Impetigo
- Very superficial skin infection.- Usually form S aureus or S pyogenes.- Highly contagious. - Honey-colored crusting. - Bullous impetigo has bullae and is usually caused by S aureus.
Erysipelas
Infection involving upper dermis and superficial lymphatics, usually from S pyogenes. - Presents with well-defined, raised demarcation between infected and normal skin.
Cellulitis
Acute, painful, spreading infection of deeper dermis and subcutaneous tissues. - Usually from S pyogenes or S aureus. - Often starts with a break in skin from trauma or another infection.
Necrotizing fasciitis
Deeper tissue injury, usually from anaerobic bacteria or S pyogenes. - Pain may be out of proportion to exam findings. - Results in crepitus from methane and CO2 production. - Causes bullae and purple color to the skin. - Surgical emergency.
Staphylococcal scalded skin syndrome
Exotoxin destroys keratinocyte attachments in stratum granulosum only (vs toxic epidermal necrolysis, which destroys epidermal-dermal junction). - Characterized by fever and generalized erythematous rash with sloughing of the upper layers of epidermis that heals completely. - ⊕ Nikolsky sign (separation of epidermis upon manual stroking of skin). - Seen in newborns and children, adults with renal insufficiency.
Herpes
Herpes virus infections (HSV1 and HSV2) of skin. May be found anywhere on the body but are most comon in the perioral and genital areas:- Herpes labialis- Herpes genitalis- Herpetic whitlow (finger) - Characterized by vesicles that ulcerate, form a crust, and gradually heal. - Histopathology is characterized by enlarged cells with intranuclear inclusions. Cell fusion produces ballooning multinucleated giant cells.
Molluscum contagiosum
Firm, flesh-colored papules on the skin and mucous membranes. - The papules are often pruritic and have umbilicated centers containing white, curd-like material.- Caused by a poxvirus and is transmitted through direct contact or fomites. - In children, lesions commonly appear on the eyelids, face, or trunk.- In adults, sexual transmission may lead to widely distributed papules, facial involvement, and lesion counts numbering in the hundreds. - Microscopic assessment of biopsy specimens will show epidermal hyperplasia along with molluscum bodies, large eosinophilic cytoplasmic inclusions made of virus particles.
Varicella zoster virus
Causes varicella (chickenpox) and zoster (shingles). - Presents with multiple crops of lesions in various stages from vesicles to crusts. - Zoster is a reactivation of the virus in dermatomal distribution (unless it is disseminated).
Hairy leukoplakia
Irregular, white, painless plaques on lateral tongue that cannot be scraped off. - EBV mediated. - Occurs in HIV-positive patients, organ transplant recipients. - Contrast with thrush (scrapable) and leukoplakia (precancerous).
Basal cell carcinoma
Most common skin cancer. Found in sun-exposed areas of body (eg, face). - Locally invasive, but rarely metastasizes. - Waxy, pink, pearly nodules, commonly with telangiectasias, rolled borders, central crusting or ulceration. - BCCs also appear as nonhealing ulcers with infiltrating growth or as a scaling plaque (superficial BCC). - Histopathology shows strands of basaloid cells with hyperchromatic nuclei. The cells at the periphery often display parallel alignment (palisading nuclei).
Verrucae
Warts; caused by low-risk HPV (16, 18) strains. - Soft, tan-colored, cauliflower-like papules. - Biopsy is characterized by undulating epidermal hyperplasia with enlarged nuclei and cytoplasmic vacuolization (koilocytosis) forming perinuclear halos, hyperkeratosis. - Condyloma acuminatum on anus or genitals.
