitative study reinforces the value of the CBT-SF and HE interventions for sleep and fatigue problems after ABI. Findings contribute to our understanding of the mechanisms driving change within these interventions and raise important considerations for clinical translation.

Concussion and the Autonomic, Immune, and Endocrine Systems: An Introduction to the Field and a Treatment Framework for Persisting Symptoms

A significant proportion of patients who sustain a concussion/mild traumatic brain injury endorse persisting, lingering symptoms. The symptoms associated with concussion are nonspecific, and many other medical conditions present with similar symptoms. Medical conditions that overlap symptomatically with concussion include anxiety, depression, insomnia, chronic pain, chronic fatigue, fibromyalgia, and cervical strain injuries. One of the factors that may account for these similarities is that these conditions all present with disturbances in the optimal functioning of the autonomic nervous system and its intricate interactions with the endocrine system and immune system-the three primary regulatory systems in the body. When clinicians are working with patients presenting with persisting symptoms after concussion, evidence-based treatment options drawn from the literature are limited. We present a framework for the assessment and treatment of persisting symptoms following concussion based on the available evidence (treatment trials), neuroanatomical principles (research into the physiology of concussion), and clinical judgment. We review the research supporting the premise that behavioral interventions designed to stabilize and optimize regulatory systems in the body following injury have the potential to reduce symptoms and improve functioning in patients. Foundational concussion rehabilitation strategies in the areas of sleep stabilization, fatigue management, physical exercise, nutrition, relaxation protocols, and behavioral activation are outlined along with practical strategies for implementing intervention modules with patients.

The beneficial effects of modafinil administration on repeat mild traumatic brain injury (RmTBI) pathology in adolescent male rats are not dependent upon the orexinergic system

The sleep-wake cycle plays an influential role in the development and progression of repeat mild traumatic brain injury (RmTBI)-related pathology. Therefore, we first aimed to manipulate the sleep-wake cycle post-RmTBI using modafinil, a wake-promoting substance used for the treatment of narcolepsy. We hypothesized that modafinil would exacerbate RmTBI-induced deficits. Chronic behavioural analyses were completed along with a 27-plex serum cytokine array, metabolomic and proteomic analyses of cerebrospinal fluid (CSF), as well as immunohistochemical staining in structures important for sleep/wake cycles, to examine orexin, melanin-concentrating hormone, tyrosine hydroxylase, and choline acetyltransferase, in the lateral hypothalamus, locus coeruleus, and basal forebrain, respectively. Contrary to expectation, modafinil administration attenuated behavioural deficits, metabolomic changes, and neuropathological modifications. Therefore, the second aim was to determine if the beneficial effects of modafinil treatment were driven by the orexinergic system. The same experimental protocol was used; however, RmTBI rats received chronic orexin-A administration instead of modafinil. Orexin-A administration produced drastically different outcomes, exacerbating anxiety-related and motor deficits, while also significantly disrupting their metabolomic and neuropathological profiles. These results suggest that the beneficial effects of modafinil administration post-RmTBI, work independently of its wake-promoting properties, as activation of the orexinergic wake-promoting system with orexin-A was detrimental. Overall, these findings highlight the complexity of sleep-wake changes in the injured brain and showcase the potential of the arousal and sleep systems in its treatment.

Delivery of Neuropsychological Interventions for Adult and Older Adult Clinical Populations: An Australian Expert Working Group Clinical Guidance Paper

Delivery of neuropsychological interventions addressing the cognitive, psychological, and behavioural consequences of brain conditions is increasingly recognised as an important, if not essential, skill set for clinical neuropsychologists. It has the potential to add substantial value and impact to our role across clinical settings. However, there are numerous approaches to neuropsychological intervention, requiring different sets of skills, and with varying levels of supporting evidence across different diagnostic groups. This clinical guidance paper provides an overview of considerations and recommendations to help guide selection, delivery, and implementation of neuropsychological interventions for adults and older adults. We aimed to provide a useful source of information and guidance for clinicians, health service managers, policy-makers, educators, and researchers regarding the value and impact of such interventions. Considerations and recommendations were developed by an expert working group of neuropsychologists in Australia, based on relevant evidence and consensus opinion in consultation with members of a national clinical neuropsychology body. While the considerations and recommendations sit within the Australian context, many have international relevance. We include (i) principles important for neuropsychological intervention delivery (e.g. being based on biopsychosocial case formulation and person-centred goals); (ii) a description of clinical competencies important for effective intervention delivery; (iii) a summary of relevant evidence in three key cohorts: acquired brain injury, psychiatric disorders, and older adults, focusing on interventions with sound evidence for improving activity and participation outcomes; (iv) an overview of considerations for sustainable implementation of neuropsychological interventions as 'core business'; and finally, (v) a call to action.

