USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 2-year-old boy comes to the emergency department because of high fever and rash for five days. His pediatrician saw him two days ago and felt his fever and rash were likely due to a non-specific viral infection. Since that appointment, he had been refusing to eat solids and had only been taking sips from his bottle and cups. His temperature is 39.6°C (103.2°F). Examination shows conjunctival injection bilaterally. His tongue is bright red and edematous. His tonsils are small but erythematous and without exudate. There is a 1.5-cm (0.59-in) lymph node along the left anterior cervical chain and a 2-cm (0.79-in) supraclavicular lymph node on the right. He has a diffuse, erythematous maculopapular type rash on his chest, abdomen, and back extending into the groin region. His hands and feet are red and painful to touch. Which of the following is the most likely diagnosis?
Kawasaki disease isn’t related to the motorcycle and engine company, other than that Japan was where the disease was first described and where the company was founded. Kawasaki disease is a vasculitis, or an inflammation of the blood vessels, that mostly affects the coronary arteries, but can also affect any large- or medium-sized arteries. With Kawasaki disease, the immune system attacks the arteries. It’s not entirely known why this happens, although some theories suggest it has an infectious cause; autoimmune reactions and genetic predisposition probably play a part as well. Whatever the case, when the endothelial cells in the blood vessels are attacked, they become damaged, exposing the underlying collagen and tissue factor found in the middle layer of the blood vessel, or the tunica media. This leads to a few serious problems.
First, these exposed materials increase the chance of blood coagulation. When blood coagulates, it forms clots that can block blood flow in the coronary arteries, leading to ischemia of the heart muscle. Secondly, damaged endothelial cells in coronary arteries mean weak artery walls, which can lead to coronary aneurysms. These aneurysms form because fibrin is deposited into the blood vessel wall as part of the healing process. Fibrin makes the vessel stiffer, less elastic, and unable to gently stretch with high arterial pressures; therefore, the arteries develop permanent bulges that we call aneurysms. Aneurysms 8mm or larger are at the most risk of rupturing, which reduces blood flow to the heart; this causes ischemia, and potentially myocardial infarction, or heart attack. Thirdly, in some cases, this fibrosis doesn’t lead to aneurysms; instead, fibrosis of the blood vessels makes the vessel walls thicker, which reduces the lumen diameter and restricts blood flow. If blood flow is restricted or reduced, again, the heart might become ischemic, leading to heart attack.
Kawasaki disease is most commonly seen in infants and children under five years old, and is more likely to affect boys. This disease is self-limited, which means that the inflammation will resolve after six to eight weeks, but if we left it untreated, there is a 20-25% risk of developing the heart complications mentioned earlier. We rarely see any cardiac symptoms in the first few weeks, unless the patient has an underlying heart condition already; otherwise, cardiac symptoms typically evolve later. These are the classic symptoms of Kawasaki disease: conjunctivitis with limbus sparing, which means red eyes with a white margin right around the iris; a rash that might extend across multiple parts of the body, which starts polymorphous, but later desquamates, or flakes off; adenopathy, or enlarged lymph nodes, especially concerning the cervical lymph nodes; a strawberry tongue, which is when the top layer of cells on the tongue slough off, giving the tongue a very red, strawberry-like appearance; their mouth and throat may also look very red; their lips might become dry and cracked; their hands and feet might develop a rash and swell; and finally, they might have five or more days of high fever that typically doesn’t resolve with antipyretics. Just remember that these patients C-R-A-S-H and burn.
Now, there isn’t a specific test to diagnose Kawasaki disease, but there are a number of lab tests that can provide clues. At the beginning of the disease, many patients are anemic and have an increased number of white blood cells with a “shift to the left,” which means that there are more immature white blood cells than normal. They’ll also have an increased C-reactive protein and erythrocyte sedimentation rate, and increased liver enzymes; these are all good clues telling us that some sort of inflammation is happening. Microscopic urinalysis will show mononuclear white blood cells in the urine without evidence of bacteria. After a few weeks, the patient’s platelet count also typically rises. That was a lot of tests, but one last test that we’d want to do is an echocardiogram to take a look at the coronary artery and the heart muscle, to see if any of the heart complications we talked about earlier are happening.