Post-craniotomy asymmetrical brain sag presumably caused by decreased cerebrospinal fluid production

Brain sag may result from post-craniotomy over-drainage or the leakage of cerebrospinal fluid. We treated a patient in whom it was caused not by the loss of cerebrospinal fluid but by its decreased production. A 72-year-old man who had previously undergone gastrectomy underwent a right superficial temporal artery to middle cerebral artery bypass. On postoperative Day 8 he lapsed into a coma and imaging showed a midline shift, and craniotomy was therefore performed again. However, there were no signs of elevated intracranial pressure, and a diagnosis of brain sag due to low cerebrospinal fluid pressure was reached. The blood concentration of retinol-binding protein was low, and decreased cerebrospinal fluid production due to vitamin A deficiency was indicated to be the cause. The patient improved after conservative treatment consisting of vitamin A supplementation. This condition should be borne in mind in the event of unexpected impairment of consciousness or if indicated by imaging findings after craniotomy.