Posterior Communicating Artery Aneurysm: A Medical Review

A posterior communicating artery (PCoA) aneurysm is a balloon-like bulge that forms at or near the junction where the internal carotid artery meets the posterior communicating artery, one of the small connecting vessels that help link the front and back halves of the brain’s blood supply. These aneurysms are among the most common intracranial aneurysms encountered in clinical practice, and they carry particular significance because of their proximity to the third cranial nerve, which controls most eye movements and pupil size. The interplay between anatomy, rupture risk, symptoms, and treatment options makes PCoA aneurysms a topic where details genuinely matter for clinical decisions.

Where the Posterior Communicating Artery Sits and Why It Matters

The brain’s blood supply relies on a ring-shaped network at its base called the circle of Willis. The posterior communicating artery is a short vessel that bridges the front circulation (fed by the internal carotid arteries) and the back circulation (fed by the vertebral and basilar arteries). Because the PCoA connects these two systems, it plays a backup role: if one system is compromised, blood can reroute through this bridge.

An important anatomical variant affects roughly one in five to one in four people. In this variant, called a fetal-type posterior cerebral artery, the PCoA is unusually large because it serves as the main blood supply to the back of the brain on that side, rather than the basilar artery doing the job. Estimates of this variant’s prevalence range from about 3% to 28% depending on the population studied and how strictly it is defined.¹ This variant is more than a curiosity: when an aneurysm sits at the origin of a fetal-type PCoA, treating it becomes trickier because accidentally blocking the vessel could cut off blood to a large territory of the brain. Fetal variant circulations have been identified at the PCoA origin in about 11% of patients with these aneurysms, and they are more commonly found in women.²

How PCoA Aneurysms Form and What Raises the Risk of Rupture

Like other intracranial aneurysms, PCoA aneurysms develop when the vessel wall weakens at a point of hemodynamic stress, typically where blood flow hits the wall at an angle. Over time, the weakened spot balloons outward. Genetic factors play a role: a large meta-analysis covering more than 116,000 individuals identified several gene variants linked to intracranial aneurysm risk, with the strongest associations on chromosomes 9, 8, and 4 near genes involved in cell cycle regulation and blood vessel signaling.³ Smoking, high blood pressure, and family history of aneurysm are the modifiable and non-modifiable risk factors most consistently identified across studies.

Not every aneurysm ruptures, and researchers have put considerable effort into figuring out which ones are more dangerous. For PCoA aneurysms specifically, several shape-based features stand out as red flags:

Irregular shape: Aneurysms with bumpy, non-smooth walls are far more likely to have ruptured. One study found irregular shape carried roughly a threefold increase in rupture odds even after adjusting for other factors.
Daughter sac (bleb): A small secondary pouch budding off the main aneurysm dome is one of the strongest morphological predictors, with odds ratios for rupture exceeding 7 in some analyses.⁴
High aspect ratio: When an aneurysm’s height relative to its neck width is large, it is more likely to rupture.⁵
Height exceeding width: When the dome is taller than it is wide, rupture risk climbs substantially.⁴

Importantly, even small PCoA aneurysms (under 5 mm) can rupture, and bleb formation and a high aspect ratio are the features most predictive of rupture in that size range.⁵ This matters because many guidelines use size thresholds to decide when to treat, and PCoA aneurysms are notorious for rupturing at sizes smaller than you might expect.

The Role of Blood Flow Patterns

Beyond shape, the way blood flows through and around the aneurysm affects rupture risk. Computational studies consistently find that ruptured PCoA aneurysms tend to have lower wall shear stress on the aneurysm sac compared to unruptured ones.⁶ Lower wall shear stress means the blood is moving sluggishly along parts of the wall, which can promote inflammation and wall degradation. Ruptured aneurysms also show larger areas of low shear stress and reduced flow velocity.⁷

When a fetal-type PCoA is present, the hemodynamic picture changes. Because more blood courses through the PCoA in these patients, aneurysms associated with fetal variants show decreased wall shear stress and increased areas of abnormal flow, features that correlate with larger or more irregularly shaped aneurysms.⁸ In this group, the inflow angle, meaning the angle at which blood enters the aneurysm, appears to be a particularly reliable predictor of rupture.⁸

How PCoA Aneurysms Present Clinically

PCoA aneurysms can announce themselves in two broad ways. The more dramatic is subarachnoid hemorrhage: the aneurysm ruptures, blood leaks into the space surrounding the brain, and the patient typically experiences a sudden, severe headache often described as the worst of their life. This is a medical emergency. Ruptured PCoA aneurysms can also trigger vasospasm, a dangerous narrowing of brain arteries days after the initial bleed, which can lead to strokes affecting both the front and back of the brain.⁹

The more subtle presentation is third nerve palsy, sometimes the only sign that an unruptured aneurysm exists. The third cranial nerve runs very close to the PCoA, and an expanding or pulsating aneurysm can compress it. Patients notice a drooping eyelid, double vision, and a dilated pupil on the affected side. About 27% of PCoA aneurysm patients in one surgical series had third nerve palsy, and interestingly, it was not the size of the aneurysm that determined whether the nerve was affected. Instead, the distance between the internal carotid artery and a bony landmark called the clinoid process was the key factor: a shorter distance left less room for the nerve and made compression more likely.¹⁰

A complicating factor with PCoA aneurysms is that some ruptured ones can evade initial imaging. Because these aneurysms tend to adhere to nearby tissue, the bleeding can stop early and produce such a small hemorrhage that CT scans miss it. In a case series of patients with confirmed subarachnoid hemorrhage from PCoA aneurysms, the initial imaging was negative and the diagnosis required a spinal fluid examination to detect blood.¹¹ This is worth knowing because a negative head CT does not completely rule out a ruptured PCoA aneurysm in someone with a suspicious headache.

