The case against the 'S': Is functional neurological disorder(s) one condition or many?

Functional neurological disorder (FND) is one of the most common, but least researched, conditions in neurology. It can cause a broad range of sensory, motor, and constitutional symptoms, with the underlying mechanism of symptom-generation proposed to be an abnormality of perceptual inference. Debate exists as to whether the clinical entity referred to as FND is truly a single disorder or is in fact multiple entities which have been erroneously amalgamated into the same condition. We sought to provide empirical evidence on this question by treating it as a problem of model comparison. We formulated statistical models equivalent to: (1) FND being a single entity with variation in phenotype, represented by latent trait models, and (2) FND being multiple discrete entities, represented by latent class models. We fitted these models to data on the symptoms experienced by 697 people with FND from the FND Research Connect database (www.fnd-research.org) and used Bayesian model comparison to compare their likelihoods. All but one of the latent trait models, representing FND as a single entity with heterogeneous phenotype, fit the data better than all the latent class models. Furthermore, secondary analysis of the latent class models showed results compatible with the models discretising continuous variation rather than capturing true discrete categories. Our results are most compatible with the hypothesis that the symptom structure of FND is the result of a single pathophysiological process, either as a single entity, or a common pathway preceded by multiple causative processes where the common pathway is solely responsible for the phenotype.