Deep Phenotyping of Musicians' Upper Limb Dystonia.

Background: Focal task-specific dystonia of the musicians' arm (FTSDma) is an unusual and challenging disorder, often causing significant disability with loss of performing careers. The etiology and optimal management of this disorder remains unclear.

Methods: We reviewed records and videos of 173 patients with FTSDma, 50 patients with writer's cramp (WC), and 16 with other forms of arm dystonia (OD), evaluated by a single examiner in clinical practice over a 25-year period. Detailed analysis of clinical features and videotaped examinations in slow motion (what we call "deep phenotyping") allowed separation of patients into four categories: "precision-grip" dystonia (groups I and III); "power-grip" dystonia (group II); and "proximal dystonia" (group IV). We compared these results to deep phenotyping of patients with FTSDma, WC and OD patients reported in the literature.

Results: FTSDma usually affects men, involves the right hand, and begins in the fourth decade. The precision hand of pianists and guitarists (digits 1, 2, 3) was preferentially affected in the right arm, and many of the remaining patients involved the power hand of either arm (digits 3, 4, 5). The dystonic phenotype of the bow arm of string players and drumming arm of stick drummers bore striking resemblance to WC and racquet dystonia, almost always involving the wrist, forearm or shoulder.

Conclusions: Deep phenotyping of FTSDma reveals similarities in dystonic phenotype between instrument classes, likely related to shared technical demands, and unexpected similarities between other forms of task-specific upper extremity dystonia. A network model to explain these findings is proposed.