Expanding the Differential Diagnosis of Ultrasonographic Flexor Digitorum Profundus–Flexor Carpi Ulnaris Dissociation of Echogenicity: Muscular Dystrophies

INTRODUCTION/AIMS: Dissociation of echogenicity of the flexor digitorum profundus (FDP) and flexor carpi ulnaris (FCU) on neuromuscular ultrasound has been reported to be a useful sign to differentiate inclusion body myositis (IBM) from more common disease mimics, but it is not clear that this finding is pathognomonic of IBM. Our study aimed to evaluate whether this sign may be present in certain muscular dystrophies. METHODS: This case series included 6 genetically confirmed muscular dystrophy patients (2 Becker muscular dystrophy [BMD], 2 myotonic dystrophy type 2 [DM2], 1 myotonic dystrophy type 1 [DM1], and 1 facioscapulohumeral muscular dystrophy type 1 [FSHD1]), whose clinical picture was notable for selective deep finger flexor weakness. Neuromuscular ultrasound had been used to image the proximal medial forearm, where echointensity (EI) of the FDP and FCU was visually graded according to the Heckmatt scale and an FDP/FCU EI ratio was generated from quantitative grayscale analysis. RESULTS: The FDP had a higher EI-by Heckmatt grade and quantitative grayscale-than the FCU in all 6 patients, indicating preferential ultrasonographic involvement of the FDP in these muscular dystrophy cases. DISCUSSION: Ultrasonographic FDP-FCU dissociation of echogenicity confirms selective deep finger flexor involvement. The differential diagnosis of this imaging sign includes IBM but also certain muscular dystrophies (BMD, DM1, DM2, and FSHD), which should be considered in uncommon presentations of suspected IBM or when additional supportive diagnostic findings thereof are lacking.