Post-COVID fatigue: Reduced quality-of-life associated with clinically relevant fatigue in mild disease courses

Fatigue is a pervasive symptom experienced by many individuals after COVID-19. Despite its widespread occurrence, fatigue remains a poorly understood and complex phenomenon. Our aim is to evaluate the subjective experience of mental fatigue after COVID-19 and to assess its significance for daily life functioning. In this online questionnaire study (N = 220), the Fatigue Severity Scale (FSS), World Health Organization Quality-of-Life assessment (WHOQoL) and a subjective severity rating of the COVID-19 disease progression were used. For our statistical analyses we utilized independent samples t-tests, one-way ANOVA with post-hoc analyses, and a multiple regression. As expected our findings revealed the COVID group reported significantly higher levels of subjective fatigue compared to the control group. Moreover, there was a significant difference between experienced fatigue across the four severity groups. Participants who had a milder course of disease also experienced severe subjective fatigue. Subjective fatigue explained 40% variance in quality-of-life. In conclusion, severe subjective fatigue appears to be associated with increased self-reported COVID-19 symptom severity and lower quality-of-life but is already observable in milder cases. This underscores, firstly, the importance of considering also less severe cases and, secondly, the need to develop rehabilitation and psychological interventions for fatigue.

Fatigue in young patients with acquired brain injury in the outpatient rehabilitation setting: A 2-year follow-up study

Acquired brain injury (ABI) may cause fatigue and participation restrictions in young patients. However, knowledge regarding the course of these problems over time is lacking. This study aims to describe the course of fatigue and participation and their relationship over time in an observational two-year follow-up study among patients(5-24 years) with ABI referred for outpatient rehabilitation and their parents. Patients/parents completed the PedsQL™Multidimensional-Fatigue-Scale(PedsQL™MFS, totalscore/3-domains) and the Child/Adolescent-Scale of Participation(CASP, totalscore/4-domains). Scores ranged from 0-100: lower scores = more fatigue/participation problems. Linear mixed models and repeated measures correlations were used to determine the course over time (change-scores/95%CI) and correlations between fatigue/participation. At baseline, 223 patients/246 parents participated with 94/104 at either T1, T2 or both. Median age was 15 years (IQR:12-17), 74% had a traumatic brain injury. Mean(SD) patient/parent-reported PedsQL™MFS totalscores(baseline) were: 50.3(17.3) and 53.8(19.1), respectively. CASP totalscores were 78.0(16.4) and 87.1(13.6). Over time, patient-reported scores improved significantly (fatigue: + 8.8 (2.9;14.7), p < 0.05)/participation: + 10.5 (6.3;14.7), p < 0.05)). Similar results were found regarding parent-reported fatigue: + 8.7 (3.4;13.9), p < 0.05 but not regarding participation. Two years later, fatigue was still considerable(patients:59.1/parents:62.5). Moderate/fair correlations between fatigue/participation over time were found. Fatigue and participation in young patients with ABI improved two years after referral to rehabilitation. However, fatigue remained a considerable problem.

Neuropharmacology in Traumatic Brain Injury

One of the primary goals in traumatic brain injury (TBI) treatment is to minimize secondary brain damage and promote neuroprotection. In TBI rehabilitation, we seek to facilitate neurologic recovery and restore what independence is possible given a patient's physical and cognitive impairments. These goals must be balanced with treatment of the various symptoms that may occur following TBI. This is challenging given the fact that many of the typical treatments for certain symptoms also come with side effects which could be problematic in the TBI population.

Non-pharmacological interventions for sleep disruptions and fatigue after traumatic brain injury: a scoping review

OBJECTIVE

The aim of this study was to conduct a scoping review to determine the nature, variety, and volume of empirical evidence on nonpharmacological interventions for sleep disturbances with potential implications for fatigue in adults sustaining a traumatic brain injury (TBI).

METHODS

A systematic literature search was conducted across four databases to identify primary studies testing a single non-pharmacological intervention or a combination of non-pharmacological interventions for sleep disturbances and fatigue in community-dwelling adults with TBI.

RESULTS

Sixteen studies were reviewed addressing six non-pharmacological interventions for sleep disruptions and fatigue after TBI including light therapy, cognitive-behavioral therapy, warm footbath application, shiatsu, and sleep hygiene protocol. Non-pharmacological interventions involving light or cognitive-behavioral therapy were reported in 75% of the studies. Actigraphy-based estimation of total sleep time and subjective level of fatigue were frequent outcomes.

CONCLUSION

While this scoping review has utility in describing existing non-pharmacological approaches to manage sleep and fatigue after TBI, the findings suggest that interventions are often developed without considering TBI individuals' source of motivation and the need for support in self-administration. Future studies may achieve greater sustainability by considering the evolving needs of TBI patients and their families and the drivers and barriers that might influence non-pharmacological intervention use at home.

