Arachnoid cysts are benign, congenital masses that are believed to form when the arachnoid membrane splits or is duplicated and the resulting space fills with fluid. Despite their potentially alarming appearance on brain imaging, congenital cysts discovered in adulthood are usually silent and do not result in functional impairment. A left-handed male veteran with mild memory complaints was discovered to have a large (16.4 cm × 7.7 cm), left-sided arachnoid cyst. Magnetic resonance imaging (MRI) revealed significant displacement of brain structures including the hippocampus, Sylvan fissure, and splenium. Viewing brain MRI images in only 1 plane was misleading and could have erroneously resulted in assuming some structures were absent. Viewing multiple planes of section revealed significant structural displacement and provided a better 3-dimensional conceptualization of an abnormal brain. A clinical interview indicated excellent premorbid functioning, and neuropsychological test results were within the normal range with the exception of mildly impaired scores on tests reliant on processing speed and lower-than-expected visual memory scores. Results were consistent with previous research noting retained verbal abilities and low-average visual skills. Low-average and mildly impaired scores were potentially secondary to microvascular changes, slowed visual scanning, psychiatric conditions, and testing base rates.
Keywords: Cognition; MRI; displacement; memory; neuroimaging.