Definition

Kawasaki disease (KD) is an acute systemic vasculitis of as yet unknown cause, first described in Japan by Dr. Tomisaku Kawasaki in 1967 [1,2]. Kawasaki disease is the most common form of acquired pediatric heart disease in the developed world and has a worldwide distribution, affecting children of all races. Kawasaki disease is a clinical diagnosis, with the complete KD criteria including fever for ≥5 days and at least four out of five of the following (Table 39.1): changes in the extremities, polymorphous skin exanthem, nonexudative conjunctivitis, changes in the lips and mucous membrane, and cervical lymphadenopathy. In addition to the diagnostic clinical criteria, there can be multiple additional associated features including (but not limited to) cardiac involvement, arthritis/arthralgia, vomiting, diarrhea, gall bladder hydrops, sterile pyuria, and irritability. A diagnosis of “incomplete” KD can be made when there is persistent fever and two or three clinical criteria plus additional laboratory data and/or echocardiographic findings that support the diagnosis [3].

Incidence

Kawasaki disease primarily affects young children, with 80% of cases reported in children less than 5 years of age, and 90% of cases occurring in children less than 8 years of age [4]. In Japan, where the prevalence of KD is the highest, the disease occurs in 134 per 100,000 children less than 5 years of age. Outside Japan, prevalence ranges from 8 to 67 per 100,000 children less than 5 years of age, depending on the ethnicity of the population being studied [5]. The disease prevalence is highest in children of Asian descent, followed by African Americans, Hispanics, and Caucasians [6]. Males are affected more frequently than females, with a ratio of 1.5 : 1.

Etiology

The etiology of KD remains unknown. An infectious trigger in a genetically susceptible patient is the most commonly proposed hypothesis. Features supporting an infectious or environmental trigger include the winter–spring predominance, a characteristic age distribution, evidence of community outbreaks, and evidence of prior epidemic periods. Genetic susceptibility to KD has increasingly been recognized and is supported by wide differences in prevalence based on ethnicity and recurrence rate in first‐degree family members (~1%) and in identical twins (~13%). Prior theories, such as toxic exposures or bacterial superantigen‐mediated inflammation, have not gained widespread acceptance.

Pathophysiology

The acute febrile phase of the illness features a diffuse vasculitis involving medium‐sized arteries, with a predilection for endarteritis and perivascular inflammation of the coronary arteries [7]. The inflammatory process may also involve pericarditis, myocarditis [8], and endocarditis. The acute phase of the disease can be associated with coronary artery changes, pericardial effusion, acute myocardial dysfunction, and valvar regurgitation. In the subacute clinical phase, following resolution of the fever, there is a persistent panvasculitis of the coronary arteries that may result in dilation, aneurysm formation, and, in cases of large/giant aneurysms, in thrombosis. Pericarditis, myocarditis, and endocarditis are less commonly seen in this phase. Three linked processes are responsible for KD vasculopathy: necrotizing arteritis, subacute chronic vasculitis, and luminal myofibroblastic proliferation. Necrotizing arteritis begins in the acute phase, is complete by ~2 weeks of illness, and is characterized by neutrophilic infiltration of the arterial wall initiated from the endothelial surface. The second process, subacute chronic vasculitis, begins within the initial 2 weeks of illness and can persist for months to years. It is characterized by infiltration of the vessel wall by lymphocytes, plasma cells, and eosinophils, starting at the adventitia and progressing inward toward the endothelium. The third process, luminal myofibroblastic proliferation, occurs in close concert with subacute chronic vasculitis and involves smooth muscle‐derived myofibroblasts and their associated matrix products laying down a concentric mass within the vessel wall. The activated myofibroblasts can persist for months or years after KD and lead to progressive coronary obstruction [9]. Coronary artery scarring, stenosis, and calcification can be seen in later phases of the illness. Coronary artery thrombosis is most common in the subacute and early convalescent phase between days 15 and 45 of illness. Approximately 60–70% of coronary artery aneurysms will demonstrate evidence of normalization of internal lumen diameter typically over the first 2 years following acute KD [10,11]; however, the mechanism for regression of aneurysms may involve myointimal proliferation and layering thrombus resulting in abnormal coronary artery architecture and, in some cases, in stenosis [12]. In patients with persistent coronary aneurysms and in some patients with aneurysm regression, coronary vascular function is abnormal, with reduced coronary artery flow reserve and abnormal vascular reactivity [13–15].