Life After Traumatic Brain Injury: Effects on the Lifestyle and Quality of Life of Community-Dwelling Patients

Persons who have experienced traumatic brain injury (TBI) may encounter a range of changes in their physical, mental, and cognitive functions as well as high fatigue levels. To gain a comprehensive understanding of the challenges faced by persons after TBI, we conducted multi-domain assessments among community-dwelling persons with a history of TBI and compared them with age- and sex-matched controls from the Northeastern Taiwan Community Medicine Research Cohort between 2019 and 2021. A total of 168 persons with TBI and 672 non-TBI controls were not different in terms of demographics, comorbidities, and physiological features. However, compared with the non-TBI group, the TBI group had a distinct lifestyle that involved increased reliance on analgesics (6.9% vs. 15.0%, respectively; p = 0.001) and sleep aids (p = 0.008), which negatively affected their quality of life. Moreover, they consumed more coffee (p < 0.001), tea (p < 0.001), cigarettes (p = 0.002), and betel nuts (p = 0.032) than did the non-TBI group. Notably, the use of coffee had a positive effect on the quality of life of the TBI group (F = 4.034; p = 0.045). Further, compared with the non-TBI group, the TBI group had increased risks of sarcopenia (p = 0.003), malnutrition (p = 0.003), and anxiety (p = 0.029) and reduced blood levels of vitamin D (29.83 ± 10.39 vs. 24.20 ± 6.59 ng/mL, respectively; p < 0.001). Overall, the TBI group had a reduced health-related quality of life, with significant challenges related to physical health, mental well-being, social interactions, pain management, and fatigue levels. Moreover, the TBI group experienced poorer sleep quality and efficiency than did the non-TBI group. In conclusion, persons who have sustained brain injuries that require comprehensive and holistic care that includes lifestyle modification, mental and physical healthcare plans, and increased long-term support from their communities. ClinicalTrials.gov (identifier: NCT04839796).

Efficient assessment of brain fog and fatigue: Development of the Fatigue and Altered Cognition Scale (FACs)

Debilitating symptoms of fatigue and accompanying "brain fog" are observed among patients with various chronic health conditions. Unfortunately, an efficient and psychometrically sound instrument to assess these co-occurring symptoms is unavailable. Here, we report the development and initial psychometric properties of the Fatigue and Altered Cognition Scale (the FACs), a measure of self-reported central fatigue and brain fog. Traumatic brain injury (TBI) was chosen to model and develop the FACs due to research team expertise and established links between TBI and the symptom complex. Potential items were generated by researchers and clinicians with experience treating these symptoms, drawing from relevant literature and review of patient responses to measures from past and current TBI studies. The 20 candidate items for the FACs-ten each to assess altered cognition (i.e., brain fog) and central fatigue-were formatted on an electronic visual analogue response scale (eVAS) via an online survey. Demographic information and history of TBI were obtained. A total of 519 participants consented and provided usable data (average age = 40.23 years; 73% female), 204 of whom self-reported a history of TBI (75% reported mild TBI). Internal consistency and reliability values were calculated. Confirmatory factor analysis (CFA) examined the presumed two-factor structure of the FACs and a one-factor solution for comparison. A measurement invariance test of the two latent constructs (altered cognition, fatigue) among participants with and without TBI was conducted. All items demonstrated normal distribution. Cronbach's alpha coefficients indicated good internal consistency for both factors (α's = .95). Omega reliability values were favorable (α's = .95). CFA supported the presumed two-factor model and item loadings which outperformed the one-factor model. Measurement invariance found the two-factor structure was consistent between the two groups. Implications of these findings, study limitations, and potential use of the FACs in clinical research and practice are discussed.

Daily patterns of fatigue after subarachnoid haemorrhage: an ecological momentary assessment study

OBJECTIVE

To examine the daily course of, and factors associated with, momentary fatigue after subarachnoid haemorrhage, and to explore subgroups of patients with distinct diurnal patterns of fatigue.

DESIGN

Observational study using ecological momentary assessment.

SUBJECTS

A total of 41 participants with subarachnoid haemorrhage.

METHODS

Patients with fatigue were included within one year post-onset. Momentary fatigue (scale 1-7) was assessed with repeated measurements (10-11 times/day) during 7 consecutive days. Multilevel-mixed-model analyses and latent-class trajectory modelling were conducted.

RESULTS

Mean (standard deviation; SD) age of the group was 53.9 (13.0) years, 56% female, and mean (SD) time post-subarachnoid haemorrhage onset was 9.3 (3.2) months. Mean (SD) momentary fatigue over all days was 3.22 (1.47). Fatigue increased significantly (p < 0.001) over the day, and experiencing more burden of fatigue and day type (working day vs weekend day) were significantly (p < 0.05) associated with higher momentary fatigue. Three subgroups could be distinguished based on diurnal patterns of fatigue. The largest group (n = 17, 41.5%) showed an increasing daily pattern of fatigue.

