A 39-year-old woman with metastatic breast cancer treated with anti-ERBB2 (formerly HER2) chemotherapy (ado-trastuzumab emtansine) presented to the emergency department with acute onset paralysis. Earlier that day she had upper-back muscle spasms while gardening. A few hours later, she developed weakness of both hands and legs, followed by urinary retention and saddle anesthesia. On arrival to the emergency department, the patient was febrile but hemodynamically stable. On physical examination, she had areflexia and weakness affecting all 4 limbs, with a sensory deficit distal to the T4 dermatome. Proprioception and vibratory sensation were intact throughout. Speech, comprehension, and cranial nerves were intact. Laboratory test results revealed elevated levels of inflammatory markers. Cerebrospinal fluid (CSF) analysis, including white and red blood cell counts, protein and glucose levels, infectious studies, and cytologic testing, was unremarkable. Magnetic resonance imaging (MRI) of the total spine showed vertebral body metastases unchanged from a computed tomography scan obtained 8 months prior (Figure 1), multiple areas of new leptomeningeal enhancement around the distal spinal cord, and tumor infiltration of multiple vertebral bodies. An electromyogram showed grossly preserved motor conduction in the upper and lower extremities. F wave latencies (a measure of nerve conduction velocity between the spine and limb) were undetectable in the upper extremities and were normal in the lower extremities, suggesting upper motor neuron disease. Sensory conduction amplitudes of upper and lower extremity peripheral nerves were mildly decreased.