Are You Confident of the Diagnosis?
What you should be alert for in the history
Pilar or trichilemmal cysts typically present as asymptomatic, cystic to firm dermal or subcutaneous nodules. The most common location is the scalp, although they can develop on other sites as well. Patients usually report a stable size or history of slow growth. If pilar cysts become inflamed due to trauma or rupture, erythema and pain may develop. Multiple pilar cysts are often present.
Characteristic findings on physical examination
On physical exam, pilar cysts are painless, firm and often mobile within the subcutaneous plane. The surface is smooth and no punctum is present. The overlying hair may be thin due to stretching of the skin (Figure 1).
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Figure 1.
Pilar cyst: dome shaped swelling on the scalp with minimal epidermal change
Expected results of diagnostic studies
Diagnosis confirmation
Skin biopsy is usually not necessary to confirm the diagnosis, but is sometimes helpful in differentiating from other nodular growths such as lipomas, epidermoid inclusion cysts, and pilomatrixomas. Biopsies demonstrate cystic structures lined with epithelium showing trichilemmal keratinization, which proceeds without the formation of a granular layer (Figure 2). The cyst is filled with brightly eosinophilic, dense keratin that may be focally calcified or may liquefy with age (Figure 3).
Trichilemmal keratinization, with the transition from squamous epithelium to keratin in the absence of a granular layer (H&E, 200X)
Histologic section showing a well-circumscribed cyst filled with brightly eosinophilic keratin (H&E, 20X)
Figure 2.
Figure 3.
Imaging (CT or MRI) is not commonly performed unless there is concern for a midline scalp developmental lesion or malignant transformation.
Who is at Risk for Developing this Disease?
Pilar cysts occur in 5-10% of the population, with a predilection for adults and women. There is autosomal dominant inheritence of a tendency to develop pilar cysts, and the putative gene (TRICHY1) has been mapped to chromosome 3.
What is the Cause of the Disease?
Etiology
Pathophysiology
Pilar cysts are thought to arise from the outer root sheath of follicular epithelium of anagen hairs. The development of pilar cysts may be partially genetic.
Systemic Implications and Complications
Pilar cysts are not associated with any known systemic syndrome.
Treatment Options
Medical treatment
Intralesional corticosteroids (if inflamed), 0.1 – 1mg.
Surgical treatment
Excision
Optimal Therapeutic Approach for this Disease
Surgical removal is curative, either by conventional linear or elliptical excision or punch excision technique. Recurrence rates are low.
Intralesional steroids may be used if there is rupture and inflammation.
Patient Management
Treatment of pilar cysts is unnecessary if the lesions are asymptomatic and stable in size. If a proliferating pilar tumor or malignant transformation is suspected, complete excision for histologic exam should be performed. If pilar cysts become inflamed or rupture, injection of intralesional corticosteroids, incision and drainage, or excision are options.
Unusual Clinical Scenarios to Consider in Patient Management
Pilar cysts can uncommonly develop into proliferating trichilemmal tumors (PTT) and rarely can show malignant transformation (mPTT). Squamous cell carcinoma can develop as well. mPTT can be locally agressive, and metastases have occurred. Large size, a history of rapid growth, ulceration, and marked atypia or infiltrative growth pattern on histopathologic examination can indicate malignant transformation.
What is the Evidence?
Eiberg, H, Hansen, L, Hansen, C, Mohn, J, Stubbe Teglbjaerg, P, Wilbrandt Kjaer, K. “Mapping of hereditary trichilemmal cyst (TRICHY1) to chromosome 3p24-p21.2 and exclusion of ß-CATENIN and MLH1”. Am J of Med Genetics. vol. 133A. 2005. pp. 44-7. (A study of a Danish family affected with trichilemmal cysts localizing the gene.)
Folpe, AL, Reisenauer, AK, Mentzel, T, Rutten, A, Solomon, AR. “Proliferating trichilemmal tumors: clinicopathologic evaluation is a guide to biologic behavior”. J Cutan Pathol. vol. 30. 2003. pp. 492-8. (A review of five cases of proliferating trichilemmal tumors identifying characteristics that may correlate with aggressive clinical behavior.)
Mehrabi, D, Leonhardt, JM, Brodell, RT. “Removal of keratinous and pilar cysts with the punch incision technique: analysis of surgical outcomes”. Dermatol Surg. vol. 28. 2002. pp. 673-77. (A description of the punch excision technique for cysts, demonstrating a low (5.1%) recurrence rate.0
Nakamura, M, Toyoda, M, Kagoura, M, Higaki, S, Morohashi, M. “Ultrastructural characteristics of trichilemmal cysts: report of two cases”. Med Electron Micros. vol. 34. 2001. pp. 134-41. (Description of the ultrastructure of pilar cysts.)
Pinkus, H. ““Sebaceous cysts” are trichilemmal cysts”. Arch Derm. vol. 99. 1969. pp. 544-55. (One of the original reports of the structure of trichilemmal (pilar) cysts.)
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