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Hirayama disease or monomelic amyotrophy

Indroduction

Hirayama disease, also known as monomelic amyotrophy, is a rare, self-limiting lower motor neuron disorder predominantly affecting young males in their second to third decades. It is characterized by focal, asymmetric wasting and weakness of the distal upper limb muscles—primarily those innervated by the C7–T1 myotomes—with relative sparing of the brachioradialis. The pathogenesis involves dynamic compression of the lower cervical spinal cord during neck flexion due to disproportionate growth between the vertebral column and dural sac, causing chronic microischemia of anterior horn cells.

Clinical Signs (7 Key Features)

  1. Unilateral or Asymmetric Distal Weakness: Insidious onset of weakness primarily in one upper limb, particularly affecting the hand and forearm.

  2. Muscle Atrophy: Progressive wasting of intrinsic hand muscles and ulnar forearm muscles, presenting as a characteristic “reverse split hand” sign with relative brachioradialis sparing.

  3. Fasciculations: Fine involuntary muscle twitches indicating active denervation.

  4. Cold Paresis: Weakness exacerbated by cold temperatures due to temperature-sensitive conduction deficits.

  5. Tremor or Polyminimyoclonus: Subtle, irregular, arrhythmic tremors in the affected hand.

  6. Absence of Sensory Deficits: Preserved sensory function despite prominent motor impairment, distinguishing it from other cervical myelopathies.

  7. Stabilization Phase After Progressive Course: Symptoms typically stabilize after a progressive phase lasting 3–5 years, though residual deficits remain.

Essential Diagnostic Studies

  1. Dynamic Cervical Spine MRI: Gold standard, showing anterior displacement of dura with epidural venous engorgement during neck flexion; neutral position may show cord atrophy.

  2. Electromyography (EMG): Shows chronic denervation signs with reinnervation patterns limited to muscles innervated by lower cervical segments.

  3. Nerve Conduction Studies (NCS): Normal sensory nerve conduction with reduced motor action potentials in affected areas.

  4. Diffusion Tensor Imaging (DTI): Provides detailed assessment of spinal cord microstructure and axonal integrity.

  5. Computed Tomography Myelography (CTM): Alternative imaging if MRI is contraindicated, assessing cervical cord compression dynamically.

Latest Treatment Options

Treatment aims to halt progression by preventing further cervical cord compression:

  1. Cervical Collar Immobilization: Early, prolonged use of rigid collars to restrict neck flexion and minimize cord trauma.

  2. Physical Therapy: Tailored strengthening exercises for neck extensors and upper limb function to stabilize symptoms.

  3. Anterior Cervical Decompression and Fusion (ACDF): Surgical option for severe dynamic cord compression unresponsive to conservative care.

  4. Posterior Laminoplasty with Duraplasty: Selected posterior decompression procedures to restore dural alignment and decompress the cord without compromising mobility.

  5. Dynamic Stabilization Techniques: Emerging minimally invasive surgical methods providing stabilization while preserving cervical spine motion.

Conclusion

Hirayama disease is a unique dynamic cervical myelopathy characterized by unilateral or asymmetric distal upper limb weakness and atrophy without sensory loss. Diagnosis relies heavily on dynamic cervical MRI and electrophysiological findings. Conservative management with cervical immobilization and targeted physical therapy remains first-line; however, progressive cases may require surgical interventions like ACDF, laminoplasty, or dynamic stabilization. Early diagnosis and intervention are critical to prevent progression and optimize patient outcomes.

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Dr. Christodoulos Amorgianos Orthopedic Trauma Surgeon – Specialist Hand Microsurgery & Interventional Pain Specialist
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