What are the MRI signal changes in the hand and forearm due to muscle atrophy in Hirayama disease?

MRI Signal Changes in Hirayama Disease

In Hirayama disease, muscle atrophy in the hand and forearm manifests on MRI as T1-weighted hypointensity (indicating chronic muscle loss and fatty replacement) and T2-weighted hyperintensity (reflecting muscle edema in active phases), though the diagnostic focus should be on the cervical cord abnormalities rather than peripheral muscle imaging. 1, 2

Cervical Cord Signal Changes (Primary Diagnostic Features)

The most diagnostically relevant MRI findings occur in the cervical spinal cord itself, not the peripheral muscles:

T2-Weighted Imaging

• Intramedullary hyperintensity in the anterior horn cells at the lower cervical cord (C4-C7 levels) is seen in 37.5-53.3% of cases 2, 3
• These signal changes reflect anterior horn cell damage and correlate with the motor neuron involvement 1
• The hyperintensity is typically located in the anterior aspect of the cord, corresponding to the distribution of affected motor neurons 4

T1-Weighted Imaging

• Localized cervical cord atrophy at C5-C7 levels is present in 25-80% of cases 2, 3
• Asymmetric cord flattening occurs in 62.5% of patients 2
• T1-weighted sequences best demonstrate chronic structural changes and atrophy 4

Peripheral Muscle Changes (Secondary Findings)

While the question asks specifically about hand and forearm muscles, these are not the primary diagnostic targets in Hirayama disease imaging:

• Muscle atrophy would theoretically show as T1 hypointensity with fatty infiltration (similar to chronic denervation patterns in other motor neuron diseases) 5
• Active muscle denervation may show T2 hyperintensity representing edema 5
• However, MRI protocols for Hirayama disease focus on cervical spine imaging, not direct hand/forearm muscle imaging 2, 3, 4

Critical Diagnostic Protocol

Flexion MRI is mandatory for diagnosis, as neutral position imaging misses findings in many cases:

• Forward displacement of the posterior dural sac with neck flexion occurs in 100% of cases 3, 4
• Laminodural space (LDS) widening to 3.1-7.0 mm (mean 5.38 mm) is pathognomonic 3
• Prominent posterior epidural venous plexus with flow voids appears in 86.6% during flexion 3
• Loss of posterior dural attachment to subjacent lamina is universal 2, 4

Optimal Imaging Protocol

• Sagittal and axial T1-weighted sequences in neutral position 2
• Sagittal and axial T2-weighted sequences in neutral position 2
• Repeat sequences with neck flexion at 30-40 degrees 3, 4
• Post-contrast imaging demonstrates enhancing posterior epidural space 4, 6
• 3D-CISS or FIESTA-C sequences may better visualize epidural flow voids 4

Common Pitfall

The major diagnostic error is performing only neutral position MRI, which can appear normal or show only subtle cord atrophy in up to 40% of cases 2, 3. The characteristic dural displacement and epidural venous congestion are only visible with flexion imaging, making this positioning essential for confirming the diagnosis 6.