Shank sign in myotonic dystrophy type-1 (DM-1)

This study aimed to examine the specific shape of the shoulders and arms as a result of selective changes in muscle bulk, which could be diagnostic of myotonic dystrophy type-1 (DM-1). Thirty-two DM-1 patients from 18 families were asked to abduct their arms to about 90° with elbows flexed to 90° in such a manner that the hands were directed upwards. Examination from behind revealed a normal appearance of the shoulder girdle and proximal upper arm musculature, and sudden tapering thereafter, due to wasting of biceps, triceps and forearm muscles. The upper limbs looked like the shanks of an animal. ‘Shank sign’ was visible in 25 (78%) patients with DM-1 but not in any with DM-2 or other common muscular dystrophies such as Becker’s dystrophy, limb-girdle syndrome or facioscapulohumeral dystrophy. Patients with positive signs included nine with very mild myotonia, suggesting its clinical utility in difficult cases.