Cutaneous cysts and pseudocysts
What is a cyst?
A cyst is a benign, round, dome-shaped encapsulated lesion that contains fluid or semi-fluid material. It may be firm or fluctuant and often distends the overlying skin. There are several types of cyst. The most common are described here.
What is a pseudocyst?
Cysts that are not surrounded by a capsule are better known as pseudocysts.
Who gets cysts?
Cysts are very common, affecting at least 20% of adults. They may be present at birth or appear later in life. They arise in all races. Most types of cyst are more common in males than in females.
What causes cysts?
The cause of many cysts is unknown.
- Epidermoid cysts are due to proliferation of epidermal cells within dermis.
- In response to an injury, a portion of the skin may get tucked in to form a sac. This is now lined by normal epidermal cells that continue to multiply, mature and form keratin. This is an epidermal inclusion cyst.
- Occlusion of pilosebaceous units (hair follicles) or eccrine sweat ducts leads to a build-up of secretions. This can present as milia.
- Occlusion of the orifice of a mucous gland can lead to a fluid-filled cyst in a mucous membrane (lip, vulva, vagina).
- A developmental error can lead to a dermoid cyst.
- Multiple cysts are often due to autosomal dominant inherited or sporadic genetic mutations.
What are the clinical features of cysts?
- The origin of an epidermal cyst is follicular infundibulum.
- Epidermoid cysts occur on face, neck, trunk or anywhere where there is little hair.
- Most epidermoid cysts arise in adult life.
- Multiple epidermoid cysts may indicate Gardner syndrome.
- They are more than twice as common in men as in women.
- They present as one or more flesh–coloured to yellowish, adherent, firm, round nodules of variable size.
- A central pore or punctum may be present.
- Keratinous contents are soft, cheese-like and malodorous.
- Scrotal and labial cysts are frequently multiple and may calcify.
- Epidermoid cyst is also called follicular infundibular cyst, epidermal cyst, keratin cyst.
Epidermal inclusion cyst
- Epidermal inclusion cyst is an epidermoid cyst that is the result of the implantation of epidermal elements in the dermis.
- The origin of a trichilemmal cyst is the hair root sheath.
- 90% occur on scalp; otherwise face, neck, trunk, and extremities.
- Most trichilemmal cysts arise in middle age.
- In 70% of cases, trichilemmal cysts are multiple.
- Inheritance is autosomal dominant (the affected gene is within short arm of chromosome 3) or sporadic.
- It presents as one or more adherent, round or oval, firm nodules.
- There is no punctum.
- The keratinous content is firm, white and easily enucleated.
- A trichilemmal cyst is also called pilar cyst.
- The origin of steatocystoma multiplex is the sebaceous duct within the hair follicle.
- A solitary steatocystoma is known as steatocystoma simplex.
- More often, multiple steatocystomas occur on chest, upper arms, axillae and neck.
- The cysts arise in the late teens and 20s due to the effect of androgens, and persist lifelong.
- Steatocystoma multiplex is sometimes an autosomal dominantly inherited disorder due to mutations are localized to the keratin 17 (K17) gene, when it may be associated with pachyonychia congenita.
- More often, steatocystomas are sporadic, when these mutations are not present.
- They are freely moveable, smooth flesh to yellow colour cysts 3–30 mm in diameter.
- There is no central punctum.
- Content of cyst is predominantly sebum.
Eruptive vellus hair cysts
- The origin of the eruptive vellus hair cysts is follicular infundibulum.
- Lesions are present in childhood if familial, and later if sporadic.
- It is an autosomal dominant, inherited disorder due to mutations in keratin gene.
- Multiple 2–3 mm papules develop over the sternum.
- The cysts contain vellus hairs.
- A cutaneous dermoid cyst is really a subcutaneous hamartoma; skin, skin structures and sometimes teeth, cartilage and bone, become trapped in subcutaneous tissue during fetal development.
- Most dermoid cysts are found on face, neck, scalp; often around eyelid, forehead and brow.
- The cyst is formed at birth but the patient may not present until an adult.
- It is a thin-walled tumour that ranges from soft to hard in consistency.
- The origin of a ganglion cyst is degeneration of the mucoid connective tissue of a joint.
- It most often involves scapholunate joint of dorsal wrist.
- Ganglions arise in young to middle-aged adults.
- They are 3 times more common in women than in men.
- It is a unilocular of multilocular firm swelling 2–4 cm in diameter that transilluminates.
- Cyst contents are mainly hyaluranic acid, a golden-coloured goo.
- Mucous/myxoid cysts arise in older adults on distal phalanx and arise from distal interphalangeal joint, associated with osteoarthritis. They often present as a longitudinal depression in the nail due to compression on the proximal matrix.
Labial mucous/myxoid cyst
- A cyst in the lip may be due to occlusion of the salivary duct.