Epithelial cell junctions
Tight junction (zonula occludens) – prevents paracellular movement of solutes; composed of claudins and occludins. Adherens junction (belt desmosome, zonula adherens) – below tight junction, forms "belt" connecting actin cytoskeletons of adjacent cells with cadherins (Ca2+-dependent adhesion proteins). Loss of E-cadherin promotes metastasis. Desmosome (spot desmosome, macula adherens) – structural support via intermediate filament interactions. Autoantibodies to desmoglein → pemphigus vulgaris. Gap junction – channel proteins called connexons permit electrical and chemical communication between cells. Hemidesmosome – connects keratin in basal cells to underlying basement membrane. Autoantibodies → bullous pemphigoid. Integrins – membrane proteins that maintain integrity of basolateral membrane by binding to collagen, laminin, and fibronectin in basement membrane.
Dermatologic microscopic terms
Hyperkeratosis: ↑ thickness of stratum corneum- Examples: Psoriasis, calluses Parakeratosis: Retention of nuclei in stratum corneum- Examples: Psoriasis Hypergranulosis: ↑ thickness of stratum granulosum- Examples: Lichen planus Spongiosis: Epidermal accumulation of edematous fluid in intercellular spaces- Examples: Eczematous dermatitis Acantholysis: Separation of epidermal cells- Examples: Pemphigus vulgaris Acanthosis: Epidermal hyperplasia of the stratum spinosum- Examples: Acanthosis nigricans
Albinism
Normal melanocyte number with ↓ melanin production due to ↓ tyrosinase activity or defective tyrosine transport. - ↑ risk of skin cancer.
Melasma (chloasma)
Hyperpigmentation associated with pregnancy ("mask of pregnancy") or OCP use.
Vitiligo
Irregular patches of complete depigmentation. Caused by autoimmune destruction of melanocytes.
Seborrheic dermatitis
Erythematous, well-demarcated plaques with greasy yellow scales in areas rich in sebaceous glands, such as scalp, face, and periocular region. - Common in both infants and adults, associated with Parkinson disease. - Sebaceous glands are not inflamed, but play a role in disease development. - Possibly associated with Malassezia spp. - Treat with topical antifungals and corticosteroids.
Acne
Multifactorial etiology – ↑ sebum/androgen production, abnormal keratinocyte desquamation, Cutibacterium (formerly Propionibacterium) acnes colonization of the pilosebaceous unit (comedones), and inflammation (papules/pustules, nodules, cysts). Risk factors:- Increased circulating androgens (eg, puberty, polycystic ovary syndrome)- Mechanical trauma/friction (eg, excessive scrubbing, tight clothing)- Comedonogenic oil-based skin & hair products- Excessive heat- Obesity Treatment includes retinoids, benzoyl peroxide, and antibiotics.
Atopic dermatitis (eczema)
Pruritic eruption, commonly on skin flexures. - Associated with other atopic diseases (asthma, allergic rhinitis, food allergies); ↑ serum IgE. - Mutations in filaggrin gene predispose (via skin barrier dysfunction). - Often appears on face in infancy and then in antecubital fossa in children and adults.
Allergic contact dermatitis
Type IV hypersensitivity reaction that follows exposure to allergen. - Lesions occur at site of contact (eg, nickel, poison ivy, neomycin).
Melanocytic nevus
Common mole. - Benign, but melanoma can arise in congenital or atypical moles. - Intradermal nevi are papular. They are considered to be older lesions in which the epidermal nests of nevus cells have been lost into the dermis. They are skin- to tan colored.- Junctional nevi are flat macules. The nevus cells are limited to the dermoepidermal junction. - Compound nevi habe both epidermal and dermal involvement. They are raised papules with uniform brown pigmentation.
Pseudofolliculitis barbae
Foreign body inflammatory facial skin disorder characterized by firm, hyperpigmented papules and pustules that are painful and pruritic. - Located on cheeks, jawline, and neck. Commonly occurs as a result of shaving ("razor bumps"). - Primarily affects African-American males.
