--- module: 034-01 language: en chapter: 34 title: "Skin Function, Wounds, Infection, Tumours, and Clinical Examination" module_title: "Foundations" source_sha256: 8e612eb3f484ac00b88c0db93e7972c7caea9419d65b19d13d48eec4aea6d242 --- # Skin function, wounds, infection, tumours ## Orientation and skin structure ### Barrier, immune, sensory, thermoregulatory organ ### Describe morphology and distribution before naming ### Colour signs differ across pigmentation ### Keratinocytes form the stratum corneum #### Corneocytes in lipid layers resist water #### Barrier disruption admits irritants and pathogens ### Melanin absorbs ultraviolet, limits DNA damage #### Colour reflects melanosomes, not melanocyte number ### Langerhans cells survey, Merkel cells aid touch ### Papillary dermis interfaces, reticular gives strength ### Subcutis cushions, insulates, stores energy ## Thermoregulation, sensation, secretion ### Vasodilation sheds heat, vasoconstriction conserves ### Eccrine sweat cools by evaporation #### Mainly sympathetic cholinergic control ### Impaired sweating or sensation raises injury risk ### Itch uses pathways distinct from pain #### Skin, kidney, liver, blood, drug, nerve causes #### Scratching damages barrier, sustains the cycle ### Sebum, apocrine secretion, vitamin D synthesis ## Describing a rash ### Site, number, arrangement, symmetry, distribution ### Primary lesions: macules to wheals and cysts ### Secondary changes: scale, crust, erosion, scar ### Diascopy tests blanching ### Dermoscopy shows subsurface pigment and vessels ### Precise description narrows mechanism ## Inflammatory dermatoses ### Atopic dermatitis: barrier, inflammation, itch #### Emollients, irritant avoidance, topical therapy #### Unsupported food avoidance harms nutrition ### Contact dermatitis: irritant or allergic #### Patch testing differs from immediate testing ### Psoriasis: demarcated scaly extensor plaques #### Systemic, cardiometabolic, psychological burden ### Urticaria: transient mast-cell wheals #### Angioedema involves deeper tissue #### Airway or anaphylaxis features need adrenaline ## Skin infection ### Impetigo, folliculitis, abscess ### Cellulitis: usually unilateral spreading infection #### Bilateral red legs often non-infectious ### Necrotising infection: pain out of proportion #### Immediate surgery; imaging must not delay ### Herpes simplex: grouped painful vesicles ### Zoster follows a sensory distribution #### Early antivirals in high-risk sites and hosts ### Dermatophytes and Candida: sample first ### Scabies: manage contacts and environment ## Wounds, ulcers, and pressure injury ### Haemostasis, inflammation, proliferation, remodelling #### Scar never matches uninjured strength ### Infection, ischaemia, oedema, pressure delay healing ### Dressing follows tissue and moisture, not brand ### Venous ulcers: compression after arterial check ### Arterial ulcers: painful, distal, poorly perfused ### Neuropathic ulcers at insensate pressure points ### Pressure and shear compromise microcirculation #### Deep tissue injury before surface breakdown #### Reposition, redistribute, off-load heels #### Healing lesions are not reverse-staged ## Burns ### Depth, area, site, inhalation, mechanism ### Superficial burns painful and blanching ### Deep burns pale, leathery, or insensate ### Circumferential burns impair circulation, ventilation ### Cool with running water, avoid ice and hypothermia ### Major burns: fluids, warmth, specialist transfer ## Drug eruptions and blistering ### Morbilliform eruption: timing, systemic findings ### Eosinophilic systemic drug reaction: organ injury ### Stevens-Johnson syndrome, toxic epidermal necrolysis #### Stop suspected drugs, urgent specialist care ### Autoimmune blistering: antibodies to adhesion #### Biopsy plus perilesional immunofluorescence #### Fluid loss, infection, temperature instability ## Skin tumours ### Basal-cell carcinoma: local invasion, rare spread ### Squamous-cell carcinoma: higher metastatic risk ### Melanoma: dangerous metastatic potential #### Evolution or outlier lesion matters most #### Nodular melanoma symmetric but enlarging #### Excision biopsy, not destructive treatment ### Prevention: shade, clothing, sunscreen ### Appearance cannot exclude malignancy