Diagnosing PCoA Aneurysms

Modern CT angiography (CTA) using current-generation scanners detects nearly all aneurysms above 3 mm, with sensitivity approaching 99% to 100% for aneurysms in that size range.¹² However, the PCoA location has historically been one of the trickier spots for CTA. An early comparison study found that while CTA matched digital subtraction angiography (DSA, the gold standard) for most aneurysm locations, posterior communicating artery aneurysms remained somewhat problematic.¹³ As a result, clinicians have historically been more likely to follow up a CTA with a catheter-based DSA when the aneurysm sits at the PCoA. In one study, about 62% of PCoA aneurysm patients went on to DSA after CTA, compared with only 34% of middle cerebral artery aneurysm patients.¹⁴

That said, CTA technology has improved considerably. With modern multi-detector scanners, the gap has narrowed, and expert-interpreted CTA or MRA is now expected to detect nearly all aneurysms causing third nerve palsy.¹² The practical takeaway: if a patient presents with a new third nerve palsy and modern CTA is negative, the clinical suspicion should remain high, and further workup may still be warranted.

Treatment Options

Two main approaches exist for securing a PCoA aneurysm: surgical clipping and endovascular treatment. The choice between them is not straightforward and depends on whether the aneurysm has ruptured, its anatomy, the patient’s age and health, and whether a fetal-type PCoA variant is present.

Surgical Clipping

In clipping, a neurosurgeon opens the skull, navigates to the aneurysm under a microscope, and places a small metal clip across its neck to seal it off from the bloodstream. Clipping achieves higher rates of complete and durable aneurysm occlusion and lower rates of recurrence compared to endovascular coiling.¹⁵ Certain PCoA aneurysm subtypes, including those with an elongated dome, true PCoA aneurysms (arising from the PCoA itself rather than the junction), and those associated with a fetal variant, tend to do better with surgical clipping in terms of complete occlusion and preservation of the PCoA.¹⁶ Adjuncts like temporary clipping of the parent artery, removal of a small bone spur near the optic nerve (anterior clinoidectomy), and real-time fluorescence angiography help surgeons confirm that the clip is properly placed and that blood flow through the PCoA is preserved.¹⁷

Endovascular Coiling and Stent-Assisted Techniques

Endovascular coiling involves threading a catheter from a blood vessel in the groin up to the aneurysm and packing it with tiny metal coils to promote clotting inside the dome. In one series of 37 patients treated with coiling for PCoA aneurysms causing third nerve palsy, the procedure was technically successful in all cases, with complete occlusion in about half and near-complete occlusion in most of the rest.¹⁸ Coiling generally carries lower short-term morbidity and a smoother early recovery compared with open surgery.¹⁵

For wide-necked PCoA aneurysms, coils alone tend to fall out of the dome, so a stent is placed in the parent artery first to hold the coils in place. This stent-assisted approach significantly reduces recurrence: at follow-up averaging over a year, recurrence was seen in about 11% of stented patients versus 28% in non-stented patients.¹⁹

Flow Diversion

Flow-diverter stents are a newer endovascular option. Rather than packing coils inside the aneurysm, a densely woven mesh stent is placed in the parent artery across the aneurysm’s neck, redirecting blood flow away from the dome and allowing it to gradually clot and scar down. A systematic review and meta-analysis of flow diversion for PCoA aneurysms found complete occlusion at final follow-up in about 73% of cases, with a retreatment rate of only 2% and a mortality rate of 1%.²⁰ A key concern with flow diversion at this location is whether the PCoA itself stays open: at final follow-up, about three-quarters of PCoAs remained patent.²⁰ When the PCoA does narrow or close after flow diversion, the brain typically compensates by increasing flow through an alternative route, and posterior circulation strokes have been rare.²¹

Choosing Between Clipping and Endovascular Treatment

A 10-year analysis of over 850 PCoA aneurysms sheds useful light on this decision. For ruptured aneurysms, functional outcomes were similar between clipping and coiling at both discharge and long-term follow-up. However, endovascular treatment carried dramatically higher recurrence and retreatment rates. For unruptured aneurysms, endovascular treatment was associated with better functional outcomes at discharge and at final follow-up, and may be particularly advantageous in older patients.²²