Psoriasis
Papules and plaques with silver scaling, especially on knees and elbows. - Acanthosis (hyperplasia of stratum spinosum) with parakeratotic scaling (nuclei still in stratum corneum), Munro microabscesses.- ↑ stratum spinosum, ↓ stratum granulosum - Auspitz sign – pinpoint bleeding spots from exposure of dermal papillae when scales are scraped off. - Associated with nail pitting and psoriatic arthritis.
Rosacea
Inflammatory facial skin disorder characterized by erythematous papules and pustules, but no comedones. - May be associated with facial flushing in response to external stimuli (eg, alcohol, heat). - Phymatous rosacea can cause rhinophyma (bulbous deformation of nose).
Seborrheic keratosis
Flat, greasy, pigmented squamous epithelial proliferation with keratin-filled cysts (horn cysts). Looks "stuck on." - Lesions occur on head, trunk, and extremities.- Common benign neoplasm of older persons. Leser-Trélat sign – sudden appearance of multiple seborrheic keratoses, indicating an underlying malignancy (eg, GI, lymphoid).
Urticaria
Hives. Pruritic wheals that form after mast cell degranulation. - Characterized by superficial dermal edema and lymphatic channel dilation.
Angiosarcoma
Rare blood vessel malignancy typically occuring in the head, neck, and breast areas. - Usually in elderly, on sun-exposed areas. - Associated with radiation therapy and chronic postmastectomy lymphedema. - Hepatic angiosarcoma associated with vinyl chloride and arsenic exposures. - Very aggressive and difficult to resect due to delay in diagnosis.
Bacillary angiomatosis
Benign capillary skin papules found in AIDS patients. - Caused by Bartonella infections. - Frequently mistaken for Kaposi sarcoma, but has neutrophilic infiltrate.
Cherry hemangioma
Benign capillary hemangioma of the elderly. - Does not regress. - Frequency ↑ with age.
Cystic hygroma
Cavernous lymphangioma of the neck. - Associated with Turner syndrome.
Glomus tumor
Benign, painful, red-blue tumor, commonly under fingernails. - Arises from modified smooth muscle cells of the thermoregulatory glomus body.
Kaposi sarcoma
Endothelial malignancy most commonly of the skin, but also mouth, GI tract, and respiratory tract. - Associated with HHV-8 and HIV. - Rarely mistaken for bacillary angiomatosis, but has lymphocytic infiltrate.
Pyogenic granuloma
Polypoid lobulated capillary hemangioma that can ulcerate and bleed. - Associated with trauma and pregnancy.
Strawberry hemangioma
Benign capillary hemangioma of infancy (vs cherry hemangioma in elderly). - Appears in first few weeks of life (1/200 births).- Grows rapidly and regresses spontaneously by 5-8 years old.
Pemphigus vulgaris
Potentially fatal autoimmune skin disorder with IgG antibody against desmoglein (component of desmosomes, which connects keratinocytes in the stratum spinosum).- Type II hypersensitivity reaction. - Oral mucosa is also involved - Flaccid intraepidermal bullae caused by acantholysis (separation of keratinocytes, resembling a "row of tombstones").- Immunofluorescence reveals antibodies around epidermal cells in a reticular (net-like) pattern.- Nikolsky sign ⊕.
Bullous pemphigoid
Involves IgG antibody against hemidesmosomes (epidermal basement membrane).- Less severe than pemphigus vulgaris.- Type II hypersensitivity reaction. - Tense blisters containing eosinophils affect skin but spare oral mucosa.- Immunofluorescence reveals linear pattern at epidermal-dermal junction.- Nikolski sign ⊝.
Dermatitis herpetiformis
Pruritic papules, vesicles, and bullae (often found on elbows). - Deposits of IgA at tips of dermal papillae. - Associated with celiac disease. Treatment: dapsone, gluten-free diet.
Erythema multiforme
Associated with infections (eg, Mycoplasma pneumoniae, HSV), drugs (eg, sulfa drugs, β-lactams, phenytoin), cancers, autoimmune disease. - Presents with multiple types of lesions – macules, papules, vesicles, target lesions (looks like targets with multiple rings and dusky center showing epithelial disruption).
