Recurrent Optic Neuritis and Perineuritis Followed by an Unexpected Discovery

We describe a woman with a history of relapsing acute optic neuritis and perineuritis. Testing failed to confirm a specific diagnosis; hence, she was diagnosed with seronegative neuromyelitis optica spectrum disorder and treated with the immunotherapy rituximab, later in conjunction with mycophenolate mofetil. She achieved a durable remission for 9 years until she presented with paresthesia affecting her left fifth digit, right proximal thigh, and left foot, while also reporting a 25-pound weight loss over the prior 3 months. New imaging demonstrated a longitudinally extensive and enhancing optic nerve, in conjunction with multifocal enhancing lesions within the spinal cord, in a skip-like distribution. The differential diagnosis is discussed.