A case report of meningeal carcinomatosis presenting as rapidly progressive dementia: cytological diagnosis with ancillary immunohistochemistry of cerebrobrospinal fluid cells.

INTRODUCTION: Meningeal carcinomatosis (MC) is a severe complication of systemic cancers. Although typical presentations involve meningeal irritation or cranial nerve deficits, its manifestation as rapidly progressive dementia (RPD) in the absence of classic signs is exceedingly rare. This complication poses a substantial diagnostic challenge, especially when the primary tumor remains occult. METHODS: We present the case of a 56-year-old man with a 4-month history of progressive cognitive decline, behavioral changes, and visual hallucinations culminating in stupor. The diagnostic evaluation consisted of a comprehensive neurologic examination, contrast-enhanced brain magnetic resonance imaging, cerebrospinal fluid (CSF) analysis, and whole-body 18F-fludeoxyglucose-positron emission tomography/computed tomography (FDG-PET/CT) imaging. RESULTS: Brain magnetic resonance imaging revealed diffuse leptomeningeal enhancement. FDG-PET/CT demonstrated abnormal FDG uptake localized to the cerebellar cortex and basal meninges; however, no primary tumor was identified. A CSF analysis showed elevated opening pressure, pleocytosis, and increased protein; cytology confirmed malignant cells. Immunohistochemical profiling (CK7+, TTF1+, MOC31+, Ki-67 40%+, BerEp4+, EMA+) indicated adenocarcinoma of probable pulmonary origin, confirming MC. DISCUSSION: This case underscores that RPD can be a rare but dominant initial feature of MC and highlights the essential role of CSF cytology combined with immunohistochemistry in achieving a definitive diagnosis, especially when neuroimaging is nonspecific and a primary tumor is not detected.