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Persistent post-concussion syndrome (PPCS) is a complex and debilitating condition that can develop after head concussions or mild traumatic brain injury (mTBI). PPCS is characterized by a wide range of symptoms, including headaches, dizziness, fatigue, cognitive deficits, and emotional changes, that can persist for months or even years after the initial injury. Despite extensive research, the underlying mechanisms of PPCS are still poorly understood; furthermore, there are limited resources to predict PPCS development in mTBI patients and no established treatment. Similar to PPCS, the etiology and pathogenesis of functional neurological disorders (FNDs) are not clear neither fully described. Nonspecific multifactorial interactions that were also seen in PPCS have been identified as possible predispositions for FND onset and progression.
- post-concussion syndrome
- functional neurologic disorders
- mild traumatic brain injury
1. Introduction
Post-concussion syndrome (PCS) is a sequela of traumatic brain injury (TBI), clinically characterized by complex symptoms that includes headache, dizziness, neuropsychiatric symptoms, and cognitive impairment [1]. Although the term “post-concussion syndrome” has been used since 1934 [2], it is more appropriate to use the term “post-TBI syndrome”, as it may also occur after moderate and severe TBI or even sub-concussive head impacts [1]. There is controversy regarding PCS when its symptoms are exhibited for longer than usual [3]. Persistent PCS is characterized by faint and subjective symptoms and having undefined underlying pathophysiology, thus not necessarily suggesting long-term effects of head trauma. Additionally, its common occurrence in the general population makes PCS easily missed by the clinicians. Moreover, as the standard procedures of PCS assessment could reveal abnormal results, they do not follow consistently defined patterns. For instance, despite the fact that the cognitive symptoms of TBI are prominent, the patient’s standard assessment for chronic PCS often fails to identify any cognitive deficits [4]. However, some studies pointed out repeated concussions as a decisive predisposing factor for chronic traumatic encephalopathy, which is characterized by significant cognitive decline and neurodegenerative state [5,6]. Furthermore, as the affected population is heterogeneous and exhibits varying degrees of injury to the head and brain, it seems that the individual characteristics of the patients may alter the expression of the injury [1].
Functional neurological disorders (FNDs), also known as conversion disorders, are characterized by limb weakness, abnormal movements, or nonepileptic seizures that a neurological disease cannot explain, yet are experienced as genuine and cause distress and/or psychosocial impairment [14]. FNDs are commonly encountered in clinical settings and are associated with poor prognosis [15,16,17,18]. The estimated incidence of conversion disorders in the general population across various geographic regions is 4–12 per 100,000 individuals per year.
2. Mechanisms and Predisposing Factors for Persistent Post-Concussion Syndrome
The distinction between persistent PCS and other medical and psychiatric disorders is vital when considering the treatment approaches that vary from one disorder to another. Similarly, the prognosis of their outcomes could vary despite the overlap of some common symptoms. In some cases, differential diagnosis can be scarce due to symptom overlap and their non-specificity. Furthermore, many other factors could contribute to the persistence of PCS symptoms. Female sex and increasing age are risk factors for PCS in patients with mild and moderate TBI [9,10,22,23].
While the nature of the head injury has not been systematically studied as a risk factor, some evidence has suggested that patients with sports-related concussions may have better prognosis than those with TBIs resulting from motor vehicle accidents, falls, or assaults [24] due to severity differences, as well as due to the physical and psychosocial impact of the injury, the differences in premorbid predisposition to PCS, and psychological factors. The relative preponderance of accidents and assaults as causes of TBI may also contribute to the observed sex differences in PCS risk. In this context, it was suggested that increased neck strength could prevent severe concussion effects [25,26].
The pathogenesis of persistent PCS is unclear. Nevertheless, it is believed to be a complex interaction of structural and functional brain changes, genetic predisposition, psychosocial factors, and healthcare solutions [11]. Studies have suggested that the pathophysiology of persistent PCS may differ from that of acute and subacute PCS [11,12,13].
A psychogenic contribution to persistent PCS is suggested by the high prevalence of comorbid depression, anxiety, and post-traumatic stress disorder (PTSD) in patients with persistent PCS [34,35,36]. Psychological factors may contribute to the persistence of symptoms through negative beliefs about recovery, increased vigilance for symptoms, illness behavior, and a focus on physical rather than emotional or cognitive symptoms [37,38].
