Superior Hypophyseal Artery Aneurysms Have the Lowest Recurrence Rate with Endovascular Therapy

Discussion

SHA aneurysms are a rare type of intracranial aneurysm. Data pertaining to the characteristics and management of these lesions are extremely scant. Their treatment has therefore been based mainly on comparison with the wider heterogeneous group of paraclinoid aneurysms and varies with the preferences and experience of different centers. Treatment options include open surgery or endovascular techniques. We have offered endovascular therapy as the primary treatment option for all patients with SHA aneurysms who presented to our institution because we believe that these lesions are effectively managed with endovascular means while being particularly challenging for open surgery. We have assessed the safety and efficacy of endovascular therapy in the largest series of SHA aneurysms to date.

An interesting finding of our study is the extreme female sex predominance (90.8%) for SHA aneurysms. This has been previously reported in several paraclinoid aneurysm series, though it was not as pronounced as in our study.^{2,4⇓⇓–7} It seems that female sex predominance is a hallmark of paraclinoid aneurysms in general and SHA aneurysms in particular. We are unable to provide a plausible explanation for this finding. The tendency toward multiple aneurysms is another unique characteristic of SHA and paraclinoid aneurysms.^{6,8⇓⇓–11} In the present series, 26.4% of patients had multiple aneurysms with most of these arising from the paraclinoid segment or the posterior communicating artery (73.5%). Most interesting, none of the 34 associated aneurysms were located in the posterior circulation. This finding could indicate that specific local factors exerting their action mainly in the anterior circulation may be implicated in the pathogenesis of SHA aneurysms. Patients presenting with subarachnoid hemorrhage accounted for only 13.8% of all those in our series. Whether SHA aneurysms have a characteristically low risk of rupture requires more investigation because the design of this study is not suitable for assessing the natural history of these lesions. The size of SHA aneurysms was quite large in our series (6 mm on average), which is a common finding for paraclinoid aneurysms in general.^11⇓–13

Surgical clipping of SHA aneurysms can be technically challenging. Obtaining proximal control, drilling of the anterior clinoid process to allow adequate exposure of the aneurysm, and opening the dural ring may add potential surgical morbidity. The procedure carries a risk of injury to the optic apparatus and other surrounding structures. It is also difficult during clip placement to preserve the patency of the small perforating branches supplying the optic chiasm and the pituitary gland, especially for large aneurysms. Despite recent advances in microsurgical techniques, the rate of complications related to surgical clipping of paraclinoid aneurysms—including SHA aneurysms—is still significant. Recent series have reported complication rates ranging from 15% to 35% with mortality rates as high as 3%–6%.^4,14⇓–16 These rates are well above those observed with endovascular therapy in this series. We found a remarkable 0% and 2.2% risk of major and minor complications, respectively. In addition, there was no procedure-related permanent morbidity or mortality.

We also found an exceptionally low recurrence rate of 3.9% at follow-up. Furthermore, the rate of recurrence requiring further intervention was as low as 1.3%. This is, to our knowledge, the lowest recurrence rate reported for any type of aneurysm with endovascular therapy. To put this into perspective, the recurrence rate is 24%–35% for posterior circulation aneurysms,^17,18 18%–32% for middle cerebral artery aneurysms,¹⁹ 37% for posterior communicating artery aneurysms,²⁰ 25% for anterior communicating artery aneurysms,²⁰ 26% for carotid ophthalmic artery aneurysms,²⁰ 40% for cavernous aneurysms,²¹ and 12%–29% for all paraclinoid aneurysms.^4,5,11,22 One plausible explanation for the observed low recurrence rate after endovascular therapy is the fact that SHA aneurysms are sidewall aneurysms projecting medially, which prevents the direct jet flow of blood from entering the aneurysm sac, hence, favoring stasis and thrombosis. This would also explain why bifurcation or end-vessel aneurysms tend to recur after endovascular treatment. In our series, no hemorrhages were observed after initial endovascular treatment, which shows that SHA aneurysms can be reliably secured with endovascular therapy.

We are aware of only 1 study that assessed the safety and efficacy of endovascular therapy for SHA aneurysms specifically. In this study, Gurian et al⁶ performed coil embolization in 11 patients with SHA aneurysms that were considered to be at high risk for surgery. They had no procedure-related complications and reported aneurysm recanalization in 1 of 7 patients at follow-up angiography. The authors concluded that coil embolization was an excellent alternative for treating SHA aneurysms. Although the results of this study are in line with our findings, the sample size was too small to draw any firm conclusions. Moreover, no patients were treated with stent or balloon-assisted coiling techniques. In other series, small samples of SHA aneurysms were included along with other paraclinoid aneurysms. The rate of recurrence at follow-up ranged from 12% to 29%, and complications were seen in 3%–7% of patients.^4,5,11,22 However, no separate analysis was done for SHA aneurysms specifically to identify this seemingly unique category in the large heterogeneous group of paraclinoid aneurysms. Furthermore, because of the nonavailability of stents at the time these studies were conducted, none of the patients were treated with stent-assisted coil embolization.

In fact, many SHA aneurysms are complex with wide necks and, thereby, require stent-assisted techniques. Besides preventing coil herniation into the parent vessel, stent deployment diverts blood flow away from the lumen and enhances thrombosis in the aneurysm.²³ In the present series, while 2 recurrences and 2 complications occurred in patients treated with coil embolization, none were noted in the stent group. Although the number of events is too small to allow any statistical analysis, stent techniques seem to have the highest occlusion rates and the lowest complication rates for SHA aneurysms. Overall, regardless of the technique, endovascular therapy has an unparalleled safety-efficacy profile and should be considered the procedure of choice for the treatment of SHA aneurysms. Surgery should be reserved for patients in whom endovascular therapy is unsuccessful.

One limitation of our study is the lack of comparison with a control group that was treated with surgery. Because most centers, including ours, prefer to manage SHA aneurysms with endovascular therapy, we believe that it is very difficult to design a study with a control surgical group. Other limitations of this study stem from its retrospective design and the lack of angiographic follow-up in 10.6% of patients. Given the relatively short-term nature of the angiographic follow-up in this series, longer follow-up periods are required to ascertain that endovascular therapy provides durable aneurysm closure.