9.4 Common Dermatologic Conditions, Cutaneous Manifestations of Systemic Disease, & Skin Cancers

Severe Cutaneous Adverse Reactions (SCARs) & Dermatologic Emergencies

Severe Cutaneous Adverse Reactions represent life-threatening, immune-mediated drug reactions requiring rapid clinical recognition, immediate drug cessation, and intensive supportive management.

Stevens-Johnson Syndrome (SJS) & Toxic Epidermal Necrolysis (TEN)

SJS and TEN exist on a severity spectrum defined by the percentage of Body Surface Area (BSA) exhibiting epidermal detachment:

• SJS: $<10%$ BSA detachment.
• SJS / TEN Overlap: $10%\text{--}30%$ BSA detachment.
• TEN: $>30%$ BSA detachment.

[Prodromal Phase (1-3 Days)]
  └─ Fever, malaise, arthralgias, upper respiratory symptoms

[Mucocutaneous Eruption Phase]
  ├─ Painful, dusky red macules & targetoid lesions spreading rapidly
  ├─ Flaccid blisters forming sheet-like epidermal sloughing
  ├─ Positive Nikolsky Sign (epidermal detachment induced by gentle lateral pressure)
  └─ Severe Mucosal Involvement (>= 2 sites: oral erosions, conjunctivitis, genital ulceration)

[Emergency Interventions]
  ├─ 1. IMMEDIATELY DISCONTINUE ALL SUSPECTED MEDICATIONS
  ├─ 2. Transfer to Burn Unit or Intensive Care Unit
  ├─ 3. Aggressive Fluid Resuscitation & Wound Management
  └─ 4. Urgent Ophthalmology Consult (prevent ocular scarring & blindness)

Common Drug Triggers

• Allopurinol (most common cause in North America).
• Antiepileptics: Lamotrigine, Carbamazepine, Phenytoin, Phenobarbital.
• Antibiotics: Sulfonamides (Trimethoprim-Sulfamethoxazole), Penicillins, Cephalosporins.
• NSAIDs: Meloxicam, Piroxicam.

---

Autoimmune & Inflammatory Dermatoses

Autoimmune Bullous Diseases

Differentiation between Pemphigus Vulgaris and Bullous Pemphigoid is a classic high-yield topic for medical examinations.

Papulosquamous & Atopic Conditions

• Psoriasis: Chronic inflammatory skin disease presenting with well-demarcated erythematous plaques covered by silvery-white scales on extensor surfaces (elbows, knees, scalp). Features include Auspitz sign (pinpoint bleeding when scale is scraped) and Koebner phenomenon (development of psoriatic lesions at sites of mechanical trauma). Treated with topical corticosteroids + calcipotriol (Vitamin D analog), phototherapy, or systemic TNF/IL-17/IL-23 inhibitors.
• Atopic Dermatitis (Eczema): Pruritic, ill-demarcated erythematous papules and plaques on flexural surfaces (antecubital and popliteal fossae). Associated with personal/family history of atopy (asthma, allergic rhinitis). Managed with frequent emollients, topical corticosteroids, or topical calcineurin inhibitors (Tacrolimus).

---

Cutaneous Manifestations of Systemic Disease

Skin lesions frequently provide the first diagnostic clue to underlying systemic pathology.

Erythema Nodosum

• Clinical Presentation: Painful, tender, warm, erythematous nodules typically located symmetrically on the anterior shins. Histology reveals septal panniculitis without vasculitis.
• Etiologic Causes:
  • Sarcoidosis: Part of Löfgren syndrome (triad of Erythema Nodosum, bilateral hilar lymphadenopathy, and migratory polyarthralgia).
  • Infections: Streptococcal pharyngitis, Tuberculosis, Coccidioides.
  • Inflammatory Bowel Disease: Crohn's disease or Ulcerative Colitis flare.
  • Medications: Oral Contraceptive Pills, Sulfonamides.

Pyoderma Gangrenosum

• Clinical Presentation: Inflammatory, non-infectious ulcerative skin condition initiating as a painful pustule that rapidly expands into an ulcer with violaceous, undermined borders and a purulent base.
• Systemic Associations: Inflammatory Bowel Disease (IBD), Rheumatoid Arthritis, Myeloproliferative disorders.
• Pathergy & Management Rule: Pyoderma gangrenosum exhibits pathergy (exacerbation of ulceration following minor trauma or surgical intervention).
• Exam Trap: Surgical debridement is strictly contraindicated as it causes dramatic enlargement of the ulcer. First-line therapy consists of systemic corticosteroids or cyclosporine.

Acanthosis Nigricans & Dermatomyositis

• Acanthosis Nigricans: Hyperpigmented, velvety hyperkeratotic plaques in flexural folds (axillae, neck). Benign form associated with Insulin Resistance, Obesity, T2DM, and PCOS. Rapid-onset, widespread malignant form associated with Gastrointestinal Adenocarcinoma (gastric cancer).
• Dermatomyositis: Autoimmune myopathy with characteristic cutaneous markers: Heliotrope rash (violaceous eruption over upper eyelids with periorbital edema), Gottron papules (erythematous/violaceous papules over MCP and IP joints), Shawl sign, and V-sign. Requires age-appropriate malignancy screening (ovarian, lung, GI, breast cancer).

---

Cutaneous Oncology

Basal Cell Carcinoma (BCC)

• Most common skin malignancy ($~80%$ of non-melanoma skin cancers).
• Presentation: Pearly, translucent papule or nodule with telangiectasias, central ulceration, and rolled borders on sun-exposed head or neck.
• Behavior & Management: Locally invasive but extremely low metastatic potential. Treated with standard excision or Mohs micrographic surgery (for high-risk facial sites).

Squamous Cell Carcinoma (SCC)

• Second most common skin cancer, arising from epidermal keratinocytes. Precursor lesion is Actinic Keratosis (rough, scaly, sand-paper-like erythematous papule).
• Presentation: Firm, hyperkeratotic, erythematous nodule or plaque with scaling, crusting, or central ulceration.
• Metastatic Potential: Higher risk of metastasis than BCC, especially when located on the lip, ear, immunosuppressed individuals, or arising within chronic scars (Marjolin ulcer). Treated with surgical excision.

Melanoma

Melanoma is the most lethal cutaneous malignancy, arising from melanocytic transformation.

ABCDE Diagnostic Criteria

• Asymmetry: One half of lesion does not match the other.
• Border Irregularity: Scalloped, notched, or blurred borders.
• Color Variation: Mixed shades of brown, black, red, white, or blue.
• Diameter: $>6\text{ mm}$ (size of a pencil eraser).
• Evolving: Changes in size, shape, color, or new onset of bleeding or itching.

Biopsy Rule & Staging

Exam Trap: In suspected melanoma, NEVER perform a partial shave biopsy or punch biopsy through the center of the lesion. Always perform a full-thickness excisional biopsy with $1\text{--}3\text{ mm}$ margins down to subcutaneous fat to preserve micro-architecture for pathological assessment.

• Prognostic Factor: Breslow depth of invasion (measured in millimeters from the epidermal granular layer to the deepest tumor cell) is the single most important determinant of survival and staging.
• Surgical Margins: Wide local excision margins are dictated by Breslow depth ($1\text{ cm}$ margin for depth $<1\text{ mm}$; $2\text{ cm}$ margin for depth $>2\text{ mm}$). Sentinel Lymph Node Biopsy (SLNB) is indicated for lesions with Breslow depth $>0.8\text{--}1.0\text{ mm}$ or ulcerated lesions.