The most important complication of KD is development of coronary artery aneurysms, which can cause angina, myocardial infarction, and sudden death. In the current era, if IVIG treatment is administered within 10 days of illness onset, coronary aneurysms are found in fewer than 5 % of children and giant aneurysms, in fewer than 1 % [56]. Other complications include peripheral artery aneurysms, which occur only among patients with giant coronary artery aneurysms, myocarditis with diminished left ventricular function, mitral and, less often, aortic regurgitation, and pericardial effusion.

The principal cause of death in KD is acute myocardial infarction (MI). In a nationwide survey in Japan, Kato and colleagues analyzed 195 KD patients with MIs [57]. They found that the relative risk of MI was highest in the first year after KD. However, MI can occur many years later, particular as coronary artery stenoses worsen. Primary manifestations in children include shock symptoms, pallor, unrest, vomiting, and abdominal pain. Chest pain was more frequently reported in children older than 4 years of age. Asymptomatic MIs were seen in 37 % of patients. Twenty-two percent with MIs died; 16 % of the survivors of a first attack had a second MI. Fatalities were 63 % in a second attack and 83 % in a third attack. From review of the coronary angiograms of patients with MIs, most who died had obstructions in the left main coronary artery or in both the right coronary artery and the left anterior descending artery. In survivors, single-vessel obstruction, particularly in the right coronary artery, was frequently recognized.

Early recognition and treatment of MIs are a challenge in children. Recurrence of MI is observed in about 20 % of children who have had a previous MI. Because the mortality of recurrent MI is high, vigilant follow-up is needed for such patients. Patients with complications after MI, such as ventricular aneurysm, papillary muscle dysfunction, heart failure, severe arrhythmias, and postinfarction angina, are managed by medical or surgical approaches.

Myocardial infarction can also occur in adults, some of whom had “missed” KD in childhood. A recent study in San Diego County observed that 5 % of myocardial infarctions in individuals under age 40 years were caused by a past history of KD [58].

Coronary artery lesions account for almost all morbidity and mortality in KD patients. However, patient with giant coronary aneurysms may also have associated peripheral artery aneurysms. Indeed, aneurysms in other arteries were observed in 1.0 % of the patients in the natural history study of Kato and colleagues [59]. His angiographic study in 22 patients with systemic artery aneurysms demonstrated that systemic artery aneurysms were seen in axillary arteries in 18 cases, common iliac arteries in 16, internal iliac arteries in 12, renal arteries in six, mesenteric arteries in two, and internal thoracic arteries in two, respectively. One patient had large common iliac artery aneurysms associated with dilatation of aorta. Although the prognosis of systemic artery aneurysms is generally favorable, renovascular hypertension may develop in a patient having a renal artery lesion, and lesions in the internal mammary arteries may complicate the choice of graft in coronary bypass surgery. Gangrene of the extremities has been reported as a rare complication of KD [60].

Mitral regurgitation, generally of mild severity, is seen by echocardiography in approximately 25 % of children with acute KD [61]. The incidence of this valvulitis declines in the convalescent phase of the disease, and late mitral regurgitation is seen primarily in patients with ischemic disease affecting papillary muscle function [62]. Aortic regurgitation by echocardiography is virtually always mild and is seen in 1 % of patients over the first 5 weeks of illness [61]. Progression to severe regurgitation over several years has been reported in some patients [63]. Aortic root dilation is a recognized complication of KD that can persist, but is usually mild [64]. Pericarditis greater than 1 mm by echocardiography occurs in fewer than 5 % of children over the first 5 weeks of illness [61]; massive pericardial effusion and cardiac tamponade are rare. There have been no reports on its progression to chronic or constrictive pericarditis. Diminished left ventricular function is observed in approximately 20 % of patients in the acute phase, especially in the first and second weeks of illness [61]. Gallop rhythms, distant heart sounds, ST-T segment changes, and decreased voltage of R waves on electrocardiogram (ECG) may suggest the presence of myocarditis. Systolic function virtually always recovers to normal in the absence of ischemic heart disease [61, 65]. Diastolic dysfunction has been noted in acute KD, but long-term diastolic abnormalities are found predominantly in patients with coronary aneurysms [66].

Coronary artery lesions in the acute stage of KD are detected by 2D echocardiography using standard views [56]. Indeed, echocardiography is highly sensitive and specific for the diagnosis of dilated lesions and even the presence of thrombus in the proximal coronary arteries. However, echocardiography is less reliable for detection of distal aneurysms or stenotic lesions. Reliable coronary imaging of young children (e.g. younger than age 2 years) generally requires the use of sedation. The diagnosis of coronary artery aneurysms may be made by two different sets of criteria. First, the 1984 Japanese Ministry of Health criteria use absolute coronary dimensions for the diagnosis of coronary aneurysms as follows: coronary arteries with lumen diameter greater than 3 mm in children younger than 5 years or greater than 4 mm in those older than age 5; lumen diameter 1.5 times the size of an adjacent segment; or irregular lumen [67]. However, this approach may underestimate the true incidence of coronary artery dilatation, so coronary artery dimensions adjusted for body surface area, i.e., so-called coronary artery z scores, have come into common use [68, 69]. Figure 22.1 depicts normal coronary dimensions according to body surface area.

The method for evaluation by echocardiography is well described in the American Heart Association scientific statement on KD published in 2004 [56] and is summarized in Table 22.2. Aneurysms are classified as saccular if the axial and lateral diameters are nearly equal or as fusiform if symmetric dilatation with gradual proximal and distal tapering is seen. If the coronary artery diameter looks larger than normal without segmental dilatation, the coronary artery is considered ectatic. Echocardiography is advised at the time of diagnosis, and at 2 and 6 weeks after illness onset. For patients with risk factors for aneurysms, such as persistent or recrudescent fever after treatment with intravenous immunoglobulin (IVIG), age less than 6 months, or coronary dilation at baseline, echocardiography should be performed more frequently. In early literature upon which much of the natural history is based, the sizes of coronary artery aneurysms are classified as small (<5 mm internal diameter), medium (5–8 mm) and giant or large (>8 mm). Some investigators classify aneurysms according to z score, with giant aneurysms defined as those with a maximum internal diameter of ≥10 [70].