Friday 23 March 2012

Middle cerebral artery

MCA is divided into four segments.
M1 - From ICA to the bifurcation.
M2 - From the MCA bifurcation to the circular sulcus of the insula.
M3 - From the circular sulcus to the superficial aspect of the sylvian fissure
M4 - Cortical branches.

M1 segment
Aka horizontal segment or sphenoidal segment.
Arises from ICA and travels in lateral direction, parallel to the sphenoid wing and terminates by dividing into the M2 segments.  The division point of the MCA main stem is considered by most clinicians to be the M1/M2 junction. The MCA bifurcates in 71% of cases, trifurcates in 20% of cases and divides into four branches in 9% of cases.
The M1 segment averages ~16 mm in length.

Lateral lenticulostriate branches, approximately 80% of the lenticulostriate perforators that arise from the MCA, arise from the M1 segment, on an average are 10 in number, enter the anterior perforated substance to supply the anterior commissure, internal capsule, caudate nucleus, putamen, globus pallidus, and substantia innominata.
Anterior temporal artery, typically arises near the midpoint of the M1 segment. Less commonly, it arises from the inferior division (an M2 segment) or as part of an M1 trifurcation, supplies the anterior temporal lobe.

MCA duplication, consists of a large MCA branch arising from the ICA proximal to the ICA bifurcation,  in 0.2–2.9% of cases, the anamolous vessel travels parallel and inferior to the main M1 segment and primarily supplies the anterior temporal lobe.
Accessory MCA, arises from the ACA and runs parallel to the M1 segment, and has a frequency of 0.3–4.0%, supplies the orbitofrontal area and should be not be confused with a large recurrent artery of Heubner.
Aplasia, is rare.

M2 segments
Aka insular segments, extend from the main division point of the M1 segment, over the insula within the sylvian fissure and terminate at the circular sulcus of the insula.
There are two, superior and inferior divisions. Cortical area supplied by the superior division usually extends from the orbitofrontal area to the posterior parietal area. Cortical area supplied by the inferior division usually extends from the temporal pole to the angular area.
The M2 segments number from six to eight vessels at the point of transition into the M3 segments.

M3 segments
Aka opercular segments, begin at the circular sulcus of the insula and end at the surface of the sylvian fissure.
These vessels travel over the surface of the frontal and temporal opercula to reach the external surface of the sylvian fissure. The M3 branches, together with the M2 vessels, give rise to the stem arteries, which in turn give off the cortical branches. There are usually eight stem arteries per hemisphere, and each one typically gives rise to one to five cortical branches.

The M4 branches
Aka cortical branches,begin at the surface of the sylvian fissure and extend over the surface of the cerebral hemisphere. The smallest cortical branches arise from the anterior sylvian fissure and the largest ones emerge from the posterior sylvian fissure.

The cortical branches of MCA can be grouped according to which lobe they supply;
1. Frontal lobe. Orbitofrontal, prefrontal, precentral, and central arteries
2. Parietal lobe. Anterior and posterior parietal arteries and angular artery
3. Temporal lobe. Temporopolar, anterior, middle and posterior temporal arteries,
and temporooccipital artery
4. Occipital lobe. Temporo-occipital artery.

The cortical branches can also be grouped according to which M2 segment they arise from;
1. Superior division. Orbitofrontal, prefrontal, precentral, and central arteries.
2. Inferior division. Temporopolar, temporo-occipital, angular, and anterior, middle, and posterior temporal arteries.
3. Dominant division (these branches may arise from either division, and usually come off of the larger of the MCA divisions). Anterior and posterior parietal arteries.

Reference: Handbook of Cerebrovascular Disease and Neurointerventional Technique.  Mark R. Harrigan, John P. Deveikis and Agnieszka Anna Ardelt.