Scalp Cysts: Medical Terminology and Types
A cyst on the scalp is most commonly called an epidermoid cyst (also known as an epidermal cyst), though the term "sebaceous cyst" is frequently used in clinical practice despite being technically inaccurate. 1, 2, 3
Primary Types of Scalp Cysts
Epidermoid Cysts (Epidermal Cysts)
- These are the most common benign cutaneous cysts found on the scalp, presenting as mobile, flesh-colored nodules with a characteristic central punctum 2, 4, 3
- They contain cheesy keratinous material and normal skin flora even when uninflamed 1, 2
- Typically range from 1 to 5 cm in size, though larger variants have been reported 3
- The term "sebaceous cyst" is a misnomer commonly applied to these lesions, but epidermoid cysts do not actually arise from sebaceous glands 1
Pilar Cysts (Trichilemmal Cysts)
- These cysts specifically arise from the hair follicle epithelium and occur predominantly on the scalp 5, 6
- They are believed to originate from the isthmus of anagen hairs or from the sac surrounding catagen and telogen hairs 5
- Occur in approximately 10% of the population and appear in regions with high concentrations of hair follicles 6
- These are slow-growing, lobulated masses most commonly found on the scalp of elderly women 7
Dermoid Cysts
- Dermoid cysts are congenital choristomas containing more complex developmental structures including hair, teeth, and cartilage 2, 4
- On the scalp, these are subcutaneous sacs lined with stratified squamous epithelium containing developmentally mature ectodermal tissues including skin, hair follicles, sweat glands, and sebaceous glands 8
- Ultrasound characteristically shows hyperechoic components with acoustic shadowing, hyperechoic lines and dots, and sometimes fluid-fluid levels 2, 4, 9
- While typically benign and limited to extracranial locations, some scalp dermoids have potential for intracranial and/or intradural extension, requiring comprehensive evaluation with MRI and CT imaging 8
Clinical Distinction
The key differentiator is that epidermoid cysts present with a central punctum and contain only keratinous material, pilar cysts arise specifically from hair follicles without a punctum, and dermoid cysts contain multiple tissue types due to their developmental origin 2, 4, 5
Important Clinical Caveat
- When a longstanding scalp nodule becomes acutely painful and inflamed, this typically represents rupture of the cyst wall with extrusion of contents into the dermis rather than true bacterial infection 2, 4
- Chronic unresponsive lesions with marked asymmetry, rapid growth, ulceration, or indurated irregular borders require biopsy to exclude malignancy, particularly squamous cell carcinoma which can arise in chronic cysts 4