Amnestic disconnection syndrome after unilateral fornix injury during pineal region tumor surgery: a case report

Abstract

Case Presentation: A 56-year-old right-handed woman with a higher education sought medical evaluation in July 2025 due to persistent memory impairment following surgical treatment for a brain tumor performed in March 2024. The investigation was initiated after the patient developed a new pattern of headache in November 2023, when a brain magnetic resonance imaging (MRI) scan revealed an expansive lesion in the pineal region, measuring 5 cm in its largest axis, suggestive of a meningioma, later confirmed histopathologically. Subtotal resection of the lesion was performed without major complications. In the following months, family members began to observe significant difficulty in retaining recent information, characterized by repetition of questions and conversations, forgetting messages, and difficulty recalling everyday events. The condition remained stable over the months and did not respond significantly to pharmacological interventions or cognitive rehabilitation strategies. On neurological examination, she was alert, cooperative, and her language fluency, naming, and repetition were preserved. The MMSE score was 22/30, with predominant impairment in temporal orientation and delayed recall. In the brief cognitive screening battery, delayed recall of 0 out of 10 figures was observed, without benefit from cues or intrusions, a pattern suggestive of diencephalic amnesia. The clock drawing test showed moderate impairment, consistent with executive dysfunction. Verbal fluency was relatively preserved, with 17 animals and 15 words beginning with the letter F in one minute. Somatic neurological examination was unremarkable. The follow-up MRI did not show tumor recurrence or structural hippocampal lesion. However, a detailed review of the imaging demonstrated a lesion of the right fornix, with interruption of its projection to the ipsilateral mammillary body. There was improvement in behavioral symptoms with desvenlafaxine, without evident cognitive benefit with donepezil. Discussion: Fornix lesions are classically associated with amnestic syndromes observed in fornix infarctions, third ventricle tumors, and neurosurgical complications, reflecting disruption of the hippocampus-mammillary bodies-anterior thalamus circuit of the Papez circuit. The present case presents atypical features, highlighting a unilateral fornix lesion, associated with structural preservation of the hippocampi, consistent with amnestic disconnection syndrome. Final Comments: The identification of this topography may help recognize diencephalic mechanisms of memory impairment in patients undergoing surgery for pineal tumors and assist in guiding cognitive rehabilitation strategies.