Trichilemmal or pilar cysts

                              Dr. KS Dhillon

                           

Introduction

Trichilemmal or pilar cysts are common intradermal or subcutaneous cysts. They occur in 5-10% of the population [1,2]. More than 90% of the cysts occur on the scalp, where trichilemmal cysts are the most common cutaneous cysts [3,4]. Trichilemmal cysts are usually benign. They can be sporadic or they may be autosomal dominantly inherited [5,6]. These cysts contain keratin and are outlined by stratified squamous epithelium similar to what we see in the outer(external) root sheath of the hair follicle.

In 2% of trichilemmal cysts, single or multiple foci of proliferating cells produce tumors which are called proliferating trichilemmal cysts [7]. Proliferating trichilemmal cysts gradually enlarge up to 25 cm in diameter. The exophytic nodules occasionally ulcerate [3]. Although these are biologically benign tumors, they may be locally aggressive. Recurrences and metastases of these tumors have been observed, with rare malignant transformation [6,7,8,9].

Pathophysiology

Trichilemmal cysts are often erroneously called sebaceous cysts. Trichilemmal cysts are lined by stratified squamous epithelium similar to that seen in the isthmus of the hair follicle [6]. This is the segment between the insertion of the erector pili muscle and the sebaceous gland duct. There is no inner root sheath here. The keratinization is the same as that which occurs in the outer root sheath [10]. The squamous epithelium undergoes “trichilemmal keratinization” or rapid keratin formation without a granular cell layer. This results in a cyst wall without a granular cell layer [3,10].

Etiology

If the trichilemmal cysts are hereditary, the inheritance pattern is usually autosomal dominant [5,6]. The hereditary trichilemmal cysts link to the short arm of chromosome 3 [11]. Patients with familial pilar cysts are often younger and present with multiple lesions at the same time.

Epidemiology

Trichilemmal cysts are common. They occur in 5-10% of the population [1].

Trichilemmal cysts have no known racial predilection. Trichilemmal cysts occur more often in women than in men [1]. Trichilemmal cysts are more common in persons of middle age than in younger persons [1]. Family history may be present in some cases since the disease follows autosomal dominant inheritance.

Prognosis

Trichilemmal cysts are biologically benign. They can, however, be locally aggressive [12]. Malignant transformation is very rare. The malignant transformation can lead to distant metastases [6,7].

There are case reports that have described Merkel cell carcinoma arising from Merkel cells in trichilemmal cysts [13,14].

Clinical presentation

Trichilemmal cysts usually occur in areas with dense hair follicle concentrations. Ninety percent occur on the scalp [3]. Sometimes they can be seen on the neck, face, trunk, and extremities [1,3,15].

Seventy percent of trichilemmal cysts are multiple and 30% are solitary [10]. Trichilemmal cysts can be swollen, tender, and red if they have ruptured or become infected.

The condition can have an autosomal dominant pattern of inheritance, hence a family history may be present [5,6]. The clinical criteria for recognizing autosomal dominant hereditary cases include [16]:

• The diagnosis of trichilemmal cyst in 2 or more first- or second-degree relatives 

• Age of diagnosis younger than 45 years

• Diagnosis of multiple or large (>5 cm) cysts

• Rare histologic features such as proliferating and ossifying cysts

A proliferating trichilemmal cyst usually presents as a slow-growing nodule [6]. They are more common in women. They occur at a mean age of 65 years [3].

Rapid growth usually does not occur. It may be a sign of infection or malignancy [1]. Other features that are suspicious include a size larger than 5 cm, a non-scalp location, and an infiltrative growth pattern [3].

Trichilemmal cysts present as smooth, mobile, skin-colored, firm, and well-circumscribed nodules [1,6]. Ninety percent of trichilemmal cysts occur on the head. About 70% occur as multiple lesions [3,10]. These cysts do not have a punctum [3].

If inflamed the cysts may be tender, erythematous, or both. Occasionally the contents may extrude to form a soft, cutaneous horn. The wall of a marsupialized cyst can fuse with the overlying epidermis to form a crypt. 

The cyst may sometimes discharge its contents and then self-resolve.

Complications

Cyst infection and rupture can occur [1]. Injury to the cysts can occur when combing or brushing hair. Trichilemmal cysts that are proliferating can invade surrounding structures and ulcerate [1]. Malignant transformation is very rare but can occur [1,17].

