Epidermoid Cyst

Epidermoid cysts (commonly misnamed sebaceous cysts) are benign, slowly growing, intradermal or subcutaneous cysts lined by stratified squamous epithelium and filled with laminated keratin. They are the commonest cutaneous cyst, and affect about 1 in 20 people, more often men and adults after puberty.

Pathogenesis

• Implantation of epidermis into the dermis after trauma, or occlusion of the pilosebaceous follicular infundibulum (follicular cyst).
• Lining epithelium has a granular layer and produces keratin, which accumulates. Rupture releases keratin into the dermis, causing an intense foreign-body inflammatory reaction that mimics infection.
• Associated conditions: acne vulgaris (open comedones), hidradenitis, and Gardner syndrome (familial adenomatous polyposis: multiple epidermoid cysts, osteomas, desmoid tumours, colorectal polyps, often on the scalp and face). Other syndromes: basal cell nevus syndrome, pachyonychia congenita type 2.
• HPV 57-related plantar epidermoid cysts.

Related cysts

• Pilar (trichilemmal) cyst: scalp (90%), lined by trichilemmal keratin with no granular layer, firmer, familial (autosomal dominant), more in women. Proliferating trichilemmal tumour is a rare variant.
• Milia: tiny superficial keratin cysts.
• Steatocystoma multiplex: multiple sebaceous duct cysts with oily fluid (KRT17 mutations).
• Dermoid cyst: congenital, along embryonic fusion lines, contains hair and adnexae.
• Complications: inflammation or secondary infection (in 10 to 15%), abscess, rupture, calcification, rare malignant transformation (squamous cell carcinoma, basal cell carcinoma in long-standing lesions, over 0.1%), and recurrence after incomplete excision.

• Firm, round, mobile, dome-shaped nodule 0.5 to 5 cm (occasionally larger), attached to the skin with a central punctum (blackhead-like opening) in many. Skin overlying is normal, movable over the cyst, but the cyst moves with the skin.
• Sites: face, neck, scalp, upper trunk, back and scrotum (multiple small scrotal cysts with calcification). Palms and soles are uncommon (traumatic implantation).
• Asymptomatic usually. Expressing the punctum releases thick, malodorous cheesy material.
• Inflamed or ruptured cyst: sudden painful red swelling, warmth, tenderness, fluctuant, and often sterile. Secondary infection with S. aureus or anaerobes is possible, with pus.
• Multiple cysts on young patients suggest Gardner syndrome or steatocystoma.
• Differential diagnosis: lipoma, abscess or furuncle, pilar cyst, dermoid cyst, ganglion, sebaceous hyperplasia, lymph node, pilomatricoma, epidermal naevus, basal cell carcinoma, neurofibroma, cutaneous metastasis, and hidradenitis.
• Red flags: rapid growth, ulceration, fixation to deep structures, bleeding, recurrent inflammation at the same site, scalp or midline lesions in children (dermoid with intracranial extension), and multiple cysts with a family history of colonic polyps.

• Clinical. Investigations are rarely needed.
• Ultrasound if diagnostic uncertainty: well-defined, hypoechoic or heterogeneous mass with posterior acoustic enhancement and a tract to the skin.
• MRI or CT for midline scalp, spine or nasal lesions in children (dermoid with extension), or deep large lesions.
• Histology of excised lesion to confirm diagnosis and exclude malignancy. Send any atypical, rapidly growing, or recurrent lesion.
• Pus culture if abscess.
• Evaluate for Gardner syndrome in young patients with multiple cysts: colonoscopy, APC genetic testing, jaw radiograph, and family history.

Asymptomatic cysts need no treatment, only reassurance. Treatment is for cosmesis, pain, repeated inflammation, or diagnostic doubt.

Excision of non-inflamed cysts (definitive)

• Perform when the cyst is quiet (ideally 4 to 6 weeks after inflammation settles). Local anaesthetic infiltration with lidocaine 1% to 2% with adrenaline around the cyst.
• Techniques:
  • Wide elliptical excision with the punctum and the entire sac (recurrence below 5%).
  • Minimal excision (punch biopsy) technique: a 4 to 5 mm punch incision over the cyst, express contents, and extract the sac wall through the opening. Smaller scar, but recurrence risk of 5 to 10%.
  • Laser-assisted or endoscopic approaches in selected sites.
• Complete removal of the sac wall is essential to prevent recurrence. Close in layers with absorbable deep sutures and nylon (5-0 to 4-0) for skin, removed at 5 to 14 days depending on site.
• Prophylactic antibiotics are not routinely needed.

Inflamed or ruptured cysts

• Early inflammation without abscess: intralesional triamcinolone 3 to 10 mg/mL (0.1 to 0.3 mL), warm compresses, and NSAIDs. Antibiotics only if there are signs of infection (cefalexin 500 mg four times daily for 5 to 7 days, or doxycycline or trimethoprim-sulfamethoxazole for MRSA).
• Fluctuant abscess: incision and drainage under local anaesthetic, expression of contents, curettage of cavity, irrigation, and loose packing. Definitive excision later after healing, since the sac often remains. Send pus for culture if recurrent or severe.
• Sinus formation: excise the tract and sac.

Special situations

• Pilar cysts: excise through a small incision, as the capsule is thick and enucleates readily ("shelling out").
• Scrotal cysts: excision or multiple punch excisions for symptomatic ones.
• Gardner syndrome: surgical care for cysts, colorectal surveillance, and genetic counselling. Prophylactic colectomy per polyposis management.
• Dermoid cysts: complete excision with imaging first for midline lesions.
• Suspected malignancy: wide excision with margins.
• Steatocystoma multiplex: incision and expression, isotretinoin 0.5 mg/kg for inflamed lesions, cryotherapy or laser for others.
• Aesthetic concerns: surgical excision remains best. Do not squeeze (promotes rupture and inflammation).

Postoperative care and follow-up

• Dressings, avoid strenuous activity for a few days, suture removal, and wound check at 1 to 2 weeks. Recurrence appears as a new nodule at the scar, requiring re-excision. Histology review. Counsel on sun protection and scar management with silicone for visible sites.