An intracranial aneurysm is a weak, bulging area in a blood vessel inside the brain that can quietly develop without warning, sometimes remaining unnoticed for years while posing a serious risk if it suddenly bursts.
Understanding Intracranial Aneurysms
An intracranial aneurysm, also called a cerebral aneurysm or brain aneurysm, happens when a weak spot develops in the wall of a blood vessel inside the brain. When this weak area exists, the constant pressure from blood flowing through the vessel pushes the weakened section outward, creating a bulge that looks like a small balloon or blister attached to the blood vessel. This is similar to how a balloon gets thinner and more fragile as it fills with air, or how a weak spot on a tire’s inner tube might bulge outward under pressure.[1][3]
The development of these aneurysms is believed to happen because blood flowing through the vessel puts continuous pressure on a weak area of the vessel wall, which can cause the aneurysm to grow larger over time. Most brain aneurysms form in the major arteries along the base of the skull, particularly in areas where blood vessels branch or connect, such as in a network of arteries called the Circle of Willis. These junction points seem to be particularly vulnerable to weakening.[1][7]
There are several types of brain aneurysms, classified by their shape and size. The most common type is called a saccular aneurysm or berry aneurysm, named because it looks like a berry hanging from a stem. This type bulges out on only one side of the blood vessel and has a narrow neck that connects it to the main artery. A less common type is a fusiform aneurysm, where the entire section of the artery is widened on both sides rather than bulging out in one direction. Brain aneurysms are also classified by size: small ones are less than 15 millimeters, large ones range from 15 to 25 millimeters, giant aneurysms measure 25 to 50 millimeters, and super-giant aneurysms are over 50 millimeters.[4][7]
How Common Are Brain Aneurysms?
Brain aneurysms are more common than many people realize. Studies suggest that up to 6% of people in the United States have an aneurysm in their brain that isn’t bleeding, meaning that millions of people are living with unruptured aneurysms without knowing it. The good news is that most of these aneurysms remain small and never cause problems.[3][12]
However, when aneurysms do rupture, the situation becomes much more serious. Approximately 30,000 people in the United States experience a ruptured brain aneurysm each year. Brain aneurysms can affect people of any age, but they are most likely to occur in adults between the ages of 30 and 60 years old. Women are more commonly affected than men, and about 10% to 30% of people who have one brain aneurysm actually have multiple aneurysms in different blood vessels.[3][12]
What Causes Brain Aneurysms?
The exact mechanisms by which brain aneurysms form and grow are not completely understood, but researchers have identified several factors that contribute to their development. Some people may be born with a weakness in their blood vessel walls, making them more susceptible to developing aneurysms later in life. This is called a congenital defect. In other cases, aneurysms develop over time due to various conditions and injuries.[4][5]
Long-standing high blood pressure, also called hypertension, is one of the major contributors to aneurysm formation. When blood pressure remains elevated over many years, it puts extra stress on blood vessel walls, potentially weakening them. Hardening of the arteries, known as atherosclerosis, occurs when fatty deposits build up in arteries and can also damage vessel walls and contribute to aneurysm development.[4][5]
Head trauma or injury to blood vessels in the brain can create weak spots that later develop into aneurysms. Certain blood infections can also injure the vessel wall and lead to what are called infectious or mycotic aneurysms. These infectious aneurysms are particularly fragile and have an increased tendency to rupture.[4][13]
Some aneurysms run in families, suggesting a genetic component. If two or more close family members have had cerebral aneurysms, other family members may have a higher risk of developing them as well. Certain genetic conditions are also associated with a higher risk of brain aneurysms, including autosomal dominant polycystic kidney disease (ADPKD), Ehlers-Danlos syndrome, and conditions involving abnormal blood vessel formations called arteriovenous malformations.[4][6]
Risk Factors for Developing Brain Aneurysms
While anyone can develop a brain aneurysm, certain factors increase the risk significantly. Understanding these risk factors can help people make informed decisions about their health and lifestyle choices.
