Functional neurological disorder: defying dualism
Authors: Jon Stone, Ingrid Hoeritzauer, Laura McWhirter, Alan Carson
Journal: World Psychiatry (2024)

Functional neurological disorder (FND) is classified in the DSM-5-TR as "functional neurological symptom disorder (conversion disorder)" and in the chapter on mental disorders of the ICD-11 as "dissociative neurological symptom disorder". Neurologists, who most commonly make the initial diagnosis, are usually barely aware of such classification systems, and use a variety of terms – such as "functional", "psychogenic" or "non-organic" – to describe symptoms of paralysis, tremor, seizures or blindness that were once encompassed under the label of "hysteria". This diversity of terms reflects a disorder that has been passed back and forward between neurology and psychiatry for 150 years. Over time, the FND pendulum has swung between a brain disorder in the late 19th century to a purely psychological condition in the 20th century. Today, FND researchers are suggesting that the pendulum rest in the middle. Defying dualism in FND may cause dissonance in clinicians, in those seeking tidy explanatory theories, and in classification systems. But it is an essential platform towards understanding FND and improving care for the millions of people around the world who have it. For those who grew up with "conversion disorder" in the DSM-IV, the idea was simple, hydraulic and comfortingly Freudian. Someone has a stressful event, which is repressed and converted to motor or sensory symptoms, that may or may not be symbolic, perhaps reducing the stress, sometimes to the point of belle indifférence. Conversion disorder was often considered a rare condition, which could only be diagnosed by exclusion, and would often respond quickly to psychological therapy. Historian E. Shorter declared that "hysteria" had largely disappeared in favour of other somatic symptoms such as fatigue1. In the last 20 years, this narrow view of the condition has been systematically dismantled by the evidence. FND is a common condition, one of the commonest seen by neurologists in both outpatient and inpatient settings, making up 5-15% of patients2. It accounts for 50% of people rushed into hospital with suspected status epilepticus, and 8% of people admitted to hospital with suspected stroke. FND symptoms are usually not transient. A 14-year study of people with functional limb weakness found that 80% still had their symptoms at follow-up. Physical disability and distress are as high as in epilepsy or Parkinson's disease2. FND is a diagnosis of inclusion, with a diagnostic stability similar to other conditions in neurology and psychiatry2. People with FND have clinical features that are characteristic of the disorder. Hoover's sign describes impairment of voluntary hip extension in the presence of normal automatic hip extension during contralateral hip flexion. A functional tremor stops or entrains to the rhythm of the examiner in the tremor entrainment test in a way that does not occur in other tremor disorders. People having a functional seizure typically experience a brief prodrome with autonomic arousal and dissociation, followed by an event in which their eyes are closed, and there are either vigorous tremor-like movements, or they fall down and lie still for more than a minute in ways that only occur in this condition. Injury, pain and infection are common triggers to functional motor and sensory disorders, and appear at least as relevant as adverse experiences2. Stressful events, adverse childhood experiences, and psychiatric comorbidity remain important in the story of many people with FND. The frequency of adverse childhood experiences (odds ratio: 3-4) and recent stress (odds ratio: 2-3) is increased, but not that different to many other conditions where they are considered a risk factor and not "the cause"3. There are patients in whom a conversion model still makes sense, but others for whom it is preposterous. The dropping of the requirement for a recent stressful event in the DSM-5, and the change of the name of the condition from "conversion disorder (functional neurological symptom disorder)" in the DSM-5 to "functional neurological symptom disorder (conversion disorder)" in the DSM-5-TR, are in keeping with that. A wider set of hypotheses, considering multiple levels from the neuron to society, is required to make sense of FND. The "predictive brain" offers a potential solution to puzzling disorders such as phantom limb phenomena, in which strong predictions that a limb "is still there" outweigh sensory input to the contrary. Similarly, in functional paralysis, one hypothesis is that the brain predicts a limb that "is not there" (and thus cannot be moved) so strongly that it outweighs sensory input telling the brain that the limb is normal4. The predictive brain builds on older notions of "ideas" or "beliefs" being important in FND, or of conditioned responses to threat, illness or injury that operate below the level of awareness. Neurodevelopmental conditions – including autism spectrum disorder, attention-deficit/hyperactivity disorder, and joint hypermobility – may be more common in people with FND because of an impairment in this predictive and interoceptive machinery. The first functional neuroimaging study of an FND patient appeared in 1997. The shock news was that FND could be seen in the brain. A number of networks have then been found to be relevant to FND, including those involved in attention, motor control, salience and emotion regulation2. Perhaps the most interesting and replicated finding is hypoactivation of the network involved in sense of agency – the parts of the brain that let you know that it is "you" who made a movement – including the right temporoparietal junction. Poor activation of this network is consistent with what we see clinically ("it looks like a voluntary movement") and what the patient is telling us ("it doesn't feel like under my control"). A diagnostic biomarker for FND may even one day become available5. For example, a study of resting state functional imaging was able to classify FND from healthy controls using brain scans alone with an accuracy of 72%6. If one considers FND a disorder of higher voluntary movement, it is hardly surprising that it has often been confused with wilful exaggeration or malingering. But a whole range of clinical and neuroscientific evidence, including geographical and historical consistency as well as remarkable responses to neurophysiological experiments, such as increased accuracy in tests of sensory attenuation, show that feigning offers a poor explanation for the clinical phenomenon of FND7. Treatment for FND reflects this new multidisciplinary approach, starting with an explanation of the disorder that emphasizes diagnosis by inclusion, mechanisms in the brain, but also relevant psychological risk factors when present. FND-focused physiotherapy promotes automatic over voluntary movement, has important differences to physiotherapy for recognized neurological conditions, and shows a lot of promise in randomised trials8. FND-focused evidence-based psychological therapy addresses adversity, but also recognizes the physiology of functional seizures and their similarity to panic9. The International FND Society, founded in 2019, embodies this co-operative approach, and is complemented by new patient-led organizations such as FND Hope and FND Action. Together they are defying the dualism which has prevented progress and understanding of this common disabling condition.