The clinical and radiological features in seven patients who had asymmetric muscular atrophy of the hand and forearm when young are reported and a new hypothesis for its aetiology is proposed. Investigation of body growth curves (a surrogate for velocity of arm growth) showed close relation between (a) the age when the body height increased most rapidly and the onset age of this disorder, and (b) the age when the rapid body growth period ended and the age when symptom progression ceased. Cervical radiological evidence is provided showing asymmetric anterior cord atrophy, disappearance of slackness of dorsal roots in neck extension, and anterior and lateral displacement of the lower cervical cord against the posterior aspects of the vertebral bodies during neck flexion. These results suggest that disproportionate shortening of the dorsal roots is further accentuated during the juvenile growth spurt, which determines the onset and self limited course of the condition, and that repeated neck flexion causes micro-trauma and relative ischaemia of anterior horn cells, which finally results in atrophy of the muscles innervated by motoneurons with long axons. Predisposing anatomical factors are a straight neck due to lack of physiological cervical lordosis and the presence of foreshortened dorsal roots.
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