Clinical History
A 59-year-old male chronic smoker and drinker presented with acute loss of memory involving events of the last few days. There was no history of trauma or drug intake, and the patient enjoyed good health in the past. Clinical examination was unremarkable apart from high blood pressure. No neurological deficits.
Imaging Findings
Non-contrast cerebral CT showed a lacunar infarct in the anterior limb of the right internal capsule. MRI showed no corresponding restricted diffusion and additional mature right thalamic infarct and bilateral periventricular T2 hyperintensities suggestive of chronic small vessel disease. MRA TOF showed complete flow void of the right distal internal carotid artery.
This was confirmed on CTA as absent right internal carotid artery; the respective carotid canal was occupied by aerated mastoid air cells. The right middle cerebral artery (MCA) was supplied by a prominent right posterior communicating artery. The A1 segment of the right anterior cerebral artery (ACA) was absent and its A2 and more distal branches were supplied by the left ACA via the patent anterior communicating artery.
CTP maps at the level of the basal ganglia showed mildly prolonged mean transit time and decreased cerebral blood flow in the right MCA territory but preserved cerebral blood volume.
Discussion
Congenital absence of the internal carotid artery encompasses the spectrum of agenesis, aplasia and hypoplasia. It is a rare anomaly typically occurring unilaterally, with even rarer bilateral involvement. The cause is believed to be mechanical and haemodynamic stress before the 4th embryonic week [1, 2].
This anatomic variant is usually discovered as an incidental finding or after a cerebrovascular episode. Patients may remain asymptomatic until the existing carotid artery or vertebrobasilar system becomes affected by atherosclerosis; or the ectatic collateral supply causes mass effect; or from rupture of associated aneurysm, which may be seen in up to 34% of cases [1, 3]. Recognition is important in thromboembolic disease, carotid endarterectomy planning, and transsphenoidal surgery particularly in patients with intercavernous collateral [1].
CT is the preferred imaging modality. A non-contrast examnation depicts an absent or small carotid canal which is the key finding in distinguishing this congenital anomaly from an acquired disease. CTA depicts absent or diminutive ICA, with collateral supply coming from intracranial arteries, from persistent foetal circulation or from branches of the external carotid artery. Associated aneurysms, likely due to increased blood flow from the altered haemodynamic state, may also be depicted and are usually found in the anterior communicating artery [4]. As expected, cerebral perfusion is preserved or only mildly affected compared with the frank perfusion abnormalities of acquired occlusion [2], although CTP is not required for diagnosis. MRI serves as a complementary tool in the identification of acute infarcts.
Clinical and radiologic follow up is encouraged to monitor development of aneurysms, but management is largely conservative, targeted towards reducing cerebrovascular risk and addressing the patient's symptoms.
Congenital absence of the ICA may be more than just an incidental finding. It can be an important anatomic variation and should be given due consideration particularly in patients presenting with stroke where it could impact medical and surgical management .
Differential Diagnosis List
Congenital agenesis of the right internal carotid artery
Atherosclerotic occlusion of the right internal carotid artery
Extensive dissection of the right internal carotid artery
Final Diagnosis
Congenital agenesis of the right internal carotid artery
