A 64-year-old woman presents with a 6-month history of a slowly enlarging bump near her left eyebrow. The patient says it appeared suddenly and became a nuisance, as well as cosmetically unacceptable as it enlarged. She denies any pain, tenderness, or oozing, and denies any other similar lesions elsewhere. Her pertinent dermatologic medical and family history are unremarkable. Physical examination reveals a 6-mm nontender subcutaneous mobile nodule on the left lateral eyebrow. There is no overlying ulceration or central punctum, and no discharge is noted. No other suspicious lesions are observed on examination of the scalp and face, aside from mild actinic damage.
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Pilar cyst, also known as trichilemmal or isthmus-catagen cyst, is a relatively common and mostly benign entity. Approximately 90% of these lesions are found on the scalp,1 an area with an obviously high concentration of hair follicles; infrequently, they develop elsewhere, including on the face, trunk, and extremities.
Frequency in the general population is approximately 5% to 10%, and lesions mostly occur in the middle-aged years. Occurrence is somewhat more frequent in women but equal across racial populations. Although pilar cysts often arise sporadically, there is a known genetic component of the autosomal dominant inheritance type and therefore, this condition is often found in many family members. The cysts can occur as a single lesion or multiple lesions can be present simultaneously.
Pilar cysts arise from the outer hair follicle sheath and fill with keratin and its breakdown products. The cyst wall is a grayish white color and is often very thick and resistant to rupture. These cysts tend to grow slowly until they become stable in size, at which time they persist indefinitely.
A proliferating trichilemmal cyst is a benign yet aggressive variant of the common pilar cyst.2 It occurs when a cyst grows aggressively at its original site, leading to very large exophytic nodules that can ulcerate. Rare cases of malignant transformation with these lesions have been reported.1
Diagnosis is typically clinically straightforward when the lesion appears on the scalp, particularly if numerous lesions are present. Pilar cysts can be differentiated from epidermal inclusion cysts by considering the location of the lesion and noting the absence of a central punctum and the presence of a thick, durable cyst wall.3 Other common conditions to consider in the differential diagnoses and to rule out include pilomatricoma, squamous cell carcinoma, nodulocystic basal cell carcinoma, Merkel cell carcinoma, and lipoma.
If the diagnosis in uncertain, a skin biopsy with hematoxylin and eosin stain can aid in the diagnosis. Stratified squamous epithelium is observed on the outer layer of the cyst wall and lacks the granular layer that is typically seen in an epidermal inclusion cyst. Densely packed eosinophilic-staining keratin debris fills the cyst and occasionally, focal calcification is observed. Observation of a pilar cyst with cyst wall rupture reveals a foreign body giant cell reaction.4 It should be noted that lesions located on the midline of the scalp or back should be radiographed prior to any invasive diagnostic procedure in order to rule out intracranial or intraspinal extension.
Patients often seek consultation for removal because the lesions are functionally or cosmetically bothersome. Some are large enough to interfere with helmet or hat use. Others may become intermittently inflamed and tender and occasionally prone to superinfection.
Treatment options are primarily surgical in nature and best considered when lesions are not inflamed or ruptured. Incision and drainage typically only provides temporary relief. If the cyst wall is not removed, the lesion will likely recur. For this reason, surgical excision with removal of the intact cyst is the most common treatment. This is usually performed in an outpatient setting with the use of local anesthesia. A skilled surgeon can minimize trauma and scarring by using a smaller punch biopsy to empty the cyst contents first and then remove the remaining cyst wall.5
As the patient in our case presented with a cystic nodule in a location that does not typically present with benign cystic lesions, a skin biopsy was necessary to confirm the correct diagnosis and rule out malignancy. A 3-mm punch biopsy confirmed the diagnosis of pilar cyst. The patient was educated regarding its benign nature and the treatment options. Because the lesion was cosmetically very bothersome to the patient, she requested surgical excision, which was successfully performed without complications. Three months later, the patient returned for a routine follow-up visit and was satisfied with the cosmetic outcome. There were no signs of recurrence of the lesion.
Esther Stern, NP, is a family nurse practitioner at Advanced Dermatology & Skin Surgery, P.C., in Lakewood, N.J.
- Epidermal nevi, neoplasms, and cysts. In: James WD, Berger TG, Elston DM, eds. Andrews’ Diseases of the Skin: Clinical Dermatology. 12th ed. Philadelphia, PA: Elsevier; 2015.
- Hendricks DL, Liang MD, Borochovitz D, Miller T. A case of multiple pilar tumors and pilar cysts involving the scalp and back. Plast Reconstr Surg. 1991;87(4):763-767.
- McGavran MH, Binnington B. Keratinous cysts of the skin. Identification and differentiation of pilar cysts from epidermal cysts. Arch Dermatol. 1966;94(4):499-508.
- Stone MS. Cysts. In: Bolognia JL, Jorizzo JL, Schaffer JV, eds. Dermatology. 3rd ed. St. Louis, MO: Elsevier Saunders; 2012;1820-1821.
- Mehrabi D, Leonhardt JM, Brodell RT. Removal of keratinous and pilar cysts with the punch incision technique: analysis of surgical outcomes. Dermatol Surg. 2002;28(8):673-677.