The practical upshot: if you have a ruptured PCoA aneurysm, clipping remains a strong option because it achieves more durable closure with fewer retreatments. If you have an unruptured aneurysm and are older or have medical conditions that make open surgery riskier, endovascular approaches are appealing. And when a fetal-type PCoA is present, the treatment decision requires extra caution because of the risk of cutting off blood supply to the back of the brain.²

Third Nerve Palsy Recovery After Treatment

For patients whose PCoA aneurysm has caused a drooping eyelid and dilated pupil, a pressing question is whether the nerve will recover after treatment. The evidence consistently shows that surgical clipping leads to faster and more complete nerve recovery than coiling, at least in the first year. A meta-analysis found that full recovery of third nerve palsy occurred in about 84% of clipped patients versus 43% of coiled patients, with clipping carrying more than four times the odds of complete recovery.²³ A second large review found a similar pattern: full recovery in about 57% of clipped versus 42% of coiled patients overall, with the gap widening in patients who had complete nerve palsy (47% clipped vs. 20% coiled).²⁴

There is an important nuance here, though. A more recent meta-analysis that tracked recovery over time found that while clipping produced faster recovery at 1, 3, 6, and 12 months, the gap closed with longer follow-up. By 18 months and beyond 24 months, recovery rates between clipping and endovascular treatment were no longer statistically different.²⁵ This suggests that coiling does not prevent nerve recovery; it just delays it. The likely explanation is that during clipping the surgeon physically moves the aneurysm off the nerve, providing immediate decompression, while after coiling the aneurysm dome remains in place and the nerve recovers more gradually as swelling subsides and the thrombosed aneurysm remodels. Early treatment of either type improved recovery at six months.²⁵

Long-Term Recurrence and Surveillance

PCoA aneurysms treated with endovascular coiling have a higher recurrence rate than most other aneurysm locations, making long-term imaging surveillance essential. One study following coiled PCoA aneurysms for an average of about six years found that nearly a third recurred, with the mean time to recurrence being almost four years.²⁶ Recurrence was especially likely when the PCoA was incorporated into the aneurysm neck, meaning coils could not fully seal the dome without also blocking the artery. Ruptured status and fetal-type PCoA were also independent predictors of recurrence.²⁶

Flow-diverter stents show promising durability, but the PCoA itself can narrow over time after flow diversion. In one series tracking PCoA patency, many initially open vessels showed progressive flow reduction over the following months.²⁷ In most patients this is clinically silent because alternative blood pathways compensate, but it underscores why follow-up imaging is not optional after any endovascular treatment of these aneurysms.

PCoA Aneurysms and the Fetal Variant Challenge

The fetal-type posterior cerebral artery deserves its own discussion because it changes nearly every aspect of management. When the PCoA is the dominant supply to the back of the brain, any treatment that risks occluding it carries a real threat of posterior circulation stroke. Surgical clipping must be done with meticulous attention to preserving the vessel. Endovascular approaches using coils risk herniation of a coil loop into the PCoA lumen. Flow diversion poses the theoretical risk of gradually sealing off the PCoA, though early experience suggests that compensatory flow through the P1 segment of the posterior cerebral artery usually prevents ischemia.²¹

In a series of 16 patients with complex PCoA aneurysms and fetal posterior circulation treated with flow diversion, all had satisfactory clinical outcomes and none developed posterior circulation ischemia, even though some showed reduced flow through the fetal PCoA on follow-up imaging.²¹ This is encouraging but reflects a small, carefully selected group. Treatment decisions for fetal variant PCoA aneurysms remain among the most debated in cerebrovascular surgery, and multidisciplinary discussion between neurosurgeons and interventional neuroradiologists is standard practice before proceeding.

Posterior Circulation Aneurysms in Children

Intracranial aneurysms in children are rare and behave differently than in adults. In the posterior circulation specifically, a 12-year single-institution review of 20 pediatric patients found that most aneurysms were dissecting rather than the saccular type seen in adults, and the most common locations were the vertebrobasilar junction and vertebral artery rather than the PCoA. Three-quarters of these children presented with subarachnoid hemorrhage. The majority were treated with endovascular techniques, and outcomes were good in 90%.²⁸ The experience highlights that posterior circulation aneurysms in children are a fundamentally different disease, with different causes and different typical locations, compared to the PCoA aneurysms that dominate adult practice.

When a Small Aneurysm Is Not a Safe Aneurysm

One of the most persistent misconceptions about intracranial aneurysms is that small means safe. Many patients learn they have a small PCoA aneurysm on incidental imaging and are told to watch and wait. While observation is appropriate for many small aneurysms, PCoA aneurysms are an exception to the general comfort zone. Multiple studies show that morphological features like blebs, irregular walls, and high aspect ratios predict rupture independently of absolute size.⁵ A scoring system combining aneurysm width, presence of a daughter sac, and height-to-width ratio showed strong predictive accuracy for rupture, performing well even in aneurysms below the traditional 7 mm treatment threshold.⁴

The message for patients and clinicians: size alone is an insufficient basis for deciding whether to treat a PCoA aneurysm. A detailed morphological assessment, ideally including three-dimensional imaging, is needed to characterize the dome shape, look for daughter sacs, and measure the aspect and size ratios that matter more than raw diameter in this location.