Sociocultural factors, such as litigation, compensation, and social support may also contribute to the persistence of PCS symptoms [39,40,41,42]. Patients with financial compensation claims, particularly those involving litigation, are more likely to have persistent symptoms [43,44]. However, it is unclear whether the litigation causes the persistence of symptoms or whether patients with persistent symptoms are more likely to pursue litigation [45].
3. Mechanisms and Predisposing Factors for Functional Neurological Disorder
Despite the etiology and pathogenesis of FND remaining unclear, biological, psychological, and social factors have been identified as probable predisposing factors of FND, triggering and/or perpetuating symptoms [58,59]. Raynor and Baslet described the historical understanding over the FND etiologies and highlighted that these heterogenic premises, as well as the variable response to treatments, could suggest that FNDs are in fact complex disorders of which clear etiology could be more related to the individual than to a pathological redundant pattern [60]. In some cases, as the diagnosis of FND could be the subject of inconsistencies and discrimination [61], efforts are currently made to diminish them.
Psychological factors, including physical or psychological trauma, interpersonal conflicts, and recent or past stressors, may be associated with the onset of FND. These factors are not always reported or specific to the disorder [63,64].
Several cognitive and neurobiological etiological models have been proposed for medically unexplained symptoms and FND. Brown and Reuber recently proposed a model that provides an integrated behavioral and psychological etiological explanation [58,59]. They used Brown’s cognitive model of unexplained illness to explain the possible sources of misleading interpretation of physical symptoms, which can be obtained through personal experience, the observation of others’ experiences or sociocultural influence about health. On the other hand, the neurobiological model of Voon and colleagues explains that FND could be characterized by conversion disorder symptoms onset coupled with increased amygdala-driven emotional arousal based on previous physical or motor experiences [64].
4. Beliefs and Expectations in Post-Concussion Syndrome
Increasing evidence indicates that PCS symptom reporting could be influenced by non-head injury factors, suggesting that symptoms typically associated with PCS may not be unique to head injury. The role of symptom expectation in PCS symptom etiology has been hypothesized, highlighting the need to compare expected symptoms for various disorders. A study of 82 undergraduates who reported their current and expected symptoms if they had suffered a head injury, an orthopedic injury, post-traumatic stress, or depression showed no significant differences in overall symptoms or symptom subscales. However, individuals simulating head injury, post-traumatic stress, and depression expected an increase in total symptoms, whereas individuals portraying orthopedic injury did not. Individuals with head injuries reported fewer affective symptoms than those portraying psychological disorders. These results indicate that illness beliefs and expectations may play a crucial role in PCS and, more specifically, its persistence [78].
5. Illness Beliefs in Functional Neurological Disorders
Beliefs and expectations about health influence functional symptoms in patients with FNDs. Studies have demonstrated that patients with FNDs have a bias toward “jumping to conclusions” and frequently changing their decision when presented with new evidence, which could be a risk factor for inappropriate updating of active inference, the theory in which the brain predicts and explains sensory input through past experiences [85]. This bias is reflected in the fact that patients with FND request less information than healthy controls before forming a decision. Furthermore, patients with functional tremors have been shown to overestimate the occurrence of tremors, reporting an occurrence rate of 80–90% of their waking day. In contrast, objective measurements indicate an average occurrence of only about 30 min daily. This overestimation is significantly more significant than in patients with organic tremors, indicating that top-down prediction of constant tremors may prevent the perception of time without tremor in patients with FND. In addition, the power of symptom expectation has been demonstrated, as those who expected to experience analgesia in certain areas of their body reported analgesia in exactly those areas. This finding has been incorporated into several etiological models for general medically unexplained physical symptoms and FND. These observations suggested that beliefs and expectations play a critical role in developing and maintaining functional symptoms, and they highlight the need for further research to elucidate the underlying mechanisms of this phenomenon [86,87].
6. Similarities in Symptoms and Underlying Mechanisms
PPCS and FND share overlapping features, with similar symptoms and potential underlying psychological and risk factors. They both include physical symptoms such as headaches, dizziness, fatigue, and cognitive symptoms, such as memory problems and concentration problems. FND may also present with motor symptoms, such as tremors, gait abnormalities, and seizures, which can also be seen in PCS [3,4,15,16]. In addition, PPCS and FND may have underlying psychological factors contributing to symptom expression. Psychological stressors, such as trauma or emotional distress have been implicated in developing both PPCS and FND. Anxiety, depression, and PTSD are common comorbidities in PPCS and FND. There is evidence for the co-occurrence of PCS and FND.
