What is the diagnosis for a patient with localized muscle wasting and weakness in the left hand, specifically affecting the flexor digitorum profundus (FDL) and first dorsal interosseous (FDI) muscles, with reduced amplitude of the left ulnar nerve recording, but no sensory involvement or other muscle wasting?

Diagnosis: Monomelic Amyotrophy (Hirayama Disease) or Benign Focal Amyotrophy

This patient most likely has monomelic amyotrophy (Hirayama disease), a benign focal motor neuron disease that characteristically presents with insidious, non-progressive muscle wasting limited to one upper limb in young adults, with normal imaging and absent sensory involvement. 1

Key Diagnostic Features Supporting This Diagnosis

Clinical Presentation Matches Hirayama Disease Pattern

• Localized, static muscle wasting affecting only the left hand for several years without progression is the hallmark of Hirayama disease 1
• Pure motor involvement (FDI wasting with index finger abduction weakness) without any sensory deficits is characteristic 1
• Unilateral hand involvement is typical, as this condition primarily affects upper limbs of young males in a focal, asymmetric pattern 1

Electrodiagnostic Findings Are Consistent

• Reduced amplitude of the left ulnar nerve recording at FDI indicates motor axonal loss, which occurs in Hirayama disease due to anterior horn cell dysfunction 1
• The nerve conduction abnormality reflects denervation of ulnar-innervated intrinsic hand muscles rather than a compressive ulnar neuropathy 1

Normal Imaging Excludes Other Pathology

• Normal cervical spine and brachial plexus MRI effectively rules out structural compression, radiculopathy, or plexopathy 1
• However, flexion MRI of the cervical spine should be performed if not already done, as Hirayama disease may show minimal posterior epidural space enlargement and anterior spinal cord displacement only during neck flexion 1

Critical Differential Diagnosis Considerations

Why This Is NOT Ulnar Neuropathy at the Elbow

• Absence of sensory symptoms or deficits argues strongly against ulnar nerve entrapment, which typically causes paresthesias in the little finger and medial half of the fourth digit 2
• Static course over several years is atypical for progressive compressive ulnar neuropathy 2
• Normal brachial plexus imaging excludes proximal ulnar nerve pathology 1
• While ulnar entrapment can coexist with Hirayama disease, the clinical picture here is dominated by the focal motor neuron disease pattern 1

Why This Is NOT Demyelinating Focal Motor Neuropathy

• Years-long static course is inconsistent with demyelinating neuropathies, which typically show either progression or improvement 3
• Reduced amplitude rather than conduction block suggests axonal loss rather than demyelination 3
• Demyelinating focal motor neuropathy would show conduction block or temporal dispersion on nerve conduction studies 4, 3

Why This Is NOT Inclusion Body Myositis

• Age and distribution are wrong—inclusion body myositis affects patients over 50 years with forearm flexor and quadriceps involvement 5
• Absence of bilateral or proximal weakness excludes inflammatory myopathies 5

Recommended Next Steps

Confirm Diagnosis with Flexion MRI

• Obtain cervical spine MRI in neck flexion to demonstrate the characteristic anterior displacement of the spinal cord and posterior epidural space enlargement that may only be visible during flexion 1
• This is the definitive diagnostic test for Hirayama disease 1

Reassure Patient About Prognosis

• Hirayama disease is benign and typically self-limited, with most cases stabilizing after an initial period of progression 1
• No specific treatment is required beyond cervical collar use during neck flexion activities in some cases 1

Avoid Unnecessary Surgery

• Do not proceed with ulnar nerve decompression or transposition, as this patient does not have compressive ulnar neuropathy 1
• Young patients with upper extremity wasting and ulnar nerve findings should have Hirayama disease carefully excluded before attributing symptoms to ulnar neuropathy 1