What is trichilemmal cyst

Trichilemmal cyst is also known as a pilar cyst, trichodermal cyst, trichodermal isthmus-degenerative cyst or wens, is a benign keratin-filled cyst that originates from the outer hair root sheath that form from hair follicles that are most often found on the scalp ¹, ², ³, ⁴, ⁵, ⁶. More than 90% of trichilemmal cysts occur on the scalp, where they are the most common type of skin cyst ⁷, ⁸. Trichilemmal cysts are smooth and mobile, meaning they can be moved slightly under the skin. They are filled with keratin (a protein component found in hair, nails, and skin). Trichilemmal cysts are usually painless but can be tender. There may be one or a few trichilemmal cysts. Trichilemmal cysts (pilar cysts) are almost always benign. Very rarely, trichilemmal cysts or pilar cysts can become cancerous. Case reports have described Merkel cell carcinoma arising from Merkel cells in trichilemmal cysts ⁹, ¹⁰, ¹¹.

Trichilemmal cysts are common in the United States, occurring in 5-10% of the population ¹². Trichilemmal cysts are more common in middle-aged persons than in younger persons, and they occur more frequently in women than in men ⁶. Trichilemmal cysts have no known racial tendency ⁶.

Trichilemmal cysts or pilar cysts may be sporadic or they often run in the family, as they have an autosomal dominant pattern of inheritance (ie, the tendency for trichilemmal cysts to be passed on by a parent to their child of either sex, and the child has a 1 in 2 likelihood of inheriting it) with incomplete penetrance ¹³, ¹⁴, ¹⁵,¹⁶, ⁵. Hörer and colleagues ¹⁵ think that Phospholipase C Delta 1 Gene (PLCD1) gene may be related to the occurrence of inherited trichilemmal cyst and recent genetic studies suggest that a somatic mutation on the same allele (a monoallelic two-hit mechanism) is the genetic trigger for cyst development ¹⁷. Patients with familial trichilemmal cysts are often younger and often present with multiple lesions at the same time. Trichilemmal cysts can also appear sporadically. They are most common on the head especially the scalp. The trichilemmal cysts rate of growth is very slow; it takes several years to grow to a big size.

In 2% of trichilemmal cysts, single or multiple foci of proliferating cells lead to tumors called proliferating trichilemmal cysts or malignant proliferating trichilemmal cysts ¹⁰. Proliferating trichilemmal cysts are gradually enlarging (up to 25 cm in diameter) exophytic nodules that occasionally ulcerate ⁷. Although these tumors are biologically benign, they may be locally aggressive ¹⁰. Recurrences and metastases have been observed, with rare malignant transformation ¹⁶, ¹⁰, ¹⁸, ¹⁹, ²⁰.

Trichilemmal cysts or pilar cysts may look similar to epidermoid cysts and are often incorrectly termed sebaceous cysts. Trichilemmal cysts present as one or more firm, mobile, subcutaneous nodules measuring 0.5 to 5 cm in diameter. There is no central punctum, unlike an epidermoid cyst. A trichilemmal cyst can be painful if inflamed.

See your doctor for an evaluation if you notice any growth you are unsure of or if a known trichilemmal cyst or pilar cyst becomes painful.

Your doctor may:

• Cut into (incise) and drain the keratin and other material inside the cyst.
• Prescribe oral antibiotics if the cyst becomes infected (a rare occurrence).
• Surgically remove (excise) the cyst.

Figure 1. Trichilemmal cyst

Footnotes: A trichilemmal cyst or pilar cyst manifest as skin-colored, smooth, mobile, firm, and well-circumscribed lump on your scalp that you first notice while washing or combing your hair. Trichilemmal cysts can crop up anywhere on your body, but you’ll usually find them on your scalp, face and neck. If you have one cyst, more may develop.

Figure 2. Trichilemmal cyst

Footnote: Trichilemmal cysts are flesh-colored nodules or lumps that usually appear on your head.

[Source ²¹ ]

Figure 3. Trichilemmal cysts

Footnotes: Trichilemmal cysts (pilar cysts) grow slowly, so you may not notice one right away. But they can grow to be quite large — sometimes as large as a baseball. While trichilemmal cysts (pilar cysts) are benign and don’t hurt, having a large cyst may make you feel self-conscious about your appearance.

