Epidermoid cyst
- Etiology & Risk Factors:
- Benign follicular infundibulum cyst containing foul-smelling keratinaceous debris (not sebum).
- Associated with Gardner syndrome (FAP variant) if multiple, unusual locations, or pediatric onset.
- Clinical Features:
- Discrete, mobile, dome-shaped, flesh-colored subcutaneous nodule (face, neck, trunk).
- Pathognomonic hallmark: Central punctum (comedo-like pore).
- Rupture: Rapid erythema, pain, and swelling (sterile foreign-body reaction mimicking an abscess).
- Diagnosis:
- Clinical; routine labs and imaging are unnecessary.
- Histopathology (post-excision): Cyst wall lined by stratified squamous epithelium with a granular cell layer; lumen filled with laminated keratin.
- High-Yield Differentials:
- Lipoma: Soft, rubbery, lobulated; deeper; lacks a central punctum.
- Pilar (Trichilemmal) Cyst: Located almost exclusively on the scalp; lacks a punctum; lacks a granular cell layer on histology.
- Dermatofibroma: Hyperpigmented nodule (typically lower extremities); (+) dimple sign on lateral compression.
- Management:
- Asymptomatic: Reassurance/observation vs. elective complete surgical excision (cyst capsule/wall must be removed intact to prevent recurrence).
- Acute Inflamed/Ruptured: Intralesional corticosteroid injection or I&D; defer definitive excision until inflammation resolves.
- Infected: I&D; systemic Abx (anti-staphylococcal) reserved only for spreading cellulitis or systemic signs.
Dermatofibroma
- Etiology & Epidemiology:
- Benign fibrohistiocytic proliferation; most common in adult females (F > M).
- Reactive response to minor trauma (e.g., insect bites, folliculitis, shaving nicks).
- Location: Most common on lower extremities (anterior legs).
- Clinical Presentation:
- Discrete, firm, nontender, hyperpigmented/pink-brown papule or nodule (< 1 cm).
- Dimple sign (Pinch sign): Lateral compression causes central depression/retraction downward (pathognomonic; due to dermal tethering).
- Diagnosis:
- Clinical diagnosis: Typical lesion + positive dimple sign requires no workup.
- Dermoscopy: Central white scar-like area with a delicate peripheral pigment network.
- Biopsy (indicated only if atypical, changing, or ruling out malignancy):
- Shows non-encapsulated spindle cells in dermis with collagen trapping at periphery.
- Differential Diagnosis:
- Melanoma: Lacks dimple sign; exhibits ABCDE features.
- Dermatofibrosarcoma Protuberans (DFSP): Indurated, growing plaque/mass (> 2 cm); locally aggressive; invades subcutaneous fat.
- Basal Cell Carcinoma (pigmented): Pearly papule with telangiectasias and rolled borders.
- Management:
- Reassurance / Observation: Best next step (benign, zero malignant potential).
- Complete surgical excision: Only if symptomatic (pruritus, pain), irritated by shaving, or cosmetically requested (warn pt about surgical scarring).