Arachnoid Cyst

Arachnoid cysts are CSF-filled, benign collections within the arachnoid membrane. They are mostly congenital, and may be intracranial or spinal.

They are found in about 1% of MRI studies in adults and in 2 to 3% of children. About 50 to 60% are in the middle cranial fossa. Males are affected more than females (about 2 to 1).

Locations. Middle cranial fossa (classified into three types by size and mass effect), retrocerebellar, suprasellar, quadrigeminal, cerebral convexity, cerebellopontine angle and spinal canal.

Causes. Congenital splitting of the arachnoid membrane. Secondary cysts follow trauma, infection, haemorrhage and surgery.

Pathophysiology. CSF accumulates by a one way valve mechanism or secretion by cyst wall cells. Larger cysts compress adjacent brain, obstruct CSF flow or bleed.

Most are asymptomatic and found incidentally. Symptoms include headache, seizures, developmental delay, macrocephaly, hydrocephalus (with suprasellar and posterior fossa cysts), focal deficit, visual loss and endocrine disturbance (suprasellar), and head bobbing in children (suprasellar and third ventricular cysts). Complications are bleeding into the cyst or a subdural collection after minor head trauma.

Red flags.

• Sudden severe headache or neurological decline after head trauma.
• Raised intracranial pressure signs, or papilloedema.
• Rapidly enlarging cyst, or visual loss.

MRI shows a smooth cyst following CSF signal on all sequences, with suppression on FLAIR and no restricted diffusion. CT shows a low density, well defined lesion. CT or MRI cisternography with flow imaging assesses communication. Check visual fields and pituitary function for suprasellar cysts.

Differential diagnosis.

• Epidermoid cyst (restricted diffusion), dermoid cyst and cystic tumours.
• Porencephalic cyst, neuroglial cyst and neurocysticercosis.
• Mega cisterna magna and subdural hygroma.

Asymptomatic. Observation. Repeat MRI at 6 to 12 months for large or growing cysts, then only for symptoms.

Symptomatic. Surgical treatment of cysts causing mass effect, hydrocephalus, seizures related to the cyst or progressive deficit.

• Endoscopic fenestration into the cisterns or ventricles is first choice.
• Microsurgical fenestration or craniotomy for complex cysts.
• Cystoperitoneal shunt when fenestration fails or recurs, with the risks of shunt dependency, infection and malfunction.

Seizures. Levetiracetam 500 mg twice daily, increased to 1500 mg twice daily (adjust to eGFR), or other antiseizure drug as indicated.

Haemorrhage into cyst or subdural. Neurosurgical assessment and evacuation if there is mass effect.

Activity. Most can do usual activities. Advise caution with contact sport for large middle fossa cysts.

Monitoring. Clinical review and imaging for symptom change.

Referral and escalation. Refer to neurosurgery for symptoms or growth. Refer urgently for acute deterioration.

Special groups. In children, assess development and head growth. In pregnancy, asymptomatic cysts need no change in care.