Images below require Macromedia's Flash Player to view

Neuroradiology Case of the Week

Case 134

Loris F. Cedeno, MD and Sven Ekholm, MD, PhD

Clinical Presentation: A 39-year-old female with a history of previous brain injury and status epilepticus.  She currently presents with left-sided weakness.

Radiological Findings: T2-weighted images demonstrate absence of the right internal carotid artery flow void, absence of the right A1 segment as well as a hypertrophied right posterior communicating artery supplying the right middle cerebral artery.  CT images with a bone algorithm demonstrate absence of the right carotid canal and foramen lacerum.  Incidental note is made of a cystic lesion in the right substantia nigra, adjacent to the enlarged PCOM, which may be secondary to continuous pulsatile impact (Figs. 1 & 2).

Discussion: Congenital absence of the internal carotid artery is a rare congenital anomaly occurring in less than 0.01% of the population. In these patients, the most common collateral pathway is through the circle of Willis. Less common collateral pathways include persistent embryonic vessels or extracranial collaterals. The exact cause of agenesis has not been established, although it is believed to be secondary to early insults to a developing fetus related to either mechanical or hemodynamic stresses. Postulated causes include exaggerated folding of the embryo towards one side or constriction by amniotic bands. Agenesis may be distinguished from acquired ICA occlusion by examining the skull base. In cases of true agenesis of the ICA, the bony canal is absent. A developmentally small ICA is associated with a small bony canal.

      There are six pathways of collateral circulation previously described in the literature in the setting of aplasia, hypoplasia or agenesis of the internal carotid artery.

• Type A consists of unilateral absence of the ICA, collateral feeding of the ipsilateral ACA through a patent ACOM and flow to the ipsilateral MCA through a hypertrophied PCOM. These are the collateral pathways seen in our patient.
• In Type B, there is unilateral agenesis of the ICA with the ipsilateral ACA and MCA supplied through a patent ACOM.
• In Type C, there is bilateral agenesis of the ICA with supply to the anterior circulation by carotid-vertebral anastomoses.
• In Type D, there is unilateral agenesis of the ICA with collateral flow to the ipsilateral ACA and MCA via an intracavernous communication from the contralateral ICA.
• In Type E, diminutive ACAs are supplied by bilateral hypoplastic ICAs. Flow is supplied via bilateral PCOM arteries.
• And lastly, Type F, the ICAs are hypoplastic bilaterally and collateral flow is provided to the ACAs and MCAs bilaterally by extracranial anastomoses, the so called rete mirabile.

      Recognition of these anomalies has clinical significance in thromboembolic disease as well as in surgical planning of carotid endarterectomies. In addition, there is increased prevalence of associated abnormalities such as intracranial aneurysms. Many cases remain asymptomatic until later in life when they present with symptoms related to cerebrovascular insufficiency.

References:

1. Given CA 2nd, Huang-Hellinger F, Baker MD, Chepuri NB, Morris PP. Congenital absence of the internal carotid artery: case reports and review of the collateral circulation. AJNR Am J Neuroradiol. 2001 Nov-Dec; 22(10):1953-9. [Medline]
2. Osborn A. Cerebral Angiography, 2nd Ed. Lippincott, Williams & Wilkins Co. Philadelphia: 1999; 63-64.