Introduction

Fusiform aneurysms are uncommon but remain challenging to neurosurgical treatment. Fusiform aneurysms of the basilar artery (BA) were first described in 1922, and since then several terms have also been used including dolichoectatic aneurysms, transitional aneurysms, and giant serpentine aneurysms. Posterior circulation fusiform aneurysms (PCFAs) are rare, with significant male predominance (~70%). PCFAs most commonly present as posterior circulation ischemic strokes. In addition, they may cause cranial nerve palsies, brainstem compression, and subarachnoid hemorrhage (SAH). Contrary to the more common saccular aneurysms, fusiform aneurysms are associated with high rates of rebleeding and morbidity. In this chapter, PCFAs are reviewed, including natural history, anatomy, pathophysiology, and treatment, with a suggested algorithm for treatment based on review of the literature.

Major controversies in decision making addressed in this chapter include:

1. 
   

   Whether treatment is indicated.

   
   
2. 
   

   Open versus endovascular treatment for ruptured and unruptured PCFA.

   
   
3. 
   

   The role of flow diverters for PCFA.

Whether to Treat

Unlike the case of saccular aneurysms, our understanding of the natural history of PCFAs is more limited. The natural history of PCFAs depends on the presenting signs and symptoms. However, it appears to be very poor in symptomatic patients, if left without treatment. In such cases, the rebleeding rate is high and ranges between 30 and 85%. In addition, untreated ruptured PCFAs are associated with high rates of mortality, ranging between 23 and 35% in a 5-year follow-up.

In a large prospective study of vertebrobasilar (VB) aneurysms over a 12-year period, the annual rupture rate of fusiform aneurysms was approximately 2%. Aneurysm enlargement is a significant predictor of lesion rupture. In a series of patients with unruptured fusiform aneurysms, a diameter of ≥ 10 mm in VB aneurysms was a significant risk factor for aneurysm growth and it was predictive of future rupture. Therefore, the vast majority of ruptured PCFAs cases should be treated ( 1 , 2, 3 in algorithm ). Additionally, unruptured PCFAs greater than 10 mm also likely warrant treatment.

Anatomical Considerations

The posterior circulation supplies the occipital lobes, cerebellum, and brainstem. The posterior circulation begins with the vertebral arteries (VAs) that join to form the BA, which then divides into the posterior cerebral arteries (PCA). These main arteries branch off several smaller vessels, including posterior inferior cerebellar arteries (PICAs), anterior inferior cerebellar arteries (AICAs), pontine branches, and superior cerebellar arteries. The VA can be divided into four segments, usually arising from the posterosuperior part of the subclavian artery and ending at lower part of pons to form the BA. The fourth segment (V4) ascends anterior to the roots of the hypoglossal cranial nerve (CN XII). The BA courses from the lower part of pons, next to the exit of the abducens cranial nerve (CN VI), the upper pontine border, and the oculomotor cranial nerve (CN III) where it bifurcates into two PCAs. From its origin at the termination of the BA, the PCA runs toward the occiput that gives several branches and perforators supplying mainly the occipital lobe and posteromedial temporal lobes.

Classification

Only few classification systems attempted to guide clinical decision in fusiform aneurysms by stratifying patients into risk groups. Mizutani et al′s classification system is composed of four types based on a combination of clinical features, imaging findings, lesional patterns of the internal elastic lamina (IEL), and the state of the intima. Type I is classic dissecting aneurysm characterized by disruption of the IEL without intimal thickening, presentation with SAH, and high rates of rebleeding. Type II is segmental ectasia, with a benign clinical course compared to type I. This type is pathologically characterized by extended and/or fragmented IEL with intimal thickening. In addition, the luminal surface is smooth without thrombus formation. Type III is dolichoectatic dissecting aneurysm. This type is distinguished pathologically from type II by dissections in the thickened intima, and organized luminal thrombus. Type III is frequently associated with hemorrhage and 50% mortality rate. Lastly, type IV is saccular aneurysm. Similar to the case of types I and III, type IV is associated with high rates of rebleeding and mortality.

Flemming′s classification for nonsaccular VB aneurysms is based on radiographic appearance. All lesions were defined as having an arterial dilation 1.5 times normal diameter: fusiform (14%)—aneurysmal dilation of the involved segment, without identifiable neck; dolichoectasia (45%)—uniformed dilation of the involved segment; transitional (19%)—uniform aneurysmal dilation with superimposed dilation of a portion of the involved arterial segment; and, indeterminate type (20%). Fusiform and transitional types were most likely to be symptomatic and dolichoectatic type. Similar to the case of dolichoectatic aneurysms (type II) in the classification of Mizutani et al, dolichoectatic aneurysms in Flemming′s classification are benign lesions with no significant neurological sequelae. ▶ Fig. 45.1 shows Flemming′s classification for nonsaccular VB aneurysms.

Workup

Treatment

Cerebrovascular Management—Operative Nuances

Microsurgical treatment of PCFAs is becoming less common given the recent advancements of endovascular therapy, especially flow diversion. In fact, microsurgical treatment is generally reserved for cases that cannot be treated with endovascular therapy. Because these aneurysms, by definition, do not have a neck and usually involve a circumferentially diseased vessel, clip reconstruction is generally not the treatment of choice (▶ Figs. 45.2 and ▶ 45.3 ). Complicating matters further, these aneurysms can be partially calcified/thrombosed and the surgical corridor is obstructed by a labyrinth of neurovascular structures. Thus, trapping with or without bypass is the main microsurgical modality. Operative nuances are discussed within each chapter for specific anatomical aneurysm location (see Chapters 37–44).

Related posts:

37 Vertebral Artery Aneurysms 38 Midbasilar Artery Aneurysms 39 Basilar Artery Apex Aneurysms 47 Traumatic Intracranial Aneurysms of the Posterior Circulation 42 Anterior Inferior Cerebellar Artery Aneurysms 46 Dissecting Intracranial Aneurysms of the Posterior Circulation

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