Dynamic Cervical MRI in Hirayama Disease: The Importance of Flexion Imaging

 In routine neuroradiology practice, evaluating juvenile-onset focal muscle wasting requires a combination of clinical context and tailored imaging protocols. Hirayama disease, first recognized in 1959, is a unique cervical myelopathy caused by dynamic compression of the anterior horn cells in the lower cervical cord. Because standard neutral-position MRI scans can appear deceptively subtle, understanding the characteristic imaging spectrum and requesting dynamic cervical flexion studies are critical for an accurate diagnosis.

Pathophysiology Behind the Imaging Signs 

During adolescent growth spurts, a disparity can develop between the growth of the vertebral column and the dural sac. In patients with Hirayama disease, the dural sac lacks sufficient elasticity. When the neck flexes forward, the posterior dural wall slides anteriorly, compressing the lower cervical cord (C7-T1) against the posterior surface of the vertebral bodies. This recurrent micro-trauma and transient ischemia selectively damage the vulnerable anterior horn cells.

Essential Radiological Markers

Radiologists look for several key findings across neutral and dynamic sequences:

• Lower Cervical Cord Atrophy: Localized thinning or asymmetrical flattening of the lower cervical cord (typically C5 to T1) seen on sagittal T2-weighted images.

• Intramedullary Signal Changes: Focal hyperintensity within the anterior horn regions on T2-weighted sequences, reflecting gliosis or chronic ischemic changes.

• Loss of Dural Attachment: On neutral T2 images, a gap or loss of normal apposition between the posterior dural sac and the adjacent laminar arch.

• Anterior Dural Displacement (Flexion MRI): On forward neck flexion, the posterior dura shifts dramatically forward, narrowing the anterior epidural space and flattening the spinal cord.

• Prominent Posterior Epidural Flow Voids: Forward shift of the dural sac creates a vacuum-like space in the posterior epidural compartment, leading to marked engorgement of the internal vertebral venous plexus. This appears as a crescentic, contrast-enhancing mass with low-signal flow voids on T2-weighted images.

Diagnostic Strategy for Clinicians 

When a young patient presents with non-sensory hand or forearm wasting, order a routine cervical spine MRI and explicitly request cervical flexion sequences. Demonstrating dynamic cord compression during neck flexion prevents misdiagnosis of progressive motor neuron disorders and allows for timely conservative interventions, such as cervical collar immobilization, to arrest further disease progression during its active phase.