Hyperemesis gravidarum can impair correct absorption of an adequate amount of thiamine and can cause electrolyte imbalance. This study investigated the neurological complications in a pregnant woman with hyperemesis gravidarum. A 29-year-old pregnant woman was admitted for hyperemesis gravidarum. Besides undernutrition, a neurological examination disclosed weakness with hyporeflexia, ophthalmoparesis, multidirectional nystagmus and optic disks swelling; the patient became rapidly comatose. Brain MRI showed symmetric signal hyperintensity and swelling of periaqueductal area, hypothalamus and mammillary bodies, medial and posterior portions of the thalamus and columns of fornix, consistent with Wernicke encephalopathy (WE). Neurophysiological studies revealed an axonal sensory-motor polyneuropathy, likely due to thiamine deficiency or critical illness polyneuropathy. Sodium and potassium supplementation and parenteral thiamine were administered with improvement of consciousness state in a few days. WE evolved in Korsakoff syndrome. A repeat MRI showed a marked improvement of WE-related alterations and a new hyperintense lesion in the pons, suggestive of central pontine myelinolysis. No sign or symptom due to involvement of the pons was present.