• NEUROPSYCHIATRY
• HYSTERIA

The diagnosis of functional neurological disorder (FND) has rested traditionally on two key features: inconsistency and incongruency.

Inconsistency usually refers to inconsistency between movement, sensory or cognitive performance in a voluntary versus an ‘automatic’ scenario. This is the principle of Hoover’s sign of functional leg weakness, the tremor entrainment test for functional tremor and a range of cognitive tests in functional cognitive disorder. It is often helpful to share these tests with patients as they show the potential for improvement, give an insight into the ‘software’ mechanism of FND in the brain and also feed into a range of FND-specific therapies. For example, a physiotherapist might actively use distraction to promote automatic movement and discourage overlearned abnormal patterns of voluntary movement.

Incongruency, on the other hand, refers to a clinical feature that is not present in other superficially similar neurological conditions, or that apparently violates laws of anatomy, biology or physics. I have always struggled with incongruency in FND but now am breaking my silence, assisted by the intriguing case of a patient with ‘functional freezing of gait’ from Jorik Nonnekes and colleagues in this issue of Practical Neurology.1

The patient described in the paper and accompanying video has a functional gait disorder. But is it reasonable, as the authors suggest, to make that diagnosis, in part, because it does not look like another disorder, principally Parkinsonism, with which we are more familiar?

In my view, incongruence needs retirement. It should be sent to the FND section of the neuromythology graveyard where it can join ‘la belle indifférence’,2 ‘non-organic’3 and the universal application of ‘conversion disorder’.4 Here is what I object to, and find incongruent, about this sign still being used in the clinical assessment of FND (box 1).