LAUSR

Internal carotid artery (ICA) aplasia is a rare clinical entity, which has been scarcely reported in the literature, with fewer than 400 cases having been presented by the end of 2021.1 The identification of this anatomical variant is equally distributed across age groups and sexes. Right and left side involvement is similar, whereas bilateral ICA absence is possible but even rarer.2 Though many patients are either asymptomatic or present nonspecific symptoms such as headaches, pulsatile tinnitus, trigeminal neuralgia, etc., more severe clinical presentations can also be encountered, such as transient ischemic attack or subarachnoid hemorrhage (SAH). The former has been identified as the most common clinical manifestation in those > 40 years old, whereas the latter among patients < 20 years.2 Concomitant intracerebral aneurysms have been reported to be common among cases with ICA agenesis.2,3 We present a case of a 65-year-old female patient, who underwent ultrasound and subsequently magnetic resonance imaging (MRI) due to pulsatile tinnitus and nonspecific symptoms of headache, who was initially diagnosed with left ICA occlusion. Similarly to the present case, color duplex imaging can provide indirect findings that point to the correct diagnosis such as the reduced lumen diameter of the common carotid artery on the affected side, the large caliber of the ipsilateral vertebral artery and the normal waveform in the common carotid, despite the absence of flow in ICA. Nevertheless, due to its rarity, the diagnosis can be initially missed.4 During evaluation of the patient and in order to determine the nature of the plaque and/or thrombus that may have caused ICA occlusion, a CT angiography was performed where agenesis of the left ICA (instead of occlusion) was diagnosed. Panel A presents a 3D reconstruction of the CT angiography (CCA: common carotid artery; ECA: external carotid artery; ICA: internal carotid artery; L: left; R: right; Vert Art: vertebral artery). The circle of Willis presented typical anatomy at the right side, whereas the left cerebral hemisphere was perfused by the basilar artery from which both the left posterior and the middle cerebral arteries arose. The left anterior cerebral artery was perfused by the right anterior cerebral through the anterior communicating branch as displayed in Panels B-1 and B-2, which demonstrate the intracranial collateral pathways (ACA: anterior cerebral artery; MCA: middle cerebral artery; PCA: posterior cerebral artery). This configuration would classify this variation as Lie Type A, according to the classification scheme that has been previously suggested, which classifies lesions in types A to F.5 Remarkably, as demonstrated in Panel C, the carotid canal was missing from the affected side, which is typical of this condition. Indeed, since the presence of the ICA is embryologically required for the formation of the bony carotid canal, its absence or hypoplasia is a prerequisite to establish the diagnosis. Taking into account the rarity of this condition, increased awareness is needed in order not to miss Images in Vascular Medicine: Internal carotid artery aplasia