Azygos ACA

Syn and alternative spellings are Azygos ACA, Azygous anterior cerebral artery, Azygous ACA,

Unpaired anterior cerebral artery. 

Definition of term 'Azygos' in the Medical dictionary is any unpaired anatomical structure.

Origin of AZYGOS  term is new Latin, from Greek, unyoked, from a- + zygon yoke —  ‘Yoke’ is a clamp or similar piece that embraces two parts to hold or unite them in position. 

First Known Use in medicine in 1646. 

The term is well known in anatomy in relation to Azygos vein in thorax. 

'Azygos' term used with ACA when A1 segments of ACA on the either side join together and continue further forming a single trunk of ACA A2 segment, without any intervening anterior communicating artery - a rare normal anatomical variation. 

Normally this term not used in relation to ACA as normally we have paired ACAs. Right and left ACA joined together by a bridge of normal traverse running single intervening artery that is Acom or ‘ Anterior communicating artery’ present in about ~40% of cases. Anomalous Acom anatomy is present in the remaining 60% of cases. Some 227 A-com 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%),  absent Acom (5%) and Azygos ACA (3%).

In absent Acom ACA remain in pair throughout, join together directly without intervening Acom whereas, in Azygos ACA the ACA A1 segment from either side join together forming a common trunk. The intervening Acom is absent here too. 

Incidence and significane:  A rare variant seen in approximately 0.4 -1% of the population.  Embryologically its significance is unclear.  

In human it has just a resemblance with the vertebrobasilar arrangement.

It is a part of “normal” anatomy in some mammals.  

There is a strong association with aneurysm formation often at the terminal end of the unpaired segment. Other associations are dysgenesis of the corpus callosum,  lobar holoprosencephaly , septooptic dysplasia,  porencephalic cysts,  arteriovenous malformations. 

References: 

1. Huh JS, Park SK, Shin JJ et-al. Saccular aneurysm of the azygos anterior cerebral artery: three case reports. 2007;42 (4): 342-5. doi:10.3340/jkns.2007.42.4.342 - Free text at pubmed - Pubmed citation

2. LeMay M, Gooding CA. The clinical significance of the azygos anterior cerebral artery (A.C.A.). Am J Roentgenol Radium Ther Nucl Med. 1966;98 (3): 602-10. Am J Roentgenol Radium Ther Nucl Med (abstract) - Pubmed citationnf.