The case report by Heper and Kose¹ in this issue raises issues surrounding the nature and clinical implications of coronary fistulae, a category that usually is considered to include multiple coronary–left ventricle microfistulae. Coronary fistulae in general are defined as a morphogenetic defect affecting the termination of the coronary arteries: normal coronary arteries end in the capillary bed. For centuries, anatomists have described the presence, in the normal human heart, of communications between coronary vessels and cardiac cavities^2,3:

• “Arterio–luminal communications” connect a precapillary arteriolar vessel to sinusoidal, ventricular (typically, left) intertrabecular spaces. They are short and feature irregular-shaped cross-sections, luminal areas of 50–200 μm, and no functional medial layer (essentially, they have only an endothelial lining).^4,5 Accordingly, arterio–luminal communications handle small amounts of blood flow and are not sensitive to vasodilatory stimuli, such as increased myocardial work or pharmacologic agents, like nitroglycerin and adenosine. Such normal communications have been observed only in specific studies, like those carried out in autopsy specimens by Baroldi and Scomazzoni,⁴ who reported finding them in 86% of left ventricles and 50% of right. Clinical angiography does not reveal such communications. They do not show a tendency to grow in size with aging.
• “Coronary–venous communications” (commonly called Thebesian veins) measure up to 2 mm in diameter and are especially frequent in the normal right atrium and ventricle. They establish a different termination point for the coronary circulation (other than the coronary sinus), but are of no significant functional consequence, because they are anatomically post-capillary and are functionally nonfistulous in character. They handle venous pressures throughout and are angiographically recognized at the right atrium and ventricle (especially at the outflow tract), where they may be the predominant, normal means of coronary drainage. No functional consequence can be expected, but trivial systemic oxygen desaturation could be demonstrated should these Thebesian vessels be located in the left ventricle. The name Thebesian veins has also been applied with some frequency in the literature to arterial coronary–left ventricular microfistulae, but in truth this is a misnomer.

We should distinguish such normal and innocuous communications from coronary fistulae. Indeed we have proposed in the recent past⁵ that the name coronary fistula be reserved for larger, abnormal communications of coronary arteries (not arterioles or veins) with a cardiovascular cavity, which feature:

• Evidence of fistulous flow and a tendency to enlarge with aging; typically, the feeding vessel is dilated to more than 50% of the expected size (secondary coronary ectasia);
• Angiographic signs of prompt run-off into the receiving cardiovascular structure, with sizable shunting of the coronary blood flow;
• Volume overload of the receiving cardiovascular cavity; and
• Potential for steal of nutrient blood flow from the feeding coronary artery, away from the myocardial capillary bed.

The condition presented by Heper and Kose¹ clearly belongs to the category coronary–left ventricular microfistulae, which features a diffuse network of multiple, angiographically evident communications arising from branches of either (or both) left and right coronary arteries, leading to the left ventricle. Such an entity is actually much more frequent than the authors state (“only 19 cases in the literature”¹). In a recent, careful, prospective review of a continuous, large series of coronary angiograms in adults,⁵ we found that more than 50% of all fistulae were of this kind, while 0.87% of all patients in the series had some kind of coronary fistulae.

The fact is that the microfistulous patches typically involved in this condition are quite variable in their extent and are frequently unreported, either because they are overlooked by the observer (these communications become evident only at the end of coronary angiography) or because they are usually considered unimportant variants. The amount of coronary–left ventricular fistulous flow is always small: consequently, these fistulae do not grow with age, nor is there ectasia of the feeding vessels. Nonetheless, these communications must be larger than the ones found in a normal heart,^6–8 because only they can be seen angiographically. Typically, the area between the diagonal and the obtuse marginal branches is involved, but examples similar to the one published in this issue¹ (with bilateral coronary origination) are not exceptional.

Several papers^6–8 have reported similar individual cases (but never large prospective series), and these cases usually have been studied because of mild reversible ischemia that was detected by nuclear myocardial scintigraphy. The discussion is still open regarding the reasons for such a circumstance and the implications of it. A positive nuclear stress test is a frequent reason for coronary angiography in general, a fact that may create a pre-test bias in favor of finding a fistulous connection within the context of a positive stress test. At the same time, the possibility exists that indeed microfistulous communications are an epiphenomenon of structural myocardial differentiation defects.

The frequent association of apical, hypertrophic cardiomyopathy noted by the authors¹ in their review of the literature could be evidence of such an association. Additionally, it may be relevant to note that the diagnosis of reversible ischemia is made on the basis of a relative (but not absolute) decreased maximum vasodilatory capacity, which could be related to the myocardial architectural defect, aside from the possibility of real steal of nutrient flow. It must be understood that exercise (increased myocardial workload) and adenosine are expected to lead to vasodilation in the competing nutrient arteriolar bed, but not likely to increased run-off into the coronary–cameral communications, which lack a muscular tunica media.^4–8 The reversible defect noted in the published reports of this condition might also be related to the specific vasodilatory behavior of such an unusual vascular bed, which can be maximally vasodilated under resting, baseline conditions (flow-mediated vasodilation). This point can be better evaluated by more refined measurement of segmental absolute blood flow. The probability is that no critical ischemia is involved in such an entity, but only a mild decrease in vasodilatory capacity. In any case, the prognostic implications are totally different from those of similar reversible scintigraphic defects due to fixed coronary obstructions.

In the condition described by Heper and Kose,¹ these defects are (typically) not progressive over time, not consistently related to angina, and not associated with an increased incidence of acute myocardial infarction. To my knowledge, no cases have ever been reported of coronary occlusion in the feeding vessels of such microfistulae, even when an apparent myocardial infarction was the clinical manifestation that prompted an angiographic study.⁹

Finally, we must recognize that small coronary–left ventricular communications can occur also in other circumstances, when “secondary” communications are formed:

• After myocardial biopsy (typically, after heart transplantation);
• After heart surgery, especially septal myomectomy;
• After myocardial infarction, especially in the presence of a mural thrombus (typically, with antero-apical aneurysms);
• In combination with pulmonary valve atresia and intact ventricular septum (ventriculo–coronary communications);
• In combination with aortic valve atresia (less frequently than in the previous category).

The literature contains many spurious reports, like the largest published series (14 cases),¹⁰ which included 7 infarct-related arterio–luminal communications (all with “positive thallium” tests).