9 months old girl with an arachnoid cyst seen on ultrasound.
Arachnoid cysts are cerebrospinal fluid-filled sacs that are located between the brain or spinal cord and the arachnoid membrane. Primary arachnoid cysts are present at birth and are the result of developmental abnormalities in the brain and spinal cord that arise during the early weeks of gestation. Secondary arachnoid cysts are not as common as primary cysts and develop as a result of head injury, meningitis, or tumours, or as a complication of brain surgery. The majority of primary arachnoid cysts form outside the temporal lobe in the middle crania fossa. Arachnoid cysts involving the spinal cord are rarer. The location and size of the cyst determine the symptoms and when those symptoms begin. Arachnoid cysts often are an incidental finding on imaging, and usually, patients are asymptomatic even if the cyst is quite large. A few individuals with arachnoid cysts develop symptoms such as headache, seizure or focal neurological signs.
Symptomatic arachnoid cyst may be treated by fenestration or cystoperitoneal shunt. Asymptomatic arachnoid cyst may be watched, or drained if expansion or haemorrhage would threaten vital structures.

National Institute of Neurological Disorders and Stroke

Credit: Dr Ken Le