At present, the pathogenesis of congenital absence of the ICA is unclear. The disease may be related to the arrested development or secondary degeneration of the ICA during embryonic development. Published studies [1–3] have reported that the incidence of congenital absence of the ICA is less than 0.01%; unilateral absence of ICA is more common, with a ratio of left absence to right absence of approximately 3:1. Bilateral absence accounts for less than 10% of all cases of congenital absence of the ICA. Patients with congenital absence of the ICA often have no clinical symptoms due to compensation by the collateral circulation of Willis circle. Some patients may experience noncharacteristic symptoms such as tinnitus, dizziness, headache, etc., and a small number of patients may be admitted to the hospital due to aneurysm rupture or epilepsy . Lie  divided congenital absence of ICA into 6 types according to the compensatory pathways of collateral circulation. In Type A (unilateral absence of the ICA), the most common type encountered in clinical practice, the affected ACA is supplied by the contralateral ACA via the ACoA, and the affected MCA is supplied by the ipsilateral posterior cerebral artery (PCA). In Type B (unilateral absence of ICA), the affected ACA and MCA are supplied by the contralateral ACA via the ACoA. In Type C (bilateral absence of ICA), the bilateral ACAs and MCAs pass through the posterior communicating artery (PCoA) to supply blood from the posterior cerebral circulation. In Type D (unilateral absence of ICA), the siphon section of the affected ICA is supplied by the contralateral ICA via the formation of an anastomotic branch between the cavernous sinuses. In Type E, the bilateral small ACAs are supplied by the bilateral maldeveloped ICAs, and the bilateral MCAs are supplied by the PCA via the PCoA. In Type F, the ACA and MCA on the affected side are supplied by anastomosis of the ECA-internal maxillary artery to the skull base, that is, the skull base microvascular network. In the present case, the RACA A1 was absent, the RACA was supplied by the LACA via the ACoA, and the RMCA was supplied by the LICA through an anastomotic branch between the cavernous sinuses; this corresponds to Type D in the Lie classification and is very rare. According to the study results of Jesse et al. , the incidence of intracranial aneurysms in the general population is approximately 2%-4%, while the incidence of intracranial aneurysms in patients with congenital absence of the ICA is as high as 25%-43%, which may be due to congenital dysplasia of the vascular wall or abnormal hemodynamics . Therefore, early and accurate diagnosis of congenital absence of the ICA and regular monitoring of the occurrence and development of intracranial aneurysms are important for preventing cerebral hemorrhage and subarachnoid hemorrhage .
CDUS is the most commonly used screening tool for cervical vascular diseases. It offers the advantages of high resolution, high safety, convenience and low cost and plays an important role in the diagnosis of congenital dysplasia of the carotid artery. Because congenital absence of the ICA is rare, most reports are individual cases, and most cases are diagnosed by CTA or DSA, there are few reports on the diagnosis of congenital absence of the ICA by CDUS. Radiologists may not have adequate knowledge about this disease and its ultrasonic characteristics, which may lead to missed diagnosis. In the present case, CDUS conducted at an outside hospital showed no abnormality. During examination, we found that the patient’s RCCA was maldeveloped, but its blood flow velocity and spectrum were normal. When we saw the bifurcated structure of the RCCA, we mistakenly thought that the thicker blood vessel that continued directly was the ICA. Pulse Doppler showed that its resistance index(RI) was significantly higher than that of the LICA and slightly higher than that of the RCCA; therefore, it was suspected to be the ECA. A superficial temporal artery tap was carried out, and the result showed sawtooth-like changes in the diastolic spectrum. Careful scanning revealed that there were two adjacent small blood vessel branches at the "bifurcation". By tracing their courses, we observed that the first branch was the superior thyroid artery, and the other branch went towards the side of the head. Therefore, it was confirmed that the RCCA continued directly to the ECA. The patient was finally diagnosed with congenital absence of the ICA.
When absence of the ICA is suspected on ultrasonography, it should be differentiated from ICA occlusion. Occlusion of the ICA is most often caused by the formation of an atherosclerotic plaque, but it can also be observed in cases of arteritis, cardiogenic embolism, etc. Ultrasound can not only show the wall structure of the occluded ICA but can sometimes be used to explore the stump of the ICA . Transthoracic echocardiogram (TTE) can be performed when cardiogenic embolism is considered. Kaya et al.  suggested that dysplasia of the CCA on the affected side is an important clue in the ultrasonic diagnosis of congenital absence of the ICA. Yilmaz et al.  found that most patients with ICA occlusion showed decreased peak systolic velocity and significantly decreased peak diastolic velocity, and some even presented with a reversed spectrum in the affected CCA. However, the peak systolic and diastolic velocity of the CCA in congenital absence of the ICA is normal. Therefore, ICA occlusion can basically be ruled out when the ICA is not visible and the CCA on the affected side shows a low-resistance spectrum. In this case, the RCCA was maldeveloped, and the spectrum, blood flow velocity and resistance of the bilateral CCAs were very similar, without the obvious wall structure of the RICA. Therefore, it was considered that the RICA was absent rather than occluded. In addition, because the ICA forms earlier than the skull base, CT of the skull base can also assist in the diagnosis of congenital absence of the ICA if the bony carotid canal is maldeveloped or not developed .
Taken together, CDUS, as a noninvasive and rapid screening tool for cervical vascular diseases, offers a new approach for the diagnosis of congenital absence of the ICA. Radiologists should improve their understanding of the disease and its corresponding ultrasonic characteristics. When maldevelopment of the CCA is found on one side and pulse Doppler shows that the blood flow velocity and spectrum are normal, and the ipsilateral ICA is not visible, the possibility of congenital absence of the ICA should be considered.