Hirayama disease, also termed monomelic amyotrophy or non-progressive juvenile spinal muscular atrophy of the distal upper limbs, is a type of cervical myelopathy related to flexion movements of the neck. It is considered a benign motor neuron disorder with a stationary stage after a progressive course 1,7.
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Epidemiology
It typically occurs mainly in young Asian males between the ages of 15-25 years 6,7.
Clinical presentation
It is often of insidious onset presenting with a predominantly unilateral upper extremity weakness and atrophy, cold paresis, and no sensory or pyramidal tract involvement. The weakness and atrophy typically spares the brachioradialis whilst affecting other muscles of the forearm and hand, giving the characteristic appearance of oblique amyotrophy that affects the C7, C8 and T1 myotomes 5,6. The amyotrophy is unilateral in most patients, asymmetrically bilateral in some and rarely symmetric 5.
Pathology
Chronic microcirculatory changes in the territory of the anterior spinal artery induced by repeated or sustained flexion account for the necrosis of the anterior horns of the lower cervical cord, which is the hallmark of pathology 5.
Radiographic features
On myelograms and flexion-extension MR images, there can be a forward migration of the posterior wall of the dura mater. The posterior epidural space becomes enlarged with flexion and is seen as a crescent.
At the site of maximal forward shift of the posterior dural sac the spinal cord is dynamically compressed with a reduction in the AP diameter of the cord compared to neutral imaging 6.
MRI
neutral position: abnormal T2-weighted signal of the spinal cord at site of maximum forward shift without an obvious cause
flexion position: the posterior dural sac crescent appears as high signal intensity on T1- and T2-weighted sequences and enhances uniformly on T1 C+ (Gd), with or without epidural flow voids
Treatment and prognosis
Management options include long-term cervical soft braces, which is the first-line management, or surgical options 7. Regardless of the management strategy, the disease usually arrests after 3-5 years 7.
History and etymology
It was initially described by Keizo Hirayama, Japanese neurologist, and colleagues in 1959 2.
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