Non-flexion sagittal T2-weighted image
A 19-year-old man with no previous significant medical history presented with worsening neck pain and progressive weakness in both hands for the last six months. No sensory loss or movement restrictions were noted. Physical examination was normal. On EMG, bilateral median and ulnar showed low amplitude waves.
Non-contrast MRI was performed, including neutral position and flexion imaging protocols. The patient refused the injection of contrast material. Non-flexion imaging showed subtle loss of the normal cervical lordosis, without any other abnormalities. Flexion imaging showed posterior detachment and forward displacement of the posterior wall of the dural canal below C2, compressing the spinal cord (C4-C7). An epidural crescent-shaped lesion posterior to the shifting dura matter was noted, with small curvilinear flow void signals inside it due to congested posterior internal vertebral venous plexus. No asymmetrical cord flattening, lower cervical cord atrophy or intramedullary high T2-signal intensity were noted.
Hirayama’s disease (HD or juvenile muscular atrophy of distal upper extremity) is a benign, self-limited and very rare variant of amyotrophic lateral sclerosis characterised by asymmetric weakness and atrophy of one or both distal upper extremities. It was first described in Japan by Hirayama et al  as a cervical myelopathy related to flexional movements of the neck. Since then many reports have been published, mostly from Asian countries [2-4]. HD has a significant male predominance  and the first symptoms appear in the 20s. Its pathogenesis is thought to be related to the relatively short and tight dura mater in patients with HD (disproportional growth between the vertebral column and the contents of the spinal canal), unable to compensate for the increased lenght of the vertebral canal during neck flexion causing forward displacement of: - the posterior wall of the dural tube; - the lower cervical cord, contacting the posterior surface of the vertebrae. Repeated neck flexion causes chronic trauma to the anterior horn cells which are vulnerable to ischaemia, leading to myelopathy and localised cord atrophy of the lower cervical region . Pathology reveals ischaemic necrosis of the anterior horn of the cervical spinal cord with degenerative changes and mild astrogliosis.  The anterior shift of the dural canal is also responsible for compression of the anterior vertebral venous plexus and for negative pressure in the posterior spinal canal, with resultant increased flow to the posterior epidural venous plexus (PEVP), also burdened by the reduction of the drainage of the jugular veins during neck flexion. Characteristic signs of HD in flexion cervical MRI include forward shifting of the posterior dural sac and engorgement of the PEVP , which appear as an enhancing crescent-shaped epidural mass in dynamic postcontrast studies. Asymmetric cord flattening and localised atrophy and parenchymal changes in the lower cervical cord appear with disease progression.  Patients typically present with insidious onset and slow progression of muscle weakness and atrophy of the distal upper limb with sparing of the brachioradialis muscles (“oblique amyotrophy”). They commonly show initial unilateral monomelic atrophy but bilateral assymetrical and symmetrical involvement have been reported.  HD has a better prognosis than other motor neuron diseases with spontaneous arrest after 3-5 years of the disease onset.  The primary principle of treatment is a restriction of neck flexion by avoiding posture with long-term neck flexion and by collar therapy, preventing progressive muscular weakness at early stage . Duroplasty with or without anterior fusion of cervical vertebrae has been performed but the methods and indications of surgery remain controversial . Written informed patient consent for publication has been obtained.
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