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Health condition · Clinically reviewed

Brain aneurysm, from silent finding to specialist neurovascular care.

Most brain aneurysms never rupture. The ones that do announce themselves with a sudden thunderclap headache. This guide covers types, imaging, PHASES-based decisions and modern treatment in UK neurovascular centres.

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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, British Association of Neurological Surgeons and peer-reviewed neurovascular sources you can see at the end.

  • 03

    Current for 2026

    Reflects modern UK neurovascular practice including flow diverters, coiling, PHASES scoring and post-SAH care.

Key facts

Brain aneurysm at a glance.

The essentials, in plain English. What it is, the main types, who is affected and how it is treated in the UK today.

  • What it is

    A weak, ballooned area of a brain artery. Around 1 to 3 per cent of UK adults harbour one, most silently.

  • Main types

    Saccular (berry) is the most common at roughly 90 per cent. Also fusiform, mycotic, dissecting, traumatic and blister aneurysms.

  • Where they form

    Saccular aneurysms cluster at Circle of Willis bifurcations. Anterior communicating around 30 per cent, posterior communicating 25 per cent, MCA 20 per cent, basilar tip 10 per cent.

  • Who is affected

    Women outnumber men. Peak diagnosis in the 40s to 60s. Hypertension, smoking and family history drive risk.

  • Rupture

    Rupture causes subarachnoid haemorrhage. A thunderclap "worst headache of life" is the classic warning and a medical emergency.

  • Treatment

    Endovascular coiling is first-line for most. Flow diverters, stent-assisted coiling and surgical clipping have defined roles in specialist neurovascular centres.

Why this guide matters

A specialist condition, met with a specialist team.

Brain aneurysm care in the UK is concentrated in neurovascular centres for good reason. The three points below shape the rest of this page.

  • Most aneurysms stay silent

    Around 1 to 3 per cent of UK adults have one. The majority are found incidentally on imaging done for another reason and never rupture.

  • Rupture is a true emergency

    A thunderclap headache is subarachnoid haemorrhage until proven otherwise. Speed to CT, transfer and treatment shape survival and recovery.

  • Treatment is a shared decision

    The PHASES score, size, site, growth, symptoms and your own values guide whether to watch, coil, divert flow or clip.

How the diagnosis is made

From first scan to a neurovascular plan.

The steps a UK emergency department and neurovascular team will normally follow, in order, so you know what to expect and why.

  1. 01

    Assessing

    History and symptom pattern

    Sudden severe headache, cranial nerve palsies, sentinel bleeds and family history all shape the working diagnosis.

  2. 02

    Assessing

    CT head, immediately if SAH suspected

    Non-contrast CT within 6 hours of headache onset has very high sensitivity for subarachnoid blood. Sensitivity drops after 24 to 48 hours.

  3. 03

    Assessing

    Lumbar puncture at 12 hours

    If CT is negative but suspicion remains high, LP at 12 hours after onset looks for xanthochromia in the CSF.

  4. 04

    Confirming

    CTA, MRA and DSA

    CT angiography and MR angiography map the aneurysm. Digital subtraction angiography is the gold standard for morphology and treatment planning.

  5. 05

    Confirming

    SAH grading

    Hunt and Hess, World Federation of Neurological Surgeons (WFNS) and Fisher grades guide prognosis and monitoring intensity.

  6. 06

    Confirming

    Cardiac and systemic screen

    ECG and troponin pick up neurogenic stunned myocardium. Chest imaging looks for neurogenic pulmonary oedema.

  7. 07

    Screening

    Family and genetic screening

    Two or more affected first-degree relatives, autosomal dominant polycystic kidney disease and connective tissue disorders warrant MRA screening, usually from age 20 with follow-up.

Typical timeline: a suspected bleed to a definitive plan within hours, not days.

Types of aneurysm

Not all brain aneurysms are the same.

Morphology and cause matter. They change rupture risk, imaging appearance and treatment.

  • Saccular (berry)

    The most common at around 90 per cent. Bulge from a bifurcation, most often on the Circle of Willis. Anterior communicating around 30 per cent, posterior communicating 25 per cent, MCA 20 per cent, basilar tip 10 per cent.

  • Fusiform

    A whole segment of artery balloons and elongates. Often related to dolichoectasia and atherosclerosis. Coiling can be difficult; flow diverters and bypass may be considered.

  • Mycotic

    Follows infective endocarditis or septic emboli. Often distal in the middle cerebral artery territory and prone to rupture. Requires infection control alongside neurovascular treatment.

