Prevalence of coronary artery fistula in adults performing coronary CT angiography among Egyptian population: a single center study

The prevalence of coronary artery fistula (CAF) among Egyptian population is increasing due to widespread availability of coronary CT angiography using high-end CT scanners. The purpose of this observational study was to determine the incidence and types of CAF detected among Egyptian population during routinely performed coronary CT angiography. Among the 2263 retrospectively evaluated CT coronary angiography studies, CAF was detected only in 17 (0.75%) patients with proximal LAD being the commonest vessel of origin; in 5 cases (29.4%) and main pulmonary artery (MPA) the commonest site of fistula termination; in 3 cases (17.6%). Indeterminate site of fistula termination was present in two cases (11.7%) due to limited scan coverage. Multislice coronary CT angiography is a useful non-invasive imaging modality for diagnosis of CAF by demonstrating vessel of origin, course and site of termination, with CAF prevalence and types among Egyptian population being reported in this study based on routinely performed coronary CT angiography.


Background
Coronary artery fistula (CAF) is abnormal connection between coronary artery and cardiac chamber or great thoracic vessel with congenital forms being the commonest so far. Its prevalence (0.05-0.25%) has been reported previously by many authors based on retrospective analysis of large amount of coronary angiography studies performed to rule-out or to treat obstructive coronary artery disease [1,2]. Different vessels of origin have been reported regarding CAF including left anterior descending (LAD) artery, left circumflex (LCx) and right coronary artery (RCA) in decreasing order, and different sites of termination were also described with cardiac chambers (coronary-cameral fistula) being the commonest most frequently into right ventricle, while superior vena cava drainage was documented in minority of patients [3].
Although coronary angiography is still considered the gold standard for diagnosis of atherosclerotic coronary artery disease, its use in CAF diagnosis is less preferable due to complex nature of CAF, lack of three-dimensional (3D) images which outline the course of CAF in relation to adjacent structures. Furthermore, the dilution of injected contrast material during coronary angiography by the shunting effect of large fistulae decreases image *Correspondence: Amr Ahmed Mubarak amr_mubarak2001@hotmail.com Diagnostic Radiology and Medical Imaging Department, Faculty of Medicine, Tanta University, Tanta, Egypt quality and interferes with accurate assessment of draining site, and here it comes the value of CT [4].
With increasing number of coronary CT angiography studies performed to rule-out obstructive coronary artery disease as well as the widespread availability of high-end CT scanners, the number of detected CAF is increasing even in asymptomatic individuals or those with atypical ischemic symptoms owing to high spatial and temporal resolution of recent CT scanners together with 3D reconstructions which facilitates the evaluation of CAF regarding site of origin, course, and termination [5].
The reported incidence of CAF in patients undergoing coronary CT angiography is higher than in those undergoing invasive coronary angiography as the latter is being replaced nowadays by multislice CT in those with low to intermediate risk of having coronary artery disease [6].
The purpose of this study was to know the prevalence of CAF among Egyptian population detected by routinely performed coronary CT angiography at our institution, and to further analyze different sites of origin and termination, and to correlate the CT findings with clinical symptoms and any associated cardiac abnormality detected at performed coronary CT study.

Study design and population
This observational study was conducted retrospectively on 2263 adult patients referred to our institution to perform coronary CT angiography, aiming to rule-out obstructive coronary artery disease or malignant course of anomalous coronary arteries in younger population. At our institution we do not perform coronary CTA routinely in infants and young children due to CT scanner limitations regarding temporal resolution which is relatively low (175 ms) being unable to freeze coronary arteries' motion in such high heart rate age groups when compared to dual source CT scanners which have better temporal resolution. So, infants and children were excluded from this study. Those included were scanned over a period of three years; from May 2019 to May 2022. However, the study itself was performed over a period of 3 months thereafter by reviewing and analyzing CT angiography (CTA) studies of those patients stored at our local picture archiving and communication system (PACS) to detect CAF. The study was approved by our institutional ethical committee and the need for patient's consent was waived due to retrospective nature of the study.

