Papulosquamous and inflammatory skin disorders: Pathology review

At the dermatology clinic, 9 month old Ethan is brought in due to recurrent skin rashes. According to his mother, he develops red, weeping lesions on his cheeks, chin, forehead, as well as the knees and elbows. His mother said these seemed to be related to the consumption of specific foods, but she couldn’t figure out what kinds. Physical exam reveals multiple erythematous papules with excoriations. Blood work shows increased IgE levels. On the same day, 68-year-old Marcia presents complaining of a pruritic rash on her wrists and elbows that has persisted for about 8 months. Her medical history is significant for chronic hepatitis C infection. On physical examination, there are multiple, flat-topped, violaceous-colored plaques, on the flexor surfaces of her upper extremities.

Based on the initial presentation, Ethan and Marcia seem to have some form of papulosquamous or inflammatory skin disorder. Okay, first, let’s talk about physiology real quick. Normally, the skin is divided into three main layers, the epidermis, dermis, and hypodermis. The hypodermis is made of fat and connective tissue that anchors the skin to the underlying muscle. Above the hypodermis is the dermis, containing hair follicles, nerve endings, glands, blood and lymph vessels. And above the dermis is the epidermis, which contains 5 layers of developing keratinocytes. Keratinocytes start their life at the lowest layer of the epidermis, so the stratum basale or basal layer. As keratinocytes in the stratum basale mature, they migrate into the next layers of the epidermis, called the stratum spinosum, stratum granulosum, stratum lucidum, and finally, the stratum corneum, which is the uppermost and thickest epidermal layer. Before we dive into the various skin disorders, there are several high yield terms to describe skin lesions. So, macules are flat, well circumcised lesions up to 1 centimeter in diameter, while patches are similar to a macule but are larger than 1 centimeter. Papules are raised bumps that are up to 1 centimeter in diameter, while plaques are like papules but larger than 1 centimeter. A smooth papule or plaque that is transient, meaning that it comes and goes, is called a wheal. Finally, scales are accumulations of thickened stratum corneum that become dry and flaky and sometimes peel off; while crusts are dry exudates like sebum, pus, or blood.

All right then, onto papulosquamous and inflammatory skin disorders! Let’s begin with atopic dermatitis, also known as eczema. This is a chronic inflammatory skin disease that’s particularly common among young children, but can last into adulthood. For your exams, remember that it can be a part of the characteristic atopic triad, which includes atopic dermatitis, allergic rhinitis and conjunctivitis, and allergic asthma. Now, the exact cause of the skin inflammation seen in atopic dermatitis is not fully agreed upon, however it seems to be a mix of dysfunction in the immune system and skin barrier abnormalities. In regards to dysfunction in the immune system, atopic dermatitis is associated with an elevated serum IgE, as well as certain foods and environmental triggers like pollen. Abnormalities of the skin barrier in atopic dermatitis are associated with a mutation in the filaggrin gene. Filaggrin is a protein that binds to keratin and contributes to the formation of the skin barrier, so if it’s abnormal, it can cause the skin barrier to become porous and leaky. This allows water to accumulate between cells, leading to edema in the epidermis, which is called spongiosis. Now, if this water escapes, it will leave the skin dry and scaly. This dry skin is extremely itchy, and that’s characteristic of atopic dermatitis. So, the rash usually presents as patches of red itchy skin that come and go, but can occasionally blister and peel. Over time, the skin can undergo lichenification, meaning that it becomes leathery and tough. Now, a high yield fact regarding the location of the rash is that in infants, it’s usually on the face and scalp. As children get older, it’s more common on flexor surfaces of extremities, like the creases of the wrists, the insides of the elbows, and the back of the knees. And in adults, it’s usually on the hands and feet. The diagnosis of atopic dermatitis through clinical examination, and lab tests might show increased eosinophils and serum IgE levels, and skin testing or allergen-specific IgE antibodies might identify the specific allergens that should be avoided. Treatment focuses on reducing symptoms by minimizing exposure to food and environmental triggers and reducing stress, as well as keeping the skin hydrated with moisturizers, minimizing itching with oral antihistamines, and dampening the immune reaction with topical medications like steroids or calcineurin inhibitors.