Objective: To delineate the characteristics and distinguishing features of functional stiff person-like disorders (fSPLD).

Background: Stiff person spectrum disorders (SPSD) are a group of rare autoimmune disorders, characterised by fluctuating stiffness, spasms, and hyperekplexia. The presentation of SPSD – the wooden, stiff-legged gait, the fear of walking unaided, the fluctuating nature of stiffness, and often the lack of other, “objective neurological signs” – can appear “bizarre” and leads frequently to an erroneous diagnosis of a functional neurological disorder. Conversely, we have seen several patients with functional “stiff-person-like” phenotypes, which were erroneously diagnosed and treated as organic SPSD.

Method: Retrospective dual-site case-control notes review of fSPLD with matched SPSD patients as controls. Participants were recruited from 2 centres (London and Heidelberg).  fSPLD patients have been referred with a suspected diagnosis of SPSD. fSPLD was defined by the diagnostic criteria for a clinically definite functional movement disorder.

Results: From 2014-2018, we (BB, KB) have seen 13 patients with a suspected diagnosis of SPSD. Four patients had in fact fSPLD, thus accounting for one third of referrals in one tertiary referral centre. Similar to organic SPSD where there are forms with dominant stiffness and predominantly myoclonic forms (“jerking stiff man”), we saw two main and equally frequent phenotypes in fSPLD: half of the patients had stiffness and spasms as main presentation; in the other half, stimulus-sensitive jerks and exaggerated startle were the dominating features. Some of the stiff-person core symptoms reported or exhibited by fSPLD patients resembled those seen in organic SPSD, such as a stiff legged-gait, trunk extension or opisthotonus during attacks, and painful spasms. Clinical characteristics particular to fSPLD included a “flexion pattern” of stiffness, functional jerks, or a co-contracting tremor. Additional functional symptoms (i.e., a functional gait disorder or speech disorder, dissociative disorder) were present in two thirds of the patients.

Conclusion: fSPLD represent a relevant differential diagnosis. Knowledge of the clinically distinguishing features and the role of ancillary testing is key for distinguishing fSPLD from SPSD, and initiating the appropriate treatment.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/functional-stiff-person-like-disorders-clinical-features-and-red-flags/