A rare, mostly benign, primary tumor of the meninges (arachnoid cap cells), usually located in the supratentorial compartment, commonly appearing in the sixth and seventh decade of life, clinically silent in most cases or causing hyperostosis close to the tumor and resulting in focal bulging and localized pain in less than 10% of cases. Additional features may include headache, seizures, gradual personality changes (apathy and dementia), anosmia, impaired vision, exophthalmos, hearing loss, ataxia, dysmetria, hypotonia, nystagmus, and rarely spontaneous bleeding.

A low-grade papillary epithelial neoplasm (adenocarcinoma) with a slow growth pattern. The endolymphatic duct emerges from the posterior wall of the saccule (of the inner ear) and ends in a blind pouch, the endolymphatic sac. Endolymphatic sac tumors (ELSTs) are known under different names in the literature (Heffner tumor, aggressive papillary middle ear tumor, and low-grade adenocarcinoma of endolymphatic sac origin).