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.