Boiled Sausage Sign: A Novel MRI Marker for Identifying Subacute Secondary Hydrocephalus in the Elderly

Ventricular enlargement is commonly caused by brain atrophy or hydrocephalus; however, distinguishing between these conditions in elderly patients is often challenging. This case report describes a patient in whom secondary hydrocephalus was suspected based on characteristic findings of ventricular enlargement, leading to the final diagnosis. A 75-year-old man with nasal cavity malignant melanoma, treated with nivolumab for six months, presented with three weeks of progressive lethargy and anorexia and subsequent mild confusion. Although MRI-negative encephalitis related to nivolumab was initially suspected, mild ventricular enlargement with a uniformly dilated anterior horn shape resembling a "boiled sausage" was noted. Subsequent gadolinium-enhanced MRI demonstrated diffuse dural thickening contiguous with the nasal cavity and meningeal enhancement, leading to the final diagnosis of intracranial invasion of malignant melanoma with secondary hydrocephalus due to meningeal carcinomatosis. Subacute hydrocephalus manifests as a uniform ventricular enlargement on MRI, a finding we propose as the novel "boiled sausage sign." This sign may help distinguish secondary hydrocephalus from physiologic atrophy, even when the Evans index remains within the normal range.