Duplicated AICAs — co-dominance of two transverse pontine arteries in supply of cerebellar territory.
Schematic of the same disposition. AICA-PICA balanceAn exceedingly well known variation — a continuum, in fact, where AICA and PICA are, respectively, larger and smaller, depending on the extent to which one can capture the territory of the other. The PICA can be conceptualized as a large radiculopial origin vessel (arising from the lateral spinal artery, a homolog of the posterior spinal system), whereas AICA originates as coronary type vessel off the basilar (homolog of anterior spinal artery).
Schematic of balanced arrangement (left image). Right image shows PICA dominance of the “right” vert and AICA dominance of the “left” vert. Below is an angiographic image of the same arrangement, except for a kind of AICA duplication on the left, and a small basilar fenestration (yellow arrow). Notice how PICA and vert change caliber when they go through the dura above foramen magnum. Not to be confused for atherosclerotic disease, of which there is also a small specimen present just above the AICA.
Superior Cerebellar Artery Dupication — same as AICA, just a balance of transverse arteries.
This remarkable case illustrates how properly interpreted pathology offers proof of “theoretical” concepts. A ruptured AVM in the ventrolateral midbrain is supplied by two hypertrophied transverse pontine arteries, rather than AICA or SCA. What an unusual picture! Why? The AVM was present, in microscopic form, in utero. Flow demand by the shunt was met by twin transverse spinal arteries, which remained the principal supply of the AVM, while AICA and SCA developed to supply normal brain territory, with minor contributions to the AVM. Interestingly, both arteries were embolized to “stumps” off the basilar artery, with no neurologic deficit. Noncontrast head CT with blood in the lateral ventricles (yellow), and CTA demonstrating nidus (red) and some associated aneurysms (purple)
Angiographic views of vertebrobasilar circulation, showing two hypertrophied transverse pontine arteries (white arrows) supplying an AVM, with associated perinidal arterial aneurysms (purple). Image on left is pre-embo, center is status-post embo of the more cranial transverse pontine pedicle, image on right is post-embo. Also shown are the SCA (pink) and AICA (black). Notice an area of spasm involving the more caudal transverse pontine artery (blue arrow) center image (the more superior transverse pontine feeder has been glued).
A schematic of this situation is shown below:
C1 origin PICA — since PICA can be regarded as a branch of the lateral spinal system, its origin may vary from usual to lower down along the lateral spinal line — C1, for example. In this case, the vertical segment of the PICA, extending from C1 through the foramen magnum, corresponds to the “lateral spinal” part of PICA.
Variation in fusion patterns of the basilar artery
As mentioned above, the basilar artery is formed via coalescence of multiple channels belonging to the longitudinal neural system, following development of the anterior circulation (see Neurovascular Evolution and Vascular Neuroembryology sections) This process sets the stage for multiple basilar artery variations, which can be considered from the standpoint of 1) extent of fusion 2) position of fusion with respect to the brainstem, and 3) completeness of fusion (i.e. fenestrations).
It is conceptually helpful to think of the basilar artery as being “zipped” in the middle, with vertebral and PCA segments being “unzipped”. The length of the “zipped” segment, and the integrity of the zipper determine the final configuration of the artery. As such, the artery may be
1) “short” — corresponding to relative lack of fusion of the caudal or rostral segments
2) Unfused at the top — corresponding to “unzipping” at the basilar tip — less common but much more confusing
3) Fenestrated — broken zipper in the middle
The first one, short basilar, is in fact a continuum in terms of fusion extent. There is no “number” to guide what short or long is, but some basilars are clearly quite foreshortened. (The tortuous basilar of the old hypertensive is a different matter from developmental variation)
The second variation, involving lack of basilar tip fusion, can generate a lot of confusion. Effectively, the top of the basilar is split in two, so that one or both superior cerebellar arteries originate from the P1 segment. This variant is not, therefore, a primary superior cerebellar artery aberration, but instead a deficiency in basilar fusion.
The third, basilar fenestration, is quite common, and usually of little clinical signficance, except when it is so short as to minic a dissection.
Basilar nonfusion — extreme of fenestration, a completely “unzipped” look. Very rare.
Unfused basilar simulating aneurysm — reported as basilar tip aneurysm on CTA, angio demonstrating an unfused basilar tip with tortuous P1 vessels simulating aneurysm, further complicated by presence of a fetal PCOM on the right.
Origin of superior cerebellar artery at “top” of the basilar on the left (yellow), and from P1 segment on the right (red arrow)– seen often, can be conceptualized as a “short” basilar which did not undergo enough coalescence at the top to incorporate the superior cerebellar artery.
