Neuroradiology

Arachnoid cyst

congenital anomaly of the arachnoid membrane that leads to CSF "sequestration" or focal impairment of CSF circulation with cystic expansion of the involved space. The cysts may be of various sizes, from very small without mass effect to very large, with mass effect and bony erosion; some are stable, others may grow and expand over time. The most common locations are the sylvian fissure and middle cranial fossa, suprasellar cistern, quadrigeminal cistern, cerebellopontine angle, posterior infratentorial midline, cerebral convexity and interhemispheric fissure.

They may be asymptomatic when small; when large they may cause intracranial hypertension, either direct or related to hydrocephalus. Quadrigeminal or paracollicular cysts more easily compress the quadrigeminal plate and aqueduct and cause hydrocephalus. Large middle cranial fossa cysts may present with seizures and headaches.

Diagnosis is easily made with CT which shows the hypodense structure distorting the adjacent nervous tissue and expanding bone (Fig.1).

MR shows CSF signal within the cyst. The main differential diagnosis, particularly for cysts in the cerebellopontine angle or suprasellar cistern, is epidermoids. Epidermoids may have a signal identical or very similar to that of arachnoid cyst. Diffusion weighted sequences are diagnostic since diffusion is reduced within epidermoids, which then appear brighter than CSF.

Arachnoid cyst may be associated with subdural haematomas since the bridging veins traversing the subarachnoid space are elongated and fragile.

GS

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Fig.1

a, b. MR, T2-weighted image. The right middle fossa is expanded and occupied by a cystic structure with the same signal as CSF, displacing posteriorly the temporal lobe and compressing medially the cortex of the insula and the right lateral ventricle. c, d. MR, T1-weighted images, coronal and sagittal. The expansion of the middle fossa and the mass effect of the cyst over the temporal and frontal lobe and the insular cortex are well appreciated. e, f. CT scan shows marked thinning of the temporal squama and of the sphenoid wing and expansion of the middle fossa.
Arachnoid cyst, Fig.1 (a)
Arachnoid cyst, Fig.1 (b)
Arachnoid cyst, Fig.1 (c)
Arachnoid cyst, Fig.1 (d)
Arachnoid cyst, Fig.1 (e)
Arachnoid cyst, Fig.1 (f)