Trichilemmal cyst
A trichilemmal cyst forms from a hair follicle, most often on the scalp. It appears as a smooth, mobile lump filled with keratin. This protein component is found in hair, nails, skin, and horns. The growth remains clinically distinct from other skin conditions. Medical professionals observe that these cysts arise preferentially in areas of high hair follicle concentration. Approximately 90% of cases occur on the scalp. They appear solitary in 30% of instances and multiple in 70% of cases.
Trichilemmal cysts may be classified as sebaceous cysts by some observers. Technically speaking they are not sebaceous. True sebaceous cysts originate from sebaceous glands and contain sebum. These true cysts are relatively rare and known as steatocystoma simplex or steatocystoma multiplex if multiple exist. Medical professionals have suggested that the term sebaceous cyst be avoided since it can be misleading. In practice however the term is still often used for epidermoid and pilar cysts. This naming confusion persists despite expert recommendations to stop using the label.
Histologically trichilemmal cysts are lined by stratified squamous epithelium. This lining lacks a granular cell layer which contrasts with an epidermoid cyst. Solid eosinophilic-staining keratin fills the interior of the growth. Swollen pale keratinocytes increase in height closer to the interior. Small cuboidal dark-staining basal epithelial cells form a palisade arrangement. There is no distinct intercellular bridging between these cells. A fibrous capsule surrounds the entire structure. Areas consistent with proliferation can be found in some cysts.
Rarely these cysts may grow more extensively and form rapidly multiplying tumors. These proliferating trichilemmal cysts are benign but may grow aggressively at the cyst site. The tumor may display nuclear atypia and dyskeratotic cells. Mitotic figures appear within the tissue samples. These features can be misleading during diagnosis. A pathologist might mistakenly render a diagnosis of squamous-cell carcinoma based on visual inspection alone. Very rarely trichilemmal cysts can become cancerous despite their generally benign nature.
Minimal excision is appropriate to treat for some trichilemmal cysts while others require formal surgical excision. Most physicians perform the procedure under local anesthetic. Others prefer a more conservative approach involving a small punch biopsy about one-fourth the diameter of the cyst. The punch biopsy enters the cyst cavity directly. The contents of the cyst are emptied leaving an empty sac. As the pilar cyst wall is the thickest and most durable it can be grabbed with forceps. It pulls out of the small incision easily if large enough. This method works best on cysts larger than a pea that have formed a thick enough wall.
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Common questions
What is a trichilemmal cyst and where does it form?
A trichilemmal cyst forms from a hair follicle, most often on the scalp. Approximately 90% of cases occur on the scalp in areas of high hair follicle concentration.
How can you distinguish a trichilemmal cyst from a sebaceous cyst?
Trichilemmal cysts are not true sebaceous cysts because they originate from hair follicles rather than sebaceous glands. True sebaceous cysts contain sebum and are known as steatocystoma simplex or steatocystoma multiplex if multiple exist.
Why do medical professionals recommend avoiding the term sebaceous cyst for this condition?
Medical professionals have suggested that the term sebaceous cyst be avoided since it can be misleading regarding the origin of the growth. The naming confusion persists despite expert recommendations to stop using the label for epidermoid and pilar cysts.
Can a trichilemmal cyst become cancerous or malignant?
Very rarely trichilemmal cysts can become cancerous despite their generally benign nature. Proliferating trichilemmal tumors may display nuclear atypia and dyskeratotic cells which can lead to a mistaken diagnosis of squamous-cell carcinoma during visual inspection.
What surgical methods treat a trichilemmal cyst effectively?
Minimal excision is appropriate to treat some trichilemmal cysts while others require formal surgical excision under local anesthetic. A small punch biopsy about one-fourth the diameter of the cyst works best on cysts larger than a pea that have formed a thick enough wall.