Summary
Perforators are especially prone to injury during endoscopic endonasal approaches (EEAs) due to their small caliber, long trajectories, and often close relationship with skull base tumors. Injury to such vessels may result in ischemia or hemorrhage in important areas of the brain, causing severe disabilities or even death. In this chapter, the anatomy of perforators is presented in a way that is logical for endoscopic skull base surgeons. Ways to prevent their injuries, measures to take when injuries happen, and a clinical example are also discussed.
Key words
Arterioles – perforators – ischemia – endoscopic endonasal approaches – skull base15 Perforator Injury During Endoscopic Endonasal Skull Base Surgery
15.1 Key Learning Points
Any vessel injury during endoscopic endonasal approaches (EEAs) may result in high morbidity or even mortality.
Perforators are particularly vulnerable during EEA due to their small caliber, long trajectories, close relationship to other important neurovascular structures, and altered anatomy in the context of skull base diseases.
Clinical consequences deriving from injuries to perforators may range from no deficit to death, depending on the brain areas nourished by these perforators, and the collateral blood supply to such areas.
Knowing the detailed anatomy of perforators from an endoscopic endonasal point of view, correctly identifying perforators during surgery, and employing microdissection techniques are all important measures to prevent vascular injuries during EEA.
The best treatment strategy for injured perforators is prevention; once perforators are injured, they cannot be sutured, repaired, or bypassed; delicate bipolar cauterization of the injured vessel is usually sufficient to avoid hemorrhage and worse consequences.
15.2 Introduction
The development of EEAs is recognized as one of the most important recent advances in the field of skull base surgery. The EEA has allowed access to lesions in the ventral skull base through a natural and minimally invasive pathway, avoiding brain retraction. 1 In spite of this, the EEA has also brought new challenges. Relevant vessels of different sizes are all in close relationship with the skull base, and, in some cases, present in the pathway to areas of surgical interest.
Injuries to large vessels like the internal carotid artery (ICA) during EEA present as surgical accidents that must be rapidly dealt with. Because this is such a feared complication, it is highly discussed worldwide and different groups have already suggested standardized management protocols for this type of injury. 2 , 3 , 4 However, the literature is scarce on injuries to perforators during EEA. Perforators are tiny arteries that penetrate the brain and some cranial nerves or nuclei, and are highly variable in number, size, and distribution. They can often be encroached or encased by tumors, making identification and preservation extremely difficult in some cases. Regardless of their caliber, injuries to such vessels can have little or no consequence or can be similarly catastrophic, resulting in severe disabilities or even death.
Compartmentalization of skull base perforators from an endonasal perspective is proposed in this chapter. Ways to prevent injuries, measures to take when inadvertent lesions happen, and a clinical example are also discussed.
15.3 Compartmentalization of Skull Base Perforators—Related Pathologies and Potential Complications
For ease of comprehension, perforators related to EEA can be localized into three different compartments: anterior, posterior, and inferior. The anterior compartment consists of small arteries branching from the ICAs and anterior cerebral arteries (ACAs). The posterior perforating system consists of perforators arising from the posterior communicating artery (PComA), posterior cerebral artery (PCA), superior cerebellar artery (SCA), and upper basilar artery (UBA). The inferior perforating system consists of perforators from the lower basilar artery (LBA), vertebral arteries (VAs), anterior inferior cerebellar artery (AICA), and posterior inferior cerebellar artery (PICA).
The diseases related to these perforators are those most often encountered in each specific skull base compartments. Likewise, the potential complications resulting from injury of such perforators are closely linked to the specific area of the brain or cranial nerves nourished by these perforators, as discussed below.
15.3.1 The Anterior Compartment
Perforators in the anterior compartment are mostly related to anterior fossa, sellar, and suprasellar diseases, such as olfactory groove and tuberculum sellae meningiomas, craniopharyngiomas, pituitary adenomas, Rathke’s cleft cysts, and others. Such arteries are often seen during transcribriform and/or transplanum-transtuberculum approaches (Video 15.1)
Anterior Cerebral Artery (ACA)
The ACA has five segments, numbered from A1 to A5. The A1 segment, extending from the ICA bifurcation to the anterior communicating artery (AComA), is the most basal segment from where most perforating branches originate. 5 , 6 , 7
The medial lenticulostriate perforators, ranging from 1 to 11 branches (average of 6), arise from the posterosuperior aspect of the proximal half of A1 segment and follow a direct posterior and superior course to enter the medial half of the anterior perforated substance (APS).
Perforators from AComA, ranging from 0 to 4 (average of 1.6), usually arise from its posterior aspect, and supply the APS, the infundibulum, the optic chiasm, the subcallosal area, and the preoptic areas of the hypothalamus.
Heubner’s recurrent artery arises in approximately 80% of individuals from the proximal A2 segment, bends backward above its parent vessel, and courses posterior to the A1 segment in 60% of individuals; it is the largest and longest branch directed to the APS. It passes above the carotid bifurcation and accompanies the M1 segment into the medial part of the sylvian fissure before entering the anterior and middle portions of the full mediolateral extent of the APS.