Smoking tobacco is one of the strongest modifiable risk factors for brain aneurysms. People who smoke not only have a higher likelihood of developing aneurysms but also face worse outcomes if an aneurysm ruptures. Smoking damages the cells that line blood vessels and contributes to the buildup of fatty deposits in arteries, weakening vessel walls over time.[4][19]
Poorly controlled high blood pressure puts continuous strain on blood vessel walls throughout the body, including those in the brain. Over time, this constant pressure can create weak spots that balloon out into aneurysms. People with hypertension should work with their healthcare providers to keep their blood pressure at healthy levels.[4][9]
Substance abuse, particularly the use of cocaine and amphetamines, dramatically increases the risk of aneurysms and their rupture. These drugs cause sudden spikes in blood pressure and can directly damage blood vessel walls. Intravenous drug use can also lead to infectious aneurysms if bacteria enter the bloodstream and infect weakened blood vessels.[4]
Alcohol consumption, especially heavy drinking, is associated with an increased risk of brain aneurysms. Similarly, high caffeine intake has been identified as a potential risk factor. People with a family history of brain aneurysms should be particularly mindful of these lifestyle factors, as they may have an inherited predisposition to vessel wall weakness.[4][6]
Symptoms: When Aneurysms Cause Problems
The majority of brain aneurysms are small and do not cause any symptoms at all. Most people discover they have an aneurysm only when they undergo brain imaging for an unrelated reason, such as a CT scan or MRI ordered to investigate headaches or another medical condition. These unruptured aneurysms often remain stable for years or even a lifetime without causing problems.[2][6]
Symptoms of Unruptured Aneurysms
When an unruptured aneurysm does cause symptoms, it’s usually because it has grown large enough to press on nearby brain tissue or nerves. The symptoms depend on which nerves or brain structures are being affected. A person with a larger unruptured aneurysm might experience pain above and behind one eye, which can be persistent and troubling. If the aneurysm presses on the nerves that control eye movement, it can cause double vision or other vision changes.[2][3]
Other possible symptoms include numbness or weakness on one side of the face, difficulty with balance or coordination, problems concentrating or speaking, and issues with short-term memory. Some people notice that the pupil in one eye becomes larger than the other, or that one eyelid droops. These symptoms develop gradually as the aneurysm slowly grows and increases pressure on surrounding structures.[2][6]
Symptoms of a Ruptured Aneurysm
When an aneurysm ruptures, the symptoms are sudden, severe, and unmistakable. The most characteristic symptom is an extremely intense headache that comes on suddenly without warning. People who have experienced this often describe it as “the worst headache of my life” or a “thunderclap headache” because it strikes like a clap of thunder, reaching maximum intensity within seconds. This headache is distinctly different from any headache the person has experienced before.[2][3]
Along with the severe headache, a ruptured aneurysm typically causes other alarming symptoms. These include sudden nausea and vomiting, a stiff neck that makes it difficult to move the head, sensitivity to light that makes it painful to be in bright environments, and blurred or double vision. The person may experience seizures, which are episodes of uncontrolled electrical activity in the brain causing jerking movements or loss of awareness.[2][3]
In some cases, people lose consciousness briefly or for a prolonged period. Confusion, extreme drowsiness, and weakness in parts of the body may also occur. In the most severe cases, a ruptured aneurysm can cause the heart to stop beating, a condition called cardiac arrest. Sometimes, a small leak occurs before a major rupture, causing what’s called a “sentinel headache” that serves as a warning sign days or weeks before the aneurysm fully bursts.[2][18]
Preventing Brain Aneurysms and Their Rupture
While not all brain aneurysms can be prevented, especially those related to genetic conditions or congenital defects, there are important steps people can take to reduce their risk of developing aneurysms or prevent existing ones from growing and rupturing.
