Arachnoid cyst (huge)
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A large left-sided arachnoid cyst measuring 5.3 x 4.3 x 10.5 cm extends from the left middle cranial fossa almost to vertex with marked distortion of the left frontal, temporal and parietal lobes and scalloping of the overlying calvarium. The midline is shifted 5 mm toward the right at the level of foramen of Monro. The midbrain is rightward and posteriorly displaced with significant rotation.
Midline shift at the level of the suprasellar cistern results in displacement of the pituitary infundibulum and optic chiasm toward the right by approximately 5 mm. The left optic nerve and tract are not focally compressed but may be stretched by the degree of displacement. Pituitary gland has normal appearances.
No brain parenchymal signal abnormality, diffusion restriction or susceptibility artefact. The left lateral ventricle appears partially effaced but there is no hydrocephalus or right ventricular entrapment.
Some arachnoid cysts are remarkably large, but only sometimes cause definite symptoms.