The word cyst is derived from the Greek word meaning ‘bladder’. The pathological term ‘cyst’ means a swelling consisting of a collection of fluid in a sac which is lined by epithelium or endothelium.
True cysts are lined by epithelium or endothelium. If infection supervenes, the true lining may be destroyed and replaced by granulation tissue. The fluid is usually serous or mucoid and varies from brown-staining by altered blood to almost colourless. In epidermoid, dermoid and branchial cysts the contents are like porridge or toothpaste, as a result of the shedding of desquamated cells. Cholesterol crystals are often found in the fluid of branchial cysts.
False cysts (pseudocysts)
Walled-off collections of fluid not lined by epithelium are not regarded as true cysts. A pseudocyst of the pancreas is an encysted collection of pancreatic enzymerich fluid lined by granulation tissue or fibrous tissue. Pancreatic pseudocysts are often in the retroperitoneum deep to but bulging into the lesser sac; they may occur anywhere in the abdominal cavity and even track into the mediastinum and pleural cavities. In tuberculous peritonitis, fluid may be walled off in cystic form by adherent coils of intestine. Fluid may collect in the centre of a tumour (cystic degeneration), due to haemorrhage or necrosis. This can also happen in the brain as a result of ischaemia, and an ‘apoplectic cyst’ is formed. In acute pancreatitis fluid collections loculated by viscera and fibrin are called ‘acute fluid collections’; these often occur in the lesser sac but are neither cysts nor pseudocysts as they are not lined by either epithelium, granulation tissue or fibrous tissue.
A classification of cysts
Congenital Sequestration dermoids
Cyst of embryonic remnants
Parasitic Hydatid, trichniasis, cysticercosis
The sequestration dermoid is due to dermal cells being buried along the lines of closure of embryonic clefts and sinuses by skin fusion. The cyst therefore is lined by epidermis and contains paste-like desquamated material. The usual sites are:
• the midline of the body — especially in the neck;
• above the outer canthus (external angular dermoid);
• in the anterior triangle of the neck (branchial cyst).
Tubuloembryonic (tubulodermoid) cysts occur in the track of an ectodermal tube used in development, e.g. a thyroglossal cyst from the thyroglossal duct or a postanal dermoid from the postanal gut. In the brain, ependymal cysts arise from the sequestration of cells of the enfolding neurectoderm.
Cysts of embryonic remnants. These arise from embryonic tubules and ducts which normally disappear or are only present as remnants. They should not be confused with teratomatous cysts, e.g. dermoid. There are many examples in the urogenital system, e.g. in the male from remnants of the paramesonephric duct (Müllerian) — the hydatid of Morgagni, or from the mesonephric body and duct (Wolffian). Cysts of the urachus and the vitellointestinal duct are other examples of cysts of embryonic remnants.
Retention cysts are due to the accumulated secretion of a gland behind an obstruction of a duct. Examples are seen in the pancreas, the parotid, the breast, the epididymis and Bartholin’s gland. A sebaceous cyst starts with the obstruction of a sebaceous gland, but this is followed by the down-growth and the accumulation of desquamated epidermal cells, thus turning it into an epidermoid cyst. In the epididymis, if the retention cyst contains sperms, it is known as a ‘spermatocele’.
Distension cysts occur in the thyroid from dilatation of the acini, or in the ovary from a follicle. Lymphatic cysts and cystic hygromas are distension cysts. Exudation cysts occur when fluid exudes into an anatomical space already lined by endothelium, e.g. hydrocele, a bursa, or when a collection of exudate becomes encrusted.
Cystic tumours. Examples are cystic teratomas (dermoid cyst of the ovary) and cystadenomas (pseudomucinous and serous cystadenoma of the ovary).
Implantation dermoids arise from squamous epithelium which has been driven beneath the skin by a penetrating wound. They are classically found in the fingers of women who sew assiduously and metal workers .
A haematoma may resolve into a cyst. This sometimes happens to haematomas of muscle masses in the loin and anterolateral aspects of the thigh or the skin. They are located between muscle, facial or subcutaneous planes and contain straw- or brown-coloured fluid containing cholesterol crystals. They become lined by endothelium and calcium salts may be laid down. Aspiration is only of temporary value, and a cure depends upon complete excision of the lining. Within the cranium, a haematogenous cyst can cause the same problems as any expanding, space-occupying lesion.
These have already been discussed under false cysts.
These are encrusted forms in the life cycle of various worms:
• Hydatid cyst of Taenia echinococcus. This is described later according to the organ involved, e.g. liver, Chapter 52; lung.
• Trichiniasis. Cysts of Trichina spiralis, affecting muscle.
• Cysticercosis. Cysts of Taenia solium. A disease of the pig, humans being rarely affected. Eosinophilia is present. The cysts occur in any organ. They calcify and may cause clinical effects according to their situation, especially in the brain. Only those cysts which are actually causing symptoms should be excised.
The swelling usually has a smooth, spherical appearance. Fluctuation depends upon the pressure of fluid within: a tense cyst feels like a solid tumour, although careful palpation between two fingers may elicit a characteristic elasticity. In addition, a solid tumour is most hard at the centre; a cyst is least hard at the centre. If fluctuation is present, a cyst may be confused with a cold abscess or a lipoma. A cold abscess usually has a peculiar rim of thickening surrounding the soft centre. A lipoma may well test clinical acumen. Transillumination,while brilliantly clear in cysts containing serous fluid, does not really distinguish between a lipoma and a dermoid or branchial cyst. There is even an old axiom that ‘when in doubt, hedge on fat’. According to circumstances, ultrasonography, computerised tomography (CT) or magnetic resonance imaging (MRI), a test aspiration or excision reveals the true nature of the swelling.
Cysts may be painful, especially when infection or haemorrhage causes a sudden increase in intracystic tension. Sometimes they change in size for no apparent reason. Occasionally, they diminish owing to rupture through a facial plane.
Effects are according to site and size. As with benign tumours, a cyst may compress ducts and blood vessels, e.g. the main bile duct may be obstructed by a choledochal cyst, a renal cyst or a hydatid cyst. The pelvic veins may be obstructed by an ovarian cyst, the patient presenting for treatment of her varicose veins. The sheer size of an ovarian cyst may so increase intra-abdominal tension as to bring the patient to hospital with symptoms of a hiatus hernia.
The cyst becomes tense and painful, and adherent to surrounding tissues. An abscess may form and discharge on the surface and result in an ulcer or a sinus (viz. Cock’s peculiar tumour). Healing will not occur until the whole lining of the cyst or the embryonic track is excised.
Sudden haemorrhage, as may occur in a thyroid cyst, causes a painful increase in size. In this particular case, breathing may be difficult because of pressure on the trachea.
Torsion may occur in cysts which are attached to neighboring structures by a vascular pedicle. Ovarian dermoids are sometimes brought to notice in this way as acute abdominal emergencies. The cyst (or cysts — they may be bilateral) turns to a purple or black colour as the venous and then the arterial supply is cut off.
Calcification follows haemorrhage, or infection, and may be the result of reaction to a parasite, e.g. hydatid cyst.
Enormous cysts are rarely seen nowadays.
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