Congenital transtentorial parahippocampal herniation presenting with new-onset seizures in a 14-year-old boy

Congenital transtentorial herniation of the parahippocampal gyrus is an uncommon developmental anomaly that can mimic pathological brain herniation. Unlike the acute form associated with raised intracranial pressure, this benign variant usually remains asymptomatic and is often discovered incidentally during neuroimaging for unrelated conditions. We report the case of a 14-year-old previously healthy boy who presented with a new-onset generalized tonic-clonic seizure followed by transient postictal confusion. Neurological and systemic examinations, as well as laboratory investigations and electroencephalography, were unremarkable. Brain MRI demonstrated a focal herniation of the left parahippocampal gyrus through the tentorial incisura with preserved cortical architecture and no evidence of mass effect, edema, or abnormal enhancement-findings consistent with a congenital (idiopathic) transtentorial herniation. The patient was managed conservatively and discharged on carbamazepine. A follow-up MRI was advised but could not be performed due to local instability. This case highlights the importance of recognizing congenital transtentorial herniation as a benign, non-emergent entity that can simulate pathological herniation or neoplasm on imaging. Awareness of its characteristic MRI features helps prevent unnecessary investigations or surgical interventions.