Arachnoid cyst

What is an arachnoid cyst?

The brain and spinal cord are covered by three protective membranes called the meninges. The middle layer is the arachnoid membrane. An arachnoid cyst is a benign, fluid-filled sac that forms between layers of the arachnoid membrane, filled with cerebrospinal fluid (CSF) rather than blood or abnormal cells. They are congenital in the vast majority of cases, meaning they develop during foetal brain development and are present from birth.

Arachnoid cysts are classified by location, which largely determines their clinical significance:

• Middle cranial fossa cysts are the most common intracranial location, occurring in the temporal region. They are the most likely to grow and cause symptoms from pressure on adjacent brain tissue or temporal lobe compression.

• Posterior fossa cysts occur near the cerebellum and can obstruct CSF flow, causing hydrocephalus in severe cases.

• Suprasellar cysts occur near the pituitary gland and hypothalamus, and can cause hormonal disturbance, visual field defects, and in children a characteristic bobbing head movement.

• Convexity cysts occur over the surface of the cerebral hemispheres and are typically small and asymptomatic.

• Spinal arachnoid cysts occur within the spinal canal and can compress the spinal cord or nerve roots, causing radiculopathy or myelopathy.

Most arachnoid cysts remain stable in size throughout life. A small proportion enlarge, either from active CSF secretion by the cyst wall or from a ball-valve mechanism trapping CSF within the cyst.

Symptoms of arachnoid cysts

The majority of arachnoid cysts cause no symptoms and are discovered entirely incidentally on MRI performed for another reason. When symptoms occur they reflect the size and location of the cyst and its effect on adjacent structures:

• Headaches, often positional or pressure-like, from mass effect on surrounding brain tissue

• Dizziness and balance difficulties, particularly with posterior fossa cysts

• Visual disturbances or visual field defects from suprasellar or temporal cysts compressing optic pathways

• Hormonal disturbances from suprasellar cysts affecting the pituitary or hypothalamus

• Cognitive difficulties, memory problems, or behavioural changes from temporal lobe compression

• Seizures from cortical irritation in some temporal lobe cysts

• Tingling, weakness, or pain in the limbs from spinal arachnoid cysts

• In rare cases of cyst rupture or haemorrhage into the cyst: sudden severe headache requiring emergency assessment

What causes arachnoid cysts?

Most arachnoid cysts are congenital, arising from abnormal splitting or duplication of the arachnoid membrane during foetal development. Contributing factors include:

• Congenital developmental anomaly is the primary cause in the vast majority of cases, with no identifiable external trigger. The cyst simply forms as the arachnoid membrane develops abnormally during early brain formation.

• Genetic factors arachnoid cysts are more common in certain genetic conditions including Marfan syndrome, neurofibromatosis, and arachnoid cyst familial clustering, suggesting a heritable component in some cases.

• Previous meningitis or brain infection can cause adhesions in the arachnoid membrane that trap CSF and form a secondary arachnoid cyst.

• Previous head trauma can disrupt the arachnoid membrane and create conditions for secondary cyst formation.

• Previous intracranial haemorrhage subarachnoid haemorrhage can cause arachnoid scarring and secondary cyst development.

How is an arachnoid cyst detected?

Arachnoid cysts are usually first identified on brain or spinal imaging, often incidentally. MRI is the preferred tool for definitive characterization, since it's far better than CT at distinguishing an arachnoid cyst from other similar-looking lesions, and provides the detail needed to assess its relationship to surrounding structures. Blood tests play a limited supporting role in assessing conditions that can coexist with or be caused by the cyst.

Brain and spinal MRI Elfcare’s full-body MRI easily identifies arachnoid cysts by mapping their exact size and location. It checks if the cyst is compressing brain tissue or blocking spinal fluid flow. Crucially, the MRI's specific imaging signals allow doctors to confirm if it is a harmless, fluid-filled arachnoid cyst rather than a tumor or abscess, which require completely different treatments.

Blood tests cannot detect arachnoid cysts but are relevant when the cyst is causing endocrine or neurological consequences, or when an underlying condition predisposing to cyst formation is suspected. Relevant markers in Elfcare's panel include:

• Hormone panel (LH, FSH, cortisol, thyroid, sex hormones): vital if a cyst is near the brain's hormone center (pituitary gland), which can disrupt your metabolism and energy.

• Vitamin B12 and folate: rule out nutritional nerve damage that might mimic the cyst's symptoms.

• Calcium and magnesium: check for mineral imbalances causing overlapping symptoms like muscle weakness or tingling.

• CRP: screens for inflammation to rule out a past brain infection as the underlying cause.

Why early detection matters

For the majority of people with an incidental arachnoid cyst, early detection provides a structural explanation for a finding that requires nothing more than reassurance and an awareness that serial MRI monitoring may be appropriate. This clarity has real value, replacing anxiety about an unexplained finding with a precise clinical assessment. For the smaller proportion in whom the cyst is causing symptoms or showing evidence of growth, early identification allows for monitoring before complications develop and, when intervention is indicated, surgical treatment at the most appropriate stage. Suprasellar cysts identified before significant pituitary or optic pathway compression has occurred respond much better to treatment than those identified after irreversible hormonal or visual consequences have developed.

How Elfcare can help

Elfcare's full body MRI includes comprehensive brain imaging as standard, which can identify arachnoid cysts, assess their size, and location, and relationship to surrounding structures. MRI can also help distinguish a typical arachnoid cyst from other cystic or structural findings, providing important information for determining whether the finding is likely incidental or may require clinical follow-up.

Our blood panel includes pituitary and hormonal markers that may be relevant when imaging identifies a lesion in or around the pituitary region, including certain suprasellar findings. These tests can provide additional clinical context when recommended by a healthcare professional.

If our MRI identifies an arachnoid cyst or other finding that may require further assessment or monitoring, we help guide you toward appropriate follow-up and specialist care where needed.