Coronary CT angiography scanning protocol
All patients were scanned using 320-row multislice CT scanner (Aquilion One, Canon Medical Systems, Otawara, Japan) installed at our institution according to our CT coronary angiography standard protocol, which involves heart rate control below 60 bpm using oral metoprolol at a dose of 50-150 mg given one hour prior to scan provided that there is no contraindication of its use, ECG-gated prospective volumetric acquisition of coronary arteries with scan range extending from carina down to cardiac apex after injection of tight bolus of non-ionic contrast material (50-90 ml) followed by 50 ml saline chaser through 18 g IV cannula inserted at right antecubital vein using dual head automatic injector (Stellant D, Medrad, USA) at injection flow rate of 4.5-6.5 ml/sec. according to kV used and patient's BMI. We usually use bolus tracking method (SureStart) to trigger CTA acquisition when contrast media arrives at descending aorta at mid-heart level using a pre-defined ROI with trigger threshold being set at 230 HU. Single heartbeat acquisition is a standard practice at our institution to get the benefit of low-dose acquisition with exposure window being set to include only 70-80% of R-R interval if heart rate is below 60 bpm and 30-80% of R-R interval if heart rate is above 60 bpm and below 70 bpm. We usually do not proceed with scan if heart rate is above 70 bpm as cardiac motion artifacts are most likely to be encountered which hinders accurate assessment of coronary arteries. Images are reconstructed thereafter using the following parameters: slice thickness: 0.5 mm, interval: 0.3 mm, smooth kernel (FC03).

Image post processing and analysis
Reconstructed axial images were transferred to workstation (Vitrea Fx, Vital Images, USA) for analysis of coronary arteries. Image analysis was done independently by one consultant radiologist who has eight years of experience in cardiac CT and blinded to previous interpretation results of all included patients. Axial images were analyzed for the presence of CAF which was defined as abnormal connection between any coronary artery segment with another vessel or cardiac chamber. After diagnosis of CAF, further analysis was performed to determine the site of origin, fistula course and site of termination together with presence of any aneurysmal dilatation or mural thrombosis along fistula course. Measurement of fistula diameters was done at coronary artery end and at the site of fistula termination using electronic caliper. CAF was considered large if its diameter exceeds 5 mm. Three-dimensional volume rendered images were created to obtain global overview of fistula regarding the size, course and site of termination. The rest of coronary arteries were evaluated as well using standard technique implicating scrolling throughout axial data set and creation of curved planar reformatted images to detected atherosclerotic plaques, measure degree of luminal stenosis to rule-out obstructive coronary artery disease. Any relevant associated findings were documented as well including cardiac chamber or great vessel dilatation.

Results
Among the 2263 patients included in this study, CAF was diagnosed only in 17 (0.75%) patients including 12 males (70.6%) and 5 females (29.4%), their age ranged from 20 to 58 years with a mean of 31.6 ± 12.3 SD.
Analysis of presenting symptoms among those diagnosed with CAF by coronary CTA revealed atypical chest pain in 7 patients (41.1%), dyspnea in 5 patients (29.4%) and typical chest pain in 4 patients (23.5%). The remaining case complained of palpitation.
Different vessels of origin were encountered during analysis of CAF, the commonest vessel of origin was proximal LAD (29.4%) followed by distal LCx (23.5%) Fig. 1 Large first diagonal branch coronary artery fistula with LV apex in 20-year-old male with typical chest pain. Semitransparent 3D volume rendered images show ectatic tortuous first diagonal branch of LAD that runs over anterior and lateral walls of LV and ends by forming large fistulous connection with LV apex (a, b). Ectatic RV acute marginal branch is evident due to intercoronary communication with first diagonal branch just proximal to its fistulous connection with LV apex (c, d) with only one case (5.8%) CAF being arising from diagonal branch of LAD (Fig. 1). The remaining vessels of origin are proximal and distal RCA as well as distal LAD.
The sites of termination of the detected CAF included main pulmonary artery (Fig. 2) being the commonest in 5 cases (29.4%), superior vena cave in 3 cases (17.6%), right atrium in 3 cases (17.6%) and right ventricle in 2 cases (11.7%). The remaining sites of termination are left ventricle (LV), and coronary sinus with indeterminate site of fistula termination noted in one case (5.8%) due to limited scan coverage where the fistulous tract extends above the proximal limit of scan range probably draining into bronchial artery.
The largest CAF as regard diameters was found among young population below 30 years of age whom typical ischemic symptoms were found mostly due to stealing effect. On the other hand smaller fistulae were observed at older age group above 40 years of age and they were complaining of atypical ischemic symptoms with their CAF being most likely discovered incidentally. The largesized CAF was found to be arising from proximal segments of LCX & RCA as well as distal segment of LCX (Fig. 3) together with the diagonal branch, whereas the small-sized CAF were arising from MPA and distal RCA. MPA was the commonest sites of termination of smallsized fistula whereas the large-sized fistulae were terminating into SVC, right atrium, RV and LV cavities. Details of the 17 cases diagnosed with CAF are demonstrated in Table 1 whereas the analysis of morphological characteristics of CAF in this study is shown in Table 2.
Aneurysmal dilatation was noted among three cases (17.6%) having large CAF all of which were arising from LCX. Subtotal thrombosis was found in one patient having proximal RCA fistula with SVC who also had anomalous origin of RCA from left coronary cusp with malignant interarterial course (Fig. 4). Significant atherosclerotic coronary artery disease was encountered among 3 out of 17 patients (17.6%) with significant atherosclerotic lesions being located at LAD & RCA, interestingly noted that all three cases with significant atherosclerotic disease had small CAF between proximal LAD & MPA. Large fistulae draining into right sided cardiac chamber or SVC was associated with right sided cardiac chambers dilatation whereas the remaining CAF was not associated with any significant cardiac chamber dilatation.

