What Is a Vertebral Artery Aneurysm?

A vertebral artery aneurysm is an abnormal bulging or ballooning in one of the two vertebral arteries that run along the back of the neck and into the skull, supplying blood to the brainstem and cerebellum. These aneurysms are uncommon compared to those that form on the more frequently discussed arteries at the front of the brain, but they carry serious risks: rupture can cause subarachnoid hemorrhage (bleeding around the brain), and even unruptured ones can trigger strokes by disrupting blood flow. Many vertebral artery aneurysms arise from a dissection, where the inner lining of the artery tears and blood seeps into the vessel wall, weakening it outward into a pouch. The condition sits at the intersection of stroke medicine, neurosurgery, and vascular biology, and its management has evolved considerably over the past two decades.

How Vertebral Artery Aneurysms Form

The vertebral arteries have a long, winding course. They begin at the subclavian arteries near the collarbones, thread upward through small bony canals in the cervical spine, then enter the skull and merge into the basilar artery at the base of the brain. Their final intracranial segment, sometimes called V4, is the stretch most prone to aneurysm formation. Anatomical variations in this segment, such as fenestrations (natural splits in the vessel), have been linked to aneurysms and dissection-related bleeding.¹

Most vertebral artery aneurysms that clinicians encounter are dissecting aneurysms. In a dissection, a tear develops in the artery’s inner layer. Blood forces its way between the wall layers, creating a pocket that balloons outward. Autopsy studies have shown what happens at the tissue level: within the first day of a dissection, clots form at the tear site, white blood cells flood in, and the arterial wall begins to die at the point of injury. In people who survive beyond the first week, the body mounts a repair response with new smooth muscle cell growth and immune cell accumulation.² That same research found no hardening of the arteries (atherosclerosis) at the dissection sites, suggesting that the process is fundamentally about structural wall failure rather than the slow cholesterol buildup most people associate with vascular disease. High blood pressure, however, does appear to contribute to the initial tear.

Who Is at Risk

Several factors raise the likelihood of developing a vertebral artery aneurysm. The most striking is connective tissue disease. A review of primary extracranial vertebral artery aneurysms found that every patient in the series had an underlying connective tissue or hereditary disorder, including Ehlers-Danlos syndrome, Marfan syndrome, and neurofibromatosis.³ Ehlers-Danlos syndrome in particular weakens the structural protein collagen throughout the body, leaving blood vessel walls fragile and prone to tearing.⁴

Trauma is another route. The vertebral arteries are partly shielded by the cervical spine, but that proximity also makes them vulnerable during neck injuries or surgery. Documented cases of iatrogenic (procedure-caused) vertebral artery injury during anterior cervical spine operations have resulted in pseudoaneurysms requiring emergency treatment.⁵ Car accidents, sports injuries, and even vigorous chiropractic manipulation have all been implicated in vertebral artery dissections that can evolve into aneurysms.

Beyond these identifiable causes, some dissecting aneurysms appear spontaneously in people with no known risk factors. Hypertension is considered a contributing factor in many of these cases, likely because chronically elevated pressure stresses the arterial wall until it gives way. Age and sex play roles too: most large treatment series report a mean patient age in the early to mid-fifties, with a roughly even split between men and women or a slight male predominance.⁶

Symptoms and Warning Signs

Vertebral artery aneurysms do not always announce themselves clearly, and that is part of what makes them dangerous. When symptoms do appear, they tend to fall into two broad categories: ischemic (related to blocked blood flow) and hemorrhagic (related to bleeding).

Ischemic symptoms show up when the aneurysm or dissection narrows the artery enough to starve the brainstem or cerebellum of blood. This can produce dizziness, difficulty walking, double vision, slurred speech, or sudden weakness on one side of the body. In a study of vertebrobasilar dissections, symptoms caused by ischemic damage to the cerebellum or brainstem were common, though headache was the single most frequently emphasized clinical sign, present in roughly half of patients.⁷ That headache is often described as sudden, severe, and located at the back of the head or neck, distinguishing it from a typical tension headache or migraine.

Hemorrhagic presentation is more dramatic. When an intracranial vertebral artery aneurysm ruptures, it spills blood into the subarachnoid space surrounding the brain. This typically causes a thunderclap headache, nausea, vomiting, neck stiffness, and sometimes loss of consciousness. A case involving bilateral vertebral artery dissecting aneurysms, for instance, presented with subarachnoid hemorrhage in a 44-year-old man, with the clinical challenge of determining which side had actually ruptured.⁸

Some vertebral artery aneurysms are found incidentally while doctors are investigating something else entirely. In the vertebrobasilar dissection study mentioned above, three cases were discovered during imaging for unrelated conditions. Giant aneurysms, though rare, can cause symptoms simply through mass effect: the aneurysm sac presses on adjacent brain structures, producing cranial nerve problems or brainstem compression without any rupture.⁹

How They Are Diagnosed

Imaging is the backbone of diagnosis. When a vertebral artery aneurysm is suspected, clinicians reach for CT angiography (CTA), MR angiography (MRA), or digital subtraction angiography (DSA). Each has strengths, and the choice depends partly on the clinical urgency.

