USMLE® Step 1 style questions USMLE
A 12-month-old boy is brought to the emergency department because of a painful vesicular rash on his face, elbows, and trunk for two days. His parent states the patient had a “flare-up of dryness and itching” in the same region for which high-potency topical corticosteroids were started five days ago. Family history is significant for asthma in his mother. Temperature is 38.05°C (100.5°F), pulse is 130/min, respirations are 20/min, and blood pressure is 110/70 mmHg. Physical examination shows an irritable child with erythematous, scaly, crusted lesions on the extensor surfaces of the elbows and on the trunk. Examination findings of the face are shown:
Reproduced from Wikimedia Commons
The presentation of this patient is most likely a complication of which of the following?
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Vincent Waldman, PhD, Tanner Marshall, MS, Kara Lukasiewicz, PhD, MScBMC
Atopic dermatitis or atopic eczema is a skin rash that’s particularly common among young children, but can last into adulthood. “Atopic” refers to an allergy, “derm” refers to the skin, and “itis” refers to inflammation.
So atopic dermatitis describes skin inflammation that results from an allergy, more specifically, it happens when the immune system attacks the skin causing a dry, itchy rash on flexor surfaces of the body, areas like the creases of the wrists, the insides of the elbows, and the backs of the knees, as well as exposed skin surfaces like on the face, the hands, and the feet.
When the immune system inappropriately starts attacking itself, we call that a hypersensitivity reaction, and there are four types.
Atopic dermatitis is a type 1 hypersensitivity reaction, and it starts off with something in the environment called an allergen, like flower pollen.
The pollen is able to travel through the slightly porous skin, where it gets picked up by an immune cell in the tissue just below.
The immune cell is called an antigen presenting cell because it presents a bit of the allergen to a naive T helper cell, activating it into a Th2 cell.
This Th2 cell then stimulates a nearby B cell to start producing IgE or immunoglobulin E antibodies specific to that pollen.
Those IgE antibodies bind to the surface of other immune cells called mast cells, as well as basophils, which can be found in the tissue layer just below the surface of the skin, and this process called “sensitization”.
Now, let’s say that there’s a second exposure to pollen that makes it’s way into the skin tissue. Now, the allergen can cross-link the IgE on these sensitized cells, resulting in degranulation or release of a number of proinflammatory molecules like histamine, leukotrienes, and proteases.
The combined effect of these molecules is to cause nearby blood vessels to dilate and become leaky as well as attract even more immune cells to the area creating inflammation of the skin tissue.
Ultimately, the inflammation makes the skin barrier more “leaky”, potentially allowing in more of the allergen, while at the same time allowing water to escape, leaving the skin dry and scaly.
Dry skin is very itchy, and scratching further damages the skin barrier, worsening the process and setting up a vicious cycle of allergy-mediated inflammation, dry skin, and itching which characterizes atopic dermatitis. Occasionally, bacteria can invade the damaged skin causing even more inflammation.
In infants, the rash is most often on the face and scalp, and in children, the rash is usually on flexor surfaces of extremities.
Atopic dermatitis can worsen in the presence of allergens like cigarette smoke, mold, and dust mites, as well as changes in the weather, and even emotional stress.
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