Utility of somatosensory evoked potentials in the early diagnosis of Bickerstaff's brainstem encephalitis

Bickerstaff's brainstem encephalitis (BBE) is characterized by diverse symptoms, and early diagnosis is often difficult, especially confusing with menigoencephalitis with cranial nerve involvements. Diagnostic role of anti-GQ1b antibody has been established, although its result is usually known late. Prompt diagnosis is preferred because immunotherapies are effective. Here we illustrate the usefulness of somatosensory evoked potentials (SEP) in the early diagnosis of BBE. Patients were a 27-year-old woman and a 28-year-old man. Both patients presented with general fatigue and common-cold-like symptoms, followed by dysarthria, gait disturbance, ophthalmoplegia, consciousness disturbance, and positive Babinski's sign. Brain MRI revealed no abnormality. ABRs were completely normal. Blink reflexes were abnormal. Median nerve SEPs registered normal P13/14 and N18, whereas N20 was lost (patient 1) or delayed (patient 2). These SEP results suggested the interruption of the somatosensory pathway within the brainstem considering their clinical signs, and the diagnosis of BBE was made. We immediately introduced immunoadsorption, which resulted in rapid resolution of symptoms. Anti-GQ1b antibody later proved to be positive. Abnormalities of evoked potentials have been reported for BBE, but these are the first cases with normal ABRs and abnormal SEPs.