Introduction

Posterior communicating artery (PCoA) aneurysms are the second most common intracranial aneurysm, representing around 20 to 25% of all aneurysms and up to half of all internal carotid artery (ICA) aneurysms. The open surgical approach to PCoA aneurysms can be considered basic, but variable projection and the presence of a hemorrhage can also make surgery very challenging. They are easily approached through endovascular means given their location on the ICA, but a tendency for long or wide necks make advanced techniques or devices such as balloon- and stent-assisted coiling or flow diversion necessary. They are of limited use in the setting of acutely ruptured aneurysms except in rare situations such as giant or large aneurysms that are calcified in an elderly or sick patient. This chapter reviews the presentation, natural history, and treatment outcomes from best-available data with a decision-making framework for management.

Major controversies in decision making addressed in this chapter include:

1. 
   

   Whether or not treatment is indicated.

   
   
2. 
   

   Open versus endovascular treatment for ruptured and unruptured PCoA aneurysm.

   
   
3. 
   

   Third nerve palsy: best treated with open versus endovascular treatment?

   
   
4. 
   

   Presence of a fetal posterior cerebral artery (PCA): contraindication for endovascular treatment?

Whether to Treat

The decision to treat or observe is difficult and should not be taken lightly. Ultimately, it emerges from a synthesis of the available but imperfect data, patient demographics, presentation and comorbidities, the skill of the surgeon, available resources, and the patient′s preferences. The American Heart Association estimates the prevalence of unruptured intracranial aneurysms around 3.2% at the age of 50 years without any predisposing comorbidities. The International Study of Unruptured Intracranial Aneurysms (ISUIA) was a multicenter study in the United States, Canada, and Europe that reported data on the natural history and treatment outcomes of unruptured intracranial aneurysms. In the most recent release (2003), aneurysms were stratified on size and location with PCoA aneurysms being included in the posterior circulation group (Post + PCoA). Those patients with no prior history of subarachnoid hemorrhage (SAH) and an aneurysm less than 7 mm in the Post + PCoA group had a 2.5% rupture rate per year that increased to 3.5% with a prior history of SAH. When considering aneurysms larger than 7 mm, history of SAH was no longer significant, but the annual rupture rate increased to 14.5, 18.4, and 50% for sizes 7 to 12, 13 to 24, and greater than 24 mm, respectively ( 1 , 2 in algorithm ). Ninety-nine of the 345 PCoA aneurysms were treated (84 surgical and 15 endovascular); outcome data are not interpretable as reported as these aneurysms were grouped with other posterior circulation aneurysms such as basilar tip. It is clear that operative morbidity increases in all location groups as age increases. In the prospective Japanese study on unruptured cerebral aneurysms, the annual rupture rate for PCoA aneurysms at the 95% confidence interval stratified by size was 0.41 (0.15–1.10) for 3 to 4 mm, 1 (0.37–2.66) for 5 to 6 mm, 3.19 (1.66–6.12) for 7 to 9 mm, 6.12 (1.66–6.13) for 10 to 24 mm, and 117.82 (16.60–836.43) for greater than 25 mm. The high rupture rate of PCoA aneurysms as compared to other locations seen in multiple studies lowers the threshold to treat when balancing against operative morbidity ( 1 , 2 in algorithm ).

In patients with multiple, large, or growing aneurysms, or family or personal history of aneurysmal SAH, more consideration should be given to treatment than those without. Patients with a personal history of SAH, sentinel headaches, third nerve palsy, growing aneurysm, or aneurysm greater than 7 mm should be treated ( 2 in algorithm ).

The Finnish study on the natural history of unruptured intracranial aneurysms showed an annual rupture rate of 1.1%. Interestingly, when patients were stratified by age, there was an independent inverse relationship of increased risk of rupture and age. The authors concluded that aneurysms of any size should be treated under the age of 50 years.

Conservative Management

The majority of aneurysms in the developed world present as incidental due to the prodigious availability and use of noninvasive imaging; as such, the question of when to observe becomes more important. Observation through computed tomography angiography (CTA) or magnetic resonance angiography (MRA) is a reasonable approach in patients with single, small (<3–4 mm) PCoA aneurysms ( 1 , 2 in algorithm ). Patient demographics including advanced age, comorbidities creating high surgical risk, and patient desire not to treat are further considerations.

