MRI Findings in Hirayama Disease: Hand and Forearm
In Hirayama disease, MRI of the hand and forearm typically shows muscular atrophy predominantly affecting the intrinsic hand muscles and distal forearm muscles, particularly on the ulnar side, corresponding to C7-T1 myotome distribution. 1, 2
Key Peripheral Findings
Hand and Forearm Muscle Changes
- Muscular atrophy is most pronounced in the intrinsic muscles of the hand and distal forearm muscles, with preferential involvement of the ulnar-sided musculature 1, 2
- The atrophy pattern corresponds to C7-T1 myotome distribution, reflecting the underlying cervical cord pathology 1
- These peripheral changes are secondary to the cervical myelopathy and represent chronic denervation changes 3, 1
Critical Diagnostic Consideration
The definitive diagnosis of Hirayama disease requires cervical spine MRI with neck flexion, not imaging of the hand and forearm alone. 3, 4, 5 While peripheral muscle atrophy can be visualized on hand/forearm MRI, this finding is nonspecific and does not establish the diagnosis.
Cervical Spine MRI Findings (The Diagnostic Standard)
Flexion MRI of the cervical spine is essential and demonstrates:
- Anterior displacement of the posterior dural sac with loss of normal dural attachment 3, 1, 4
- Widened laminodural space (LDS) measuring 3.1-7.0 mm (mean 5.38 mm) at maximum forward shift 3, 5
- Prominent crescent-shaped posterior epidural space with flow voids from venous plexus congestion (seen in 86.6% of cases) 3, 1
- Localized cervical cord atrophy at C4-C7 levels (80% of cases) 3, 1
- T2-weighted hyperintensities in the cord (53.3-37.5% of cases) 3, 1
- Loss of normal cervical lordosis (93.3-100% of cases) 3, 1
Critical Imaging Pitfall
Neutral position MRI may miss the diagnosis entirely, as characteristic findings are often only visible with neck flexion 2, 4. One case report documented completely normal neutral MRI with diagnostic findings only appearing on flexion imaging 2. This is why flexion MRI at 30-40 degrees is mandatory for suspected Hirayama disease. 3, 4, 5
Clinical Context
The peripheral muscle changes visible on hand/forearm MRI correlate with the clinical presentation of asymmetric weakness and atrophy affecting one or both distal upper extremities 3, 1. The disease predominantly affects young males (mean age 21 years) with unilateral involvement in 93% of cases, though bilateral asymmetric involvement can occur 3, 1.
Bottom line: While hand and forearm MRI may show nonspecific muscle atrophy, cervical spine MRI with neck flexion is the definitive imaging study required to diagnose Hirayama disease. 3, 4, 5