CT angiography tends to outperform MRI-based techniques for vertebral artery problems specifically. A comparative study found that CTA identified more intimal flaps, pseudoaneurysms, and high-grade narrowing than MRA, and reviewers showed a statistically significant preference for CTA when evaluating vertebral artery dissections.¹⁰ CTA is also faster and more widely available in emergency departments, making it the go-to choice when subarachnoid hemorrhage is suspected.

MRI with MRA still has a role, particularly for extracranial vertebral artery dissections and for follow-up imaging that avoids repeated radiation. One pediatric case demonstrated how MRI with MRA confirmed both the diagnosis and the size and extent of an extracranial vertebral artery dissecting aneurysm at the C3–C4 level, and guided ongoing treatment decisions including the need for anticoagulation.¹¹

Digital subtraction angiography remains the gold standard for detail and is often performed before or during treatment. It involves threading a catheter into the blood vessels and injecting contrast dye to create high-resolution images of the artery’s interior. It is more invasive than CTA or MRA, but it provides the clearest view of aneurysm shape, blood flow patterns, and the relationship between the aneurysm and nearby branch arteries, all of which matter when planning treatment.

Treatment Options

Treating a vertebral artery aneurysm is a balancing act. The goal is to eliminate the risk of rupture or re-rupture while preserving blood flow to the brainstem and cerebellum. Three broad categories of treatment are used, and the best choice depends on whether the aneurysm has ruptured, where it sits, which branches it involves, and whether the opposite vertebral artery can compensate if flow through the affected one is sacrificed.

Flow Diversion

Flow-diverting devices, or flow diverters, have become increasingly prominent. These are fine-mesh stents deployed inside the parent artery across the mouth of the aneurysm. Rather than physically filling the aneurysm sac, they redirect blood flow past it, causing the blood inside the sac to stagnate and gradually clot. Over months, the aneurysm typically shrinks and the artery wall heals behind the device. A systematic review and meta-analysis of flow diverters for vertebral artery aneurysms reported favorable clinical outcomes in about 95% of patients, with complete aneurysm closure in roughly four out of five cases. Serious complications were uncommon: ischemic complications occurred in about 4% and hemorrhagic complications in about 1%.¹²

One important consideration with flow diverters is the posterior inferior cerebellar artery, or PICA, a critical branch that feeds the cerebellum. When the aneurysm sits at or near the PICA’s origin, covering it with a flow diverter could theoretically block that branch. Reassuringly, the meta-analysis found PICA preservation in about 95% of treated cases. A study using Pipeline embolization devices for unruptured vertebral artery aneurysms involving the PICA confirmed that 28 of 30 PICAs remained open during follow-up, with the two closures occurring only in cases where the parent artery itself clotted off.¹³

Flow diverters are particularly useful for unruptured but growing aneurysms, where there is time to plan and where the device can be deployed electively. A case report described successful flow-diverter placement for an unruptured but enlarging vertebral artery dissecting aneurysm, with satisfactory occlusion confirmed at six months and preserved flow through both the PICA and the anterior spinal artery.¹⁴ A pilot study of the Tubridge flow diverter for large vertebral artery dissecting aneurysms also found encouraging results, with five of six aneurysms completely occluded and the sixth nearly so at a median follow-up of about two years.¹⁵

Parent Vessel Occlusion and Trapping

When the aneurysm cannot be treated while keeping the artery open, or when the anatomy makes flow diversion impractical, deliberately closing the affected artery is sometimes the safest option. This approach, called parent vessel occlusion (PVO), works because most people have two vertebral arteries, and if one is sacrificed, the other can often supply enough blood to the brainstem through the basilar artery. In an analysis of endovascular treatment approaches over 20 years, vessel sacrifice was the most commonly used strategy, applied in about half of all treated cases. It also had the lowest rates of procedural complications and retreatment compared to stent-assisted coiling or flow diversion.¹⁶

Outcomes tend to be better when the aneurysm involves only one vertebral artery, because complete clotting of the sac can usually be achieved. When aneurysms extend to the basilar artery or involve both vertebral arteries, sacrifice becomes riskier since there is less room to reroute blood flow.¹⁷ In a pediatric case of a giant vertebral artery aneurysm causing brainstem compression, parent artery occlusion was used deliberately without filling the sac with coils initially, to avoid adding to the mass effect on the brainstem.¹⁸

Open Surgery and Bypass

Microsurgical options include direct clipping of the aneurysm, trapping (clipping the artery on both sides of the aneurysm), and bypass surgery to reroute blood around the diseased segment. Open surgery is less common today than endovascular approaches, but it remains essential in certain situations, particularly when both vertebral arteries are affected. A reported case of bilateral vertebral artery dissection required surgical trapping on one side, then clipping on the other when the second aneurysm enlarged, combined with a bypass from the superficial temporal artery to the superior cerebellar artery to maintain blood flow to the back of the brain. The patient recovered well with no new deficits over 18 months of follow-up.¹⁹

A comparison of endovascular and microsurgical treatment for aneurysms at the junction of the vertebral artery and PICA found that endovascular treatment carried a higher recurrence rate: three major recurrences among eleven endovascular patients versus a small remnant in two of ten surgical patients. The recurrences after endovascular treatment tended to occur specifically when the PICA originated at the aneurysm’s neck.²⁰ This highlights that no single approach works best for every case; the anatomy of each aneurysm largely dictates the strategy.

