Overview

A cyst is a closed sac-like structure that is separated from surrounding tissue by a distinct capsule or membrane. It may contain air, clear fluid, semi-solid material or, in some cases, more complex contents. Cysts are a common finding in medicine and can occur in the skin, organs and deeper tissues. They differ from an abscess in that an abscess is a localized collection of pus due to infection, whereas a cyst is typically lined by cells or a capsule and is not defined by pus alone.

Structure and characteristics

Most cysts have a wall or lining that distinguishes them from surrounding tissue; this lining may be true epithelial tissue or a fibrous capsule. Some so-called pseudocysts lack a true epithelial lining and result from injury or inflammation. Contents vary: serous fluid, keratin and sebum (as in epidermal cysts), blood, or materials derived from included tissues (as in dermoid cysts). Their size and behavior depend on origin and location.

Causes and development

Cysts arise by several mechanisms. They may form when ducts or gland openings become blocked, causing secretions to accumulate; when embryonic tissue becomes trapped during development; as a response to chronic inflammation; or in association with neoplastic processes. Some cystic conditions have a genetic basis, for example polycystic changes in the kidney and liver, while many skin and ovarian cysts are acquired.

Common locations and examples

  • Skin: epidermoid, sebaceous and pilonidal cysts.
  • Reproductive organs: functional ovarian cysts and dermoid cysts (mature cystic teratomas).
  • Joints and bursae: Baker's cyst behind the knee.
  • Internal organs: simple renal cysts and hepatic cysts; polycystic kidney disease involves many cysts.
  • Glands: Bartholin cysts in the vulva and retention cysts of salivary glands.

Diagnosis and treatment

Diagnosis relies on clinical examination and, if needed, imaging such as ultrasound, computed tomography (CT) or magnetic resonance imaging (MRI). Small, asymptomatic cysts are often observed. Treatment choices include needle aspiration for temporary relief, antibiotic therapy if infected, or complete surgical excision when recurrence, symptoms, diagnostic uncertainty or risk of complications are present. In some cases, a biopsy or laboratory analysis of aspirated fluid is used to exclude malignancy or infection.

Notable distinctions and clinical significance

Important distinctions include cyst versus abscess (the latter contains pus and usually requires drainage and antibiotics) and benign cysts versus cystic tumors (which may require more extensive evaluation). Complications of cysts can include infection, rupture, bleeding into the cyst, compression of adjacent structures and, rarely, malignant change depending on the type. When a cyst changes rapidly, becomes painful, or interferes with organ function, medical assessment is recommended. For additional resources see institutional references: further reading and clinical guidelines at professional sources or specialty pages.