Hirayama disease is a rare juvenile amyotrophy that is often misdiagnosed as an unrelated, relentlessly progressive disease. We present the case of an 18-year-old man who presented with weakness and atrophy of the right forearm and hand. Dynamic cervical magnetic resonance imaging was used, revealing the classic findings of epidural venous plexus dilation and anterior displacement of the dural sac. In addition, dilation of the external vertebral venous plexus was visualized. We discuss the clinical utility of dynamic magnetic resonance imaging and the underlying pathophysiology of these findings in Hirayama disease.