| Discussion: |
Congenital lesion of the arachnoid membrane which expands by secretion of CSF. The cysts are intra-arachnoid and lined by arachnoid membrane. The cysts arise in both intracranial and intraspinal locations. In the brain they are most commonly found in the middle cranial fossa and are associated with partial or complete temporal lobe agenesis. In this location, spontaneous hemorrhage is a recognized complication. They may also be found in the suprasellar cistern, quadrigeminal plate and cerebellopontine angle cisterns, in the midline of the posterior fossa, velum interpositum, over the cerebral surface and in the interhemispheric fissure. When small the cysts may be asymptomatic. When large they may cause mass effect on adjacent structures and obstruct the flow of CSF causing hydrocephalus. Patients may also present with seizures. Less frequently they may be associated with precocious puberty and other endocrine dysfunction. Minor trauma may cause bleeding into a cyst particularly those in the middle cranial fossa. Subdural hematoma or hygroma may also occur in association with trauma.
Cysts may be diagnosed on antenatal and postnatal ultrasound, CT and MRI. They are smooth-walled nonenhancing lesions of CSF density. The major differential diagnosis is cystic astrocytoma cerebral, which may be distinguished by the presence of a solid enhancing nodule in the wall of the cyst, and epidermoid cyst, which may be distinguished by its irregular outline, the presence of fine septations and a tendency to envelope adjacent structures.
Arachnoid cysts in the spine may arise anterior, posterior or posterolateral to the spinal cord and may be intra- or extradural in location or a combination of both. They may be unilocular or multilocular. Symptoms and signs depend on the size and location of the cyst. If large they will cause mass effect on the spinal cord or nerve roots. In the spine the differential diagnosis include dural ectasia as seen in neurofibromatosis and epidermoid. Acquired arachnoid cysts may be seen in the spine after repair of myelomeningocele. These are not true arachnoid cysts but are caused by arachnoid adhesions. They produce mass effect on the spinal cord and may be the cause of deterioration in neurological function.