Features of Kawasaki Disease Myocarditis in Children
Myocarditis occurs in all children with Kawasaki disease on histologic examination, though only a minority are clinically symptomatic, presenting with findings ranging from subtle echocardiographic abnormalities to overt heart failure with hemodynamic instability. 1
Clinical Features
Symptomatic Presentations
- Gallop rhythm or distant heart sounds on auscultation 2
- Congestive heart failure with decreased ejection fraction (can drop to as low as 28%) 3
- KD shock syndrome with hemodynamic instability due to decreased systolic function and vasoplegia 1
- Striking irritability beyond what is typical for febrile illness 2
- Chest pain, particularly in older children or those with coronary involvement 4
Asymptomatic/Subclinical
Most children have subclinical myocarditis detectable only by advanced imaging, without overt clinical symptoms 1
Laboratory Findings
Acute Phase Markers
- Elevated acute phase reactants: ESR often >40 mm/hr (commonly ≥100 mm/hr), CRP typically ≥3 mg/dL 2, 5
- Leukocytosis with left shift 2
- Hypoalbuminemia 2
- Mild anemia in acute phase 2
- Elevated cardiac biomarkers (troponin) when myocardial injury is present 4
Important Caveat
In patients with coronary artery involvement, ESR, lymphocyte count, and platelet count are significantly higher before treatment, but these inflammatory differences disappear after appropriate therapy 6
Imaging Features
Echocardiography
Systolic Dysfunction:
- Decreased shortening fraction and ejection fraction 1
- Increased end-systolic and end-diastolic dimensions 1
- Decreased contractility on 2D echo 2
- Cardiomegaly on chest X-ray 2
Advanced Strain Imaging Findings:
- Reduced global longitudinal strain (GLS): Most sensitive marker with AUC 0.953 for detecting acute KD myocarditis (cutoff -19.89%, sensitivity 95.8%, specificity 83.2%) 7
- Reduced global circumferential strain (GCS) 8, 7
- Transmural gradient abnormalities between endocardium and epicardium 7
- Lower early diastolic mitral annular velocity (e') 8
- Reduced left atrial reservoir strain (LASr) 8
Diastolic Dysfunction:
- Strain abnormalities persist even when systolic function appears normal 1
- Subendocardial myocardium remains damaged even in chronic stages 7
Electrocardiography
- Arrhythmias 2
- Abnormal Q waves 2
- Prolonged PR and/or QT intervals 2
- Low voltage (occasionally) 2
- ST-T wave changes suggesting ischemia 2, 4
Cardiac MRI (Advanced Imaging)
- Late gadolinium enhancement indicating myocardial scarring 9
- Assessment of active myocarditis in subacute phase 9
- Regional wall motion abnormalities 10
- Pericardial effusion 2
Associated Cardiac Findings
- Mitral and/or aortic valvular insufficiency 2
- Pericardial effusion 2
- Coronary artery abnormalities (aneurysms, ectasia) - present in ~20% untreated, ~5% with treatment 2, 5
Temporal Evolution
Acute Stage
All myocardial layers show decreased systolic function to varying degrees, with LVGLS being the most sensitive early marker 7
Convalescent Stage
LVGLS, GLS across all layers, LVGCS, and mid/epicardial GCS remain decreased 7
Chronic Stage
Systolic function typically normalizes over long-term follow-up, but subtle abnormalities (strain, diastolic function) may persist indefinitely 1. Subendocardial myocardium remains damaged even when global function appears recovered 7
Critical Clinical Pitfalls
Age-Related Risk: Infants <6 months and children <1 year have paradoxically higher rates of coronary artery aneurysms and are more likely to present with incomplete KD, making myocarditis harder to recognize 5
Myocarditis Without Coronary Involvement: Myocarditis can occur in the absence of coronary artery abnormalities and still result in long-term sequelae 1
Treatment Response: Echocardiographic signs of myocarditis improve more quickly in patients receiving IVIG compared to aspirin alone, emphasizing the importance of prompt immunoglobulin therapy 1
Younger Patients at Higher Risk: Children with coronary artery involvement are significantly younger, have longer hospitalization, prolonged fever before IVIG treatment, and delayed defervescence 6