CONCLUSION

Momentary fatigue in patients with subarachnoid haemorrhage increases over the day, and diurnal patterns of fatigue differ between  participants. In addition to conventional measures, momentary measures of fatigue might provide valuable information for physicians to optimize personalized management of fatigue after subarachnoid haemorrhage.

Stability and Change in Biopsychosocial Factors Associated With Fatigue 6 and 12 Months After Traumatic Brain Injury: An Exploratory Multilevel Study

OBJECTIVE

To explore factors associated with stability and change in fatigue from 6 to 12 months following traumatic brain injury (TBI).

SETTING

Combined in- and outpatient acute care and postacute rehabilitation settings.

PARTICIPANTS

A total of 103 patients with confirmed intracranial injury were assessed 6 and/or 12 months following TBI.

DESIGN

A prospective observational study with repeated measures at 2 time points, analyzed with a hybrid mixed-effects model.

MAIN MEASURES

Primary outcomes were the fatigue factor derived from items from several fatigue patient-reported outcome measures (PROMs; Fatigue Severity Scale, Chalder Fatigue Scale, Giessen Subjective Complaints List-fatigue subscale, and Rivermead Post-Concussion Symptoms Questionnaire-fatigue item) Secondary outcomes were PROMs relating to pain, somatic and psychological distress, insomnia, sleepiness, personality traits, optimism, resilience, behavioral activation and inhibition, and loneliness, as well as neuropsychological measures. Demographic variables and injury severity characteristics were included as covariates.

RESULTS

In multilevel regression, female sex, years of education, and 3 factors related to injury severity, somatic vulnerability, and psychosocial robustness were all significantly associated with variation in fatigue between subjects, and explained 61% of the variance in fatigue that was due to stable between-subject differences. Fatigue levels declined significantly over time. Changes in pain severity, somatic symptom burden, psychological distress, and behavioral inhibition were positively associated with changes in fatigue, explaining 22% of the variance in fatigue within subjects.

CONCLUSIONS

The study demonstrated that several previously implicated factors show robust effects in distinguishing individuals with TBI on levels of fatigue, but only a few show additional within-subject associations across time. Pain severity, somatic symptom burden, psychological distress, and behavioral inhibition correlated with fatigue across time, implicating these factors as crucial targets for rehabilitation of patients with TBI who suffer from persistent fatigue.

Defining a Treatment Model for Self-Management of Fatigue in Rehabilitation of Acquired Brain Injury Using the Rehabilitation Treatment Specification System

Systematic treatment descriptions to standardize and evaluate management of fatigue after acquired brain injury (ABI) are lacking. The purpose of this multi-phase qualitative study was to formulate a treatment model for promoting self-management of fatigue in rehabilitation of ABI based on practice-based understandings and routines. The study was conducted in a community-based rehabilitation center in Denmark. The model was defined using the Rehabilitation Treatment Specification System. Phase 1 comprised co-production workshops with five service providers (occupational therapists, physiotherapists, and a neuropsychologist) to elicit preliminary treatment theories. In Phase 2, four case studies were conducted on management of fatigue in vocational rehabilitation. Interviews (n = 8) and treatment log entries (n = 76) were analyzed thematically to specify treatment targets and active ingredients. The treatment model comprised five main components: (i) Knowledge and understanding of fatigue, (ii) Interoceptive attention of fatigue, (iii) Acceptance of fatigue, (iv) Activity management, and (v) Self-management of fatigue. For each component, lists of targets and active ingredients are outlined. In conclusion, management of fatigue includes multiple treatment components addressing skills, habits, and mental representations such as knowledge and attitudes. The model articulates treatment theories, which may guide clinical reasoning and facilitate future theory-driven evaluation research.

Traumatic Brain Injury, Sleep, and Melatonin-Intrinsic Changes with Therapeutic Potential

Traumatic brain injury (TBI) is one of the most prevalent causes of morbidity in the United States and is associated with numerous chronic sequelae long after the point of injury. One of the most common long-term complaints in patients with TBI is sleep dysfunction. It is reported that alterations in melatonin follow TBI and may be linked with various sleep and circadian disorders directly (via cellular signaling) or indirectly (via free radicals and inflammatory signaling). Work over the past two decades has contributed to our understanding of the role of melatonin as a sleep regulator and neuroprotective anti-inflammatory agent. Although there is increasing interest in the treatment of insomnia following TBI, a lack of standardization and rigor in melatonin research has left behind a trail of non-generalizable data and ambiguous treatment recommendations. This narrative review describes the underlying biochemical properties of melatonin as they are relevant to TBI. We also discuss potential benefits and a path forward regarding the therapeutic management of TBI with melatonin treatment, including its role as a neuroprotectant, a somnogen, and a modulator of the circadian rhythm.