AICA

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 sysem, a forerunner of the basilar artery.  One can think of the early vertebrobasilar system as a longtitudinal vessel (basilar) with a myriad trasverse 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 transvese vessels remain confined to suppy 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 dominantes 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 postuated to traverse the inner ear to connect with the carotid — but has never been concusively 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 collegues view arterial system of the brainstem and cerebellum as a natural extention 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.
 
The following diagrams serve to illustrate this concept.  On a personal note, I genearally find anatomical diagrams to be at least somewhat wanting; when applied to the living body, they too often suffer from both rigidity and inconsistency, and almost universally fall short of the predictive potential for which their creation was originally intended.  In this case, however, I believe that the genius of Lasjaunias (and supporting giants), may prove an exception.  It is not, by any stretch, The Periodic Table, but some time investment into a bit of theoretical discussion is likely to produce major dividends.
 
Below is a diagram of cervical spinal vasculature (left), and brainstem vasculature (right), without the cerebellum.
 
 
The image on the LEFT represents cervical spinal cord arterial supply, which consists of the anterior spinal artery and a paired, loose network of posterolateral vessels known as the posterior spinal arteries, and which are conceptually represented here as contiguous vessels (which is at least mostly true in the cervical spine).  The anterior and posterior spinal systems are connected by anastomoses running along the circumference of the cord, although known as “coronary” arteries, are conceptually quite clear.  A number of perforating arteries into the substance of the cord exist; when arising from the anterior spinal artery and penetrating through the ventral cord sulcus, they are named “sulco-comissural” arteries.  The entire spinal cord system is supplied via segmental radiculomedullary arteries, which connect the vertebral artery to the anterior spinal artery.  In practice, as you know, the radiculomedullary and radiculopial arteries are fewer, and may arise from longitudinal vessels other than the vert.  Radiculopial arteries are those which supply the posterior spinal system.  Radiculomedullopial arteries are those which happen to supply both anterior and posterior spinal systems simultaneously, sometimes via a coronary artery or via separate connections. For a more complete discussion of spinal vasculature (essential, I believe), see Spinal Vascular Anatomy section, particularly Spinal Arterial Anatomy.
 
Now, lets add the brainstem to the spinal cord, and use existing arterial vascular networks to furnish its supply.  Think of the brainstem as just a somewhat larger diameter biomass than the spinal cord, and things start to make sense. The unapaired basilar artery is a homolog of the equally unpaired anterior spinal artery.  The intracranial vertebral arteries, although obliquely oriented, are essentially homologs of the radiculo-medullary arteries, inasmuch as they serve as trasverse connections between the extraspinal vertebral system and the anterior spinal axis.  The transverse pontine arteries are homologs of the coronary arteries.  The basilar perforators are homologs of the sulco-comissural arteries.  The posterior spinal arteries, in the superior cervical spine, are sometimes termed “Lateral spinal arteries”.  This creates much unnecessary confusion, but the posterior spinal system and lateral spinal system are one and the same longitudinal arrangement. 
 
 
 
 
Now, add a cerebellum to the back of the brainstem, – again, simply more biomass — and use existing arteries to supply it.  As the cerebellum develops, some of the transverse pontine perforators are recruited to capture the cerebellar hemispheric territory.  Superiorly, this happens repatively consistently, and produces what is known as the Superior Cerebellar Artery.  At the mid to lower basilar segment, a homologous enlarging channel is the AICA.  At the bottom, the Posterior Inferior Cerebellar Artery (PICA) is the latest addition to cerebellar supply,  Unlike AICA and SCA, it seems to arise from the lateral spinal system (yet nervertheless also a coronary artery homolog).  The vermian arteries (of which only inferior is shown here) may be regarded as homologs of the sulco-comissural vessels. 
 
There are many advantages to viewing the vertebrobasilar system in this way.  All kinds of variants become quite predictable.  For example, duplicated and triplicated SCAs and AICAs are simply persistence of adjacent transverse pontine (or midbrain) arteries in supply of the cerebellar hemisphere.  AICAs arising higher or lower along the basilar are either results of dominance of higher or lower transverse arteries, or consequent to a relatively “short” basilar artery fusion.  C1 origin of PICA reflects dominance of the C1 radiculopial artery, which via the C1 segment of the lateral medullary artery, gives rise to the PICA.  The AICA-PICA balance in extent of cerebellar territory capture is a consequence of either anterior spinal (AICA) or lateral spinal (PICA) dominance.  All of these cases are given angiographic illustrations below. 
 

Basilar artery perforators
Classical dispostion depicts AICA and SCA arising from the basilar artery, in addition to multiple short basilar perforators whose supply is limited to the brainstem.  Understanding the embryology of vertebrobasilar circulation helps explain many variations seen in this pattern.  As explained above, one can think of the basilar as a longitudinal channel, with multiple trasverse channels.  As the cerebellum develops, some of these transverse channels enlarge to capture the cortical territory of the cerebellum — thus becoming AICAs and SCAs.  Others stay relatively small and are thus “limited” to brainstem supply.  This simple concept explains ALL variations seen in the area.  For example, duplicated SCAs and AICAs are just two adjacent perforators, which persisted in co-dominance of cerebellar supply.  Dominant AICAs and PICAs and SCAs are simply variations in extent of cerebellar surface capture by one vessel, with corresponding dominance or hypoplasia of the others.  “Larger” vs. smaller basilar perforators are but transverse channels which may capture some small segment of anterior cerebellar surface that is not taken by adjacent AICAs or SCAs.
 
Also important to recognize is the fact that brainstem perforators usually “arise” from SCA and AICA — or, rather, that AICA and SCA in fact “arise” from such perforators.  Important, though very small and thus typically angiographically invisible, collaterals exist between these perforators.  It is probable that extent of such collateral supply underlies the tremendous clinical recovery occasionally observed following brainstem infarcts.
 
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 Ponine Artery”).  Occlusion of these transverse pontine arteries typically corresonds to ventral pontine infarction, and may lead to the ischemic cause of dreaded “locked in” syndrome.
 
 
Diagram of the above concept.  Image on left, with brainstem alone, shows schematic of vertebrobasilar system with numerous “transverse” perforators supplying the brainstem (transverse pontine arteries).  Image on right, with cerebellum in place, depicts SCA, AICA, and PICA as perforators which elnarged to capture cerebellar hemispheric territory.  This viewpoint allows for ready conceptualization of multiple SCA, AICA, and PICA variations observed within the vertebrobasilar system.
 
 
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 slighly 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 signficance, 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 (meanging 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 juction 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)