Kista Atheroma

01Pathophysiology

The epidermoid cyst is the most common cutaneous cyst. Understanding its origin is the key to understanding both its presentation and its management.

The cyst arises from the infundibular portion of the hair follicle, which is the upper segment of the follicle that opens to the skin surface. The lining of this infundibulum is essentially identical to the surface epidermis: stratified squamous epithelium complete with a granular layer. When this epithelium becomes enclosed within the dermis, it continues to do what epidermis always does: it keratinizes. However, because the keratin has nowhere to shed, it accumulates within the cyst cavity, forming the characteristic laminated, cheese-like material. This is why the cyst slowly enlarges over time.

A second mechanism of formation is traumatic implantation. When the surface epidermis is driven into the deeper dermis through a penetrating injury, a surgical procedure, or even chronic friction, a small fragment of epidermal tissue can establish itself beneath the surface and begin producing keratin in the same manner. These are sometimes referred to as "epidermal inclusion cysts," and the mechanism is identical.

The critical clinical event occurs when the cyst wall ruptures. When keratin escapes into the surrounding dermis, the immune system recognizes it as foreign material and mounts a foreign body-type inflammatory reaction. This produces acute redness, swelling, warmth, and tenderness around the cyst. This inflammatory response is commonly mistaken for bacterial infection, but it is sterile inflammation in the majority of cases. True secondary bacterial infection can occur but is less common than rupture-induced inflammation. Distinguishing between these two is a frequently tested concept.

It is also important to note that the commonly used term "sebaceous cyst" is a misnomer. Epidermoid cysts do not contain sebum and do not arise from sebaceous glands. True sebaceous cysts (steatocystomas) are a separate, rare entity lined by sebaceous ductal epithelium. On exams, the term "sebaceous cyst" may appear in the stem, but the correct histopathologic and clinical entity being tested is almost always the epidermoid cyst.

02Classification and Clinical Manifestation

03Diagnostic Workup

The diagnosis of an epidermoid cyst is overwhelmingly clinical. In a typical exam vignette, you will be given a patient with a slowly enlarging, nontender, mobile subcutaneous nodule that has a visible central punctum. When this description is present, no further workup is needed before proceeding to management. The central punctum is the discriminating physical exam finding that separates this entity from lipomas, dermoid cysts, and other subcutaneous masses.

Histopathology becomes relevant in two scenarios: when the clinical diagnosis is uncertain, or when the excised specimen is sent for pathologic examination (which is standard practice). The hallmark finding is a cyst lined by stratified squamous epithelium with a granular layer that produces laminated keratin. The presence of the granular layer is the distinguishing histologic feature that separates an epidermoid cyst from a pilar (trichilemmal) cyst, which lacks the granular layer. This distinction is tested frequently.

Ultrasound is not routinely needed but can be helpful when the lesion is in a deep or atypical location, or when you need to confirm it is cystic rather than solid before planning excision. The characteristic appearance is a well-demarcated hypoechoic mass with posterior acoustic enhancement.

If there is clinical concern for true bacterial infection (fluctuance, purulent drainage, surrounding cellulitis, or systemic signs such as fever), wound culture of the drained material should be sent. However, do not routinely culture the contents of an inflamed but non-purulent cyst; the inflammation is most likely sterile and driven by ruptured keratin.

04Management and Treatment

The cornerstone of definitive treatment is complete surgical excision with removal of the entire cyst wall. If any fragment of the epithelial lining remains, the cyst will recur. This is the single most important management principle and the most commonly tested point. The excision is typically performed as an outpatient procedure under local anesthesia with an elliptical incision that includes the central punctum.

For an acutely inflamed cyst that is not infected, the next best step is not excision. Attempting excision during active inflammation is associated with a significantly higher recurrence rate because the inflamed, friable cyst wall is difficult to dissect cleanly and fragments are often left behind. Instead, inject intralesional triamcinolone acetonide (commonly 10 mg/mL) to reduce inflammation, apply warm compresses, and schedule elective excision once the inflammation has fully resolved, usually in 4 to 6 weeks.

If the cyst has progressed to a true abscess with fluctuance and purulent material, the immediate step is incision and drainage. Empiric antibiotics targeting Staphylococcus aureus should be added if there is surrounding cellulitis. A common oral regimen is cephalexin 500 mg four times daily for 7 to 10 days; if there is concern for methicillin-resistant S. aureus (MRSA), use trimethoprim-sulfamethoxazole (160/800 mg twice daily) or doxycycline 100 mg twice daily. After the infection clears and the tissue heals, schedule definitive excision to remove the remaining cyst wall and prevent recurrence.

A key contraindication to remember: do not prescribe antibiotics alone for a fluctuant abscess. Antibiotics without drainage will not resolve a walled-off collection. The tested principle here is that abscesses require source control.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students lose points on epidermoid cyst questions is by confusing sterile rupture-induced inflammation with bacterial infection. When a vignette describes a previously stable cyst that suddenly becomes red, swollen, and tender without fever, purulent drainage, or cellulitis, the correct answer is intralesional corticosteroid injection or warm compresses and delayed excision. Jumping to incision and drainage or antibiotics in this scenario is the classic trap.

The second major trap involves attempting excision during acute inflammation. Even when a student correctly identifies the diagnosis, choosing immediate excision for an inflamed cyst is penalized because it leads to incomplete wall removal and recurrence. The tested principle is that definitive surgery should be deferred until the acute process has settled.

A third commonly tested angle is the association between multiple epidermoid cysts and Gardner syndrome. If a vignette presents a young patient with several epidermoid cysts along with osteomas or a family history of colon polyps, the question is testing whether you recognize the syndrome and recommend colonoscopy as the next step.

Finally, always remember the histologic distinction: epidermoid cysts have a granular layer, pilar cysts do not. This single detail is the most reliable way the exam differentiates between the two, and it is tested repeatedly. The core competency here is pattern recognition: identifying the central punctum on physical exam and the granular layer on pathology, then applying the correct management sequence based on the clinical state of the cyst.