What are unruptured intracranial (brain) aneurysms?

Unruptured intracranial aneurysms are balloon-like swellings of the blood vessels in the brain, found in about 3% of people in the UK today. For most people, they don’t cause problems and people live well with one. However, in rare cases (about 1%), they can rupture and bleed. 

The ROAR study is trying to find out why some aneurysms rupture and some don’t. The groundbreaking study is analysing data from more than 20,000 patients here in the UK to better predict which aneurysms may rupture. In turn, this will help guide how treatment can be improved and personalised. The study has the potential to impact how every patient in the UK with an unruptured aneurysm will be treated and cared for on the NHS. 

The vast majority of unruptured aneurysms don’t burst and don’t cause any issues. When they do, it’s called a subarachnoid haemorrhage (SAH). This is a rare type of stroke caused by bleeding into the space around the brain. It is a medical emergency that can lead to serious complications or even death. 

The ROAR study aims to save lives by improving early identification of people at risk, helping to prevent these life-threatening bleeds through better prediction and treatment strategies.

Treatment decisions are based on the person and their circumstances. They also depend on the aneurysm’s size, location, and the patient’s overall health risk. Larger aneurysms, or those in high-risk areas of the brain, are more likely to rupture. Risk factors like high blood pressure, smoking, or previous bleed from another aneurysm also increase the risk of rupture. While most unruptured aneurysms cause no symptoms, they can occasionally—but rarely—press on nearby nerves and cause problems like double vision or drooping eyelids. Doctors carefully assess the risk of rupture against treatment risks, with some cases best managed through regular monitoring.

Doctors use the PHASES score to estimate an individual’s risk of aneurysm rupture. This tool which is used by every neuro centre in the UK today combines key factors—Population, Hypertension, Age, Size of Aneurysm, Earlier SAH, and Site—to guide treatment decisions. A higher PHASES score suggests a greater risk of rupture and may support intervention, while a lower score might mean that monitoring is preferable.  The ROAR study is validating and improving this model to support more personalised decisions. For example, the current model misses out some known risk factors such as being female and having a strong family history of the condition. Our new model will look at a wider set of risks to ensure patients and clinicians can make informed choices about their treatment pathway and ongoing monitoring. 

If treatment is needed, options include surgical clipping, where a metal clip is placed across the aneurysm to stop blood flow, or endovascular treatment, which involves inserting coils or a flow-diverting stent through the blood vessel to prevent rupture. While both approaches are effective and tried and tested, they are not without risk and potential complications including bleeding, stroke, or damage to surrounding brain tissue. The risk of complications are between 5% and 8% but can be higher for more complex aneurysms. The decision to treat is carefully balanced against the individual’s risk of rupture and their overall health. Often, a specialist will decide that the best mode of care is to continue to watch and monitor the unruptured aneurysm. In this case, it’s monitored through regular MRI or CT scans. 

Currently, the risk prediction model used in the NHS for unruptured intracranial aneurysms are limited in their scope. These models, unfortunately, rely on a small number of variables to estimate rupture risk. As a result, we are left with many aneurysms where the exact risk of rupture remains unclear. This limitation means that while some aneurysms are accurately assessed, others, potentially high-risk aneurysms, may be overlooked. The challenge lies in identifying which aneurysms will go on to rupture and which will remain stable. The ROAR study will address these challenges and lead to better care for patients.

Being diagnosed with an unruptured brain aneurysm can have a significant emotional and psychological impact on patients. Although most aneurysms never rupture, living with uncertainty can cause ongoing anxiety and fear. Many patients worry about the risk of rupture during everyday activities, leading to them making changes in lifestyle, work, or social behaviour.  Some may feel overwhelmed by the need to make complex decisions about monitoring or treatment—especially when the risk is unclear. Improved risk prediction models and clearer guidance can help reduce this burden by supporting more confident, personalised decisions based on robust evidence.