  • Dissecting

    A tear in the arterial wall creates a false lumen. Can cause ischaemia, mass effect or rupture. Often vertebral or basilar.

  • Traumatic and iatrogenic

    Follows head injury or interventional procedures. Presentation may be delayed. High rupture risk once identified.

  • Blister aneurysm

    A small, fragile bulge on the internal carotid artery. Difficult to treat because the wall is thin. Often needs flow diversion or wrapping.

Symptoms

What a brain aneurysm actually feels like.

Most are silent. The ones that speak up tend to do so loudly, often with a headache unlike any before.

  • Silent, incidental aneurysm

    Most unruptured aneurysms cause no symptoms and are found on imaging done for another reason.

  • Third nerve palsy

    A posterior communicating aneurysm can compress cranial nerve III, giving a dilated fixed pupil, ptosis and a "down and out" eye.

  • Sentinel headache

    A small warning leak days or weeks before major rupture. Sudden, severe and different from the usual pattern of headache.

  • Thunderclap headache

    The classic rupture symptom. Peak intensity within seconds, described as the worst headache of life.

  • Neck stiffness, photophobia and vomiting

    Meningism from subarachnoid blood. Often paired with reduced consciousness or a seizure at onset.

  • Focal neurological deficit

    Weakness, speech disturbance or visual loss can point to the site of the ruptured aneurysm or an associated haematoma.

  • Reduced GCS and collapse

    High-grade SAH may present as sudden collapse, coma or cardiac arrest at the scene.

  • Red flag - any thunderclap headache

    Assume subarachnoid haemorrhage until proven otherwise. Call 999 and request emergency imaging.

Treatment

How brain aneurysms are treated in the UK.

Care is concentrated in specialist neurovascular centres including the National Hospital at Queen Square, King's, St George's, Southampton, Bristol, Cambridge, Oxford, Sheffield, Newcastle, Manchester, Salford, Liverpool, Leeds, Edinburgh and Glasgow.

  • Endovascular coiling

    First-line for most ruptured and many unruptured aneurysms. Platinum coils are packed into the sac via a femoral or radial catheter, triggering thrombosis.

  • Stent-assisted coiling

    A stent across the aneurysm neck holds coils in place for wide-necked lesions. Requires dual antiplatelet therapy.

  • Flow diverter stent

    Devices such as Pipeline, Silk, Surpass and FRED divert flow past the aneurysm, driving thrombosis and endothelial healing across the neck.

  • Surgical clipping

    Craniotomy with a titanium clip across the aneurysm neck. Preferred for some MCA and complex anterior circulation aneurysms.

  • Wrapping and parent vessel sacrifice

    Reserved for aneurysms not amenable to standard clipping or coiling, often with a bypass to protect distal territory.

  • Nimodipine and vasospasm care

    Oral nimodipine 60 mg every 4 hours for 21 days after SAH. Hypertensive and hypervolaemic therapy, intra-arterial vasodilators and balloon angioplasty treat established vasospasm.

  • ICU-level supportive care

    Blood pressure control, euvolaemia, management of hyponatraemia (SIADH and cerebral salt wasting), external ventricular drain for hydrocephalus and DVT prophylaxis.

  • Rehabilitation

    Cognitive, physical, speech and psychological rehab, often extensive and long-running. Family support and return-to-work planning matter as much as the medicine.

Watch, treat or refer

Unruptured aneurysms are a shared decision.

The PHASES score combines population, hypertension, age, aneurysm size, earlier SAH and site to estimate 5-year rupture risk. Small anterior circulation aneurysms under 7 mm are often watched with MRA every 6 to 12 months. Aneurysms over 7 mm in the posterior circulation, any that grow, and symptomatic aneurysms are usually treated.

What this guide is based on

The sources behind every claim on this page.

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

Key references

Guidelines and trials we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your neurovascular team knows your imaging and history and can tell you which parts apply to you. In an emergency, call 999.

  • NICE. Subarachnoid haemorrhage caused by a ruptured aneurysm: diagnosis and management (NG228).

  • British Association of Neurological Surgeons. Neurovascular service standards.

  • International Study of Unruptured Intracranial Aneurysms (ISUIA) and PHASES score literature.

  • International Subarachnoid Aneurysm Trial (ISAT) 2002 and long-term follow-up.

  • Neurocritical Care Society. Guidelines for the management of aneurysmal subarachnoid haemorrhage.

