DEVELOPMENTAL VARIATIONS AND VASCULAR ANOMALIES
Arterial Agenesis, Aplasia, and Hypoplasia
Developmental abnormalities that result in absence or narrowing of intracranial vascular structures are common. However, collateral pathways prevent functional brain alterations in most individuals with these anomalies. Agenesis refers to complete failure of embryological development. Aplasia indicates lack of development despite the presence of a precursor. Hypoplasia is incomplete development. Agenesis and aplasia of vascular structures usually cannot be differentiated radiographically or clinically, and the 2 terms are generally used interchangeably.
Agenesis or hypoplasia of an internal carotid artery occurs in less than 0.01% of the population. There is a 3:1 left-sided predominance. The anomaly is most often unilateral. Absence of the internal carotid artery is usually an isolated lesion, but can also occur in association with other cervicocerebral vascular anomalies, as in patients with PHACES (posterior fossa malformations, hemangiomas, arterial anomalies, cardiac anomalies and aortic coarctation, eye abnormalities, and a sternal cleft or suprapubic raphe) syndrome (see Figure 13-16). Most patients are asymptomatic or have nonspecific complaints, such as headache. Rare potential clinical manifestations include transient ischemic attacks, hemiplegia, and congenital Horner syndrome. Patients with absence of an internal carotid artery are at increased risk for cerebral aneurysms, although these are usually not evident until adult life.1,2
There are 3 basic patterns of collateralization in patients with congenital absence or substantial hypoplasia of a carotid artery; the developmental stage at which the insult occurs determines the specific pattern. (1) By far the most common pattern of collateralization is by way of the circle of Willis. Typically, the contralateral internal carotid artery provides flow to the anterior cerebral artery via a prominent anterior communicating artery. An enlarged posterior communicating artery supplies flow to the middle cerebral artery ipsilateral to the absent carotid artery. (2) If the embryonic insult occurred prior to completion of the circle of Willis, primitive pathways of collateral circulation prevail. This most often consists of an intercavernous communication between the ipsilateral carotid siphon and the cavernous segment of the contralateral internal carotid artery. (3) Rarely, collateral flow occurs into a reconstituted internal carotid artery via skull base collaterals from the external carotid artery system. Potential pathways for arterial supply to the ophthalmic artery in patients with absence of the internal carotid artery are external carotid branches, a residual hypoplastic segment of the internal carotid artery, and various anomalous origins of the ophthalmic artery (e.g., from the posterior communicating, middle meningeal, or anterior cerebral artery).3–6
Techniques for the diagnosis of congenital absence of the internal carotid artery include angiography, CT angiography (CTA), and MR angiography (MRA). The presence of the developing internal carotid artery is a prerequisite for formation of the carotid canal (5 to 6 weeks of gestation). Therefore, an absent carotid canal on CT or MR confirms the diagnosis of internal carotid ...