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Health condition · Clinically reviewed

Arachnoid cyst, what it means, when to watch and when to operate.

Most arachnoid cysts are found by chance on MRI and never cause a symptom. This guide explains the ones that do, and how UK neurosurgical teams decide what to do next.

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A radiographer guides a patient onto the bed of an advanced 3 Tesla MRI scanner in a London imaging suite

Why trust this guide

  • 01

    Clinically reviewed

    Written by our editorial team and reviewed by a registered UK clinician before publication.

  • 02

    Sourced from guidance

    Checked against NICE, NHS England neurosurgical standards and peer-reviewed neuroradiology literature.

  • 03

    Current for 2026

    Reflects modern UK practice including endoscopic cystoventriculostomy and cautious observation of incidental cysts.

Key facts

Arachnoid cyst at a glance.

The essentials, in plain English. What it is, where it sits, and how it is treated in the UK today.

  • What it is

    A cerebrospinal fluid filled sac sitting between the arachnoid membrane and the surface of the brain or spinal cord.

  • Usually congenital

    Most are present from birth and form as the arachnoid membrane splits during early development. A minority are acquired after trauma, infection or surgery.

  • Commonest location

    The middle cranial fossa accounts for roughly half of all intracranial arachnoid cysts. Galassi types I to III grade their size and mass effect.

  • Often incidental

    Many are found by chance on a brain MRI ordered for another reason. Most cause no symptoms and never will.

  • When it matters

    Headache, seizures, focal weakness, hydrocephalus, macrocephaly in babies or developmental concerns can prompt surgical review.

  • First line is watch

    Asymptomatic cysts are usually monitored with interval MRI. Surgery is reserved for symptomatic, progressive or complicated cysts.

Why this guide matters

A cyst on a scan is rarely a crisis.

Being told you have a cyst on your brain is frightening. In practice, most are benign, stable and safely watched. The three points below shape everything else on this page.

  • Most are incidental

    Arachnoid cysts are common findings on brain MRI. The default position is careful observation with interval imaging, not immediate surgery.

  • Symptoms have to fit

    Before blaming a cyst for a headache or a memory concern, the pattern of symptoms and the location of the cyst have to match up clinically.

  • Modern surgery is targeted

    When surgery is needed, endoscopic fenestration has largely replaced open procedures for many cases, with shunts reserved for selected patients.

How the diagnosis is made

From first scan to a clear plan.

The steps a UK neurology or neurosurgery team will normally follow, in order, so you know what to expect and why.

  1. 01

    Assessing

    History and neurological exam

    A careful account of headache pattern, seizures, focal symptoms and any developmental delay, followed by a structured neurological examination.

  2. 02

    Assessing

    MRI brain or spine

    The gold standard. Cyst signal follows cerebrospinal fluid on every sequence, helping to characterise size, location and mass effect.

  3. 03

    Assessing

    Rule out lookalikes

    Diffusion weighted and FLAIR sequences separate an arachnoid cyst from an epidermoid, dermoid, porencephalic cyst or chronic subdural collection.

  4. 04

    Confirming

    CT if bleeding suspected

    A non contrast CT looks for cyst haemorrhage or an associated subdural haematoma after even minor head trauma.

  5. 05

    Confirming

    Neurosurgical opinion

    Symptomatic, large or progressive cysts, and any cyst causing hydrocephalus, warrant a paediatric or adult neurosurgical review.

  6. 06

    Preparing

    Developmental and cognitive review

    In children a developmental assessment is standard. In adults formal neuropsychology helps when cognitive change is suspected.

  7. 07

    Preparing

    Genetic counselling if syndromic

    When cysts sit within a wider clinical picture such as glutaric aciduria type 1 or a connective tissue disorder, clinical genetics is offered.

Typical timeline: MRI to a settled follow up plan within a few weeks in most UK centres.

Symptoms

What an arachnoid cyst can look like.

Symptoms depend on where the cyst sits and how big it is. Many cause none at all. Others announce themselves gradually, or occasionally after minor head injury.

  • Incidental on MRI

    The commonest presentation. A cyst is spotted on a scan ordered for headache, trauma or an unrelated indication.

  • Headache

    Often dull and positional. Sudden severe headache with vomiting can suggest bleeding into the cyst or a subdural haematoma.

  • Seizures

    Focal or generalised seizures can occur when a cyst irritates the overlying cortex, especially in the middle cranial fossa.

  • Focal neurological deficit

    Weakness, sensory change, cranial nerve palsies or ataxia depending on where the cyst sits and what it compresses.

  • Hydrocephalus

    A cyst blocking the flow of cerebrospinal fluid can cause raised pressure with headache, vomiting, drowsiness and visual change.

  • Macrocephaly in infants

    A rapidly enlarging head circumference or bulging fontanelle in a baby is a red flag for a large or obstructing cyst.

  • Developmental or cognitive change

    Delayed milestones in children, or subtle memory, mood and attention changes in adults, sometimes accompany large lesions.

  • Red flag - sudden deterioration

    Sudden severe headache, new focal deficit or reduced consciousness after minor head injury needs urgent imaging for haemorrhage or rupture.

Treatment

How arachnoid cysts are managed in the UK.

Watchful observation for most, focused surgery for the minority who need it, and prompt treatment of any complications along the way.

  • Watchful observation

    Most asymptomatic incidental cysts are simply monitored. Interval MRI at 6 to 12 months then annually confirms stability.

  • Symptomatic assessment

    Attributing symptoms to a cyst requires careful correlation. Not every headache in someone with a cyst is caused by the cyst.

