Clinical History
A 45-year-old woman was referred for MRI of the brain with a five day history of headache and fever. Although no parenchymal abnormality was noted on MRI an incidental rare vascular anomaly was detected. The patient's symptoms were attributed to systemic viral illness. She recovered with symptomatic treatment.
Imaging Findings
The flow void of the left internal carotid artery (ICA) in the upper neck and within the cranium was not visualised on MRI. The right ICA flow void appeared normal. No significant cerebral or cerebellar parenchymal abnormality was noted.
MR angiography confirmed absence of the left ICA. A normal calibre left middle cerebral artery was noted to be formed through the left posterior communicating artery. The left vertebral artery appeared prominent in comparison to the right side. The left anterior cerebral artery was formed through the anterior communicating artery. Absence of the A1 segment of the left anterior cerebral artery was also seen. Correlative axial CT sections confirmed the absence of the bony left carotid canal.
Discussion
Absence of the internal carotid artery (ICA), a term that encompasses agenesis, aplasia and hypoplasia of the ICA, is a rare congenital anomaly seen in less than 0.01% of the population [1]. Agenesis refers to complete failure of development. Aplasia refers to lack of development with evidence to suggest existence of a precursor sometime during development. Hypoplasia refers to incomplete development [1, 2]. Unilateral absence of ICA is postulated to occur due to haemodynamic and mechanical stresses on the developing embryo [1]. Most cases are detected incidentally but some may present with features of cerebrovascular insufficiency, mass effect from enlarged collaterals or complications related to coexisting aneurysms [1, 3].
The collateral blood flow to the distal ICA or its intracranial branches may occur through the circle of Willis or through other primitive collateral pathways depending on whether disruption to the ICA development occurred before completion of the circle of Willis or after it [1, 4]. Lie described six types of collateral circulation associated with absence of ICA [2].
Type A – unilateral absence of ICA with formation of ipsilateral anterior cerebral artery (ACA) through the anterior communicating artery (ACOM) and middle cerebral artery (MCA) through the posterior communicating artery (PCOM).
Type B – unilateral absence of ICA with formation of ipsilateral MCA and ACA through a patent ACOM.
Type C – bilateral agenesis of ICA with anterior circulation supplied by carotid-vertebrobasilar anastamoses.
Type D – unilateral agenesis of cervical portion of ICA with reconstitution of ipsilateral ICA through an intercavernous communication from the contralateral cavernous ICA.
Type E – bilateral small ACA’s supplied by bilateral hypoplastic ICA’s with MCA’s supplied by PCOM’s.
Type F – flow to the distal ICA achieved through trans-cranial collateral pathways from ECA system.
On imaging, agenesis of ICA is characterised by absence of the ICA with associated absence of the carotid canal. In cases of aplasia a thin fibrous band may remain as a remnant of the ICA. Hypoplasia is characterised by a small calibre ICA. Presence of an associated small carotid canal is however necessary to differentiate a hypoplastic ICA from other acquired causes of narrowing [1].
The clinical implication of absence of the ICA lies in the higher incidence of cerebral vascular insufficiency and associated cerebral aneurysms [1, 3]. Recognition of an intercavernous collateral associated with absence of the cervical ICA (Type D) is extremely important prior to trans-sphenoidal hypophyseal surgery to avoid the risk of serious haemorrhage [1].
Differential Diagnosis List
Agenesis of the left internal carotid artery.
Aplasia of internal carotid artery
Hypoplasia of internal carotid artery
Thrombosis of internal carotid artery
Final Diagnosis
Agenesis of the left internal carotid artery.
