Crucial differential diagnosis of rapidly progressive dementia: A case of leptomeningeal metastasis

Dementia that advances subacutely without accompanying neurological symptoms can often be misdiagnosed as a psychiatric condition. Leptomeningeal metastasis (LM), caused by the spread of malignant cells to the leptomeninges and the subarachnoid space, is a relatively unfamiliar condition to psychiatrists in this context. The diagnosis of LM remains challenging due to the scarcity of diagnostic tools possessing high sensitivity and specificity.We present the clinical presentation of a male in his seventies with LM secondary to gastric ring cell carcinoma. The patient exhibited an acute confusional state, visual hallucinations, irritability, and cognitive impairments over a 3‐week period. Initially, the patient was misdiagnosed with several conditions, including alcohol withdrawal syndrome, psychosis, and delirium associated with dementia, as there were no noteworthy findings on neurological examination or the head magnetic resonance imaging (MRI). Given the rapidly progressive cognitive decline, we maintained vigilance for potential neurological conditions, and a repeat investigation using head MRI and cerebrospinal fluid analysis led to the diagnosis of LM.This critical case report underscores the rarity of psychiatric‐onset LM originating from gastric cancer and highlights the importance of comprehensive neurological evaluations.