Fatal ornithine transcarbamylase deficiency presentation postresection of fourth ventricle ependymoma: illustrative case.

backgroundFourth ventricular ependymomas are rare glial neoplasms that commonly present with obstructive hydrocephalus, headache, and cerebellar symptoms. Ornithine transcarbamylase deficiency (OTCD), the most common urea cycle disorder, typically presents in infancy but may remain latent until triggered by physiological stressors such as surgery. OBSERVATIONS: A 59-year-old right-handed previously healthy woman presented to the emergency department with subacute nonspecific neurological symptoms. Initial imaging showed a large mass centered along the inferior aspect of the fourth ventricle with extrusion through the foramina of Luschka and Magendie and into the foramen magnum. She underwent a posterior fossa craniectomy for tumor resection without any intraoperative complications. The focus shifted in the postoperative period to managing emerging hyperammonemia. Complications, including cerebral edema, status epilepticus, and recurrent hyperammonemia, revealed an undiagnosed metabolic condition. LESSONS: This case describes a novel presentation of late-onset OTCD triggered by posterior fossa craniectomy for resection of a fourth ventricular ependymoma-an association not previously reported in the literature. It emphasizes the importance of maintaining an index of suspicion for metabolic disorders in the setting of postoperative hyperammonemia. Future steps should focus on early testing of ammonia levels in patients with unexplained neurological decline to be proactive with multidisciplinary management. https://thejns.org/doi/10.3171/CASE25324.