Recurrence After Treatment

Even after apparently successful treatment, vertebral artery aneurysms can come back. A study following patients after endovascular treatment of intracranial vertebrobasilar dissecting aneurysms found a recurrence rate of about 13%. Among 97 aneurysms with follow-up imaging, 13 recurred, and six of those recurrences involved rebleeding. Every hemorrhagic recurrence occurred in an aneurysm that had initially presented as ruptured. The strongest predictor of recurrence was involvement of the PICA origin, which increased the odds roughly eightfold.²¹

Another case series with two years of follow-up reported a recurrence rate of 16%, though most of those patients were successfully retreated with further endovascular procedures and experienced no additional complications afterward.²² The message for patients is that regular imaging follow-up after treatment is not optional. Most treatment centers schedule repeat angiography or MRA at intervals over the first one to two years and sometimes beyond, specifically to catch any re-expansion early while retreatment is still straightforward.

Vertebral Artery Aneurysms in Children

Although rare, vertebral artery dissecting aneurysms do occur in the pediatric population, and they present some distinctive challenges. A study comparing children with vertebral artery dissection to those with dissecting aneurysms found that children with aneurysms were more likely to present with stroke, presented at a younger age, and had a significantly higher rate of recurrent stroke compared to children with dissection alone. Importantly, nearly 40% of vertebral artery dissecting aneurysms in children in that study were initially missed on the first diagnostic evaluation. After surgical treatment, however, no child with a vertebral artery dissecting aneurysm experienced further stroke.²³

The clinical picture in children can be confusing. One case involved a 12-year-old boy who presented with double vision, vomiting, difficulty coordinating movement, and severe headache. The aneurysm was extracranial, at the C3–C4 spinal level, and the initial challenge was distinguishing it from inflammatory neurological conditions that can produce similar symptoms.²⁴ Males appear to be affected more frequently than females in the pediatric population. Endovascular treatment has shown promise in children, with case reports documenting full recovery and no neurological deficits after treatment.²⁵

Long-Term Outlook and Quality of Life

For patients who survive the initial event and receive appropriate treatment, the long-term outlook is generally encouraging. A study with a mean follow-up of nearly five years after endovascular internal trapping of vertebral artery dissections found that while complications such as cranial nerve weakness, spinal cord infarction, and brainstem ischemia did occur, most of these symptoms were temporary. Cranial nerve weakness resolved in all affected patients, and most of those with partial brainstem stroke symptoms also recovered. Despite residual symptoms in a minority, clinical scores indicated that most patients were in good general condition at long-term follow-up.²⁶

Even patients with recurrence can maintain a reasonable quality of life. In one case series, a patient whose aneurysm recurred four months after surgery but who declined reoperation continued to live without notable symptoms and maintained normal physical activity.²⁷ That said, such a wait-and-watch approach carries real risk and is not standard practice; retreatment is generally recommended when recurrence is detected.

Patients treated for ruptured aneurysms tend to have a rougher road than those treated electively for unruptured ones. The damage from the initial hemorrhage or stroke often determines more of the long-term picture than the treatment itself. For unruptured aneurysms caught before they cause harm, outcomes are substantially better, which underscores the value of prompt and thorough imaging when suspicious symptoms like sudden-onset posterior headache, unexplained dizziness, or new neurological deficits appear, especially in people with connective tissue disorders or a history of neck trauma.

Why the PICA Keeps Coming Up

If you read through the medical literature on vertebral artery aneurysms, you’ll notice one anatomical landmark mentioned again and again: the posterior inferior cerebellar artery. The PICA branches off the vertebral artery and supplies the lower part of the cerebellum and portions of the brainstem. Vertebral artery aneurysms frequently sit at or near the point where the PICA takes off, and this creates a treatment dilemma: any intervention that blocks the aneurysm risks blocking the PICA too, potentially causing a cerebellar stroke.

PICA involvement also drives recurrence. As noted in the recurrence data, the PICA origin being involved was the single strongest predictor of an aneurysm coming back after treatment.²⁸ Similarly, recurrences after endovascular treatment were concentrated in cases where the PICA arose from the aneurysm’s neck.²⁹ The reason is mechanical: when the PICA shares real estate with the aneurysm, it is harder to fully seal off the aneurysm without either compromising the branch or leaving a gap through which blood continues to enter the sac. This is one reason surgeons and interventionalists spend considerable time mapping the PICA’s exact origin before choosing a treatment strategy, and why patients with aneurysms in this zone tend to need closer follow-up imaging afterward.