[Source ²² ]

Can trichilemmal cysts become cancerous?

For the most part, trichilemmal cysts are benign, meaning they don’t become cancerous. About 2 to 3% trichilemmal cysts become proliferating trichilemmal tumors or malignant proliferating trichilemmal cysts. They’re called proliferating tumors because they grow quickly, unlike slow-growing pilar cysts. Like pilar cysts, doctors treat proliferating trichilemmal tumors by removing them.

What’s the difference between a trichilemmal cyst and a sebaceous cyst?

People frequently confuse sebaceous cysts and trichilemmal cysts, but there are several differences between the two:

• Trichilemmal cysts grow from your hair follicles, most frequently on your scalp. Sebaceous cysts grow from sebaceous glands (oil producing glands) that can appear almost anywhere on your body. Sebaceous glands secrete a substance called sebum that makes up some of the oil on your skin. Trichilemmal cysts are filled with keratin, a protein that makes hair, skin and nail cells.
• Pilar cysts are round and smooth and can grow quite large. Sebaceous cysts are tiny domes with a minuscule opening on top that will ooze sebum if you squeeze the cyst.

Trichilemmal cysts causes

Pilar cysts or trichilemmal cysts are intradermal cysts that arise from the epithelium located between the sebaceous gland and the arrector pili muscle. They are lined by stratified squamous epithelium without a granular cell layer similar to what is seen in the outer root sheath of the hair follicle and filled with keratin and its breakdown products. Trichilemmal cysts happen when old skin cells and keratin begin to pile up under the surface of your skin. Your skin is constantly creating new skin cells and getting rid of old ones. Usually old skin cells stop multiplying and fall off your skin’s surface. But sometimes they get trapped under your skin’s epidermis, or top layer, where they keep on multiplying. At the same time, your scalp is creating keratin, a protein that makes skin, hair and nail cells. Your old skin cells and your keratin cells combine to create layer after layer of cells. Over time, and many layers later, your old skin cells and keratin cells become pilar cysts. These cysts push up your outer skin, which is when you may begin to notice the cysts.

Pilar cysts or trichilemmal cysts may be inherited as an autosomal dominant trait with incomplete penetrance ¹³, ¹⁴, ¹⁵. Hörer and colleagues ¹⁵ think that Phospholipase C Delta 1 Gene (PLCD1) gene may be related to the occurrence of inherited trichilemmal cyst and recent genetic studies suggest that a somatic mutation on the same allele (a monoallelic two-hit mechanism) is the genetic trigger for cyst development ¹⁷. Patients with familial trichilemmal cysts are often younger and often present with multiple lesions at the same time ²³. Trichilemmal cysts can also appear sporadically. Patients with hereditary trichilemmal cysts typically develop multiple lesions that start at an earlier age than those with sporadic lesions. They are most common on the head especially the scalp. The trichilemmal cysts rate of growth is very slow; it takes several years to grow to a big size.

There are rare cases of malignant transformation in trichilemmal cyst ²⁴ and if cancerous transformation occurs, it can lead to distant metastasis. trichilemmal cyst lesions are often multiple, and there are a few single lesions ²⁵, ¹⁷, ¹⁵.

What are trichilemmal cysts pathology?

The pathology of a trichilemmal cyst is characteristic. The wall of the pilar cyst is stratified squamous epithelium (skin) that has a palisaded outer layer, which resembles the that of outer root sheath of a hair follicle. The inner layer does not have a granular layer. The cyst shows very dense pink keratin on hematoxylin and eosin staining. Keratin in the pilar cysts stains with antikeratin antibodies similar to that seen in keratin derived from human hair. Trichilemmal cysts might rupture and of their components will leak into the dermis leading to the formation of foreign-body reaction. Rare malignant transformation in the form of increase mitotic figure along with cell atypia and necrosis might be seen in cases of proliferating trichilemmal cysts.

Trichilemmal cysts signs and symptoms

Pilar cysts or trichilemmal cysts are usually found on the scalp (over 90% of cases) in areas with dense hair follicle concentrations, but they can also be found on the face, head, and neck ⁷, ¹. 90% of trichilemmal cysts occur on your head, and 70% occur as multiple lesions, but sometimes, single lesions might be seen ⁷, ²⁶. Pilar cysts or trichilemmal cysts appear as smooth, movable bumps under the skin. There is often more than one in an area, and they may become quite large. Occasionally, they are tender to the touch.

