Pilar cysts, also known as trichilemmal cysts, are common, non-cancerous skin growths that form beneath the skin. They represent one of the most frequent types of cutaneous cysts, though they affect less than ten percent of the general population. These dome-shaped lumps are benign and slow-growing.
Identifying Pilar Cysts
Pilar cysts possess distinct physical characteristics. They present as smooth, firm, mobile, and well-circumscribed nodules. These cysts are typically flesh-colored, though they can sometimes appear slightly white or yellow, and they range in size from a few millimeters up to several centimeters.
The overwhelming majority (approximately 90 percent) are found on the scalp, where hair follicles are densely concentrated. This location sets them apart from the closely related epidermal cyst, which is most often seen on the face, neck, or trunk. Pilar cysts are filled with a thick, pasty material composed almost entirely of keratin, the structural protein found in hair and nails. In contrast, epidermal cysts generally contain a mixture of broken-down skin cells and sebum.
Pilar cysts are generally asymptomatic and non-tender unless they become inflamed or rupture. They do not feature a central dark plug, or punctum, which can sometimes be seen on an epidermal cyst. Their presence on the scalp can sometimes cause discomfort if they are repeatedly snagged by a hairbrush or pressed upon.
Understanding the Cause: Hair Follicle Origin and Genetics
The cause of pilar cysts lies in the abnormal proliferation of cells within the hair structure. These cysts originate from the outer root sheath of the hair follicle, a layer of tissue surrounding the lower part of the hair shaft. Due to cellular dysfunction, the cells in this sheath multiply inward instead of shedding normally, forming a closed sac deep beneath the skin surface.
This sac continues to fill with keratin, the protein produced by the lining cells, creating the characteristic firm lump. This results in a keratin-filled capsule without a connection to the skin’s surface. The cyst wall itself is thick and durable, distinguishing it from other cyst types.
A significant factor determining who develops these cysts is heredity, as they often have a strong genetic component. Pilar cysts are frequently inherited in an autosomal dominant pattern, meaning a person only needs to inherit the gene from one parent to have a chance of developing the condition. Individuals with a strong family history are at a higher risk and often present with multiple cysts at a younger age.
The presence of a strong family history is the primary risk factor and suggests a predisposition to this form of cellular overgrowth. Recent research has pointed toward variants of the PLCD1 gene in some families, indicating a biological mechanism tied to this genetic inheritance. The underlying cause is a combination of a localized cellular abnormality in the hair follicle and a genetic tendency passed down through generations.
Medical Management and Removal
Diagnosis is typically made through a clinical examination. The characteristic feel, appearance, and location on the scalp are usually enough to distinguish it from other skin lesions. Rarely, a biopsy might be performed if the diagnosis is uncertain or if the cyst exhibits unusual growth characteristics.
If a pilar cyst is small and asymptomatic, intervention is unnecessary. Treatment is appropriate if the cyst is painful, infected, ruptures, or causes cosmetic concern. When a cyst becomes inflamed, it is treated with antibiotics or corticosteroid injections to reduce swelling before definitive removal is attempted.
The most effective treatment for permanent resolution is complete surgical excision. This procedure is performed under local anesthetic and involves removing the entire cyst wall, or sac, intact. Removing the entire sac is important because leaving any part of the lining behind significantly increases the chance of recurrence. Techniques like punch excision, which use a small circular blade, may be used for smaller lesions to minimize scarring.