Pilar Cyst
(Trichilemmal Cyst)
Pilar cysts are relatively common benign growths that occur most frequently
on the scalp. They go by many names, including wen, trichilemmal cyst, and
isthmuscatagen cyst. Most are solitary, but it is not uncommon to see multiple
pilar cysts in a single individual. Their appearance is similar to that of
epidermal inclusion cysts, but the pathogenesis is completely different. There
is a malignant counterpart called a metastasizing proliferating trichilemmal
cyst. The malignant trans- formation of a pilar cyst is exceedingly rare.
Subsets of these growths are inherited.
Clinical Findings: Pilar cysts occur most frequently on the scalp.
They can be mistaken for epidermal inclusion cysts. The main clinical
differentiating points are that pilar cysts do not have an overlying central
punctum, and they tend to be a bit firmer to touch. These cysts occur more
commonly in adults, and they have a tendency to affect women more often than
men. They typically manifest as slowly growing, firm dermal nodules with no
overlying epidermal changes and no central punctum. These cysts do not drain,
as epidermal cysts sometimes do. They also rarely get inflamed. Almost
exclusively found in the scalp, they are for the most part asymptomatic.
Patients present to the clinician because of an enlarging nodule. As opposed to
the epidermal inclusion cyst, which essentially has no malignant potential, the
pilar cyst does have a small proliferating and malignant potential. This risk
is very low.
Some families show an autosomal
dominant inheritance pattern. The exact gene defect has yet to be determined,
but a possible gene has been mapped to chromosome 3. Most patients with the
hereditary version of this condition have solitary lesions. Numerous lesions
are infrequently encountered in the inherited form.
Pathogenesis: Pilar cysts are also called trichilemmal cysts,
because they are derived from the outer root sheath of the hair follicle, which
undergoes trichilemmal keratinization. This form of keratinization is unique in
that there is no granular layer. The hereditary version of this disease was
originally thought to be caused by a defect in the gene encoding β-catenin.
This has been disproven, and the familial gene has been mapped to the short arm
of chromosome 3, although the exact genetic defect has yet to be elucidated.
These cysts are believed to be derived from the isthmus of anagen-type hairs.
They are formed from deeper elements of the hair shaft apparatus than the
epidermal inclusion cyst are.
Histology: Pilar cysts are composed of compact layers of
stratified squamous epithelium without a granular cell layer. The cysts are
found within the dermis, and the overlying epidermis is unaffected. These cysts
show an absence of intercellular adhesion molecules. The cysts can become
calcified or ossified. The cysts have a unique peripheral rim of keratinocyte
nuclei, which is very helpful in classifying them. The central aspect of the
cyst contains homogenous pale, eosinophilic, compressed keratin.
Treatment: Simple surgical excision is curative. The
recurrence rate is minimal. These cysts typically are removed very easily after
excision through the overlying skin into the cyst wall. The cyst almost always
“pops” out with slight lateral pressure, and only a small incision is needed.
After removal, care needs to be taken to decrease the amount of dead space
left, to avoid seroma formation. This can be prevented by removing some of the
redundant overlying epidermis and suturing the deeper tiss es together to close
the space left by the removed cyst.
