Neutral Cervical Spine MRI is Not Enough: The Critical Role of Flexion Imaging in Hirayama disease in Pediatric Patients.

Hirayama disease is a rare flexion-induced cervical myelopathy that predominantly affects adolescents and young adults and is frequently overlooked, as its key flexion MRI features are often absent on routine neutral-position MRI. The aim of this study is to raise awareness of this disease and to highlight the diagnostic value of dynamic flexion and extension MRI in establishing Hirayama disease as a differential diagnosis in patients presenting with suspected cervical spinal cord pathology. We retrospectively analyzed three adolescent patients diagnosed with Hirayama disease at a quaternary academic pediatric hospital. Clinical presentation, electrophysiological findings, and cervical spine MRI studies (including neutral, flexion, and extension views) were reviewed. All patients presented clinically with progressive distal upper extremity weakness and electrophysiological findings suggestive of lower cervical cord pathology. Neutral MRI showed insufficient findings to diagnose Hirayama disease, while flexion MRI revealed an imaging pattern characterized by anterior displacement of the posterior dural sac, marked enlargement of the posterior epidural space, and posterior epidural venous plexus prominence. These dynamic changes resulted in flexion-induced spinal cord compression. In this case series, the diagnosis of Hirayama disease relied on flexion MRI, while neutral imaging alone would have been insufficient. Recognition of this characteristic, position-dependent imaging pattern is essential in young patients with unexplained distal upper limb weakness. Early consideration of dynamic imaging in patients clinically suspected of Hirayama disease helps to avoid diagnostic delays.