Friday 23 March 2012

Anterior cerebral artery

On either side ICA divides into ACA which travel anteromedially and MCA which travel laterally.
ACA includes three segments A1, A2 and A3.
A1. From ICA to anterior communicating artery.
A2. From anterior communicating artery to the origins of the pericallosal and supramarginal arteries.
A3. Distal ACA or cortical branches.

A1 segment and Acom complex
The A1 segment also known as precommunicating segment, extends from the ICA bifurcation to its junction with the anterior communicating Artery. It travels superior to the optic chiasm or optic nerves and inferior to the anterior perforated substance.

In most cases, the A1- Acom complex assumes one of the four configurations.
A) Single or duplicated Acom forms a bridge between the ACAs.
B) A single large branch arising from the A com
C) Absent A-com and the two ACAs join together directly.
D) Azygos ACA.
Single or duplicated Acom forms a bridge between the ACAs. 
A single large branch arising from the A com 
Absent A-com and the two ACAs join together directly 
Azygos ACA.

A1 Branches: 
Perforating branches of A1 divided into superior and inferior branches.
i. Approximately 2–15 superior branches are medial lenticulostriate arteries that travel superiorly and posteriorly into the anterior perforated substance and supply the anterior hypothalamus, septum pellucidum, anterior commissure, fornix, and the anterior striatum.
ii. Inferior branches supply the optic chiasm and optic nerves.
Acom branches
Perforating branches of the Acom divided into subcallosal, hypothalamic and chiasmatic branches, according to their vascular territories.  The subcallosal branch is usually single and the largest branch of the A com, supplies the septum pellucidum, columns of the fornix, corpus callosum and lamina terminalis.
Recurrent artery of Heubner, most often an A2 branch, may arise from the A1 segment in upto 17% of cases and from the ACA-A-comm junction in 35% of cases.

A1 variants
Asymmetry:The left and right A1 segments are asymmetric in size in upto 80% of cases. About 10% of the A1 vessels are hypoplastic.
Absence:Absence of one A1 segment is seen in 1–2% of cases.
Infraoptic ACA:  The A1 segment may travel inferior to or through the optic nerve, rare.
Fenestration of A1 segment.
Accessory ACA: An atypical branch of the ICA courses under the optic nerve and ACA to give rise to the orbitofrontal and frontopolar arteries.
Anomalous origin of A1 from the cavernous ICA or from the contralateral ICA.
Acom variants
A normal Acom, is a single vessel which forms a link between two ACAs, is present in only about 40% of cases.
Anomalous Acom anatomy is present in the remaining 60% of cases. Some 227 A-comm artery complex variations or pattern have been described. These patterns included plexiform (i.e., multiple complex vascular channels, 33%), dimple (i.e., incomplete fenestration, 33%), fenestration (21%), duplication (18%), string (18%), fusion (12%), median artery of the corpus callosum (6%), and azygos ACA (3%). The Acom is absent in some 5% of cases.

The A2 segment travels in a vertical direction, adjacent to the genu of the corpus callosum, extend from the Acom to its division into the pericallosal and callosomarginal arteries. Averages 43 mm in length.
The left and right A2 segments usually travel together in the interhemispheric fissure. Right A2 is more often (~72% of cases) anterior to the left A2 in the sagittal plane.

A2 Branches
Perforating branches penetrate the gyrus rectus and olfactory sulcus.
Recurrent artery of Heubner, also known as the medial striate artery or long central artery.  Thelargest medial lenticulostriate perforator branch from ACA. Arises from the A2 segment in most (57–78%) cases, may arise from A1 segment in up to 17% of cases and from the ACA-A-comm junction in 35% of cases. Called ‘Recurrent’ because it takes U turn after origin, goes ‘back’ laterally over A1, parallel to A1, in opposite direction of A1, towards the terminal ICA, so it often looks like a smaller artery running alongside and above the relatively bigger A1.
Orbitofrontal artery, this artery runs close to the midline in an anterior direction to the gyrus rectus, olfactory bulb and medial aspect of the inferior frontal lobe.
Frontopolar artery, this artery travels anteriorly and superiorly towards the frontal pole.

Azygos ACA, a single unpaired A2 segment that arises from the junction of the A1s. It is present in < 1% of the general population. Commonly associated with terminal aneurysm in ~41% of cases.  Associated with holoprosencephaly.
Duplicated A2.
Superior anterior communicating artery, an anomalous communicating vessel between the ACAs near the corpus callosum.

‘A3’ include all the ACA cortical branches distal to the origin of the pericallosal and callosomarginal arteries. Some authors have further subdivided the distal ACA into A4 and A5 segments.
The distal ACA branches have extensive anastomoses with distal branches of the MCA and PCA with a watershed zones inbetween which is most vulnerable to ischemia during hemodynamic failure.

A3 Branches 
Pericallosal artery, the pericallosal artery comprises the main trunk of the ACA as it passes posteriorly over the corpus callosum, gives off multiple small branches 'short callosal arteries'  that travel laterally along the corpus callosum and anastomoses with the splenial artery the 'posterior pericallosal branch', a branch of the PCA.
Callosomarginal artery, is the second largest distal branch of the ACA, after the pericallosal artery. It travels superiorly over the cingulate gyrus to run in a posterior direction within the cingulate sulcus.
Frontal branches are Anterior frontal, middle frontal and posterior frontal. These branches arise from the pericallosal or the callosomarginal artery, are identified according to which part of the superior frontal gyrus they supply.
Paracentral artery, supply the paracentral lobule.
Parietal arteries, the final and most distal branches of the ACA, anastomose with the parietooccipital branch of the PCA. They can be divided into Superior parietal and inferior parietal artery.

Cortical ACA Branches:
Orbitofrontal Artery.
Frontopolar Artery.
Internal Frontal Branches (Anterior, Middle, and Posterior).
Paracentral Artery.
Parietal Arteries (Superior and Inferior).
Reference: Handbook of Cerebrovascular Disease and Neurointerventional Technique.  Mark R. Harrigan, John P. Deveikis and Agnieszka Anna Ardelt. 

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