Trichilemmal cysts are usually asymptomatic unless they calcify or rupture their contents leading to inflammatory process and pain in the affected site. Sometimes, the presence of pilar cyst overpressure or bony prominence might lead to pain. Trichilemmal cysts usually present as flesh-colored, smooth, mobile, firm, and well-circumscribed nodules.

Usually, they are slow-growing nodules but sometimes they increase in size rapidly and it would indicates infection or malignant transformation. Young females are affected more than males.

Pilar cysts may spontaneously burst open (rupture), usually causing intense redness and irritation.

Pilar cysts complications

Trichilemmal cyst or pilar cyst might cause pain especially over pressure areas; other complications include inflammation, cosmetic disfigurements, infection, and calcification ¹². A complication from surgical removal may include bleeding, pain, infection, and scarring ²⁷.

Trichilemmal cyst rapid growth is unusual and may be a sign of infection or cancer ¹², ²⁸. Other suspicious features include the following ⁷:

• Non-scalp location
• Size larger than 5 cm
• Infiltrative growth pattern

Trichilemmal cysts signs diagnosis

Diagnosis of pilar cysts is mainly clinical, based on signs and symptoms and additional investigations rarely need to be done. Family history is very important since this condition can have an autosomal dominant pattern of inheritance. Radiological studies sometimes are 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 (CNS). CT scan is the best way to determine the extent of bone invasion. MRI can be used for more deeper soft tissue involvement and to visualize very tiny invasion.

Trichilemmal cyst differential diagnoses

Trichilemmal cyst differential diagnoses include ²⁹:

• Acne Keloidalis Nuchae (AKN). Acne keloidalis nuchae (AKN) is a skin disorder involving inflammation of the hair follicles (folliculitis) on your neck and back of your scalp. Acne keloidalis nuchae is often mistaken for acne or razor bumps, but it’s not actually acne and the scars it forms aren’t true keloids. Acne keloidalis nuchae causes red patches on your skin, which can develop into papules, plaques, pustules, infections, scarring and hair loss. Acne keloidalis nuchae usually begins with red, itchy bumps (papules) or patches of skin. The papules tend to get infected because of scratching, leading to pustules (pockets of pus, almost like pimples). Acne keloidalis nuchae is most common among men with curly hair. There isn’t a cure for acne keloidalis nuchae (AKN), so treatment usually involves medications to ease symptoms, such as: topical or oral antibiotics to treat or prevent bacterial infection; retinoids (which come from vitamin A) to improve symptoms; steroids (in creams, pills or injections) to help calm inflammation.
• Cutaneous Lipomas. Cutaneous lipomas are noncancerous, fatty tumors of mature adipocytes (fat cells) that grow under the skin in the subcutaneous tissue and are usually soft, mobile, and superficial; rarely, it may expand deep to the fascia or intramuscularly ³⁰. Single or multiple lipomas may occur. Although lipomas may occur anywhere on the body, they most commonly are found on the trunk and upper extremities ³⁰. Rarely, large lipomas may compress nerves or other adjacent structures, leading to pain, numbness, or other symptoms. Lipomas typically present as solitary, painless, soft or rubbery, often lobulated nodules, ranging from 1 to >10 cm in diameter. The overlying epidermis is unchanged and not tethered to the lesion. While most lipomas are solitary, some 5-10% of patients will have multiple lesions, usually associated with an underlying syndrome or genetic disorder (e.g., familial multiple lipomatosis, Proteus syndrome, PTEN hamartoma syndrome, Gardner syndrome, or multiple endocrine neoplasia type 2B). In the case of angiolipomas, pain is a frequent feature. Multiple painful lesions may be seen in Dercum disease. While lipomas typically affect patients between 40 and 60 years of age, they can occur at any age and are more common in patients with hyperlipidaemia, type 2 diabetes mellitus, and obesity. Both sexes are equally affected. Surgical excision is curative, provided the thin capsule is completely removed. Larger lipomas, or those associated with multiple lipomatosis for example, can be treated with liposuction. Laser lipolysis is also an effective treatment. Liposuction or laser are associated with improved cosmetic outcomes compared with that of excision. However, they also have a significantly higher recurrence rate as the lipoma capsule is not completely removed.
• Dermoid Cyst. A dermoid cyst is a choristoma, a benign tumor consisting of microscopically normal tissue derived from germ cell layers foreign to that body site ³¹. In the skin, dermoid cysts are made up of epidermal and dermal components: keratinocytes, hair follicles and hair, and sweat glands ³¹. Dermoid cyst of the skin appears in early childhood, with about 40% present at birth and most appearing by the age of 5 years. Dermoid cyst develops during skin tissue development in the embryonic stage. They occur on fusion lines. Dermoid cysts in other parts of the body, such as within the skull, spine or abdominal cavity particularly the ovaries, may present at any time in people of all ages. A dermoid cyst is usually removed surgically by excision biopsy. Prior to surgery, it may be investigated with ultrasound scan to determine the extent of underlying tissue involvement. If there is a nearby sinus tract, this is often explored using a sinogram. Dermoid cysts in other parts of the body such as intracranial cysts, intraspinal cysts, intra-abdominal cysts and cystic tumors of the ovary may be difficult to treat and require special surgical techniques to remove the lesion and prevent possible complications.
• Epidermoid Cyst. Epidermoid cysts also called sebaceous cysts are the most common benign skin cysts derived from the infundibulum or upper portion of a hair follicle ³², ³³. Historically, epidermoid cysts have been referred to by various terms, including follicular infundibular cysts, epidermal cysts, and epidermal inclusion cysts ³². The term epidermal inclusion cyst refers specifically to an epidermoid cyst that is the result of the implantation of epidermal elements in the dermis ³². Because most lesions originate from the follicular infundibulum, the more general term epidermoid cyst is favored. The term sebaceous cyst should be avoided because it implies that the cyst is of sebaceous origin and epidermoid cysts do not involve sebaceous glands, nor do they contain sebum. Epidermoid cysts contain soft “cheesy” like material that’s composed of keratin, a protein component of skin, hair, and nails. Epidermoid cysts form when the top layer of skin (the epidermis) grows into the middle layer of the skin (the dermis) ³⁴. This may occur due to injury or blocked hair follicles. An epidermoid cyst may have no pain or other symptoms associated with it, but if it becomes inflamed or infected, it may grow, become painful and red, and may rupture.
• Favre-Racouchot Syndrome (Nodular Elastosis With Cysts and Comedones). Favre-Racouchot syndrome is a disorder consisting of multiple open and closed comedones in the presence of sun damaged skin ³⁵. Although the precise pathogenesis of Favre-Racouchot syndrome is unknown, it develops in individuals with a heavy smoking history and chronic exposure to UV light ³⁶. Vogel et al ³⁷ described a case of unilateral Favre-Racouchot syndrome indicating the etiologic role of chronic solar damage in this disease. Owing to the strong association with smoking, and the name “smokers’ comedones” has been proposed ³⁸. Favre-Racouchot syndrome may represent as a sign of chloracne (a rare skin condition that appears as a result of exposure to dioxins or other polyhalogenated aromatic hydrocarbons), although both ultraviolet (UV) and infrared light exposure may contribute. Favre-Racouchot syndrome may also follow exposure to radiation therapy ³⁹, ⁴⁰. This was illustrated in a paper by Hoff et al ⁴¹, in which a 71-year-old man treated with radiation therapy for a malignant melanoma of the right paranasal sinus. He subsequently developed elastosis, open and closed comedones, and cysts in the radiation field ⁴². Lim et al ⁴³, in a discussion of the adverse effects of UV radiation from the use of indoor tanning equipment, has cited Favre-Racouchot syndrome as one of these adverse effects.
• Pilomatrixoma (Pilomatricoma). A pilomatrixoma, also known as pilomatricoma or calcifying epithelioma of Malherbe, is a slow-growing, noncancerous (benign) skin tumor that originates from the hair follicle. It’s usually a small, hard lump under the skin that’s painless unless it becomes infected. The skin over the lump may look normal or have a blue hue. A pilomatrixoma is most common on the face and neck, but it may be on other parts of the body. A pilomatrixoma is often a single lump. But sometimes there may be more than one. Pilomatrixomas are more common in children and young adults than in older adults. Treatment may include surgery to cut out the lump. Surgery may be done if the pilomatrixoma is uncomfortable, is cosmetically bothersome, or to confirm the diagnosis. Multiple pilomatrixomas have been observed, mainly in association with myotonic dystrophy ⁴⁴, but also in association with other syndromes, including Rubenstein-Taybi syndrome ⁴⁵ and constitutional mismatch repair deficiency (CMMRD) syndrome ⁴⁶. Pilomatrix carcinoma is a rare development ⁴⁷, ⁴⁸, ⁴⁹, ⁵⁰. Appropriate surgical interventions for pilomatrix carcinoma include Mohs micrographic surgery to prevent any recurrences or metastases ⁵¹.
• Steatocystoma Multiplex. Steatocystoma multiplex is a rare inherited skin disorder in which numerous noncancerous (benign) hamartomatous malformations of the pilosebaceous duct junction (hair follicle unit) known as steatocystomas ⁵², ⁵³. These growths begin in the skin’s sebaceous glands, which normally produce an oily substance called sebum that lubricates the skin and hair. Steatocystomas are filled with sebum. If a single cyst of this type is found, it is called steatocystoma simplex. In affected individuals, steatocystomas typically first appear during adolescence and are found most often on the torso, neck, upper arms, and upper legs. These cysts are usually the only sign of the condition. However, some affected individuals also have mild abnormalities involving the teeth or the fingernails and toenails. In steatocystoma multiplex, the tendency to develop cysts is inherited in an autosomal dominant fashion, so one parent can be expected to also have steatocystoma multiplex. It may also occur sporadically. Both males and females may be affected. Steatocystoma multiplex can be caused by mutations in the KRT17 gene. This gene provides instructions for making a protein called keratin 17, which is produced in the nails, the hair follicles, and the skin on the palms of the hands and soles of the feet. It is also found in the skin’s sebaceous glands. Keratin 17 partners with a similar protein called keratin 6b to form networks that provide strength and resilience to the skin, nails, and other tissues.