  • Endoscopic cystoventriculostomy

    A minimally invasive keyhole approach that fenestrates the cyst into a ventricle. Often the first choice for suitable intracranial cysts.

  • Microsurgical fenestration

    Open craniotomy with fenestration into the basal cisterns or subarachnoid space, used when endoscopy is not feasible.

  • Cystoperitoneal shunt

    A permanent tube drains cyst fluid to the abdomen. Effective but carries a higher long term risk of blockage, infection and revision.

  • Treatment of complications

    Cyst haemorrhage or an associated subdural haematoma is managed on its own merits, with evacuation and observation as needed.

  • Tarlov cyst care

    Sacral perineural cysts are usually incidental. When they cause pain or radiculopathy, selective CSF aspiration with fibrin glue may be considered.

  • Genetic and syndromic care

    Where a cyst is part of a wider syndrome, coordinated care with clinical genetics, paediatrics or neurology guides long term follow up.

What this guide is based on

The sources behind every claim on this page.

UK national guidance and specialist society standards, current at the time of last review.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your GP, neurologist or neurosurgeon knows your scans and history and can tell you which parts apply to you. If in doubt, get seen.

  • NHS England. Neurosciences service specification and paediatric neurosurgery standards.

  • Society of British Neurological Surgeons. Guidance on intracranial cyst management.

  • British Paediatric Neurology Association. Guidance on incidental intracranial findings in children.

  • European Association of Neurosurgical Societies. Consensus statements on arachnoid cyst surgery.

Red flags

When an arachnoid cyst needs urgent attention.

Most cysts are quiet. These situations are the exceptions and mean you should seek urgent medical review.

  • Sudden severe headache

    A thunderclap headache in someone with a known cyst can indicate acute cyst haemorrhage or rupture and needs urgent imaging.

  • Head injury with a known cyst

    Even minor head trauma can trigger a subdural haematoma when a large arachnoid cyst is present. Have a low threshold for assessment.

  • New focal weakness or numbness

    New neurological deficit needs same day review with an urgent scan and neurosurgical input.

  • Rapidly enlarging head in a baby

    A fast rising head circumference, bulging fontanelle or downcast eyes suggest hydrocephalus and needs paediatric emergency review.

  • Reduced consciousness or vomiting

    Persistent morning vomiting, drowsiness or confusion may reflect raised intracranial pressure and warrants immediate assessment.

  • Progressive visual change

    Blurred vision, double vision or visual field loss can accompany compression or hydrocephalus and needs prompt review.

  • New seizure

    A first seizure in an adult or child with a known cyst deserves urgent neurology assessment and imaging.

  • Progressive gait or balance change

    A cerebellar or brainstem cyst may cause worsening ataxia, needing neurosurgical review before falls occur.

  • New sacral pain with bladder change

    Symptomatic sacral Tarlov cysts with bowel or bladder symptoms need urgent spinal assessment to exclude cauda equina compromise.

Living with it

A stable finding, with a clear plan.

Four things that make the biggest day to day difference. Reassurance where it is warranted, awareness of red flags, sensible activity and sticking to the follow up plan.

A quiet reminder

Stability, not silence, is what matters.

A cyst that has been the same size for years is unlikely to suddenly change. Follow up scans confirm the picture and keep the plan honest.

  1. 01 Reassurance

    Most cysts stay quiet

    The great majority of incidental arachnoid cysts never change and never cause trouble. Follow up imaging usually shows stability.

  2. 02 Awareness

    Know your red flags

    Sudden severe headache, new neurological symptoms or a head injury with vomiting or confusion means same day review.

  3. 03 Activity

    Sport and contact activities

    For most small stable cysts, normal activity is fine. Discuss high impact and contact sports with your neurosurgical team.

  4. 04 Follow up

    Keep to the review plan

    Attend scheduled scans and clinic reviews. Any new or changing symptoms between visits are worth flagging early.

Frequently asked

Everything we get asked about arachnoid cysts.

Quick answers on what a cyst is, how it is diagnosed, and when surgery is really needed.

  • What is an arachnoid cyst?

    An arachnoid cyst is a benign sac of cerebrospinal fluid that sits between the arachnoid membrane and the underlying brain or spinal cord. Most are congenital and form when the arachnoid layer splits during early development.

  • Are arachnoid cysts dangerous?

    Most are not. The majority are found by chance on brain imaging and never cause symptoms. A minority cause headache, seizures, focal deficits or hydrocephalus, and a small number can bleed or trigger a subdural haematoma after head injury.

  • Where do arachnoid cysts usually sit?

    The middle cranial fossa is by far the commonest site, accounting for around half of intracranial cysts. Others sit in the suprasellar region, the cerebellopontine angle, over the cerebellum or along the spine, including sacral Tarlov cysts.

  • How is an arachnoid cyst diagnosed?

    MRI is the gold standard. The cyst follows cerebrospinal fluid signal on every sequence, and diffusion weighted imaging helps distinguish it from lookalikes such as epidermoid or dermoid cysts. CT is used when acute bleeding is suspected.

  • Do arachnoid cysts always need surgery?

    No. Most are observed with interval MRI. Surgery is offered for symptomatic, enlarging or complicated cysts and typically involves endoscopic cystoventriculostomy, microsurgical fenestration or a cystoperitoneal shunt.

  • Can an arachnoid cyst come back after surgery?

    Cysts can partially recur, particularly after fenestration if the new communication scars over. Shunts avoid this problem but carry their own long term risks of blockage and infection. Regular MRI review is usual after any procedure.

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