Cyst

A cyst is a pathological, closed, sac-like structure within the body that is separated from surrounding tissues by a distinct wall or lining. It may contain fluid, semi-solid material, or gas and can develop in almost any organ or tissue. Cysts vary widely in size, origin, and clinical significance, ranging from harmless incidental findings to lesions associated with infection, inflammation, or neoplasia. The defining characteristic of a true cyst is the presence of an abnormal cellular lining that is structurally and functionally different from adjacent normal tissue.

Definition and Cellular Characteristics

A cyst consists of a distinct envelope of abnormal cells enclosing its contents. These lining cells differ in appearance, growth behaviour, and biological regulation compared with neighbouring cells at the same anatomical site. Unlike normal tissue structures, cysts represent a localised pathological organisation of cells, forming a sac-like configuration.
Cysts may contain:

• Clear serous fluid
• Mucus or other viscous secretions
• Semi-solid material such as keratin
• Gas or air in rare cases

A key distinction must be made between a cyst and an abscess. An abscess is a collection of pus caused by infection and inflammation, whereas a cyst is not primarily defined by infection and does not necessarily contain pus.
Once formed, some cysts may resolve spontaneously without intervention. Others persist and may enlarge, become symptomatic, or cause complications depending on their size, type, and anatomical location.

Pathogenesis and Biological Behaviour

The mechanisms underlying cyst formation vary considerably. Some cysts arise from developmental abnormalities, others from obstruction of ducts or glands, degeneration of tissue, inflammation, or trauma. In certain cases, cysts are associated with neoplastic processes.
In cancer-related contexts, cyst-like structures may develop when the body attempts to encapsulate rapidly dividing abnormal cells following genetic mutations that disrupt normal cell cycle control. Initially, this encapsulation may limit local spread. However, malignant cells can acquire further mutations that enable angiogenesis, allowing them to form their own blood supply. At this stage, the containment mechanism fails, and the lesion may progress from a benign tumour to a malignant cancer.
Not all cysts are neoplastic. Many are associated with dysplasia or metaplasia, representing abnormal but non-cancerous cellular changes.

True Cysts, Pseudocysts, and Related Terms

Cysts are classified based on their structural features:

• True cysts possess a lining of epithelial or endothelial cells.
• Pseudocysts resemble cysts in appearance and contents but lack a true epithelial lining.
• Microcysts are very small cysts visible only under magnification.
• Macrocysts are larger-than-usual cysts or cysts that are large relative to others in the same tissue.

Certain structures are sometimes inaccurately labelled as cysts. For example, a syrinx in the spinal cord or brainstem is a fluid-filled cavity but is not a true cyst.

Cysts by Anatomical Location

Cysts can occur throughout the body and are often categorised by their anatomical location.

Female Reproductive System

Cysts in the female reproductive tract are common and include:

• Cervical cysts on the surface of the cervix
• Ovarian cysts
• Paraovarian cysts located near the fallopian tubes
• Vaginal wall cysts
• Cysts at the vaginal introitus
• Periurethral cysts and urethral diverticula
• Ectopic ureterocoeles near the urinary meatus

These cysts vary in clinical relevance, with some being incidental findings and others causing pain, infection, or reproductive complications.

Male Reproductive System

In males, cysts may occur in:

• The rete testis
• The epididymis
• The testis, often containing clear fluid
• The cavum vaginale

Most are benign but may require differentiation from solid testicular masses.

Cutaneous and Subcutaneous Cysts

Cysts of the skin and subcutaneous tissues are among the most frequently encountered types. These include:

• Epidermoid and pilar cysts
• Familial scalp cysts
• Cysts associated with cystic acne, often presenting as inflammatory nodules
• Pilonidal cysts near the tailbone

Such cysts may become infected or rupture, leading to pain and inflammation.

Head and Neck

Cysts in this region may involve:

• The ear
• The eyelid
• Scalp structures

Some are congenital, while others arise later in life due to obstruction or degeneration.

Thoracic Cysts

Within the chest, cysts may include:

• Breast cysts
• Air-filled cystic spaces within the lung
• Abnormal dilatations of the pericardium

These lesions may be asymptomatic or detected during imaging for unrelated conditions.

Abdominal and Pelvic Cysts

Abdominal cysts encompass a wide range of entities:

• Simple cysts of solid organs
• Biliary cystadenomas and cystadenocarcinomas
• Adrenal cysts, which are rare but account for a significant proportion of adrenal gland pathologies

Adrenal cysts are classified into endothelial, epithelial, pseudocystic, parasitic, and unclassified types, with a small percentage showing malignant potential.
Other notable abdominal cysts include:

• Renal cysts
• Peritoneal inclusion cysts, typically affecting women of reproductive age with a history of pelvic surgery or inflammation

These may present with chronic abdominal, pelvic, or lower back pain.

Central Nervous System

Cysts of the central nervous system include:

• Pineal gland cysts
• Glial cysts
• Cystic lesions within the spinal canal

While often incidental, some may cause neurological symptoms depending on size and location.

Musculoskeletal System

Cystic lesions affecting bones and joints include:

• Benign bone cysts with radiographic cystic appearance
• Popliteal (Baker’s) cysts behind the knee
• Ganglion cysts associated with joints and tendons
• Subchondral cysts near articular surfaces

Some of these represent anatomical variants rather than true pathological cysts.

Infectious Cysts

Certain cysts arise from parasitic infections, such as:

• Cysticercosis caused by the larval stage of Taenia species
• Hydatid cysts caused by Echinococcus granulosus, commonly affecting the liver and other organs

These cysts have significant clinical implications and often require specialised treatment.

Neoplastic Cysts

Some cysts are classified as cystic tumours, where cyst formation is an integral feature of a neoplasm. These may be benign or malignant and require careful histological evaluation.

Treatment and Management

Management of cysts depends on their type, size, location, symptoms, and underlying pathology. Treatment options include:

• Observation and monitoring for asymptomatic cysts
• Surgical enucleation
• Curettage
• Segmental resection