Teaching Case

66-year-old lady presenting with complaints of paresthesia in both arms, predominantly on the right side, for the past 6 months.

Axial and coronal T2w images (Figure 1a, b) shows a left parafalcine extra-axial cyst of CSF intensity with imperceptible walls. Mass effect is noted adjacent to the left superior parietal gyri. Sagittal T1w image (Figure 1c) shows the cyst with hypointense signal. It is adjacent to the precuneus. Axial FLAIR image (Figure 1d) shows a left parafalcine hypointense extra-axial cyst confirming it to be of CSF fluid signal intensity. On DWI, no restriction is noted, ruling out an epidermoid cyst (Figure 1e).

Arachnoid cysts are common benign CNS (intracranial and spinal canal compartment) lesions. They contain CSF. Arachnoid cysts account for ~ 1% of all intracranial masses [1]. The clinical presentation of arachnoid cysts are small and asymptomatic. Symptoms are expected only if the lesions are large with mass effect on adjacent structures.
Arachnoid cysts are thought to arise due to splitting of the arachnoid layer with accumulation of CSF within this formed space [1]. The cyst wall is comprised of flattened arachnoid cells forming a thin translucent membrane. There is no solid component and no epithelial lining. Arachnoid cysts are generally stable over time [1].
Most arachnoid cysts are located in the middle cranial fossa, where they invaginate into and widen the sylvian fissure. The best diagnostic clue is a sharply demarcated extraaxial cyst that can displace or deform the adjacent brain tissue. If it is located adjacent to bone, then scalloping of the inner table occurs [1].
They can occur anywhere as long as a subarachnoid lining is present. They never communicate with the ventricular system. This imaging finding helps to rule out many intracranial cysts and the MRI feature of low signal on FLAIR and absence of restricted diffusion on DWI excludes an epidermoid cyst [2, 3, 4]. They follow CSF signal intensity on MRI [2, 3, 4].
Other differential diagnoses include mega cisterna magna (communicates with IV ventricle in posterior fossa), neuroglial cyst (will be surrounded by brain parenchyma), porencephalic cyst (lined by white matter), astrocytomas (presence of solid component) and chronic subdural haematomas (do not show classical CSF intensity on MRI). Arachnoid cysts are excised only if they are known to cause symptoms and are seen to enlarge on follow-up imaging [1, 2].