Key Clinical Formulas
Body Surface Area (BSA) — Du Bois:
BSA (m²) = 0.007184 × height(cm)0.725 × weight(kg)0.425
Used for chemotherapy dosing, burn assessment, and drug dosing
BSA (m²) = 0.007184 × height(cm)0.725 × weight(kg)0.425
Used for chemotherapy dosing, burn assessment, and drug dosing
Rule of Nines (Burn Area):
Head = 9%, each arm = 9%, anterior torso = 18%, posterior torso = 18%, each leg = 18%, groin = 1%
Total = 100% — guides fluid resuscitation
Head = 9%, each arm = 9%, anterior torso = 18%, posterior torso = 18%, each leg = 18%, groin = 1%
Total = 100% — guides fluid resuscitation
Parkland Formula (Burn Fluid):
Fluid (mL) = 4 × weight(kg) × %TBSA burned
Half given in first 8 hours, half in next 16 hours (LR solution)
Fluid (mL) = 4 × weight(kg) × %TBSA burned
Half given in first 8 hours, half in next 16 hours (LR solution)
PASI Score (Psoriasis Area Severity Index):
PASI = 0.1(Eh+Ih+Dh)Ah + 0.2(...torso) + 0.3(...legs) + 0.4(...head)
E=erythema, I=induration, D=desquamation, A=area; 0-72 scale
PASI = 0.1(Eh+Ih+Dh)Ah + 0.2(...torso) + 0.3(...legs) + 0.4(...head)
E=erythema, I=induration, D=desquamation, A=area; 0-72 scale
ABCDE Melanoma Detection:
Asymmetry — one half ≠ other half
Border — irregular, ragged edges
Color — varied shades within lesion
Diameter — >6 mm (pencil eraser)
Evolving — changing in size, shape, color
Asymmetry — one half ≠ other half
Border — irregular, ragged edges
Color — varied shades within lesion
Diameter — >6 mm (pencil eraser)
Evolving — changing in size, shape, color
Core Concepts
1. Skin Layers
Epidermis (outermost, keratinocytes, melanocytes, Langerhans cells) → Dermis (collagen, elastin, blood vessels, nerves, hair follicles) → Hypodermis (subcutaneous fat). Each layer has distinct pathology.
Epidermis (outermost, keratinocytes, melanocytes, Langerhans cells) → Dermis (collagen, elastin, blood vessels, nerves, hair follicles) → Hypodermis (subcutaneous fat). Each layer has distinct pathology.
2. Inflammatory Skin Conditions
Atopic dermatitis (eczema): pruritic, erythematous patches; Type I hypersensitivity. Psoriasis: well-demarcated scaly plaques; T-cell mediated, hyperproliferation (cell turnover 3-5 days vs 28).
Atopic dermatitis (eczema): pruritic, erythematous patches; Type I hypersensitivity. Psoriasis: well-demarcated scaly plaques; T-cell mediated, hyperproliferation (cell turnover 3-5 days vs 28).
3. Skin Cancer Types
Basal cell carcinoma (most common, rarely metastasizes, pearly papule). Squamous cell carcinoma (scaly nodule, can metastasize). Melanoma (most deadly, arises from melanocytes, ABCDE criteria).
Basal cell carcinoma (most common, rarely metastasizes, pearly papule). Squamous cell carcinoma (scaly nodule, can metastasize). Melanoma (most deadly, arises from melanocytes, ABCDE criteria).
4. Infectious Diseases
Bacterial (impetigo — Staph/Strep; cellulitis — deeper infection). Viral (herpes simplex, varicella-zoster, warts — HPV). Fungal (tinea corporis, candidiasis). Parasitic (scabies, lice).
Bacterial (impetigo — Staph/Strep; cellulitis — deeper infection). Viral (herpes simplex, varicella-zoster, warts — HPV). Fungal (tinea corporis, candidiasis). Parasitic (scabies, lice).
5. Autoimmune Skin Diseases
Lupus (malar rash, photosensitivity). Bullous pemphigoid (tense bullae, older adults). Pemphigus vulgaris (flaccid blouses, positive Nikolsky sign). Dermatomyositis (heliotrope rash, Gottron papules).
Lupus (malar rash, photosensitivity). Bullous pemphigoid (tense bullae, older adults). Pemphigus vulgaris (flaccid blouses, positive Nikolsky sign). Dermatomyositis (heliotrope rash, Gottron papules).