Red flags

When a brain aneurysm needs urgent attention.

These are the situations that need same-day imaging, an ambulance or a specialist neurovascular opinion.

  • Thunderclap headache

    Sudden, severe headache peaking within seconds. Assume subarachnoid haemorrhage until imaging proves otherwise. Call 999.

  • Sentinel bleed

    A short-lived severe headache days or weeks earlier can be a warning leak. Report it - urgent imaging is warranted.

  • New third nerve palsy

    Ptosis, dilated fixed pupil and a "down and out" eye can signal a posterior communicating aneurysm compressing the nerve. Emergency neurovascular review.

  • Reduced consciousness or seizure

    Any collapse, coma or new seizure with headache is a high-grade SAH pattern. Blue-light transfer to a specialist centre.

  • Family history - two or more first-degree relatives

    Warrants MRA screening from around age 20 with follow-up. Discuss with a neurovascular team.

  • Polycystic kidney disease

    Around 10 per cent of people with autosomal dominant PKD harbour a brain aneurysm. Screening should be discussed.

  • Connective tissue disorders

    Ehlers-Danlos type IV, Marfan, fibromuscular dysplasia and aortic coarctation increase risk and change treatment planning.

  • Infective endocarditis with new headache

    Consider mycotic aneurysm, often distal and rupture-prone. Needs urgent vascular imaging and infection control.

  • Cocaine or methamphetamine use with headache

    Stimulant-associated hypertensive surges are well-recognised triggers for aneurysmal rupture.

Living with it

A specialist condition, with a clear pathway.

Four things that make the biggest difference day to day. Keep surveillance scans, treat the modifiable risks, respect the length of rehab and check on the family.

Support and charities

You are not the first to walk this path.

The Brain and Spine Foundation, Different Strokes and Brain Aneurysm UK Support offer peer support, information and a shared vocabulary for the road through diagnosis, treatment and recovery.

  1. 01 Follow-up

    Keep every surveillance scan

    For unruptured aneurysms on watch, MRA every 6 to 12 months tracks size and morphology. Missed scans hide growth.

  2. 02 Risk

    Blood pressure and smoking are the levers

    Tight BP control and stopping smoking are the two biggest modifiable rupture-risk factors. Alcohol moderation matters too.

  3. 03 Recovery

    Rehab takes months, not weeks

    Fatigue, cognitive slowing, low mood and headaches are common after SAH. A structured neurorehabilitation programme helps.

  4. 04 Family

    Screen the right relatives

    If you have two or more first-degree relatives with an aneurysm, or PKD, ask your team about MRA screening from age 20.

Frequently asked

Everything we get asked about brain aneurysm.

Quick answers on types, warning signs, imaging, PHASES-based decisions and modern treatment.

  • What is a brain aneurysm?

    A brain aneurysm is a weak, ballooned area of an artery inside the skull. Around 1 to 3 per cent of UK adults have one. Most are silent. The concern is rupture, which causes subarachnoid haemorrhage.

  • What are the different types?

    Saccular (berry) aneurysms account for about 90 per cent and arise at Circle of Willis bifurcations. Fusiform aneurysms involve a whole segment. Mycotic aneurysms follow infective endocarditis or septic emboli. Dissecting, traumatic, iatrogenic and blister aneurysms round out the picture.

  • What are the warning signs of rupture?

    A sudden thunderclap headache described as the worst of your life, often with neck stiffness, photophobia, vomiting, reduced consciousness or a focal deficit. A smaller sentinel bleed can precede major rupture by days or weeks and deserves the same urgency.

  • How is a brain aneurysm diagnosed?

    Non-contrast CT within 6 hours of a suspected bleed is highly sensitive. If negative and suspicion is high, a lumbar puncture 12 hours after onset looks for xanthochromia. CT angiography, MR angiography and digital subtraction angiography then map the aneurysm.

  • Do all aneurysms need treatment?

    No. Many small unruptured aneurysms are watched with MRA every 6 to 12 months. The PHASES score, aneurysm size and site, growth, symptoms and patient preference guide shared decision-making. Aneurysms over 7 mm in the posterior circulation and any that grow or become symptomatic are usually treated.

  • What is the difference between coiling and clipping?

    Coiling is an endovascular procedure done through a catheter, packing platinum coils into the aneurysm to thrombose it. Clipping is open microsurgery with a titanium clip across the neck. The ISAT trial favoured coiling for morphologically suitable ruptured aneurysms, but both remain essential options in specialist neurovascular centres.

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