Ischemic events in this compartment, most often due to injuries to A1 or AComA perforators, may result in personality disorders, intellectual deficits, and altered level of consciousness. Occlusion of A2 branches, especially Heubner’s recurrent artery, may cause hemiparesis with brachial predominance or dysphasia due to caudate, putamen, and internal capsule infarcts. Emotional changes, personality disorders, and intellectual deficits may also be ascribed to ischemia in this area. 6
Internal Carotid Artery (ICA)
The supraclinoid portion of ICA is usually divided into three segments based on the origin of its major branches: the ophthalmic segment extends from the ophthalmic artery origin to the posterior communicating artery (PComA), the communicating segment extends from the PComA origin to the anterior choroidal artery (AChA), and the choroidal segment extends from the AChA origin to the ICA bifurcation. The first two segments are more related to EEA as the choroidal segment has a posterolateral course. The PComA has a posteroinferior course and will be discussed later in this chapter. 7 , 8
Perforators from the ophthalmic segment usually comprise four rami (ranging from 1 to 7) arising from the posteromedial aspect of the ICA and are distributed to the pituitary gland, stalk, optic chiasm, and, less commonly, optic nerve, premamillary bodies, third ventricle floor, and optic tract. The superior hypophyseal arteries, which can range from 1 to 5 in number and can be either uni- or bilateral, run medially to supply the optic chiasm, pituitary stalk, and anterior lobe of the pituitary gland (Fig. 15.1). Bilateral injury of superior hypophyseal arteries may increase the risk of stalk and pituitary dysfunction. Chiasmal dysfunction (visual field defects), in turn, may occur with even a unilateral superior hypophyseal artery lesion. 9
15.3.2 The Posterior Compartment
Perforators in the posterior compartment are more related to the transsphenoidal-transclival approaches for removal of upper clival chordomas and meningiomas, and some pituitary adenomas or large craniopharyngiomas with inferior extension. 10
The posterior compartment has perforators arising from the PComA, UBA, PCA, and SCA. In the posterior compartment, perforators comprise a less intricate vascular network, which supplies the posterior perforated substance (PPS) to irrigate the upper brainstem and the posterior diencephalon (thalamoperforating arteries). Although these perforators are smaller in number when compared to the anterior compartment, ischemic injuries resulting from their lesions may cause much more severe neurological impairment due to the involvement of white matter tracts and nuclei within the brainstem and diencephalon.
Posterior Cerebral Artery (PCA)
The PCA is divided into four segments, from P1 to P4, with P1 and P2 being the most important sites for perforators, and P1 being the most common site of perforators seen during EEA. P1 extends from the basilar bifurcation to the site where the PComA joins the PCA. 11 , 12
The main branches arising from the PCA that are relevant to EEA are the posterior thalamoperforating arteries, and the short and long circumflex arteries. The posterior thalamoperforating arteries, which arise from P1 and enter the brain through the PPS, interpeduncular fossa, and medial crus cerebri, supply the anterior and part of the posterior thalamus, hypothalamus, subthalamus, substantia nigra, red nucleus, oculomotor and trochlear nuclei, oculomotor nerve, mesencephalic reticular formation, pretectum, rostromedial floor of the third ventricle, and the posterior portion of the internal capsule (Fig. 15.2). The short and long circumflex arteries often arise from P1 and less frequently from P2; the short circumflex artery courses around the midbrain and terminates at the geniculate bodies, whereas the long circumflex artery courses around the midbrain and reaches the colliculi.
Interruption of these arteries, especially the thalamoperforating pedicle, may lead to a paramedian thalamic infarct, which results in cognitive impairment, executive dysfunction, memory impairment, aphasia, decreased vigilance, and vertical gaze paresis. The symptoms can be more severe if the left and right paramedian arteries originate from a single pedicle (artery of Percheron), leading to bilateral ischemia (Fig. 15.3). 13
Posterior Communicating Artery (PComA)
Perforators do not arise from the communicating segment of the ICA in 60% of individuals. 7 , 8 Perforators arise mainly from the PComA and range from 1 to 14 in number, predominantly from the proximal half of the artery. They course superiorly and terminate on the third ventricle floor. The largest branch from the PComA is the premammillary artery, also called anterior thalamoperforating artery. Its interruption may cause anterior thalamic ischemia with cognitive and consciousness impairment and vertical gaze paresis (Fig. 15.2 and Fig. 15.4). The infundibular arteries are another group of arteries that arise from the PComA and supply the same area as the superior hypophyseal artery. 11 , 12
Basilar Artery (BA)
A mean of eight branches usually arise from the lateral and dorsal aspect of the BA (Fig. 15.5). There are no perforators on the ventral aspect of the vessel. Perforators irrigate the lateral pons, mesencephalon, and PPS. Those arising close to the origin of the SCA intermingle with the direct perforating branches arising from the proximal SCA. Those arising above the origin of the SCA enter the interpeduncular fossa. Their interruption will cause motor deficits due to cerebral peduncle ischemia.
Superior Cerebellar Artery (SCA)
The SCA is the most rostral of all infratentorial vessels; it arises close to the BA apex and encircles the pons and the lower midbrain. It supplies the tentorial surface of the cerebellum, the upper brainstem, the deep cerebellar nuclei, and the inferior colliculi.
Perforators arising from the SCA are divided into direct and circumflex types. The direct type follows a straight course to enter the brainstem while the circumflex type winds around the brainstem before perforating it. The circumflex perforating arteries are subdivided into short and long types. The short circumflex type travels up to 90 degrees around the circumference of the brainstem. The long circumflex type travels a greater distance to reach the dorsal surface. Both types of circumflex arteries send branches into the brainstem along their course. Inferolateral territory ischemia may present with hypoesthesia, ataxia, hemiparesis, cognitive impairment, executive dysfunction, and aphasia. 13
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