Thursday, 22 March 2012

Carotid - Vertebrobasilar Anastomoses

These are transient connections which appear during embryonic development between the carotid and vertebro basilar circulations. These embryonic anastomotic connections usually disappear as the posterior communicating arteries develop.
In rare cases these vessels persist into adulthood.
From superior to inferior, these persistent fetal vessels are:
1. Fetal PCA, the most common of all with prevalence of ~20% of genernal population.
2. Trigeminal, named after the cranial nerve it parallels.
3. Otic,
4. Hypoglossal, named after the cranial nerve it parallels.
5. Pro atlantal intersegmental artery.

1. Persistent Fetal PCA
Normally the posterior communicating artery arises from the ICA just proximal to the ICA bifurcation joins PCA at the junction of p1 and p2 segments. Fetal PCA or  Persistent fetal origin of PCA defined as a prominent P com that gives rise to or continues as PCA p2 segment and onwards with same diameter.  Ipsilateral PCA p1 segment is usually hypoplastic or may be absent.

2. Persistent Trigeminal Artery
The next most common.
Extends from the cavernous ICA to the upper part of the basilar artery and often perforates the dorsum sella.
The vertebrobasilar system proximal to the upper basilar artery may be hypoplastic, with the primitive trigeminal artery supplying most of the flow to the PCAs and the SCAs.
Two main variants.
Saltzman Type I. The persistent trigeminal artery supplies the PCA and SCA territories. The posterior communicating arteries and the basilar artery proximal to the anastomosis are hypoplastic.
Salzman Type II. The PCAs are supplied by the posterior communicating arteries, and the persistent trigeminal artery joins the basilar artery at the level of the SCAs.
Clinical significance is association with intracranial aneurysms. May have an intrasellar component and should not be mistaken for a pituitary mass.

3. Persistent Otic aArtery
Rarest of all carotid-basilar anastomosis.
Extends from the petrous ICA to the basilar system via the internal auditory canal.

4. Persistent Hypoglossal Artery
Common carotid-basilar anastomosis next to trigeminal.
Extends from the cervical ICA to the basilar artery via the hypoglossal canal.
The ipsilateral vertebral artery is usually hypoplastic.
May be associated with an aneurysm.

5. Proatlantal Intersegmental Artery
Extends from the cervical ICA or ECA to the vertebrobasilar system via the foramen magnum.
Relatively rare.
Associated with aplasia or hypoplasia of the vertebral arteries in 50% of cases.

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

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