What is a Pilar (Trichilemmal) Cyst?
A pilar cyst, also known as a trichilemmal cyst, is a benign skin lesion that originates from the outer root sheath of the hair follicle and is characterized by a unique type of keratinization without a granular layer. 1, 2
Clinical Characteristics
Location and Demographics
- Pilar cysts occur most commonly on the scalp (90% of cases), presenting as firm, mobile nodules that can be solitary or multiple 1, 2, 3
- These lesions show a female predominance and can occur at any age, though they are often discovered in middle-aged adults 2, 3
- Multiple pilar cysts can occur in a familial pattern, suggesting a genetic component in some cases 3
Physical Appearance
- The cysts present as subcutaneous, dome-shaped nodules that are typically smooth, firm, and freely movable within the dermis 1
- Size ranges from 1-3 cm on average, though they can gradually enlarge over years 1
- The cyst contents are characteristically cheese-like white material with smooth, shiny inner and outer walls 1
Histopathological Features
Distinguishing Characteristics
- The cyst wall consists of epidermal tissue with intact spinous and basal layers but notably lacks a granular layer, which distinguishes it from epidermoid cysts 1, 2
- Trichilemmal keratinization is the hallmark feature, with dense protein accumulation within the cyst cavity 1, 2
- This unique keratinization pattern mimics the outer root sheath of hair follicles 2, 3
Clinical Pitfall
- Many pilar cysts are clinically misdiagnosed as "sebaceous cysts", and their true nature is only revealed on histopathological examination 2
- Recognition of the specific keratinization pattern is essential to avoid misdiagnosis 2
Clinical Variants and Complications
Benign Forms
- Simple pilar cysts represent the vast majority (>94%) and are entirely benign 2
- Calcified trichilemmal cysts can occur, particularly in multicentric distributions 3
Proliferating and Malignant Forms
- Proliferating trichilemmal cysts (PTCs) are rare variants that show increased cellularity and mitotic activity but remain largely benign 2, 4
- Malignant pilar tumors are exceedingly rare but carry an aggressive clinical course with potential for nodal and distant metastases 5, 4
- PTCs have high recurrence rates and potential for local invasion, requiring careful histological analysis 4
Management
Definitive Treatment
- Complete surgical excision is the standard and curative treatment for simple pilar cysts 1, 2
- The prognosis after total excision is excellent, with minimal recurrence risk for benign lesions 1
Follow-up Considerations
- Routine follow-up at 1 year is appropriate to confirm no recurrence after excision of simple cysts 1
- For proliferating or atypical variants, more aggressive surgical treatment with wider margins and closer surveillance is warranted due to recurrence potential 5, 4
- Adjuvant radiotherapy may be considered for malignant variants, particularly when nodal metastases are present 5
Association with Cowden Syndrome
Important Distinction
- Trichilemmomas (benign tumors of the outer root sheath) should not be confused with trichilemmal cysts 6
- The presence of 2 or more trichilemmomas is considered pathognomonic for Cowden syndrome, a hereditary cancer predisposition syndrome associated with PTEN mutations 6
- This association requires histologic confirmation of trichilemmoma diagnosis, as clinical distinction from other mucocutaneous lesions is difficult 6