What is a Trichilemmal Cyst?
A trichilemmal cyst (also called a pilar cyst) is a benign skin tumor that originates from the outer root sheath of hair follicles, most commonly occurring on the scalp (90% of cases), presenting as a firm, mobile, subcutaneous nodule filled with cheese-like keratinous material 1, 2.
Key Clinical Characteristics
Location and Appearance:
- Predominantly affects the scalp, though can rarely occur on the neck, trunk, groin, thigh, or inframammary regions 3, 4, 5
- Presents as a firm, mobile, subcutaneous lump that is typically painless unless infected
- Contains dense, cheese-like white keratinous material with smooth, shiny inner and outer cyst walls 2
- Female predominance (approximately 59% of cases) 1
Natural History:
- Slow-growing lesions with median duration from first awareness to excision of 18 months 1
- Median size at presentation is 21 mm, though can become quite large (giant variants reported) 1, 3
- Usually benign, though malignant transformation can rarely occur 1
Risk Factors for Larger Lesions and Complications
Age and comorbidities significantly influence cyst behavior:
- Older patients tend to have multiple lesions and larger tumors (median age 54 years for multiple lesions vs. 38.5 years for single lesions) 1
- Diabetes mellitus independently predicts larger lesion size and shows trends toward increased infection risk 1
- Hypertension paradoxically shows negative association with lesion size but may correlate with infection 1
Pathological Features
The cyst wall consists of:
- Epidermal tissue with intact spinous and basal layers
- Absence of granular layer (key distinguishing feature)
- Dense protein content within the cyst cavity 2
- Rare variants may show ossification or divergent ductal differentiation (sweat duct features in <1% of cases) 6, 7
Clinical Management
Definitive treatment requires complete surgical excision:
- Simple incision and drainage is insufficient and leads to recurrence
- Total excision provides excellent prognosis with minimal recurrence 2
- Multiple or large lesions may require extensive scalp excision with flap reconstruction 7
- Infected cysts require the same surgical approach as other cutaneous abscesses: incision, thorough evacuation, and probing to break up loculations 8
Important clinical pitfall: These cysts are often mislabeled as "sebaceous cysts" (which are actually epidermoid cysts), but trichilemmal cysts have distinct histological features and originate from different structures 8.