What is functional neurological disorder?

Functional neurological disorder (FND) is a topical area of interest in the field of medicolegal neurology. FND is a disabling condition which predominately affects young and middle-aged adults. FND is thought to affect how the brain sends and receives information, resulting in heterogenous clinical presentations impacting motor and sensory function. It has been understood for hundreds of years under many different diagnostic labels but in recent years driven by the work of Stone and colleagues, it has settled under the label of FND. FND describes neurological symptoms and disability which arises in the absence of any trauma to the brain. However, like psychiatric conditions, FND has been linked to alterations in both brain structure and brain function. FND is therefore thought to sit at the intersection between neurology and psychiatry.

How is FND diagnosed?

Diagnostic manuals including the DSM-5 and ICD-11 classify this disorder in different ways which is unhelpful for clinical service provision as well as medicolegal practice. The DSM-5 classifies this disorder as a “conversion disorder/functional neurological symptom disorder”, whereas the ICD-11 classifies this disorder as a “dissociative neurological symptom disorder”, respectively.

A diagnosis of FND using the DSM-5 criteria requires one or more symptoms related to voluntary motor or sensory function. This can include symptoms related to weakness, a lack of movement, as well as tremors, jerks or dystonia. The symptoms should not be better explained by another recognised medical or neurological condition, and they should

cause significant distress or impairment in social or occupational functioning.

What are the challenges in diagnosing FND?

Diagnostic manuals do not provide clear descriptions of the clinical signs or symptoms of FND to support the clinical diagnosis of FND, nor do they distinguish between motor type FND or seizure type FND. This presents a significant challenge for clinicians and legal professionals who are not given detailed clinical symptom profiles to look for. This also contributes to the stigma felt by patients with FND who may feel that this is a “catch-all” diagnosis for patients who do not neatly fit into other diagnostic categories.

The complexity of diagnosing FND and the variability in the clinical presentation of both motor FND and seizure type FND is highlighted by the fact that there are over 37 clinical tests for motor function in FND, and 23 clinical tests to support the diagnosis of seizure type FND (Aybek and Perez, 2022). FND can present alongside other neurological conditions, and so it can be very challenging to accurately diagnose the condition, particularly in the context of comorbidities. For example, it is common for a formal diagnosis of epilepsy to be made alongside a diagnosis of seizure type FND. Whilst recent work has aimed to characterise clinical signs and symptoms of FND, further work is needed to integrate this evidence base into diagnostic manuals to support the accurate diagnosis and clinical management of FND to support clinical practice and the medicolegal field.

The key skill set of a practicing neurologist is to recognise FND early and to accurately differentiate between an organic presentation of a neurological disease, such as epilepsy, from a neuropsychiatric disorder, such as seizure type FND.

What are the symptoms of FND?

Whilst FND is an incredibly complex disorder to diagnose, common symptoms seen in both motor type [paralysis or abnormal movements] and seizure type FND include abnormalities in oculomotor function, such as eye spasms. Other commonly observed symptoms to support the diagnosis of motor function FND are gait abnormalities, such as slowness, knee buckling, dragging legs and falling towards support, as well as facial spasms lasting 3 or more seconds, involving lip pulling or eyebrow movement. Another distinguishing feature of motor FND is that patients exhibit an asymmetry in the strength of muscles in the trunk, arms, hips and legs.

Key defining features of seizure type FND which differ from epileptic seizures are that the seizures have a duration of more than two minutes and patients show a rapid recovery following seizures. These signs are highly specific to seizure-type FND supporting the clinical diagnosis of the condition. As an epilepsy specialist, I recommend attempting to recognise the pattern of how epilepsy presents compared with FND. Though this can be very challenging, a quarter of all patients referred to my difficult-to-treat epilepsy clinic had FND as their disabling problem.

Recent findings on the neurobiology of FND have shown that FND is related to a disruption in large-scale neural networks in the brain related to attention, motor function, emotion regulation and homeostatic balance (Aybek and Perez, 2022). This means that FND relates to an alteration in how neurons communicate across many areas of the brain.

Whilst epilepsy is also linked to changes in the way that neurons communicate, the pattern of functional activity that emerges is far more pronounced. In epilepsy, neuronal hyperexcitability leads to the synchronous depolarisation of many neurons. This widespread functional change in brain activity results in a seizure which can lead to brain damage.

This highlights the complexity as the mechanisms underlying symptoms are very different from organic diseases such as epilepsy and extend into the domain of psychiatry and psychology.

How is FND treated?

The most important treatment is an early diagnosis, a clear explanation of the disorder, avoiding incorrect diagnosis and inappropriate medical treatment that causes physical and psychological harm. Evidence for the effectiveness of early psychological intervention, and the prognosis can be excellent (Goldstein et al., 2020). Dr McCorry can state, however, that longstanding FND can be very hard to treat. There are limited treatment options available to treat chronic FND. Whilst medicines acting on the serotonergic system are often prescribed to help manage mental health symptoms, these medicines do not treat FND. There are currently no licensed treatments available to support the clinical management of FND.

Physiotherapy is the first line treatment for motor type FND, where there is a focus on retraining the muscles in the context of diverted attention. Randomised trials have shown that five days of consecutive treatment versus a waiting list improve symptoms and social functioning by 72% (Aybek and Perez, 2022).

Preliminary studies are also being conducted to investigate the therapeutic potential of mindfulness-based therapy, botulinum neurotoxin and transcranial magnetic stimulation for the clinical management of FND (Aybek and Perez, 2022).

However, a key challenge hindering advances in the development of new treatments for this condition is the ongoing debate and lack of clarity surrounding the diagnosis of the condition. Reaching consensus within the field on this important issue is critical to support advances in research which are needed to test and develop more efficacious treatments for the clinical management of FND.

Within medicolegal practice the issue of causation of a single life event to cause a complex condition is always subject to a range of views. Having led a specialist FND clinic in a psychiatric unit, I cannot stress the importance of effective communication and collaboration between multi-disciplinary teams including neurologists and psychiatrists to ensure that an accurate diagnosis and treatment plan is made. Often the elephant in the room can be the importance of the claim as a driver of the condition, but every case is different. Protracted and highly disputed claims after what appears to be a relatively minor injury are often not in the interests of the Claimant, and intervention is best undertaken after the claim has ended. However, the extent of improvement that can be expected is very difficult to predict and Dr McCorry is grateful it’s for a Judge to decide!

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