Monomelic amyotrophy (MMA), also known as Hirayama disease, is a sporadic juvenile muscular atrophy in the distal upper extremities. This disorder rarely involves proximal upper extremities and presents minimal sensory symptoms with no upper motor neuron (UMN) signs. It is caused by anterior displacement of the posterior dural sac and compression of the cervical cord during neck flexion. An 18-year-old boy visited our clinic with a 5-year history of left upper extremity pain and slowly progressive weakness affecting the left shoulder. Atrophy was present in the left supraspinatus and infraspinatus. On neurological examination, positive UMN signs were evident in both upper and lower extremities. Electrodiagnostic study showed root lesion involving the fifth to seventh cervical segment of the cord with chronic and ongoing denervation in the fifth and sixth cervical segment innervated muscles. Cervical magnetic resonance imaging (MRI) showed asymmetric cord atrophy apparent in the left side and intramedullary high signal intensity along the fourth to sixth cervical vertebral levels. With neck flexion, cervical MRI revealed anterior displacement of posterior dural sac, which results in the cord compression of those segments. The mechanisms of myelopathy in our patient seem to be same as that of MMA. We report a MMA patient involving proximal limb with UMN signs in biomechanical concerns and discuss clinical importance of cervical MRI with neck flexion. The case highlights that clinical variation might cause misdiagnosis.
Citations
MethodA retrospective analysis of the medical records, electrodiagnostic, and imaging findings of 12 patients (4 HD, 2 LMA, 6 BAD) was done. For patients whose last clinic follow-up exceeded 6 months a telephone survey was done to see if there were any symptom changes.
ResultsThe clinical, electrodiagnostic, and imaging findings of the HD and BAD patients were similar to previous studies. Except for a later onset, age disease duration was too short to distinguish LMA from HD or other motor neuron diseases. One patient in the BAD group progressed to amyotrophic lateral sclerosis (ALS) and another died due to undetermined respiratory failure. These two patients showed abnormalities in their lower extremities, thoracic paraspinal, and craniocervical muscles on needle electromyography. Except for another patient, none of the other three patients showed abnormalities in their lower extremities, thoracic paraspinals, or craniocervical muscles on needle electromyography.
ConclusionHD and BAD can be considered as separate disease entities. However, a longer follow-up period than previously recommended is necessary to differentiate BAD from ALS. Follow-up period was too short to determine whether LMA can also be considered as a separate disease entity.
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