Case 10: Diagnosis and Discussion
Diagnosis: Monomelic amyotrophy (Hirayama disease)
MRI of the cervical spine at flexion and extension. Note the presence of focal cord atrophy (large arrows) and signal intensity abnormality in anterior part of the spinal cord, likely in the vicinity of anterior horns (narrow arrow in the axial T2).
Hirayama disease was first described in 1959 by Hirayama et al , as “ Juvenile muscular atrophy of the unilateral upper extremity”. It predominantly affects young man, in second and third decades of life, with Male/female ratio of 2.8:1. The hotspots of this disease include Japan and India. Hirayama disease is characterized by weakness and wasting usually starts in one hand (predominantly in C7,C8, and T1 myotomes), very commonly the contralateral upper limb is affected subsequently. The disease course is progressive for 1-3 years, does not spread to the non-cervical myotomes. A common symptom is irregular coarse tremors (minipolymyoclonus) in the fingers of the affected hands. Hyperreflexia is reported in about a fifth of the cases.
It is proposed that at least in some cases of Hirayama disease the disease results from forward displacement of the posterior wall of the lower cervical dural canal when the neck is in flexion. This results in marked, often asymmetric, flattening of the lower cervical cord. The mechanical compression may lead to chronic microcirculatory changes in the territory of the anterior spinal artery. Dynamic compression of the cervical spinal cord may result from disproportional growth between the vertebral column and the contents of the spinal canal during the juvenile growth spurt. In the cases reported from Japan, the age of onset is 2 years after the peak of longitudinal growth curve. Hirayama disease is to be distinguished from the ALS variant “flail arm syndrome” , bilateral brachial plexopathy and multifocal motor neuropathy. In contrast to Hirayama disease which is monophasic, flail arm syndrome has a progressive course. Diagnosis is often made by cervical spinal MRI in neutral and during neck flexion, which shows localized lower cervical cord flattening / atrophy (C4-C7), as well as anterior shifting of the posterior wall of the cervical dural canal during neck flexion. Other reported findings include enhancing epidural component with flow voids and intramedullary signal hyperintensity.
Suggested readings:
1. Hassan KM, Sahni H, Jha A. Clinical and radiological profile of Hirayama disease: A flexion myelopathy due to tight cervical dural canal amenable to collar therapy. Annals of Indian Academy of Neurology. Apr 2012;15(2):106-112.
2. Hirayama K. Juvenile muscular atrophy of distal upper extremity (Hirayama disease): focal cervical ischemic poliomyelopathy. Neuropathology : official journal of the Japanese Society of Neuropathology. Sep 2000;20 Suppl:S91-94.
3. Huang YL, Chen CJ. Hirayama disease. Neuroimaging clinics of North America. Nov 2011;21(4):939-950, ix-x.