Stevens-Johnson syndrome
Characterized by fever, bullae formation and necrosis, sloughing of skin at dermal-epidermal junction, high mortality rate. - Typically 2 mucous membranes are involved, and targetoid skin lesions may appear, as seen in erythema multiforme. - Usually associated with adverse drug reaction. - A more severe form of SJS with >30% of the body surface area involved is toxic epidermal necrolysis (TEN). - 10-30% involvement denotes SJS-TEN.
Acanthosis nigricans
Epidermal hyperplasia causing symmetric, hyperpigmented thickening of skin, especially in axilla or on neck. - Associated with insulin resistance (eg, diabetes, obesity, Cushing syndrome), visceral malignancy (eg, gastric adenocarcinoma).
Actinic keratosis
Premalignant lesions caused by sun exposure. - Small, rough, erythematous or brownish papules or plaques. - Risk of squamous cell carcinoma is proportional to degree of epithelial dysplasia.
Erythema nodosum
Painful, raised inflammatory lesions of subcutaneous fat (panniculitis), usually on anterior shins. - Often idiopathic, but can be associated with sarcoidosis, coccidioidomycosis, histoplasmosis, TB, streptococcal infections, leprosy, inflammatory bowel disease.
Lichen planus
Pruritic, purple, polygonal planar papules and plaques. - Mucosal involvement manifests as Wickham striae (reticular white lines) and hypergranulosis.- Sawtooth infiltrate of lymphocytes at dermal-epidermal junction. - Associated with hepatitis C.
Pityriasis rosea
"Herald patch" followed days later by other scaly erythematous plaques, often in a "Christmas tree" distribution on trunk. - Multiple pink plaques with collarette scale. - Self-resolving in 6-8 weeks.
Sunburn
Acute cutaneous inflammatory reaction due to excessive UV irradiation. - Causes DNA mutations, inducing apoptosis of keratinocytes. - UVB is dominant in sunburn.- UVA is dominant in tanning and aging. - Exposure to UVA and UVB ↑ risk of skin cancer. - Can also lead to impetigo.
Burn classifications
First-degree burn: Superficial, through epidermis (eg, common sunburn).- Painful, erythematous, blanching Second-degree burn: Partial-thickness burn through epidermis and dermis. Skin is blistered and usually heals without scarring.- Painful, erythematous, blanching Third-degree burn: Full-thickness burn through epidermis, dermis, and hypodermis. Skin scars with wound healing.- Painless, waxy or leathery appearance, nonblanching
Squamous cell carcinoma
Second most common skin cancer. - Associated with excessive exposure to sunlight, immunosuppression, chronically draining sinuses, and occasionally arsenic exposure. - Commonly appears on face, lower lip, ears, hands. - Locally invasive, may spread to lymph nodes, and will rarely metastasize.- Ulcerative red lesions with frequent scale.- Histopathology: keratin "pearls" Actinic keratosis, a scaly plaque, is a precursor to squamous cell carcinoma. Keratocanthoma is a variant that grows rapidly (4-6 weeks) and may regress spontaneously over months.
Melanoma
Common tumor with significant risk of metastasis. S-100 tumor marker.- Associated with sunlight exposure and dysplastic nevi. Fair-skinned persons are at ↑ risk.- Often driven by activating mutation in BRAF kinase - Depth of tumor (Breslow thickness) correlates with risk of metastasis. AsymmetryBorder irregularityColor variationDiameter >6 mmEvolution over time 4 types:- Superficial spreading- Nodular- Lentigo maligna- Acral lentiginous (highest prevalence in African-Americans and Asians) - Metastatic or unresectable melanoma in patients with BRAF V600E mutation may benefit from vemurafenib, a BRAF kinase inhibitor.
Abscess
Collection of pus from a walled-off infection within deeper layers of skin. Offending organism is almost always S aureus.
