Trichilemmal cyst

(Ref. Pathology of the skin, PH McKee, 3rd Ed. Elsevier-Mosby)

The outer root sheath of the hair follicle at the level of the follicular isthmus is reproduced in the wall of trichilemmal (pilar) cysts. Their origin is unknown, but it bas been suggested that they are produced by budding off from the external root sheath as a genetically determined structural aberration. Familial occurrence is seen in 75% of patients, in a pattern suggesting autosomal dominant inheritance.

 

Clinical features

Trichilemmal cysts are found on the scalp in 90% of cases; they are solitary in 30% and multiple in 70%. They present as smooth, yellowish, dome-shaped intradermal swellings and are more common in females. In contrast to epidermoid cysts, they are characteristically devoid of a punctum. It should be noted that the term sebaceous cyst favored by many clinicians is a misnomer because such lesions represent either epidermoid or trichilemmal cysts. Typically, the cyst is encapsulated and uncomplicated lesions readily shell out at surgery. Acute inflammation is uncommon and when it does occur is of non-bacterial origin; its presence makes excision more difficult, with an increased likelihood of rupture.

 

Histological features

The cyst is surrounded by a fibrous capsule against which rests a layer(s) of small dark-staining basal cells. These merge with characteristic squamous epithelium composed of pale keratinocytes, which increase in height as they mature and transform abruptly into solid eosinophilic-staining keratin without forming a granular cell layer. Occa­sionally small foci of epidermal keratinization (with a granular cell layer) may also be identified. Calcification occurs in 25% of lesions, regardless of the age or size of the cyst, and cholesterol clefts occur in 90%. Secondary inflammation is manifest as an influx of inflammatory cells into the lumen of the cyst, in contrast to the granulomatous response that may surround an epidermoid cyst. In a small percentage of cases there is budding of tiny daughter cysts from the parent. Very rarely, sebaceous and apocrine differentiations are found in the cyst wall.