Trichilemmal cyst treatment

It is not necessary to remove trichilemmal cysts or pilar cysts if they are not causing symptoms. However, incision and drainage under local anesthesia provide comfort, and elective excision before rupture prevents scarring. The mainstay of treatment is radical surgical excision of the cyst including the wall of the cysts to avoid the recurrence.

Surgical treatment involves either of the following methods:

• Enucleation of the pilar cyst, ie removal without cutting into it and leaving surrounding skin intact
• Incision followed by expression of contents and removal of cyst wall – this is often best achieved with a surgical punch with the cyst expressed through the hole. Then the content should be sent to the pathology department to confirm the diagnosis.

After surgical removal of trichilemmal cyst or pilar cyst, it is very important to take care of the surgical site. Daily dressing is advisable along with normal saline and disinfectant solution, covering the sutures with gauze is recommended for the first very days. Typically, sutures should be removed in 7 to 10 days depending on the site of the cyst and status of the wound ⁵⁴.

Acute inflammation after rupture of trichilemmal cyst or pilar cyst is often misdiagnosed as a bacterial infection. Antibiotics are of little value unless an actual infection is present.

If trichilemmal cysts or pilar cysts return or become inflamed, then it is not recommended to remove surgically, and it is better to wait until the inflammation subsides to proceed to the option of surgical removal. Wound swab and culture sensitivity for the inflamed lesion is mandatory to exclude infection and to guide treatment options. Proliferating trichilemmal cysts might need multiple surgical removal sessions along with radiation therapy and/ or chemotherapy.

Trichilemmal cyst prognosis

Generally, trichilemmal cysts have good prognosis regardless of the presence of complications, but may be locally aggressive in some cases ²¹ Malignant transformation is very rare but may lead to distant metastases ¹⁶, ¹⁰.

Complications of trichilemmal cysts include inflammation, infection and malignant transformation (which is rare). Counseling is needed to assure the patient and his or her family since trichilemmal cysts or pilar cysts can have an autosomal dominant pattern of inheritance ⁵⁵.

Proposed clinical criteria for recognizing autosomal dominant hereditary trichilemmal cyst cases include the following ²³:

• Diagnosis of trichilemmal cyst in two or more first- or second-degree relatives
• Age younger than 45 years at diagnosis
• Diagnosis of multiple or large (>5 cm) cysts or rare histologic features such as proliferating and ossifying cysts.

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