Quitting smoking is one of the most important things a person can do to protect their blood vessels. Smoking cessation not only reduces the risk of developing new aneurysms but also lowers the chance that an existing aneurysm will rupture. People who stop smoking allow their blood vessels to begin healing and reduce ongoing damage to vessel walls.[6][16]
Keeping blood pressure at a healthy level is crucial for brain aneurysm prevention. People with high blood pressure should work with their doctors to manage it through lifestyle changes and medication if necessary. Regular monitoring of blood pressure, either at home or at a pharmacy or doctor’s office, helps ensure it stays within a safe range. Reducing salt intake, maintaining a healthy weight, exercising regularly, limiting alcohol consumption, and managing stress all contribute to better blood pressure control.[6][15]
Making healthy lifestyle changes supports overall vascular health. Eating a balanced diet rich in fruits, vegetables, and whole grains while limiting saturated fats helps prevent atherosclerosis, which can weaken blood vessels. Regular physical activity strengthens the cardiovascular system and helps maintain healthy blood pressure. Losing weight if overweight reduces strain on the heart and blood vessels.[6][15]
Avoiding illegal drugs, particularly cocaine and amphetamines, is essential for preventing aneurysms and their rupture. These substances cause dangerous spikes in blood pressure and can directly damage blood vessel walls. People who use these drugs should seek help to stop, as continued use dramatically increases the risk of life-threatening bleeding in the brain.[16]
For people who already have an unruptured brain aneurysm, regular follow-up scans are important to monitor whether the aneurysm is growing or changing. Doctors typically recommend periodic imaging tests to check the aneurysm’s size and shape. If significant changes are detected, treatment options can be discussed before a rupture occurs.[11][15]
How the Body Changes: Understanding Aneurysm Pathophysiology
To understand what happens in the brain when an aneurysm forms and potentially ruptures, it helps to know how blood vessels normally function. The brain relies heavily on a steady supply of oxygen and nutrients delivered through blood vessels. Two major sets of vessels supply the brain: the internal carotid arteries, which run up the front of the neck, and the vertebral arteries, which travel along the spinal column. These vessels connect through a network of arteries at the base of the brain called the Circle of Willis, from which other major arteries branch out to supply all parts of the brain.[7]
Blood vessel walls are made up of three layers. The innermost layer, called the intima, is a thin lining of cells that come in direct contact with blood. The middle layer, called the media, contains smooth muscle cells and elastic tissue that give the vessel strength and flexibility. The outer layer provides additional support and protection. When an aneurysm forms, there is a weakening in these layers, particularly in the inner muscular layer, which causes the vessel wall to become thin and bulge outward under the constant pressure of blood flow.[9]
As blood rushes into the bulging area with each heartbeat, the aneurysm wall stretches further and becomes thinner, much like a balloon that becomes more fragile as it inflates. The walls of the aneurysm can become so thin that they are at risk of rupturing. When a rupture occurs, blood spills out into the space around the brain, most commonly into the area between the brain and the thin tissues that cover it, called the subarachnoid space. This type of bleeding is called a subarachnoid hemorrhage (SAH), and about 90% of SAHs are caused by ruptured brain aneurysms.[1][3]
When blood accumulates in the subarachnoid space or within brain tissue, it creates several problems. The blood itself puts excess pressure on brain tissue, causing the brain to swell. This swelling, combined with the blood taking up space inside the rigid skull, increases pressure inside the head, which can damage delicate brain cells and reduce blood flow to areas of the brain. The bleeding can also lead to a hemorrhagic stroke, which occurs when blood vessels rupture and cause bleeding in the space between the skull and brain.[3][12]
After a ruptured aneurysm, several dangerous complications can develop. One of the most serious is vasospasm, which happens when blood vessels in the brain suddenly narrow or clamp down, usually between 3 and 14 days after the initial bleeding. This narrowing reduces oxygen delivery to brain tissue and can cause additional brain damage. Another complication is hydrocephalus, which occurs when cerebrospinal fluid or blood accumulates around the brain, further increasing pressure inside the skull.[2][3]
Seizures can also occur after an aneurysm rupture, representing a temporary surge of uncontrolled electrical activity in the brain. These seizures can worsen brain damage. In severe cases, people may fall into a coma, a state of prolonged unconsciousness that can last from days to weeks. Delayed cerebral ischemia (DCI) is one of the biggest causes of complications and death after a ruptured aneurysm, occurring between 3 and 14 days after bleeding and causing major damage to brain cells and neurological function.[2][3]
Unfortunately, ruptured brain aneurysms result in death in about 50% of cases, even with medical treatment. This high mortality rate, combined with the fact that many survivors face permanent brain damage or disability, makes brain aneurysms one of the most serious neurological conditions.[3][12]