One argument for classifying PPCS as an FND is the lack of objective evidence for structural brain damage in most cases of PCS, as well as the presence of psychological or functional factors that may play a role in the expression of PCS symptoms. Furthermore, evidence suggests that psychological factors can contribute to the development and persistence of PCS symptoms. Individuals who experience high levels of stress or anxiety before or after a concussion may be more likely to develop PCS symptoms, even if the initial injury was relatively mild. This is consistent with the idea that FNDs manifest psychological distress rather than a result of structural or physiological abnormalities. One would argue that PCS shares significant similarities with FND.
7. FND Overlay Model of PCS
Symptoms of PCS, particularly persistent ones, may be related to underlying psychological factors, such as anxiety, depression, or PTSD. These factors may contribute to the development and maintenance of PCS symptoms. One would argue that FND and PCS share many features with FND, such as the presence of physical symptoms that are not fully explained by underlying structural or physiological abnormalities and the high prevalence of comorbid psychiatric disorders, such as depression and anxiety, in individuals with PCS.
One potential model for conceptualizing PCS as an FND is the “functional overlay” model. This model suggests that PCS symptoms may be influenced by psychological and behavioral factors, which can exacerbate underlying neurological impairments and contribute to persistent symptoms. In this context, individuals with PCS may experience a range of neurological symptoms, such as changes in cognitive function, mood, and sensory processing, which are related to the underlying brain injury. These impairments may be exacerbated in some cases by psychological and behavioral factors, such as anxiety, depression, or maladaptive coping strategies. Thus, the psychological and behavioral factors could contribute to the development of PPCS by amplifying or maintaining neurological impairments. For example, anxiety or stress may increase physiological arousal, leading to changes in sensory processing or attentional focus. Similarly, maladaptive coping strategies, such as avoidance or overexertion, may exacerbate neurological impairments and contribute to the development of chronic symptoms.
8. Limitations of FND Diagnostic Criteria
The DSM-V diagnosis criteria [14] of FND require one or more symptoms that affect body movement or senses that cannot be explained by a neurological or other medical condition or another mental health disorder, but that cause significant distress or problems in social, work, or other areas, or that are significant enough for medical evaluation to be recommended. Patients with PPCS may exhibit one or more symptoms of cognitive impairment that may cause distress or impairment of functioning. Can their presentation be attributed to another medical condition? Data failed to establish a direct link between the severity of an mTBI and the duration or the severity of cognitive symptoms. The initial symptoms could be attributed to the head impact and the physiological cascade that follows; however, persistent cognitive symptoms are mainly related to certain psychological factors, similar to the predisposing and risk factors for FND.
9. Critiques and Limitations of the FND Model for PPCS
The main arguments against the FND model would be that PCS is a distinct clinical entity with clear neurobiological underpinnings, and that there is no evidence to suggest that psychological factors play a primary role in the development of PCS symptoms. The FND model is too broad and lacks specificity, which may lead to overdiagnosis and overtreatment of individuals with PCS. The controversy surrounding whether PCS is an FND highlights the need for continued research to better understand the underlying mechanisms of PCS and develop effective treatments that target the condition’s physical and psychological aspects. One of the main arguments against PCS as an FND would be that it has distinct underlying neurological factors that are not present in other FNDs. Unfortunately, the pathophysiology of FNDs is still not fully understood; thus, further research would help in formulating more relevant observations regarding the overlap between PCS and FNDs.
Moreover, as stated before, the current model meets some methodological limitations that are due to the multifactorial aspects of both PCS and FNDs. In this way, it was suggested that one solid source of non-specificity could be the fact that most health impairments that include a psychological component should be regarded as an interaction between the physiological response and the psychological status.
On the other hand, most of the current data were obtained from the evaluation of PCS cases that were the result of sports-related and military-related concussions, along with only a few cases that were due to car accidents. In this context, further research should channel their attention to other causes in order to test the whole patient care approach in PCS versus FNDs.
This entry is adapted from the peer-reviewed paper 10.3390/brainsci13071028
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