Notice P1 origin of the right superior cerebellar artery (red) with contralateral classical disposition in yellow
The same unfused upper basilar, with the seemingly opposite appearance of the right P1 (red arrow) originating from the SCA. Both cases are in fact variants of deficient upper basilar fusion. P2 wash-in (purple) is present, as well as some reflux into a dominant left PCOM (yellow). Notice abundant pontine perforators (within red oval) in setting of bilateral PICA dominance. The smaller the AICAs, the more perforators will be.
Diagram of P1 origin SCA, as related to deficient fusion of the upper basilar.
Another illustration, on opposite sides, in a patient with a complete circle of Willis, displayed to advantage by a generous injection.
“Normal” P1 segment origin from distal basilar (in a patient with duplicated AICAs)
Basilar Artery Fenstration — “broken zipper in middle”. Seen quite often, of litte clinical significance, but illusrative of the fusion concept.
Another fenestration, on CTA
Again, a rather large fenestration (brown arrow), in a patient with fetal PCOM. Notice, how on the right, a small AICA (with dominant ipsilateral PICA) is accompanied by a number of basilar perforators between the AICA and SCA (not marked with any arrows)
In this patient, a basilar fenestration is associated with an AVM:
A few years later, the AVM is gone, and the fenestration can be better appreciated thru lack of competing inflow from the right vert.
While the fenestration might be sporadic, one might also postulate that dual channels persisted secondary to AVM-related flow demand during fetal period. This is a tempting, though likely erroneous assumption. Here is another one, courtesy of Dr. Daniel Sahlein:
An ex vivo specimen, courtesy of Peggy Mason, PhD, Department of Neurobiology, University of Chicago, Chicago, IL. Notice also a nice anterior spinal artery (yellow arrow). Also to be seen are petrosal vein, vein of the lateral recess, and nice pontine perforators.
Basilar NonfusionThe extreme form of fusion variation is non-fusion. As the basilar forms by fusion of the longitudinal neural axis vessels, a complete deficiency in fusion is not unreasonable, and very rare cases of it do exist. Longitudinal non-fusion refers to a case of “twin basilars” — two parallel channels, which can also be regarded as the extreme form of a fully “unzipped basilar”. Transverse, or axial non-fusion describes a basilar artery which is disconnected in its mid-portion, usually below the AICAs. It is not so uncommon to see diminished flow in the same segment on MRA, which can be appreciated as a “pseudo-stenosis” of mid-basilar (see below). Lasjaunias felt that this basilar segment is an embryologic boundary of sorts, and therefore non-fusions happened there with at least some frequency. Longitudinal Nonfusion, as seen by MRI, electronically reproduced with kind permission of ASNR and First Author: Hoh BL, Rabinov JD, Pryor JC, Hirsch JA, Dooling EC, Ogilvy CS. AJNR Am J Neuroradiol. Persistent nonfused segments of the basilar artery: longitudinal versus axial nonfusion. Aug;25(7):1194-6. 2004, © by American Society of Neuroradiology
Axial non-fusion from the same source, electronically reproduced with kind permission of ASNR and First Author: Hoh BL, Rabinov JD, Pryor JC, Hirsch JA, Dooling EC, Ogilvy CS. AJNR Am J Neuroradiol. Persistent nonfused segments of the basilar artery: longitudinal versus axial nonfusion. Aug;25(7):1194-6. 2004, © by American Society of Neuroradiology
Pseudostenosis of the basilar artery — in some degree possibly related to embryologic boundary in the mid-basilar, as above.
This patient underwent an angiogram for evaluation of a midbasilar stenosis as seen on the MRA. No stenosis was found. This is occasionally seen with time-of-flight MRA imaging — a flow-based technique. If flow is diminished in a segment of a vessel, an erroneous appearance of stenosis can result. It is particularly common when decent-size posterior communicating arteries are present — thus generously supplementing posterior cerebral and even superior cerebellar arteries, and secondarily diminishing the amount of forward flow in the basilar distal to the AICAs. Even though as a radiologist you have to call them like you see them, reasonable scepticism may be useful here, and a CTA rather than an angiogram can be of value also. Notice also an incidental duplicated left SCA (orange)
Pontine Perforators:Fronal projection view of the basilar artery, with several transverse pontine “ziggii” perforators (yellow and brown) between the SCA and the duplicated AICAs (red arrows). A loose correlation exists between the number and size of these perforators and distance between the SCA and the AICAs — the longer the distance, the more and larger the perforators tend to be.
Submental view of the basilar artery (below), 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-riding basilar — the position of the basilar tip (red) in relation to the posterior clinoid (white) and less importantly to the petrous apex (blue) is critical in pre-surgical assessment for those who still clip basilar tip aneurysms. An aneurysm neck below the posterior clinoid tip is difficult or impossible to reach and therefor to adequately clip even with a subtemporal appoach. This is a high-riding basilar. Also notice downward sweep of the PCAs relative to basilar tip — another indication of high basilar positon. The opposite istrue of the low basilar