trichilemmal cyst (pilar cyst, isthmus catagen cyst)

Introduction

2nd most common cutaneous cyst (after epidermoid cyst) generally occurring on the scalp in middle-aged individuals.

Epidemiology

• middle-aged individuals
• more common in females
• often familial
• occurs in ~5-10% of the population
• most common cutaneous skin on the scalp

Pathology

• cyst is not connected to epidermis
• cyst wall
  • thick & can be removed intact
  • stratified squamous epithelium
  • palisading outer layer resembling outer root sheath of hair follicle
  • corrugated inner layer without a granular layer
• cyst contains keratin & its catabolites which appears pink & homogeneous
• cysts often calcify & form cholesterol clefts
• almost always benign
• malignant transformation extremely rare

Genetics

• sporadic
• autosomal dominant

Clinical manifestations

• firm, dome-shaped 0.5-5 cm nodules or tumors
• no central punctum as seen in epidermoid cysts
• frequently occurs as multiple lesions
• > 90% are on scalp
• hair overlying cyst is generally normal, but may be thinned
• cyst may become inflammed & painful if ruptured

Laboratory

• bacteria may be cultured from cyst, but appear to be incidental, unrelated to inflammation

Management

• excision & drainage of inflamed cysts
• intralesional injection with triamcinolone
• in the absence of inflammation, cysts may be excised

More general terms

• cyst
• scalp disorder

Additional terms

• epidermoid cyst (wen, infundibular cyst)
• punctum