The Middle Cerebral Artery is, phylogeneticaly, the yongest of all cerebral vessels (see dedicated Neurovascular Evolution and Vascular Neuroembryology for more detail). This situation corresponds with relatively recent development of the neocortex in the “higher species” Briefly, the MCA arises from a phylogenetically far older anterior cerebral artery (older because it vascularizes, among other things, the more primitive olfactory lobe). A collection of “perforating” vessels, such as the future artery of Heubner, are developed in the earlier versions of the neocortext to supply the growing cerebral hemispheres. As the cortex continues to enlarge, the “perforator” system evolves from several competing channels into a dominant vessel (the MCA) which by definition is that branch from the ICA which captures the cortical territory of the cerebrum. The MCA, in its current form, has multiple perforator vessels of its own. The origins of the MCA help explain some of the variants we have come to appreciate — the duplicated MCA is nothing more than a disposition of two “perforator” vessels remaining in balance instead of selection of a dominant single trunk. Relatively few MCA variants are present, the accessory MCA being the most important one. A nomenclature system of Manelfe (type I, II, and III) was used in the past to describe the relative sizes of the two MCAs but it has not evolutionary or functional significance. The important thing to know is from which of the MCAs the perforators come from, as this has the most clinical implication. The type III accessory MCA is instructive however since it represents not an accessory MCA but in fact an enlarged artery of Heubner which has taken over some the surface cortical territory. This again underscores the origin of MCA as a perforator vessel, much like the Heubner, the medial or lateral MCA perforators, etc.
The MCA has two divisions — superior and inferior. Again, there is plenty of nomenclature and discussion about bifurcations, trifurcations, etc. These are helpful only to the extent of aiding communication and reporting. The only issue of importance is from where, in each individual case, the various MCA candelabra vessels arise, and where they go. Broadly speaking, there is usually a dominance of either superior or inferior division. What that means is the following — the superior division will always by definition be responsible for the frontal convexity. The inferior division will supply the temporal lobe, in balance with temporal branches of the PCA. The parietal lobe is variable — when supplied by branches originating from inferior division, we see larger inferior trunk and therefore the inferior division is dominant. Same goes for superior division dominance. Trifurcation may reflect a separate parietal lobe trunk or any sort of combination / permutation of the many MCA branches. There is really not much more there to discover. Also to mention, the anterior temporal branch (or frontopolar branch) often comes from the M1 segment before its bifurcation or trifurcation, so don’t call the first branch point an MCA bifurcation. Look for the big split.
What the MCA lacks in evolutionary importance it more than makes up in clinical relevance — and in that sence the study of and “feel” for all the branches when treating a stroke or operating on a tumor are obviously critical. For example, pioneering work by Dr. Kelly of our own NYU fame has emphasized thorough knowledge of MCA perforator location with respect to target surgical lesion to minimize operative morbidity — tumors which displace the perforators medial to the mass have the comparatively lowest incidence of postoperative deficit secondary to perforator injury. (Ref: Resection of insular gliomas: the importance of lenticulostriate artery position Moshel, Yaron A; Marcus, Joshua D S; Parker, Erik C; Kelly, Patrick J
2008 Nov;109(5):825-34, Journal of neurosurgery)
2008 Nov;109(5):825-34, Journal of neurosurgery)
The M1 segment can be long, short, or anything inbetween. Again, no particular anatomical interest but has definite surgical implications.
Dominant inferior divsion
Short M1 segment (red) with smaller superior division (yellow) supplying the frontal convexity, and larger inferior division (orange) ointo the the temporal lobe (purple, subdividing into black anterior and white posterior temporal and white parieto-occipital) and parietal lobe (blue) feeders.
Dominant Superior Division, “early” bifurcation
Early Bifurcation with dominant inferior division.
The superior division (red) can be traced to the frontal lobe (purple). The inferior division (yellow) is dominant.
Heuber and M1 perforators
Again, 3-T MRA is very nice for small arteries like Heubner. This is a very clear demonstration that Heubner and medial perforators off the MCA serve the same function. In this patient, a large Heubner on the left serves the same function as the medial M1 perforators on the right, supplying the putamen, caudate, and parts of the anterior limb of the internal capsule.
Right MCA, same patient
Lateral perforators coming off the M1 segment.
There can be one, two, three, nine visible M1 segment perforators, and yet more present at surgery than meets the angiographic eye. A common trunk for many of these may be present — as in the above example with red arrows of the Heubner and yellow of the MCA. The more laterally disposed perforatos typically have a “stretched” apperance originating medial to their eventual entry through the perforated substance, as most perforatos come from the M1 segment, the shorter the segment the more laterally directed corse the perforator may assume within the subarachnoid space before diving through the perforated subsetance. Unless they happen to come off the M2, which happens occasionally. So, dealers choice — and it pays to get an angiogram before tumor resection in locations where perforator issues are important; MRI/A is at present of insufficient resolution for imaging the perforators, although 3T MRA can arguably settle the issue in some cases.
MCA branch nomenclature — vessels are named for the cortical territory they supply, regardless of divisional origin. This nomenclature is adopted from P. Lasjaunias and A. Berenstein, Surgical Neuro-angiography, in turn referencing a series of dissection by Michotey.
To restate the aformentioned, MCA arises as a dominant vessel from a number lateral striate perforators of the anterior cerebral artery in the reptile. As such, a disposition may arise where two perforators remain dominant, thus resulting in the “accessory MCA” The accessory vessel is, in fact, either a hypertrophied Recurrent Artery of Heubner (RAH) – a medial ACA perforator, or another perforator-like vessel. To qualify as an MCA, the vessel must have cortical territory. In this cartoon, the schematic on the LEFT shows aMCA configuration where both branches (purple) appear to originate proximal to the A1 complex (which is here defined as segment past the more “distal” MCA branch. These are known as Manelfe type 1 or 2 – depending on which branch is larger. The important feature however is to note from which vessel the perforators originate, and whether they are medial or lateral. The schematic on the RIGHT shows the Heubner-type aMCA, known as “Manelfe Type 3.
Type I/II accessory MCA on MRA
Catheter angiogram of the same patient. Notice how well MCA perforators can be seen, originating from the smaller and more distal of the two MCA vessels.
Type III (Heubner-like) aMCA originating from the ACOM complex
This series of images shows a prominent RAH, as a corollary to the aMCA case above. The bottom set of images (with the blue eyes) is a stereo pair.
Practically any vessel can have a fenestration. There is an opinion that fenestrations arise when something is physically present in the way of the vessel — for example, ACA fenestrations can be associated with the optic nerve passing through the fenestrated portion. There are some issues with this idea, and clearly there are times when nothing of particular note is present within the fenestration. This is an incidental MCA fenestration.