Seven patients with extrinsic midline tumors of the posterior fossa had normal-sized lateral ventricles by radiographic contrast study or at autopsy. A small number of patients harboring such neoplasms may not show evidence of ventricular dilatation. This is emphasized because of the erroneous but common belief that midline posterior fossa neoplasms cause lateral ventricular enlargement in all cases. In the seven patients, two tumors were misdiagnosed and one may have been incidental. Proposed mechanisms for failure of the lateral ventricles to enlarge are patency of cerebrospinal fluid flow or a rapid clinical course in which there is no time for ventricular enlargement to occur although cerebrospinal fluid flow is obstructed.
Yashon D, White R, Croft TJ, Becker DP, Jane JA. Midline Posterior Fossa Neoplasms Without Lateral Ventricular Enlargement. JAMA. 1971;215(1):89–93. doi:10.1001/jama.1971.03180140053009
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