Diagnosis is difficult and infants with three or more of the above symptoms with unexplained fever for more than 2 weeks should have an echocardiographic evaluation to exclude coronary artery aneurysms. Damage to the coronary arteries by vasculitis occurs in more than 25% of infants and in about 10% of children age 2–5. Coronary artery aneurysm and thrombosis may cause myocardial infarction and sudden death. Similar aneurysms may be found in the worst affected cases in the renal, cerebral, and abdominal arteries. Children frequently died during the acute phase of Kawasaki disease, but if they recover they may present with symptoms of aneurysms during adolescence and rupture of an aneurysm may cause sudden death. Coronary-artery aneurysms occur as a sequela of the vasculitis in 20–25% of untreated children. The syndrome may remain silent until the third or fourth decade of life, when patients are present with an acute myocardial infarction. Cardiac complications include myocarditis, pericarditis with effusion; mitral valvular lesions occur in about 1% of patients.
Nonspecific findings include anemia, increasing white blood cell count, sediment rate and C-reactive protein, and mild elevation of liver enzymes. The electrocardiogram may show nonspecific ST-T wave changes caused by myocarditis.