Core Definitions
Pruritus: Itching — the most common dermatologic symptom. Mechanisms: histamine, opioids, serotonin, interleukins. Can be localized or generalized; may indicate systemic disease.
Erythema: Redness of skin due to vasodilation and increased blood flow. Blanches with pressure (diascopy). Seen in inflammation, infection, and allergic reactions.
Desquamation: Shedding of the stratum corneum (outermost epidermal layer). Normal process (invisible) or pathological (visible peeling as in scarlet fever, Kawasaki disease).
Bulla (Bullae): A fluid-filled blister >5 mm in diameter. Tense bullae: bullous pemphigoid. Flaccid blisters: pemphigus vulgaris. Both are autoimmune conditions.
Nikolsky Sign: Gentle lateral pressure on skin causes separation of epidermis. Positive in pemphigus vulgaris and toxic epidermal necrolysis. Indicates loss of epidermal adhesion.
Worked Examples
Example 1: Rule of Nines
A patient has burns on anterior torso (18%) and both arms (9% + 9%): Total = 18 + 18 = 36% TBSA. Using Parkland: 4 × 70kg × 36 = 10,080 mL. First 8 hours: 5,040 mL. Next 16 hours: 5,040 mL.
A patient has burns on anterior torso (18%) and both arms (9% + 9%): Total = 18 + 18 = 36% TBSA. Using Parkland: 4 × 70kg × 36 = 10,080 mL. First 8 hours: 5,040 mL. Next 16 hours: 5,040 mL.
Example 2: ABCDE Assessment
A mole is asymmetric, has irregular borders, shows 3 shades of brown, measures 8 mm, and has grown over 3 months. This meets all 5 ABCDE criteria — high suspicion for melanoma, requires urgent biopsy.
A mole is asymmetric, has irregular borders, shows 3 shades of brown, measures 8 mm, and has grown over 3 months. This meets all 5 ABCDE criteria — high suspicion for melanoma, requires urgent biopsy.
Example 3: BSA Calculation
Patient: 170 cm, 70 kg. BSA = 0.007184 × 1700.725 × 700.425 = 0.007184 × 49.8 × 6.06 = 2.17 m². Used for chemotherapy dosing.
Patient: 170 cm, 70 kg. BSA = 0.007184 × 1700.725 × 700.425 = 0.007184 × 49.8 × 6.06 = 2.17 m². Used for chemotherapy dosing.
Practice Problems
Problem 1: Calculate BSA for a patient 180 cm tall weighing 80 kg.
Answer: BSA = 0.007184 × 1800.725 × 800.425 = 0.007184 × 53.7 × 6.49 = 2.50 m²
Answer: BSA = 0.007184 × 1800.725 × 800.425 = 0.007184 × 53.7 × 6.49 = 2.50 m²
Problem 2: A burn patient (80 kg) has 25% TBSA burned. Calculate Parkland fluid requirements.
Answer: 4 × 80 × 25 = 8,000 mL. First 8 hrs: 4,000 mL. Next 16 hrs: 4,000 mL.
Answer: 4 × 80 × 25 = 8,000 mL. First 8 hrs: 4,000 mL. Next 16 hrs: 4,000 mL.
Problem 3: What are the ABCDE criteria for melanoma detection?
Answer: Asymmetry, Border irregularity, Color variation, Diameter >6mm, Evolving (changing). Meeting any 2+ criteria warrants biopsy.
Answer: Asymmetry, Border irregularity, Color variation, Diameter >6mm, Evolving (changing). Meeting any 2+ criteria warrants biopsy.
Problem 4: What is the most common type of skin cancer?
Answer: Basal cell carcinoma (BCC). Appears as pearly papule or nodule with telangiectasias. Rarely metastasizes but can be locally destructive.
Answer: Basal cell carcinoma (BCC). Appears as pearly papule or nodule with telangiectasias. Rarely metastasizes but can be locally destructive.
Problem 5: What does a positive Nikolsky sign indicate?
Answer: Epidermal detachment with lateral pressure. Indicates pemphigus vulgaris or toxic epidermal necrolysis — both life-threatening blistering conditions.
Answer: Epidermal detachment with lateral pressure. Indicates pemphigus vulgaris or toxic epidermal necrolysis — both life-threatening blistering conditions.