Differential Diagnoses

The differential diagnosis for trichilemmal cyst includes:

• Acne Keloidalis Nuchae (AKN)

• Pilomatrixoma

• Steatocystoma Multiplex

• Cutaneous Lipomas

• Dermoid Cyst

• Epidermal Inclusion Cyst

• Favre-Racouchot Syndrome (Nodular Elastosis With Cysts and Comedones)

Imaging Studies

Diagnosis of trichilemmal cysts is mainly clinical, based on symptoms and signs, and additional investigations are rarely needed. Radiological studies are sometimes needed to exclude other differentials, especially with midline head and neck lesions, and to check for the extent of the lesion and the involvement of the underlying central nervous system. CT scan is useful to determine the extent of bone invasion. MRI can be used for deeper soft tissue involvement and to visualize very tiny invasions.

Procedures

Excision of the cyst and histopathologic examination can confirm the diagnosis. Trichilemmal cysts can usually be removed more easily than epidermoid cysts [3].

If the cyst becomes inflamed and ruptures, the cyst is excised and submitted for histopathology to exclude carcinoma, particularly nodular or nodulocystic basal cell carcinomas [19]. Inflamed, ruptured cysts are usually infected. A wound culture should be taken to elucidate the infection and guide therapy [1].

Histologic Findings

Trichilemmal cysts are surrounded by fibrous capsules. Layers of small cuboidal, dark-staining basal epithelial cells in a palisade arrangement with no distinct intercellular bridging rest against the fibrous capsules. These merge with squamous epithelium composed of swollen pale keratinocytes. They increase in height as they mature and transform abruptly into solid eosinophilic-staining keratin without the formation of a granular cell layer. Often they have foci of calcification [10]. 

The trichilemmal cyst keratin stains with antikeratin antibodies derived from human hair [20]. Very rarely, apocrine or sebaceous differentiation occurs in the cyst wall [21]. Calcification can be seen, and there are a few reports that have described ossification [22,23]. If the cyst wall ruptures, a foreign body giant cell reaction may surround the cyst.

Proliferating trichilemmal cysts have lobules of squamous epithelium in the cyst wall [1]. Other histologic characteristics of proliferating trichilemmal cysts can include cell atypia, mitoses, and necrosis. Tumor cell penetration through the cyst lining suggests malignancy [3].

Treatment

Swollen, painful, erythematous, and/or purulent cysts usually indicate the presence of infection. In such cases, a wound culture will help to direct therapy [1].

The definitive treatment is complete excision of the cyst. There are several methods that can be used to surgically remove these cysts. An elliptical excision, a small linear incision, and a circular dermal punch incision can be used to remove the cysts [24].

The surgical procedure involves anesthetizing the area with a local anesthetic. Over the center of the cyst, a small linear incision, an elliptical excision, or a punch incision is made using a dermal punch biopsy tool.

If the fibrous capsules of trichilemmal cysts are thick enough then the cyst can be removed intact via blunt dissection. Otherwise, the contents of the cyst are expressed, and then using a curette, the outer side of the cyst is dislodged from the surrounding tissue. The edge of the cyst is held with forceps and the cyst wall is separated from the surrounding connective tissue via blunt dissection.

Bleeding is controlled and the wound is sutured or dressed as necessary.

If the cyst is ruptured or infected, excision is deferred until the inflammation is reduced to prevent the likelihood of spreading the infection and wound healing problems [1].

Most proliferating trichilemmal cysts are cured with complete surgical excision [3]. Very occasionally multiple proliferating trichilemmal cysts require several local excisions [8]. Sometimes additional radiotherapy and/or chemotherapy may be needed [9].

Conclusion

Assessment of any patients with trichilemmal cysts begins with taking a thorough history and careful physical examination which is usually enough to reach the diagnosis in most cases. Rarely, a biopsy of the lesion is required for the diagnosis. Trichilemmal cysts are benign lesions. They might transform into malignancy on rare occasions. The head and neck are the commonest sites to be affected. Recurrences and metastases have been seen in some patients. If the sac is not completely removed the recurrence rate can be high. Merkel cell carcinoma can sometimes arise from pilar cysts [6].

References

1. Cuda JD, Rangwala S, Taube JM. Benign Epithelial Tumors, Hamartomas, and Hyperplasias. Kang S, Amagai M, Bruckner AL, et al, eds. Fitzpatrick's Dermatology in General Medicine. 9th ed. New York, NY: McGraw-Hill; 2019.

2. Albasri AM, Ansari IA. The histopathological pattern of benign and non-neoplastic skin diseases at King Fahad Hospital, Madinah, Saudi Arabia. Saudi Med J. 2019 Jun. 40 (6):548-554. 

3. James WD, Elston DM,Treat JR, Rosenbach MA, Neuhaus IM. Epidermal Nevi, Neoplasms and Cysts. James WD, Elston DM,Treat JR, Rosenbach MA, Neuhaus IM, eds. Andrews’ Diseases of the Skin: Clinical Dermatology. 13th ed. Elsevier; 2020. 636-685.

4. Al-Khateeb TH, Al-Masri NM, Al-Zoubi F. Cutaneous cysts of the head and neck. J Oral Maxillofac Surg. Jan 2009. 67(1):52-7. 

