Juvenile muscular atrophy of the distal upper limb (Hirayama disease) is a rare disease predominantly affecting the anterior horn cells of the cervical spinal cord in young men in their 2nd or early 3rd decade of life.

Hirayama et al (1) first described this disease in 1959. Typical clinical features are muscular weakness and atrophy in the hand and forearm. There is unilateral involvement in the majority of patients, but asymmetric and symmetric bilateral involvement is also observed (2). The onset of this disease is insidious, and the clinical course is steadily progressive. After a period of initial deterioration, a stable stage is reached (3).

The diagnostic criteria of Hirayama disease include the following:

• Chronic weakness and atrophy of the distal upper limb(s).
• Insidious onset in the 2nd or 3rd decade of life.
• Absence of substantial sensory deficits, reflex abnormalities, and cranial nerve, pyramidal tract, lower limb, sphincter, or cerebellum involvement; no history of poliomyelitis, toxin exposure, or periodic paralysis; and no evidence of an underlying disease causing peripheral neuropathy of axonal motor type shown at biochemical and neurophysiologic work-up.

Although considered to be a type of cervical myelopathy related to the flexional movements of the neck, the pathogenesis of this entity is debated. Hirayama and Tokumaru (2) concluded that dynamic spinal cord compression at neck flexion with forward displacement of the posterior dura is an unequivocal finding in the progressive stage of this disease. The mechanism of this anteriorly displaced dural canal has been explained by Kikuchi et al (4) as a “tight” dural canal in flexion, caused by a disproportional length between the vertebrae and the dural canal.

The mechanism of myelopathy may involve ischemic changes or chronic trauma inflicted by repeated neck flexion (5). The asymmetric cord compression can be explained by the hypothesis of a “posterior epidural ligament factor” proposed by Shinomiya et al (6).

Among the MR imaging findings, localized lower cervical cord atrophy, asymmetric cord flattening, and loss of attachmentwith the subjacent lamina had accuracy of more than 80% in identification of the disease. Although diagnosis at flexion MR imaging is straightforward, loss of attachment between the posterior duralsac and subjacent lamina was the only significantly important predictor of the disease at neutral position MR imaging as reported by Chen et al (7).