21 Superior Hypophyseal Artery Aneurysms
Abstract
Superior hypophyseal artery aneurysms are usually described as part of paraclinoid aneurysms; therefore, their incidence and prevalence are not clear. Their anatomical location in proximity to the proximal dural ring, in the posteromedial part of the internal carotid artery (ICA), and close to the optic nerve make their surgical approach difficult. Symptomatic patients present with subarachnoid hemorrhage; they rarely present with optic nerve or pituitary stalk compression. Digital subtraction angiography is the gold standard for aneurysm visualization and characterization. Asymptomatic and small aneurysms are treated conservatively. Large or ruptured aneurysms should be treated. Nowadays, microsurgical clipping is rarely performed due to the anatomical complexity and the need for optic nerve or stalk manipulation. The majority of these aneurysms are treated endovascularly with coiling, stent-assisted coiling, or flow diversion techniques with good to excellent results. Appropriate clinical and radiological follow-up is mandatory in all intracranial aneurysms.
Introduction
The superior hypophyseal artery (SHA) is a complex summary of about one to five vessels, which arise from the paraclinoid segment of the internal carotid artery (ICA) between the distal dural ring of the ICA and the posterior communicating artery. Located in the medial wall of the ICA, this artery supplies the pituitary stalk and both optic nerves and the chiasm. Both the right- and left-sided SHA form a circumferential anastomosis to adequately supply these important pathways.
SHA aneurysms are rarely described in the literature and are mostly summarized together with other paraclinoid aneurysms. This makes it difficult to give clear evidence-based treatment recommendations for this rare disease. A female predominance is reported. Further, there is a tendency of multiple aneurysms when SHA or paraclinoidal aneurysms are reported.
Major controversies in decision making addressed in this chapter include:
Whether or not treatment is indicated.
Open versus endovascular treatment for ruptured and unruptured SHA aneurysms.
Management of complex SHA aneurysms using advanced surgical or endovascular techniques.
Whether to Treat
Ruptured SHA aneurysms are clearly recommended for immediate treatment as with other intracranial ruptured aneurysms to prevent the risk of rebleeding. Also, unruptured but symptomatic SHA aneurysms including patients with visual deficits or pituitary insufficiency should be considered for treatment to improve the symptoms or prevent further decline.
In asymptomatic patients with unruptured SHA aneurysms (incidental SHA aneurysms), the decision for treatment is based on aneurysm size, patient age, previous aneurysmal subarachnoid hemorrhage (SAH) from another aneurysm, and cardiovascular risk factors. As with other intracranial aneurysms, the PHASES or the UIATS scores may help for the decision-making process to weigh up aneurysm rupture risk with treatment risk. There is no specific treatment regimen proposed for SHA aneurysms compared to all other aneurysms. No increased risk of hemorrhage is reported specifically.
Ruptured and symptomatic SHA aneurysms need immediate treatment ( 1 in algorithm ). Unruptured SHA aneurysms should be treated if the rupture risk is higher than the treatment risk. Besides aneurysm size, patient age is important to be considered in the decision-making process. Aneurysms greater than 7 mm and young patients should be treated, rather than patients with short remaining life expectancy ( 2 in algorithm ).
Anatomical Considerations
The close location of the SHA to the proximal dural ring in the posteromedial part of the ICA makes a direct surgical approach sometimes difficult for proximal control. The optic nerve is always very close to these aneurysms and sometimes hides the complete aneurysm from an ipsilateral approach. Therefore, a contralateral approach, which gives the surgeon a direct view between the optic nerves, should be considered as well. The circumferential anastomosis made by both SHA is another important anatomical consideration. In case the SHA is at risk to be occluded during aneurysm treatment, each patient′s SHA anatomy needs to be understood or at least estimated before treatment to secure a retrograde flow in case of SHA occlusion. Insufficient anastomosis may lead to visual deficits or pituitary insufficiency ( 2 in algorithm ).
Classification
There are many classifications of paraclinoid aneurysms, which all include SHA aneurysms at least in one group. Horiuchi et al classified SHA aneurysms based on the relationship between aneurysm and the SHA. The SHA can originate from the proximal neck, the medial neck, the distal neck, and the aneurysm body or has no relation to the aneurysm. Mostly SHA can be found in the proximal neck of the aneurysm.
Workup
Clinical Evaluation
Clinical evaluation is important in both ruptured and unruptured SHA patients. In ruptured aneurysms, the clinical status as summarized in the World Federation of Neurological Surgeons (WFNS) score gives the treating surgeon an idea about the prognosis and influences the decision to treat or not to treat.
In unruptured aneurysms, an ophthalmologic and endocrinologic workup is mandatory to rule out any visual deficits such as vision loss or visual field deficits as well as deficits in one of the hormonal axes.
Imaging
Angiography such as computed tomography angiography (CTA), magnetic resonance angiography (MRA), and/or digital subtraction angiography (DSA) are important to understand the exact origin, projection, size, and relationship to neighboring structures. Since SHA aneurysms are located close to the skull base, the quality of CTA might be diminished by artifacts but demonstrates the close relationship to the bone better than most MRIs. The DSA clearly has an advantage in this location, since treatment of aneurysm might be possible in the same session. Magnetic resonance imaging (MRI)/MRA might be important to rule out differential diagnoses.
Differential Diagnosis
If the aneurysm in this location is clearly detected by angiography, differentials are rare. Located closely to the pituitary stalk/gland and especially in patients with pituitary dysfunction, a pituitary adenoma/cyst needs to be excluded in case of doubt (e.g., with MRI of the sellar region). Other tumors including tuberculum sellae meningiomas or optic nerve tumors are rarely but sometimes misdiagnosed with SHA aneurysms.
Treatment
Choice of Treatment and the Influence of Intracerebral Hematoma
The choice of treatment in SHA aneurysms depends on many factors. Similar to aneurysms in other locations, surgical hematoma evacuation and clipping is indicated in a space occupying intracerebral hemorrhage (ICH) in ruptured SHA aneurysms, but this is very rare for SHA aneurysms.
In all other scenarios (ruptured SHA aneurysm without ICH and unruptured SHA aneurysms with indication for treatment), endovascular DSA and coiling should be considered first, if feasible. The complication rates of surgically treated SHA aneurysms (>15%) are reported higher than embolized ones in larger series (2%). However in experienced surgical hands, those complication rates should be much lower but still higher than 2%, due to the close proximity of these aneurysms with the optic nerve and the clinoid process. In case of an SHA aneurysm with a wide neck or incorporation of the SHA, which makes the endovascular approach difficult and surgical reconstruction possible, stent-assisted coiling or flow diversion should be considered as well as surgical reconstruction ( 3 in algorithm ).
If aneurysm occlusion is not possible with standard techniques and occlusion is highly recommended, then more advanced endovascular or surgical techniques should be considered. For instance, an extracranial–intracranial (IC-EC) high-flow bypass or flow-diverter devices could be considered in large and/or fusiform aneurysms incorporating the whole wall of the ICA segment ( 4 in algorithm ).
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