- It is a firm, 5–15 mm diameter, semi-translucent nodule.
- A hidrocystoma is derived from eccrine or apocrine duct.
- It is a translucent jelly-like cyst arising on an eyelid.
- It is also known as cystadenoma, Moll gland cyst, and sudoriferous cyst.
- A milium is a pseudocyst due to failure to release keratin from an adnexal structure.
- The origin of primary milium is infundibulum of vellus hair follicle at the level of the sebaceous gland; a tiny version of an epidermoid cyst.
- The origin of secondary milium is a retention cyst within a vellus hair follicle, sebaceous duct, sweat duct or epidermis.
- Primary milia arise in neonates (50%), adolescents and adults; it is rarely familial and sometimes eruptive.
- Primary milia occur on eyelids, cheeks, nose, mucosa (Epstein pearls) and palate (Bohn nodules) in babies; and eyelids, cheeks and nose of older children and adults.
- Transverse primary milia are sometimes noted across nasal groove or around areola.
- Secondary milia arise at the site of epidermal repair after blistering or injury, eg epidermolysis bullosa, bullous pemphigoid, porphyria cutanea tarda, thermal burn, dermabrasion.
- Secondary milia are reported as an adverse effect of topical steroids, 5-fluorouracil cream, vemurafenib and dovitinib.
- They are 1–2 mm superficial white dome-shaped papules
- Milia contain keratin
- In milia en plaque, multiple milia arise on an erythematous plaque on face, chin or ears.
Vulval mucous cyst
- A vulval mucous cyst is due to occlusion of Bartholin or Skene duct.
- It presents as a soft swelling in the introitus of vagina: a posterior swelling is a Bartholin cyst and a periurethral swelling is a Skene cyst.
Comedo and acne pseudocyst
- Comedones are pseudocysts formed by occlusion of follicle by keratin and sebum.
- The open comedo (whitehead) and closed comedo (blackhead) are small, superficial papules typical of acne vulgaris
- Solar comedones arise in sun-damaged skin and are associated with smoking.
- Large uninflamed pseudocysts accompany inflammatory nodules in nodulocystic acne and hidradenitis suppurativa.
Pseudocyst of auricle
- Pseudocyst of auricle (external ear) follows trauma.
Complications of cysts
Rupture of a cyst
- The contents of the cyst may penetrate the capsular wall and irritate surrounding skin.
- The area of tender, firm inflammation spreads beyond the encapsulated cyst.
- Sterile pus may be discharged.
- A ruptured cyst may infrequently become secondarily infected by Staphylococcus aureus, forming a furuncle (boil).
- A dermoid cyst can cause pressure on underlying bony tissue.
- A ganglion cyst can cause joint instability, weakness, limitation of motion and may compress a nerve.
- A digital mucous cyst may place pressure on the proximal matrix and cause malformation of the nail.
- Cutaneous cysts and pseudocysts are non-proliferative benign lesions.
- Nodulocystic basal cell carcinoma is a common skin cancer that presents as a rounded nodule and may initially be mistaken for a cyst, but steady enlargement, destruction of the epidermis with ulceration and bleeding occur eventually.
- Malignant proliferative trichilemmal cyst is actually a misnomer. It is an extremely rare tumour.
How are cysts diagnosed?
Cysts are usually diagnosed clinically as they have typical characteristics. When a cyst is surgically removed, it should undergo histological examination. The type of lining of the wall of cyst and the cyst contents help the pathologist classify it.
- Epidermoid cysts are lined with stratified squamous epithelium that contains a granular layer. Laminated keratin contents are noted inside the cyst. An inflammatory response may be present in cysts that have ruptured.
- Trichilemmal cysts have a palisaded peripheral layer without granular layer. Contents are eosinophilic hair keratin. Older cysts may exhibit calcification. The proliferating variety is considered a tumour.
- Steatocystoma has a folded cyst wall with prominent sebaceous gland lobules.
- Dermoid cyst contains fully mature elements of the skin including fat, hairs, sebaceous glands, eccrine glands, and in 20%, apocrine glands.
- The lining of the wall of a ganglion cyst or digital mucous cyst is collagen and fibrocytes. It contains hyalin material.
- Hidrocystoma has a thin lining wall of eosinophilic bilaminar cells.
What is the treatment for cysts?
Asymptomatic epidermoid cysts do not need to be treated. In most cases, attempt to remove only the contents of a cyst is followed by recurrence. If desired, cysts may be fully excised. Recurrence is not uncommon, and re-excision may be surgically challenging.
Inflamed cysts are sometimes treated with:
- Incision and drainage
- Intralesional injection with triamcinolone
- Oral antibiotics
- Delayed excision biopsy
How can cysts be prevented?
What is the outlook for cysts?
Cysts generally persist unless surgically removed.