Teaching NeuroImages: Onion-skin pattern facial sensory loss

A 42-year-old woman presented with acute-onset facial numbness. Physical examination revealed graded loss of pain and temperature over the left half of her face with central sparing (figure 1A)—an onion-skin distribution. Corneal reflex was sluggish on the left, but results for the rest of the clinical examination, CSF studies, and visual evoked potential were normal. MRI was suggestive of acute demyelination (figure 1, B and C), and the patient's condition improved with steroids. This segmental pattern of trigeminal sensory loss, in contrast to the divisional pattern (figure 2, A and B), reflects the rostral-caudal somatotopic arrangement in the spinal tract nucleus of the trigeminal nerve with the perioral area represented rostrally and the lateral face caudally (figure 2C)^1,2 and helps in anatomic localization.