Coronary artery aneurysm with an associated fistula (CAAAF) is a rare congenital malformation. Neufield et al.  first reported a surgical case of CAAAF in a 10-year-old girl in 1961. In 1963, Habermann et al.  reported a CAAAF in an adult patient who died of aneurysm rupture. The number of cases of CAAAF has gradually increased with the widespread adoption of coronary angiography, but reports of this disease are still limited. Coronary artery branch aneurysm with a fistula is extremely rare. Hirose et al.  performed a literature review of 45 reports of CAAAF, of which 23 were written in Japanese, and 12 of the 22 reports written in English were from Japan. The authors found that CAAAFs occurred more often in female than in male patients. One third of the patients with CAAAFs were asymptomatic, and were usually diagnosed incidentally by cardiac murmur or abnormalities on chest radiographs. The clinical presentation depends on the severity of the left-to-right cardiac shunt. Clinical symptoms, such as fatigue, dyspnea, angina, myocardial ischemia, or myocardial infarction, may appear as the shunt enlarges. The LCX is the origin of the fistula in 18.3% of cases with a coronary artery fistula, with the left anterior descending artery and the right coronary artery accounting for 42% and 50% – 60% of cases, respectively. The drainage site of the fistula is the CS in 7% of cases .
Libertini et al.  and Kemmochi et al.  reported cases of giant LCX aneurysms with CS fistulas. Almansori and Tamim  also reported a case of a giant coronary artery fistula from the LCX to the CS. However, these cases involved a main LCX aneurysm, whereas our case involved a branch aneurysm arising from the LCX. Edwards et al.  reported a case of a giant CS with a focal aneurysm secondary to multiple fistulous connections arising from a dilated tortuous LCX. However, the LCX in our patient was of normal caliber and the branch of the LCX was aneurysmal, which were different from these former cases. To our knowledge, this is the first report of the combination of a giant coronary artery branch aneurysm with a fistula to the CS.
Previous reports have also suggested that surgery should be performed for all patients with a CAAAF, even when asymptomatic, due to the risk of future development of cardiac complications, such as rupture, ischemia, or thrombosis . Indeed, Yoshino et al. reported rupture of a coronary artery fistula to the coronary sinus with giant aneurysm of the coronary artery . We followed the strategy recommended in these previous reports. El Nihum et al. recently reported transcatheter embolization of a coronary artery fistula . However, we did not select this procedure as our case involved a coronary branch aneurysm and we considered that cannulation of the fistula would have been difficult and embolization of the LCX main trunk would associated with a risk of ischemia. Moreover, transcatheter embolism would not prevent aneurysm rupture, and coil embolization could affect the blood supply to the normal coronary artery.
The precise mechanisms underlying aneurysm development with a coronary artery fistula are unknown. In our case, pathological examination of both the fistula and aneurysm revealed congenital structural weakness of the coronary artery wall, which may have contributed to the development of the CAAAF. Genetic factors may also be associated with the process of aneurysm formation .
We reported the successful surgical treatment of a giant LCX branch aneurysm with a fistula to the CS. Surgical aneurysmectomy should be considered in such cases to avoid fatal aneurysmal complications.