Anterior Inferior Cerebellar Artery (AICA)

Evolution and Embryology: Unlike PICA, which can be conceptualized as a cervical artery impressed into posterior fossa service by expanding needs of the cerebellum, the AICA is a true cerebellar and brainstem artery.  It develops as a branch of the longitudinal neural system, a forerunner of the basilar artery.  One can think of the early vertebrobasilar system as a longitudinal vessel (basilar) with a myriad transverse vessels, each having the potential to become SCA or AICA by capturing the cortical territory of the developing cerebellum.  It seems that hemodynamics favor selection of a dominant vessel (or two) for this role, and so most transverse vessels remain confined to supply of the brainstem (perforators).  The dominant vessels which emerge are thus named AICA, SCA, and PICA.  This concept helps explain most variations seen within the arrangement — a duplicated AICA, for example, represents co-persistence of two adjacent trasverse vessels with cortical territory.  The same goes for the cortical territory as well, where hemodynamic balance exists between SCA, AICA, and PICA in extent of cortical supply.  Any variation is possible, depending on which trunk dominates the cortex.  Dominance of AICA produces the well-known AICA-PICA variant.  The reverse (PICA-AICA) is just as legitimate, with a small AICA and large PICA cortical supply.  SCA can dominate as well.  An “otic” artery — segmental anteroposterior anastomotic artery similar to persistent trigeminal and hypoglossal — has been postulated to traverse the inner ear to connect with the carotid — but has never been conclusively demonstrated to exist.  

Origin and course:   The AICA is a highly variable artery in its position along the basilar axis, and extent of cortical territory.  Classically, it arises from the mid-basilar, and sweeps posterolaterally, covering ventral to pons in the prepontine cistern, to head posterolaterally within the cerebellopontine angle along the anterior cerebellar surface.  Its supply, therefore, includes portions of pons and the anterior cerebellum.  It gives a branch to the inner ear (Internal Auditory Canal branch), which will form a loop within the IAC more often than not.   Depending on hemodynamic balance, a more prominent AICA will continue inferiorly to capture the inferior cerebellar hemisphere and, potentially, the inferior vermis, due to relative hypoplasia of the PICA, corresponding to the aforementioned AICA-PICA variant.  It does not, to my knowledge, ever extent its inferior cortical supply to the posterolateral medulla.  Thus, even in extreme dominance of AICA, a small and, perhaps unrecognizable as such, “PICA” exists, limited to its medullary territory.  This has important clinical implications, in that even dominant AICA strokes do not produce lateral medullary infarcts (Wallenberg and other similar syndromes), whereas vert occlusions at the level of small PICAs typically do.  The inferior vermian territory can also be supplied by the SCA, which typically vascularises the superior vermis.  

Conceptual homology of vertebrobasilar and spinal arterial anatomy 

The basilar artery is formed by fusion of the longitudinal neural system, which in its most primitive form consists of loosely connected channels running along the undersurface of the brainstem. Lasjaunias and his colleagues view arterial system of the brainstem and cerebellum as a natural extension of the segmental arrangement found in the spinal cord.  The conceptual brilliance of this view allows one to understand all the myriad variations to which the basilar artery and its daughter vessels are subjected. In other words, if you consider the basilar artery to be a continuation of the anterior spinal artery, and its named branches and perforators as homologs of the coronary and sulco-comissural arteries (see Spinal Vascular Anatomy section), then the overall arrangement and its possible variations make perfect sense.








 Basilar artery perforators

Almost always, one or more “large” perforator is present between the AICA and the SCA — so consistently visualized, in fact, that it might almost deserve a unique name (we agreed to name it “ziggy” — or rather “Arteria ziggus basilarius” — until we found out that it already has a name — “Transverse Pontine Artery”).  Occlusion of these transverse pontine arteries typically corresponds to ventral pontine infarction, and may lead to the ischemic cause of dreaded “locked in” syndrome.

Duplicated AICAs: Bilateral AICA duplications (red arrows), in setting of AICAs dominance (AICA-PICA).  Notice the low position of the AICAs with respect to vertebrobasilar confluence.  Two groups of “ziggii” perforators are seen superior to the AICAs — yellow and brown arrows — yellow ones being located in position where “classical” AICA origin might be expected.  This disposition illustrates the above concept of AICAs arising as a dominant perforator which captures the cortical territory of the cerebellum (similar to how MCA develops from ACA perforators, see Vascular Neuroembryology).  In this case, it so happened that two of of the more inferior (with respect to basilar artery) perforators developed into “AICAs”, whereby the more superior (yellow and brown) perforators are slightly larger than usual due to hemodynamic need for supply of the brainstem and, probably, some anterior cerebellum, typically addressed by branches of the more rostrally located “classical” AICA.  In other words, the yellow perforators might be what typically becomes AICA, but in this patient the lower ones did, also capturing PICA territory.




 Schematic of AICA duplication (right), as co-dominance of adjacent perforators in capture cerebellar territory.  Classical dispostion is on the left.

The perforators between AICA and SCA are of great functional significance, typically supplying the ventral pons.  Occlusion of these often manifests as a hemiparesis.  

Duplicated, dominant AICA

Bilateral duplicated AICAs.  The superior ones (blue) supply the anterolateral cerebellar hemispheres, while the inferior ones (yellow) have a more medial territory, in this case.  A relatively small, C1 origin right PICA (red) is limited to medullary and inferior vermian territory. 

Earlier phase of the same patient, showing AICA duplications somewhat better.  Notice how left lower AICA (yellow) curves medially to balance the vermian territory of the right C1 origin PICA (red). 

Submental view of the basilar artery, showing different positions of left left and right AICA (red arrow) origins — again supporting the notion that AICAs develop from a number of possible choices — representing basilar artery perforators.  Notice also the perforators themselves (yellow arrows), of which there are two visible ones on the right (likely because of larger distance between SCA and AICA) and one on the left (shorter SCA/AICA distance).  Both AICAs are about equal in size, vis-a-vis AICA/PICA balance. The “Post Rx” refers to stenting of right superior cervical vert dissection/pseudoaneurysm, below the field of view.


Diagram of the same disposition (right image), depicting origin of the AICAs from a somewhat lower right and higher left perforators, as compared with the classical disposition (left image) 

 High vertebrobasilar junction (meaning short basilar, in contrast with the one immediately above), such that right AICA comes off the distal vert, in a patient with right vert spinal fistula (brown).  Again, its just a matter of which perforator gets selected for capture of cerebellar territory.  Short basilar means a VB junction somewhere around mid-pons (classically, it is at the pontomedullary junction), and so the AICA comes off where it does in relation to the cerebellum.




Diagrammatic representations of this disposition (right), with foreshortened basilar artery leading to origin of the AICAs from distal vertebral arteries, as compared with the classical arrangement (left)