5. Leppard BJ, Sanderson KV, Wells RS. Hereditary trichilemmal cysts. Hereditary pilar cysts. Clin Exp Dermatol. 1977 Mar. 2(1):23-32. 

6. Stone MS. Cysts. Bolognia JL, Schaffer JV, Cerroni L, eds. Dermatology. 4th ed. Elsevier; 2017. Section 18. Neoplasias of the Skin: 1920-21.

7. Weiss J, Heine M, Grimmel M, Jung EG. Malignant proliferating trichilemmal cyst. J Am Acad Dermatol. May 1995. 32(5 Pt 2):870-3. 

8. Ibrahim AE, Barikian A, Janom H, Kaddoura I. Numerous recurrent trichilemmal cysts of the scalp: differential diagnosis and surgical management. J Craniofac Surg. Mar 2012. 23(2):e164-8. 

9. Satyaprakash AK, Sheehan DJ, Sangueza OP. Proliferating trichilemmal tumors: a review of the literature. Dermatol Surg. 2007 Sep. 33(9):1102-8.

10. Kirkham N. Tumors and Cysts of the Epidermis. Elder DE, Elenitsas R, Johnson BL, Murphy GF, Xu X, eds. Lever's Histopathology of the Skin. 10th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2009. 801-3.

11. Eiberg H, Hansen L, Hansen C, Mohr J, Teglbjaerg PS, Kjaer KW. Mapping of hereditary trichilemmal cyst (TRICY1) to chromosome 3p24-p21.2 and exclusion of beta-CATENIN and MLH1. Am J Med Genet A. 2005 Feb 15. 133A (1):44-7. 

12. Valerio E, Parro FHS, Macedo MP, Pinto CAL. Proliferating trichilemmal cyst with clinical, radiological, macroscopic, and microscopic correlation. An Bras Dermatol. 2019. 94 (4):452-454. 

13. Ivan D, Bengana C, Lazar AJ, Diwan AH, Prieto VG. Merkel cell tumor in a trichilemmal cyst: collision or association?. Am J Dermatopathol. 2007 Apr. 29(2):180-3. 

14. Su W, Kheir SM, Berberian B, Cockerell CJ. Merkel cell carcinoma in situ arising in a trichilemmal cyst: a case report and literature review. Am J Dermatopathol. 2008 Oct. 30(5):458-61.

15. Melikoglu C, Eren F, Keklik B, Aslan C, Sutcu M, Zeynep Tarini E. Trichilemmal cyst of the third fingertip: a case report. Hand Surg. 2014. 19(1):131-3. 

16. Seidenari S, Pellacani G, Nasti S, Tomasi A, Pastorino L, Ghiorzo P, et al. Hereditary trichilemmal cysts: a proposal for the assessment of diagnostic clinical criteria. Clin Genet. 2012 Oct 15. 

17. ElBenaye J, Elkhachine Y, Sakkah A, Sinaa M, Moumine M, Jakar A, et al. [Malignant proliferating trichilemmal cyst of the scalp: A case report]. Ann Chir Plast Esthet. 2018 Feb. 63 (1):97-101.

18. He P, Cui LG, Wang JR, Zhao B, Chen W, Xu Y. Trichilemmal Cyst: Clinical and Sonographic Features. J Ultrasound Med. 2019 Jan. 38 (1):91-96. 

19. Abreu Velez AM, Brown VM, Howard MS. An inflamed trichilemmal (pilar) cyst: Not so simple?. N Am J Med Sci. 2011 Sep. 3(9):431-4. 

20. Cotton DW, Kirkham N, Young BJ. Immunoperoxidase anti-keratin staining of epidermal and pilar cysts. Br J Dermatol. 1984 Jul. 111(1):63-8. 

21. Hanau D, Grosshans E. Trichilemmal cyst with intrinsic parietal sebaceous and apocrine structures. Clin Exp Dermatol. 1980 Sep. 5(3):351-5. 

22. Pusiol T, Morichetti D, Zorzi MG, Piscioli F. Ossifying trichilemmal cyst. Am J Dermpathol. Dec 2011. 33(8):867-8. 

23. Mommers XA, Henault B, Aubriot MH, Trost O, Malka G, Zwetyenga N. Kystes trichilemmaux ossifiants multiples du cuir chevelu : un cas familial [Multiple ossifying trichilemmal cysts of the scalp: a familial case]. Rev Stomatol Chir Maxillofac. 2012 Feb. 113(1):53-6.

24. Mehrabi D, Leonhardt JM, Brodell RT. Removal of keratinous and pilar cysts with the punch incision technique: analysis of surgical outcomes. Dermatol Surg. 2002 Aug. 28(8):673-7.