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Malassezia (Tinea versicolor and Seborrhoeic dermatitis)
Pediculus humanus and Phthirus pubis (Lice)
Sarcoptes scabiei (Scabies)
Human herpesvirus 6 (Roseola)
Varicella zoster virus
Herpes simplex virus
Poxvirus (Smallpox and Molluscum contagiosum)
Human herpesvirus 8 (Kaposi sarcoma)
Acneiform skin disorders: Pathology review
Bacterial and viral skin infections: Pathology review
Papulosquamous and inflammatory skin disorders: Pathology review
Pigmentation skin disorders: Pathology review
Skin cancer: Pathology review
Vesiculobullous and desquamating skin disorders: Pathology review
Viral exanthems of childhood: Pathology review
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21 year old Holly comes to her primary care provider's office complaining of pigment changes on her skin for the past year. She denies any history of trauma or recent inflammation of the skin. Her past medical history is significant for autoimmune thyroiditis. Physical examination shows several sharply demarcated depigmented patches on the dorsum of both her hands and wrists. On the same day, 30 year old Maria comes due to some tan spots that recently appeared on her cheeks. Maria tells you she is pregnant and of hispanic descent. Physical examination reveals that no other body area is involved. Based on the initial presentation, Holly and Maria seem to have some form of pigmentation skin disorder.
All right, so the skin is divided into three main layers, the epidermis, dermis, and hypodermis. Melanocytes are located in the stratum basale layer of the epidermis and they produce a pigment called melanin from tyrosine. Melanin is then taken up by surrounding keratinocytes, and it contributes to the color of our skin, hair, and eyes. Now, what’s high yield is that melanin acts as a natural sunscreen that absorbs and dissipates, or scatters, UV radiation from the sun or other sources such as tanning booths, preventing it from damaging the keratinocytes. Now, 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 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.
Okay, now, let’s start with pigmentation skin disorders. First, there’s hypopigmentation, which refers to any form of decreased or lost skin pigmentation compared to the baseline skin color. Sometimes, this can progress to depigmentation, in which there’s total absence of all pigment. These are in contrast to hyperpigmentation, which refers to darkening or increase in the natural color of the skin.
Let’s start with hypopigmentation disorders. One of the most common and well known disorders is vitiligo, which is characterized by well defined, irregular shaped macules or patches of skin depigmentation. Lesions can range in size from millimeters to centimeters, and can sometimes expand and merge with other lesions over time. Vitiligo can affect any area of the body, but the most commonly affected body areas include the face, genitals, and body surfaces subjected to repeated trauma like the hands, wrists, and extensor forearms. Vitiligo can be classified according to the location affected. The most common type is non segmental vitiligo, which occurs at various locations that are often symmetrical on both sides of the face and body, and can affect any age group. On the other hand, segmental vitiligo occurs in segments along a single spinal nerve or dermatome typically on only one side of the body, and mostly affects children. Now, the exact cause of vitiligo isn’t known, but it is thought to be an autoimmune disorder where immune cells attack and destroy melanocytes. For your exams, remember that this is why vitiligo is often associated with other autoimmune disorders, like systemic lupus erythematosus and autoimmune thyroiditis. Now, the diagnosis of vitiligo is mainly clinical, and a Wood’s lamp may be used for diagnostic aid. Regarding treatment, topical corticosteroids are commonly used as first line therapy of smaller or more limited vitiligo, and if not effective then topical calcineurin inhibitors can be used. On the other hand, when the affected area is large, systemic immunosuppressants, UV phototherapy, skin bleaching, and in severe cases, skin grafts, can all be tried. Whatever the course of therapy, sunscreen is recommended to supplement the protection melanin would have provided.
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