Presentation
Persistent headaches and heaviness of the head for a year duration
Patient Data
There is an extra-axial oval-shaped unilocular CSF density lesion measuring 24x18x17mm seen within the left ambient cistern with mass effect on the medial aspect of the left temporal lobe. The rest of the cerebral hemispheres are preserved. No brain contusion/ intracerebral/ subarachnoid hemorrhage is demonstrated.
Features are highly suggestive of arachnoid cyst of the left ambient cistern
A well-circumscribed ovoid expansion of the left ambient cistern is seen as having the same signal as CSF in all sequences. No restricted diffusion on DWI/ADC. No evidence of susceptibility on GRE to suggest hemorrhage. Mass effect is seen in form of compression of the grey matter of the hippocampus confirming an extra-axial location. The surrounding brain tissue shows no evidence of gliosis.
Features are consistent with arachnoid cyst of the ambient cistern.
Case Discussion
Differential Diagnosis.
The differential diagnosis of arachnoid cysts includes epidermoid cysts, chronic subdural hematoma, and porencephalic cysts.
Epidermoid cysts are often hyperintense on FLAIR sequence and show restricted diffusion on DWI/ADC while arachnoid cysts have low signal intensity on both FLAIR and diffusion sequences.
Chronic subdural hematomas do not show the same signal intensity with CSF on MRI examination.
Porencephalic cysts are CSF-filled cavities lined by white matter which may show gliotic changes whereas arachnoid cysts are lined by grey matter.
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