What are the characteristics of a sebaceous cyst?

Characteristics of Sebaceous Cysts

Sebaceous cysts, also known as epidermoid cysts, are benign, encapsulated subepidermal nodules filled with keratin material that typically form on areas with hair follicles. 1

Clinical Presentation

• Appearance: Usually presents as an indurated nodular keratinizing or crusted lesion that may ulcerate 2
• Size: Typically small, but can grow to larger sizes (>5 cm would be considered giant) 3
• Location: Commonly found on the scalp, face, neck, trunk, and other hair-bearing areas 1, 4
• Growth pattern: Slow-growing, may be present for years before reaching significant size 4
• Surface features: May have a central punctum (opening) visible on the skin surface 5
• Consistency: Firm, mobile, dome-shaped nodule beneath the skin

Diagnostic Features

Physical Examination

• Round or oval, well-circumscribed nodule
• Mobile when palpated (not fixed to underlying tissues)
• Central punctum (blackhead) often visible
• Non-tender unless infected or ruptured
• Expressible cheesy, foul-smelling material (keratin)

Ultrasound Characteristics

• Morphology: Typically oval-shaped (96.6% of cases) 5
• Echogenicity: Predominantly hypoechoic (91.5% of cases) 5
• Appearance: 50.8% show homogeneous "pseudotestis" pattern 5
• Key diagnostic feature: Visible skin pore (epidermal punctum) on high-frequency ultrasound 5

Types of Cysts

Sebaceous cysts can be categorized based on their characteristics:

1. Simple cysts:

   • Well-circumscribed
   • Round or oval with well-defined walls
   • Anechoic (cystic) with posterior enhancement 2

2. Complicated cysts:

   • Contains most but not all elements of simple cysts
   • May contain low-level echoes or intracystic debris
   • No solid elements, thick walls, or thick septa 2

3. Complex cysts:

   • Contains discrete solid components
   • May have thick walls, thick septa, and/or intracystic mass
   • Both anechoic (cystic) and echogenic (solid) components 2

Pathological Considerations

• Contents: Filled with keratin material, not actual sebum (despite the name) 1
• Wall structure: Lined by stratified squamous epithelium 1
• Malignant transformation: Extremely rare but possible, particularly in long-standing or large cysts 6, 3

Warning Signs Requiring Further Evaluation

Suspicious features that warrant pathological examination include:

• Size greater than 1.5 cm
• Rapid growth
• Nodular mass
• Ulceration
• Extensive scarring
• Fixation to surrounding tissues 1

Management Approach

1. Non-inflamed cysts:

   • Complete surgical excision with removal of the entire cyst wall is recommended
   • Should include a minimum 4-mm margin around the cyst border 1

2. Inflamed/infected cysts:

   • Incision and drainage (I&D) is the cornerstone of treatment
   • Antibiotics reserved for specific situations (systemic inflammatory response, impaired host defenses)
   • Complete excision should be delayed until acute infection resolves 1, 7

Prevention of Recurrence

• Complete removal of the cyst wall is essential to prevent recurrence
• Recurrence rates range from 2.2-8.3% depending on the technique used
• Main cause of recurrence is incomplete removal of the cyst wall 1

Complications

• Infection
• Rupture leading to inflammation
• Rare malignant transformation to squamous cell carcinoma 6
• Cosmetic concerns, especially with large or multiple cysts 4

Proper identification and complete surgical excision are key to successful management and prevention of recurrence of sebaceous cysts.