Discussion
This study revealed that the prevalence of CAF among Egyptian patients underwent coronary CT angiography was 0.75% which is higher than the prevalence reported during invasive coronary angiography by many authors. This could be explained that many of CAF are most likely to be asymptomatic particularly the smaller ones and discovered incidentally during coronary CT angiograms done to rule-out atherosclerotic CAD. Such incidence is higher than what was reported by Zhou et al. [5] who reported CAF in 0.19% of their studied population using dual source CT scanner. Higher prevalence of CAF was detected by Ouchi et al. [6] among patients undergoing CT coronary angiography, they reported a prevalence of 0.91% among larger population (6789) over a period of six years. All major coronary arteries have been reported to give rise to CAF. The commonest vessel of origin in this study was LAD which was involved in 6 cases (35.2%) having CAF. Such observation goes well with studies done by Ouchi et al. [6] who reported the same issue in 67.7% of their CAF cases.
As regard the site of CAF termination, the commonest drainage site in this study was found to be the main pulmonary artery (MPA) in 5 cases (29.4%), which is in agreement with the studies done by Ouchi et al. and Pan et al. who also found that MPA was the commonest site of CAF drainage being involved in 82.3% and 73.5% of their CAF cases, respectively [2,6].
Indeterminate site of CAF termination was found in one case in this study, the fistula arises from proximal segment of LAD and ascends superiorly extending outside the upper limit of scan coverage probably ending at bronchial artery which has been reported by different authors [7,8]. This issue requires further increase in the upper limit of scan range to include at least the aortic arch if there is high clinical suspicion of CAF to ensure optimal coverage of the site of CAF termination.  Atherosclerotic coronary artery disease has been reported by Zho et al. [5] to be present among patients diagnosed with CAF as it was present in 18.2% of their cases. Our study showed that atherosclerotic coronary artery disease was present in 3 out of 17 cases (17.6%) with atherosclerotic lesions being located close to fistula origin only in one case (Fig. 2) and at non-involved vessels in the remaining two cases. Different authors had reported significant atherosclerotic lesions close to origin of CAF, this could be due to the rapid shunting of blood across the fistula specially the larger ones which results in endothelial injury close to fistula origin and intimal hyperplasia with resultant luminal stenosis [9,10].
Subtotal thrombosis of CAF was noted in one case in this study with proximal RCA being the vessel of origin. Interestingly noted that RCA in the same patient had anomalous origin arising from left coronary cusp with malignant interarterial course (Fig. 4). We believe that this is an extremely rare association and to the best of our knowledge there is no reported similar case in the literature.

Limitations
The major limitation of this study is data collection was done only from a single center lacking pediatric population, who were scanned at other centers having dual source CT scanners which have better temporal resolution overcoming challenges of high heart rate in children. Combining data obtained from those centers together with our data would yield a higher prevalence of CAF among Egyptian population considering CAF are common in those population. Moreover, asymptomatic individuals who did not have coronary CT angiography were not included in this study who may have small CAF that could also increase the overall prevalence as well. Furthermore, standard coronary CTA protocol used in this study missed fistula drainage site in one patient which would be possible to know if scan protocol was tailored for coronary fistula detection by increasing upper extend of scan range. The absence of gold standard coronary angiography correlation is another limitation to know the accuracy of CTA results regarding CAF detection. Fig. 4 Subtotally thrombosed large proximal RCA fistula with right atrium together with anomalous origin of RCA in 21-year-old male with palpitations. Semitransparent 3D volume rendered image (a) showing anomalous origin of ectatic RCA arising from left coronary cusp with faintly opacified coronary artery fistula draining into right atrium due to subtotal thrombosis. Axial oblique MIP image (b) at the level of aortic root shows the anomalous origin of RCA and its malignant interarterial course with subtotal thrombosis of proximal RCA fistula. Axial oblique MIP image (c) at higher level reveals the remaining length of the subtotally thrombosed proximal RCA coronary fistula with its draining site into right atrium with possible extension of thrombus tail inside right atrium