The anterior inferior cerebellar artery (AICA) is one of the most vital arteries. It supplies structures of the posterior fossa. The posterior fossa is bounded by the foramen magnum and tentorium cerebelli and houses structures including the medulla, pons, midbrain, and cerebellum. Arterial supply to these structures are branches of the vertebrobasilar circulation. The vertebral arteries typically arise from the subclavian arteries bilaterally. They ascend and enter the cranium through the foramen magnum where they fuse at the caudal border of the pons to form the basilar artery.
Major branches from the vertebral arteries include the anterior spinal artery which supplies the medial medulla, the posterior inferior cerebellar arteries (PICA) which supply the rostral lateral medulla, and the posterior spinal artery (PSA) which supplies the caudal medulla.
The anterior inferior cerebellar artery is one of the major branches of the basilar artery. It is the most caudal branch and arises from the lateral wall of the caudal third of the basilar artery. Anterior inferior cerebellar artery usually originates as a single trunk and supplies the ventral and inferior surface of the cerebellum and the lower lateral pons. Anterior inferior cerebellar artery courses laterally and posteriorly, commonly bifurcating into the superior and inferior trunk at the pontomedullary junction near where the facial and vestibulocochlear nerves exit the brain stem. The length of the anterior inferior cerebellar artery can be divided into four segments including the anterior pontine (close to the abducent nerve), lateral pontine (close to the facial and vestibulocochlear nerves), floccu peduncular, and cortical segments. The lateral pontine segments can be further divided into pre-meatal, meatal, and postmeatal segments. The segments are named because of their relationship with the internal auditory meatus.
Other arteries that branch off the basilar artery includes the pontine arteries which supply both the upper and lower medial pons. The labyrinthine artery is also called the internal auditory artery (in most cases usually branched from the anterior inferior cerebellar artery). It supplies the labyrinth and cochlea of the inner ear. The superior cerebellar arteries branch off the basilar artery immediately before it bifurcates and supplies the rostral pons, caudal midbrain and superior part of the cerebellum. Finally, the posterior cerebral artery (PCA) is the last branch of the basilar artery, and it becomes part of the Circle of Willis. Some posterior cerebral artery branches supply the midbrain, thalamus, temporal lobes, medial and inferior occipital lobes of the cerebral cortex.
The anterior inferior cerebellar artery gives rise to the labyrinthine arteries which supply the vestibulocochlear and facial nerves. The anterior inferior cerebellar artery also supplies the lower lateral pons, the choroid plexus of the lateral ventricle, the middle cerebellar peduncle and the inferior anterior area of the cerebellum. Infarction of the anterior inferior cerebellar artery or any of its branches will lead to neurological deficits corresponding to the region its branches supplies. An elaboration of this impairments is explained in the clinical significance section below.
The embryological development of the anterior inferior cerebellar artery, just like other blood vessels, occurs in several phases. The process of angiogenesis begins with the angioblast, which arises from the mesoderm. The embryonic development of blood vessels occurs through three primary mechanisms. The large blood vessels, such as the dorsal aorta, are formed from the coalescence of angioblast in situ. Other large blood vessels, such as the endocardium, are formed by the migration of angioblasts from other sites. Vessels of the central nervous system, like the anterior inferior cerebellar artery, arising from the sprouts of existing larger blood vessels. The vertebral arteries, which give rise to the basilar artery, are a derivative from connections of the lateral branches of the dorsal intersegmental arteries. These are paired embryonic branches of the aorta part of which later become cervical intersegmental. The lateral branches of the cervical intersegmental join to become vertebral arteries.
With advancements in Computed Tomography (CT) technology, it has become easier to more accurately visualize the vascularization of small vessels, making it simpler to note variations that were typically seen only during an autopsy. Some of these advances have been useful studying the variations of the arteries of the posterior fossa including the anterior inferior cerebellar artery. In about 99% of patients, the anterior inferior cerebellar artery arises from the basilar artery with about 75% of cases originating from the lower third. It may also stem from the vertebrobasilar junction as seen in about 9% of cases. The anterior inferior cerebellar artery has also been cited to give rise to the labyrinthine artery in about 90% of individuals. The most common variation noted is the presence no anterior inferior cerebellar artery, with its absence more common on the left side. In this variation, the anterior inferior cerebellar artery will either originate from a common trunk of the vertebral artery or the posterior inferior cerebellar arteries directly. Duplication of the anterior inferior cerebellar artery has also been noted but is rare. The double origin anterior inferior cerebellar artery has also been pointed out in the right anterior inferior cerebellar artery in which one branch originates from the basilar artery and the other from the vertebral artery. Some anastomoses have also been noted between the rostral part of the anterior inferior cerebellar artery and the superior cerebellar artery (SCA) and between the caudal part of the anterior inferior cerebellar artery and the posterior inferior cerebellar arteries. Other common variations that occur in the branches of the basilar artery that may affect the anterior inferior cerebellar artery are the agenesis of the right posterior inferior cerebellar arteries or the left posterior inferior cerebellar arteries. The agenesis of the posterior inferior cerebellar arteries may create an inverse relationship between the anterior inferior cerebellar artery and posterior inferior cerebellar arteries. In this case of the agenesis of the posterior inferior cerebellar arteries, there will be a well-developed anterior inferior cerebellar artery with the posterior inferior cerebellar arteries arising from an anterior inferior cerebellar artery/posterior inferior cerebellar arteries common trunk. The prevalence of this variation as reported in the literature is about 20% to 24%.
