Aetiology

Functional neurological disorder

Risk factors are heterogeneous and non-specific, and the underlying cause is still not fully understood. Although psychological stressors can be important risk factors and may perpetuate symptoms, patients may present without identifiable stressors.[30] In some patients, symptoms are triggered by physical injury, surgery, or another neurological disorder (e.g., migraine).[31][32]

Historically the aetiology has been explained as physical symptoms being a manifestation of psychological distress; however, there is ongoing debate about the extent to which psychological/psychiatric stressors act as drivers for both conditions.[33][34][35]​​​​ Empirical data on causation is currently insufficient. See Pathophysiology.

The biopsychosocial model (with consideration of predisposing vulnerabilities, acute precipitants, and perpetuating factors) is a useful framework to consider with respect to aetiology, and can assist with treatment planning. Factors to consider include pain, fatigue, psychiatric comorbidity, active psychosocial stressors, unhelpful illness beliefs, and unhelpful behavioural responses to symptoms.[3]

Somatic symptom disorder

Aetiology is unclear; somatic symptom disorder occurs due to heightened awareness of bodily sensations, in association with interpretation of sensations as denoting medical illness.

Risk factors are heterogeneous and include:​[23][27][36][37]

  • History of childhood illness

  • Family history of chronic illness

  • History of emotional trauma, including sexual abuse

  • Female sex

  • Health anxiety

  • Concurrent psychiatric disorder (e.g., depression or anxiety).

Pathophysiology

Functional neurological disorder

The pathophysiology of functional neurological disorder is the subject of intense study. Relevant processes include:[38][39][40][41][42][43]

  • Selective/biased attention

  • Dissociation

  • Abnormalities in sense of agency (subjective sense of control) for self-generated movements

  • Abnormalities in interoception (perception of sensations from inside the body)

  • Emotional dysregulation

  • Alexithymia (difficulty in identifying one's own emotions)

  • Differences in connectivity between the limbic system and motor cortex.

Functional neurological disorder involves abnormalities within several brain networks. The mechanism is not fully established, but functional neuroimaging studies suggest there is disruption in neural circuits linking volition, movement, and perception.[44][45][46]​​​ There is evidence to suggest dysfunction affecting pre-conscious aspects of motor planning; in particular, there is a strengthened connection between the (emotionally oriented) limbic system and motor networks.[42] Greater functional connectivity of limbic regions with motor preparatory regions in patients with functional neurological disorder may explain why physiological or psychological stress can trigger functional neurological symptoms.[47][48]​​​ There is evidence to suggest that the brain may involuntarily generate faulty predictions about the body, which override genuine incoming sensory information.[41]​ However, one caveat to highlight is that the above studies looking at changes in brain structure/function in functional neurological disorder include healthy controls (and not people with psychiatric conditions) as the control group. Consequently it is not possible to definitively establish based on the current literature whether the changes seen within the studies are specific to functional neurological disorder, or are instead related to changes seen in the brain in people with psychiatric conditions/psychological distress.

Somatic symptom disorder

  • May arise from generalised sensory amplifications of bodily symptoms involving the insula.[49] Preliminary neuroimaging evidence suggests increased activity of limbic regions in response to painful stimuli.[50]

  • Somatic amplification may occur when previously sensitised brain cytokine systems are reactivated by infectious or non-infectious trauma.[51]

  • Cytokines acting on the brain are likely to be involved in a variety of sickness behaviours.[52] Chronic activation of the immune system in response to stress may sensitise the cytokine response.

  • Central sensitisation may play an important role in symptom production and may be a useful pathophysiological model for how symptoms develop.[53]

Classification

Functional neurological (symptom) disorder (previously conversion disorder)

Although 'la belle indifference' (exhibition of apparent lack of concern about symptoms) is classically described, often the symptoms are distressing to the patient, as well as physically, socially, and occupationally disabling.[1] Functional neurological disorder often presents with discrete neurological symptoms to neurology speciality clinics; clinical findings provide evidence of incompatibility between the symptom and recognised neurological or medical conditions.[2]

Can be classified according to the type of symptoms present, such as:

  • Functional limb weakness

  • Functional movement disorder (e.g., dystonia, tremor)

  • Functional sensory signs

  • Functional seizures

  • Functional cognitive disorder.

  • Persistent postural perceptual dizziness.

Note that mixed functional motor symptoms are common (50% to 75%).[3]

Somatic symptom disorder

Somatic symptom disorder occurs due to heightened awareness of bodily sensations, in association with interpretation of sensations as denoting medical illness. It is represented by physical symptoms that are persistent and can affect any organ system. Patients with somatic symptom disorder often seek treatment from primary care providers.

Other specified somatic symptom and related disorder

Applies to "presentations in which symptoms characteristic of a somatic symptom and related disorder that cause clinically significant distress or impairment in social, occupational, or other areas of functioning predominate but do not meet full criteria for any of the disorders in the somatic symptom and related disorders diagnostic class".[2]

Unspecified somatic symptom and related disorder

In this category, “symptoms characteristic of a somatic symptom and related disorder that cause clinically significant distress or impairment in other areas of functioning predominate but do not meet full criteria for any of the disorders in the somatic symptom and related disorders diagnostic class. The unspecified somatic symptom and related disorder category should not be used unless there are decidedly unusual situations where there is insufficient information to make a more specific diagnosis.”[2]

Comorbidities

Typical comorbid diagnoses for both functional neurological disorder and somatic symptom disorder include mood disorders, panic disorder, generalised anxiety disorder, post-traumatic stress disorder, dissociative disorders, social or specific phobias, obsessive-compulsive disorders, and personality disorders.[4][5][6][7] Near relatives with psychiatric illness or severe somatic disease are also common.[4]

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