Anatomical Considerations

The PCoA most commonly originates from the posterior medial or posterior wall of the ICA, approximately 10 mm on average distal to the ophthalmic artery, and marks the beginning of the communicating segment of the carotid. The PCoA courses posterior medial for approximately 12 mm to terminate at its connection with the PCA. Embryologically, the PCoA continues as the PCA and is a major source of posterior circulation; if this configuration persists in the adult, it is termed a fetal PCoA and is seen in 10 to 30% of patients (▶ Fig 23.1 ). In fetal origin of the PCA, the PCoA will terminate lateral to the oculomotor nerve and the P1 segment may be hypoplastic with some persistent flow or absent altogether. Each case should be defined based on dynamic cerebral angiography. The superior surface of the PCoA has approximately eight small perforating branches originating from it supplying the floor of the third ventricle, optic chiasm and tract, thalamus, hypothalamus, internal capsule, and pituitary stalk. The largest perforator to the floor of the third ventricle is termed the anterior thalamoperforating artery. The anterior choroidal artery (AChA), usually originating from the posterior wall of the communicating segment distal to the PCoA, is a major consideration as it partially supplies the optic tract and portions of the internal capsule and thalamus. The AChA can often be found close to the distal neck of a PCoA aneurysm and must be preserved. A cone-shaped origin of the PCoA from the ICA with the PCoA arising from the apex is termed an infundibulum, present in 10% as a normal anatomical variant, and is neither pathologic nor pre-aneurysmal. Fenestrations or duplications of the PCoA are rare but have been reported. The PCoA aneurysm can arise in many locations with the neck originating from the ICA proximal or distal (most common) to the PCoA, incorporated with the proximal or distal origin of the PCoA, or as a “true PCoA aneurysm” (rare) where the neck originates directly from the PCoA. The direction of the aneurysm fundus has clinical implications and is well described in four variants by Dr. Yasargil. The variants include anterolateral where the neck can be hidden by larger aneurysms, superolateral, posterolateral superior where the fundus is superior to the tentorium projecting into the temporal lobe, posterolateral inferior where the fundus is inferior to the tentorium directed to the interpeduncular cistern, and posteromedial inferior. Sometimes the dome of the aneurysm can be indented by the tentorium with parts being found both above and below. Posterolateral superior projecting aneurysms tend to cause temporal lobe and/or ventricular hemorrhage when ruptured. Posterolateral inferior aneurysms are most likely to cause oculomotor nerve palsy (OMNP) given its projection into the interpeduncular cistern.

Workup

Treatment

Choice of Treatment and the Influence of Intracerebral Hematomas

PCoA aneurysm surgery can be basic and gratifying to treat or extremely complicated and unnerving depending upon the anatomy of each case. Microsurgical clipping is historically the treatment of choice and well described ( 3, 5, 7 in algorithm ; ▶ Fig. 23.2 ). However, given the right conditions, endovascular management can have clinical equipoise.

If PCoA aneurysm rupture has resulted in a large intracerebral hemorrhage (ICH) requiring urgent evacuation, craniotomy and microsurgical clipping should be performed with consideration given to craniectomy ( 3 in algorithm ). Some may consider and have reported endovascular management followed by craniotomy and evacuation or a minimally invasive approach to evacuation. If there is no ICH present after rupture, the patient can be treated by clip ligation ( 5 in algorithm ) or endovascular means provided the morphology of the aneurysm and anatomy of the approach vessels are amenable to an endovascular approach ( 4 in algorithm ).

PCoA aneurysms are much easier to treat with endovascular methods compared with an anterior communicating or middle cerebral artery aneurysm. An angiographic working angle can usually be found and narrow necks are amenable to simple coiling. Balloon- or stent-assisted coiling can be used in many cases with unfavorable necks. Cerebral flow diversion may also be an option for cases that are difficult to coil for a variety of reasons ( 4, 6 in algorithm ; ▶ Fig 23.3 ). A significant drawback to cerebral flow diversion is the need for dual antiplatelet therapy, generally a contraindication in acutely ruptured aneurysms. Some authors have published series using flow diversion in this clinical situation; however, it is not standard. If a sufficient working angle is unobtainable for endovascular management, clip ligation must be performed ( 4, 7 in algorithm ).

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19 Cavernous Carotid Artery Aneurysms 24 Anterior Choroidal Artery Aneurysms 26 Middle Cerebral Artery Aneurysms 25 Internal Carotid Artery Bifurcation Aneurysms 31 Giant Aneurysms of the Anterior Circulation 33 Dissecting Intracranial Aneurysms of the Anterior Circulation

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