Juvenile Segmental SMA (Benign Focal Amyotrophy; Hirayama Disease)
This disease was originally described by Hirayama as a slowly progressive focal motor neuron disease affecting the upper extremities. Most cases occur on a sporadic basis. The onset of this syndrome is typically between 15 and 25 years, with a range of 2 to 30.
Wasting and weakness develop segmentally in C8-T1 hand and forearm muscles and unilaterally and often but not always in the dominant extremity. Sensation is completely normal. The disease progresses to more proximal upper extremity muscles. The lower extremities are never affected, and typically the disease pro- gession plateaus after two to six years. Symptoms worsen in the cold (“cold paresis”). Tremor may occur due to distal weakness. Hyperhidrosis of the involved limb is a common complaint. Reflexes are typically spared but not brisk. EMG studies are consistent with an anterior horn cell disorder. MR imaging abnormalities of the cervical spinal cord (segmental atrophy, stenosis, or foraminal narrowing) have been described in a proportion of patients. The disease is more common in Asian populations.
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