Introduction

Intracranial aneurysm treatment has seen many changes over the past two decades with the advent of detachable coils, flow-diverting stents, and other endovascular techniques. Although aneurysm recurrence has always been an important issue for vascular neurosurgeons, more attention has been placed on recurrence due to a higher rate with endovascular treatment. In this chapter, we will briefly overview the natural history of ruptured and unruptured posterior circulation intracranial aneurysms and the treatment modalities currently employed. We will also discuss the natural history of treated aneurysms and the rates of recurrence with surgical and endovascular techniques. Lastly, we will discuss the importance of clinical and radiological follow-up, nuances of endovascular and open surgical treatment of recurrent aneurysms, and the durability of each treatment algorithm.

Major controversies in decision making addressed in this chapter include:

1. 
   

   Whether or not treatment is indicated.

   
   
2. 
   

   Recurrence rate after endovascular or surgical techniques.

   
   
3. 
   

   Open versus endovascular treatment for ruptured and unruptured recurrent aneurysms of the posterior circulation.

Whether to Treat

Aneurysmal subarachnoid hemorrhage (SAH) affects 9 to 15 per 100,000 persons per year. The incidence varies between different populations, but overall life risk is approximately 0.5%. Due to improvement in noninvasive imaging modalities, the incidence of incidental aneurysms has increased. Treatment of unruptured aneurysms is complex and dependent on patient′s and aneurysm′s factors. Based on the data collected in the International Study of Unruptured Intracranial Aneurysms (ISUIA), posterior circulation aneurysms of less than 7 mm in diameter carried a 5-year rupture risk of less than 5% compared to 0% for anterior circulation aneurysms. Similarly, and of particular importance for this chapter, posterior circulation aneurysms including the posterior communicating artery increased the relative risk of rupture by a factor of 2.3. These data must be kept in mind when deciding whether to treat or to follow a recurrence of an unruptured posterior circulation aneurysm.

Multiple studies have demonstrated improved outcomes of ruptured aneurysms with endovascular therapy. The 10-year data from the International Subarachnoid Aneurysm Trial (ISAT) demonstrated greater independence (82 vs. 79%, modified Rankin Scale [mRS] score 0–2) and greater average survivability (82 vs. 78%) in those patients treated by endovascular embolization. In the Barrow Ruptured Aneurysm Trial (BRAT), similar findings were noted for 3-year post-bleed mRS between endovascular treatment and surgical clipping: 35.8% for clipping and 30% for endovascular therapy with mRS greater than 2. While outcomes are improved with endovascular therapy, recurrence rate is higher than with surgical clipping ( 1 , 2 in algorithm ).

Rerupture rates in the recent literature following treatment are low. In a study of over 1,000 patients treated at multiple centers over a 2-year period, rerupture rate was found to be 1.8%, similar to the 1-year rerupture risk (1.8%) published in ISAT ( 1 , 2 in algorithm ). With respect to posterior circulation aneurysms, less than 3% of the aneurysms in ISAT and 17% of the aneurysms in BRAT were located in the posterior circulation. This is consistent with the commonly published incidence of aneurysms within the posterior circulation and explains the lack of literature regarding posterior circulation aneurysm recurrence. These studies briefly emphasize the problem that faces vascular neurosurgeons frequently; aneurysm remnants are common and require continued observation at a minimum ( 2, 8 in algorithm ). More importantly, it is the role of the vascular neurosurgeon to determine the patients who are at high risk for rebleeding from the index aneurysm. In other words, who should be retreated and who can be safely observed?

Anatomical Considerations

Initial angiographic occlusion comes from patience, diligence, and experience in the angiography suite and operating room and is the goal whenever possible and safe. However, there are multiple studies that demonstrate low rebleed risk despite the presence of a neck remnant/recurrence after endovascular therapy and even lower risk of recurrence and rebleeding after clip ligation. Based on this published data, certain anatomical factors and other risk factors can guide the neurosurgeon as to the aneurysm remnants that should be retreated and those that can be followed.

History of rupture is one of the most important factors to consider when deciding whether or not to treat a recurrence ( 1 , 2 in algorithm ); however, anatomical features guide whether or not the recurrence is significant or worrisome. The initial angiographic result, aneurysm type (saccular, fusiform, dissecting), aneurysm location (e.g., at a bifurcation), large aneurysm size, and wide neck are factors that correlate with aneurysm recurrence ( 6, 7 in algorithm ). However, not all recurrences are created equally and do not always correlate with the risk of rebleeding. For example, aneurysm filling into the dome of a ruptured aneurysm intuitively is more dangerous than filling of an electively treated unruptured aneurysm. That said, a large dome recurrence of a 10-mm incidental aneurysm is likely to be retreated given the fact that the aneurysm was treated due to its higher risk of rupture in the first place. The angiographic location of the recurrence is an important factor when reviewing the angiographic remnant. A small, stable neck remnant, without filling of the dome, could be followed with serial magnetic resonance angiography (MRA), and with formal angiography at 6 months or with changes in MRA (▶ Fig. 48.1 ) ( 8 in algorithm ). However, a side-wall recurrence, meaning filling of the aneurysm between the coil mass and the side wall of the aneurysm, should not be followed and should be considered for treatment. Sidewall filling can signal an endovascular leak and portends a higher rate of rerupture by allowing filling to reach the dome (▶ Fig. 48.2 ) ( 6, 7, 9 in algorithm ). Sidewall recurrences should be treated diligently. Posterior circulation dissecting aneurysms carry a high risk of rupture and any recurrence should be treated.

Length of follow-up and stability of the remnant on angiographic or noninvasive imaging are also factored into whether an aneurysm remnant requires treatment. For example, an aneurysm remnant that has remained unchanged on MRA and 5-year follow-up angiography is unlikely to change and rarely requires intervention. Finally, patient comorbidities and history, such as smoking, poorly controlled hypertension, and strong family history of aneurysms should be observed more closely and for a longer period of time or should be treated early, depending on the other factors mentioned earlier.

Pathophysiology/Classification

Posterior circulation aneurysms are typically classified as saccular or berry aneurysms, fusiform aneurysms, and dissecting aneurysms. The initial treatment differs based on aneurysm classification, anatomical considerations, and symptomatology. This means that treatment of the recurrence will differ as well based on the pathophysiology of the different aneurysm types. The treatment for recurrence of a saccular basilar terminus aneurysm and a dissecting vertebrobasilar junction aneurysm will likely be approached differently. This requires a case-by-case discussion and evaluation as the previous treatment employed will certainly dictate what options are available for addressing any recurrence that may occur in the future.

The Raymond and Roy aneurysm recurrence classification is purely descriptive, but not prognostic. The classification is not meant to predict which aneurysm remnants are of clinical significance or should be retreated, but rather was developed to illustrate angiographic follow-up for endovascularly treated unruptured aneurysms. Class 1 was described as no residual aneurysm, class 2 has angiographic filling at the neck, and class 3 describes residual aneurysm filling beyond the neck. While this classification is simple, it is too simple to use as any sort of guideline for treating patients. Therefore, an individualized approach is more appropriate.