Role of Dynamic Magnetic Resonance Imaging in Hirayama Disease, a Rare Motor Neuron Disease.

Introduction Hirayama disease (HD) is a benign self-limiting motor neuron disease, most commonly occurring in young males. The disease has an insidious onset that affects T1, C8, and C7 myotomes. HD is more common in Japan and Asian countries, and rare in the western population. Magnetic resonance imaging (MRI) is the best technique for the diagnosis of this entity. Early diagnosis is important as the patients can be advised to limit neck flexion movements to arrest the progression of the disease. Any clinically suspected case of Hirayama disease should undergo flexion MRI as conventional neutral MRI may miss findings in a few cases. The purpose of the present study is to evaluate the usefulness of flexion MR imaging and laminodural space (LDS) measurement in young patients with clinical and electroneuromyography (ENMG) definite Hirayama disease. Materials and methods This is a retrospective observational study of 15 patients with clinical and ENMG definite Hirayama disease who were referred to the Department of Radiology. These patients underwent MRI of cervical spine in neutral position and with neck flexion of 30°-40°. In neutral MRI, atrophy and T2-weighted hyperintensities in the cord were noted. In flexion MRI, the maximum forward shifting of the posterior dural sac, also known as the LDS, was noted along with other parameters. Observation The mean age of the study population was 21 ± 3.36 years. Out of 15 patients, 14 were males and one was female; 14 patients (93.3%) had an involvement of unilateral upper extremity while one patient (6.6%) had asymmetric bilateral involvement. Straightening of cervical spinal curvature and cord atrophy was seen in 14 (93.3%) and 12 (80%) patients, respectively, on neutral position MRI. Intramedullary cervical cord T2-weighted hyperintensities were noted in eight patients (53.3%). Loss of the dural attachment and forward shifting of the posterior dural sac with prominent posterior epidural space was noted in all patients (100%). At the maximum forward shift of cord, the LDS ranged from 3.1 to 7.0 mm, with a mean of 5.38 ± 1.13 mm. Epidural flow voids were noted in 86.6% of cases. Conclusion Flexion MRI plays a very important role in confirming the diagnosis of Hirayama disease in clinically suspected cases. Anterior displacement of posterior dura matter and widening of LDS is noted in all cases in our study. Even though findings like cord atrophy and T2 hyperintensities are seen in conventional neutral MRI, these findings are not seen in all cases. So flexion MRI increases diagnostic confidence by showing increased LDS, which is a characteristic finding in Hirayama disease.