General principles

Kawasaki disease presents acutely (and subacutely) as a pancarditis, hence echocardiographic assessment should focus on the endocardium, myocardium, and pericardium in addition to systematic assessment of coronary arteries. Identification of pericardial effusion, mitral regurgitation, or myocardial dysfunction is more common in Kawasaki shock syndrome and associated with an elevated risk of coronary artery changes [16,17]. These findings may also increase clinical suspicion of KD when clinical criteria are incomplete [3]. Abnormalities in left ventricular diastolic function, based on tissue Doppler assessment, have also been reported in acute and subacute KD, suggesting that many patients have subclinical myocardial inflammation even in the absence of systolic dysfunction [18]. In patients with large or giant aneurysms, the focus of the echocardiographic assessment shifts to assessment of coronary artery thrombosis and/or stenosis, regional and global myocardial function, and ischemia‐related atrioventricular valve dysfunction.

Consensus recommendations for echocardiographic imaging in Kawasaki disease

Coronary artery changes typically are present within the first 2 weeks of illness, with most patients who go on to develop aneurysms having coronary changes present within the initial 10 days of illness [19]. For patients with normal coronary arteries at diagnosis, the 2017 American Heart Association guidelines [3] recommend echocardiographic imaging at the time of diagnosis, at 1–2 weeks after diagnosis, and again at 6–8 weeks after diagnosis. In patients with coronary artery changes at baseline, serial echocardiographic evaluation every 2–4 days is recommended until coronary dimensions stabilize. During this period, acute changes in valvar or myocardial function, pericardial effusion, and acute coronary artery changes should be noted. In addition, imaging in the first 6–8 weeks of illness encompasses the period during which transient changes in coronary artery dilation and ectasia will often resolve. Alternatively, this is the period during which coronary artery aneurysms attain their maximum size [20]. In complex cases where coronary artery aneurysms and/or thrombosis, significant valvar regurgitation, effusion, or myocardial dysfunction occur, echocardiography during the acute and subacute phases should be performed as indicated by clinical circumstances. For patients with chronic coronary changes, annual follow‐up with echocardiography is recommended. The focus of these follow‐up evaluations is to assess for resolution of coronary artery abnormalities, evidence of coronary artery thrombosis/stenosis, valvar dysfunction, or myocardial dysfunction/ischemia. The proposed algorithm for long‐term follow‐up echocardiography, myocardial perfusion imaging, and angiography is summarized in Table 39.2.

General principles for coronary artery assessment

Coronary artery abnormalities are the most significant short‐ and long‐term sequelae of KD. Echocardiography is a highly sensitive, specific, and noninvasive means of assessing the coronary arterial system, and thus is the central modality in the cardiac evaluation of children with KD. In general, coronary artery imaging should be performed at the highest feasible transducer frequency. Reducing 2D gain and dynamic range (i.e., compression) settings will often improve demonstration of the endovascular lumen and thereby improve coronary artery imaging. Imaging at lower depth will also enhance anatomic visualization.

Imaging of the left coronary artery system is best performed in the parasternal and apical windows. The left main coronary artery (LMCA) is most readily imaged in the parasternal short‐axis window at the level of the aortic root. In the parasternal short‐axis view, the left anterior descending artery (LAD) and proximal left circumflex artery (LCx) can also be demonstrated. Often, a slight clockwise rotation of the transducer from the standard parasternal short‐axis view will increase visualization of a greater length of both LAD and LCx (Figure 39.1, Video 39.1). In the setting of coronary artery aneurysms, the LMCA, LAD, and LCx are easily identifiable in the parasternal short‐axis view (Figure 39.2, Video 39.2). In the parasternal long‐axis view, sweeping the plane of sound between aorta and pulmonary artery – from right to left – will often demonstrate the proximal left coronary artery, LAD, and LCx (Figure 39.3, Video 39.3). In addition to imaging in the parasternal views, the apical four‐chamber view with anterior angulation of the imaging plane will demonstrate the more distal LCx in the anterior atrioventricular groove (Figure 39.4). Although technically more challenging, the LCx can also be demonstrated in subxiphoid windows, particularly when aneurysms are present (Figure 39.5).

The right coronary artery (RCA) system can be visualized in several imaging windows. In the parasternal short‐axis view at the aortic root, the proximal RCA can be imaged (Figure 39.6, Video 39.4). A slight clockwise rotation of the imaging plane in this view will usually result in better visualization of a greater length of the proximal and mid RCA (Figure 39.7, Video 39.5). The proximal RCA can also be visualized in the anterior atrioventricular groove on either apical imaging with anterior angulation of the plane of sound or in the subxiphoid long‐axis (coronal) view (Figures 39.8 and 39.9, Video 39.6

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