Surgical procedures in the arteries of the posterior fossa require a great fund of anatomical knowledge. Understanding anatomical variations are the first step in carrying out successful procedures. For example, the position (proximal or distal anterior inferior cerebellar artery) and nature of an aneurysm (ability to maneuver micro-catheter into an aneurysm) are important factors that determine if surgery or endovascular treatment will be used. With a proximal anterior inferior cerebellar artery aneurysms, a skull base surgery is mostly used with an aneurysm being clipped or trapped and resected. A skull base approach can be very challenging because large exposures are needed and due to the critical relationship between anterior inferior cerebellar artery other neurovascular structures like abducents, facial, vestibulocochlear nerves, perforating pontine arteries, and the trigeminal, glossopharyngeal and vagus nerve may be in the way.
Endovascular treatment is mostly used in distal anterior inferior cerebellar artery aneurysms and has been discouraged in proximal anterior inferior cerebellar artery aneurysms because of unrelieved brain stem compression by the stent, recurrence of an aneurysm after coiling, and antiplatelet therapy that is used with stent-assisted coiling. Also, due to insufficient cases of proximal anterior inferior cerebellar artery aneurysms treated endovascularly, its indications are currently not quite clear. Endovascular treatment has become very useful in treating aneurysms, for example, using coiling embolization to treat aneurysms with narrow necks or parent vessel occlusion in anterior inferior cerebellar artery aneurysms that arise pass the brainstem in patients with good collateral circulation.
Some clinical significance that may result from the anterior inferior cerebellar artery may be secondary to either an infarction or might be secondary to the formation of an aneurysm that may have a mass effect compressing surrounding neurovascular structures or may rupture leading hemorrhage that may be fatal. In exceptionally rare cases, a hemangioblastoma may also present with mass effect compressing surrounding structures.
Patients presenting with anterior inferior cerebellar artery infarction may show a cardiogenic origin, as in an emboligenic heart disease or atherosclerotic/arteriosclerotic plaque. These patients may have cerebellar ataxia (secondary to impairment of anterior inferior cerebellar artery branches supplying the middle cerebellar peduncle), vertigo that may be present with hearing loss and tinnitus (secondary impairment of labyrinthine artery supply the cochlea and labyrinth). Since branches of the anterior inferior cerebellar artery supply the lower lateral pons that houses the nuclei of the abducent, facial, and vestibulocochlear nerves, infarction of these branches may lead to patients presenting with symptoms that include facial nerve palsy, ipsilateral hearing problems superficial ipsilateral sensory abnormalities, and contralateral body.
As reported in the literature, aneurysms of the posterior fossa account for 8% to 12% of all a cerebral aneurysm, with aneurysms of anterior inferior cerebellar artery accounting for about 1% to 2% of all aneurysms. The most common site of an aneurysm formation is the basilar (BA)-anterior inferior cerebellar artery junction. The cause of an aneurysm in the anterior inferior cerebellar artery can be due to hemodynamic stress on the weak arterial wall at a branch point, due to a high flow arteriovenous malformations (AVM) or due hemangioblastoma or a location that was exposed to radiation treatment. As stated above, the anterior inferior cerebellar artery can be divided into four segments. The anterior pontine (close to the abducent nerve), lateral pontine (close to the facial and vestibulocochlear nerves), floccu peduncular segment, and cortical segment. Anterior inferior cerebellar artery aneurysms, are usually saccular and located in the anterior pontine segments.
In very rare case reports, aneurysms have been noted in the distal anterior inferior cerebellar artery with the aneurysms mainly being dissecting or fusiform and rarely associated with bifurcation with about 55.5% located near the meatal loop. Patients with an anterior inferior cerebellar artery aneurysm that is more peripherally located may have a cerebellopontine angle mass effect (sudden hearing loss, tinnitus, vertigo due to the involvement of the labyrinthine artery, facial paresis due to involvement of the facial nerve, gait ataxia, dysmetria due to the involvement of the middle cerebellar peduncle with subarachnoid hemorrhage, acute severe headache, nausea, vomiting, or sudden coma.
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