A surgical lesion which may be discussed in general, for the same principles may apply to almost any affected tissue, is the cyst. A cyst is an abnormal collection of fluid in a cavity surrounded by a definite limiting wall. Cysts in general they fall into four main categories:
- retention cysts,
- blood and serous cysts,
- congenital cysts,
- parasitic cysts.
Retention cysts are so named because they are formed from a surface gland by obstruction of the outlet with retention of the normal secretions. The glands of the skin which secrete oils and are called sebaceous glands give rise to sebaceous cysts, and the mucous glands of the mouth, intestines, and other mucosal surfaces give rise to mucous cysts. Retention cysts also occur in the breasts, kidneys, and internal glands. The main feature of all of these is the retention of an abnormally large amount of their own secretion. The steps in the development of this type of cyst are first the plugging of the duct of the gland, secondly the continued secretion of the gland’s cellular lining, and finally compression of living cells so that progression is halted. The gland wall then becomes the wall of the cyst. The very common sebaceous cyst of the skin occasionally becomes infected, and an inflammatory process is superimposed.
Cysts within tissues may develop through normal body fluids escaping from their normal channels to occupy space within certain tissues. These are of two types. The blood cyst occurs by the escape of blood from a blood vessel and seepage into a tissue where it accumulates. The serous cyst occurs by the leakage of serum and lymph from their normal channels into a tissue. The mechanism of production is the same for both. Since the blood and the serum are foreign substances in these new locations, a foreign body type of inflammation develops around the fluid, with production of a scar-tissue limiting wall. These may occur in any tissue throughout the body. They are initiated most often by direct injury to a tissue. Many hematomas (blood-swellings) develop into blood cysts when the blood is not absorbed from the area.
Congenital cysts are those which have been present since birth. They may not appear until later in life, but from the microscopic structure it can be ascertained that the cells making up the cyst wall have been present since birth. Embryology (the study of development of the embryo and fetus) is a complicated subject and cannot be given in detail here. It involves the rapid growth in some areas with relatively slower growth of other tissues, which produces various folding-in and folding-out of certain structures; there are other tissues which are formed on the sides and grow toward the center where they arc to unite; there is also union of many internal structures; many structures develop buds which go on to become an entirely different type of structure; there are certain organs which lose their connections to other parts and become completely separate; and there are other parts which after serving their purpose atrophy and disappear.
Through these complicated processes there may be slight errors, where structures did not quite meet their intended related tissue, or there may be a few cells caught in the processes and left behind included in an abnormal place. The abnormal space may later collect fluid, or the cells may begin to grow or secrete. Cysts are thereby formed. They usually come to attention when an inflammatory process, most often infection, stimulates the collection of fluid and cell production so that the cyst enlarges.
There are numerous types of congenital cysts. A common one is the pilonidal cyst, which occurs in the soft tissues overlying the lower end of the spinal column. This is due to inclusion of a few cells in this region during fetal development. Dermoid cysts may occur any place in the body; these are produced by inclusions of skin and related structures in an abnormal place. Dental cysts about the teeth in the jaw bones are occasionally encountered from developmental defects in this region. Bone cysts occasionally occur and are usually congenital in origin. Branchial cysts and thyroglossal duct cysts occur in the neck from minor errors in the complicated development of this region. There are several other congenital cysts; the mechanism of production is by the same processes, and they may occur in any organ. Multiple congenital cysts of the kidney are not infrequent.
Parasitic cysts arc those formed around a parasite which has gained entrance to the body. The commonest portal of entry is by mouth, with the ingestion of food. From the intestine the organisms may pass to almost any other region of the body by means of the circulation. The most common site for parasitic cysts is the liver, but no organ is exempt. It is the cystic phase (larval stage) of the parasite’s life which takes place in man usually, so the cyst is present from the start. The cyst then grows and more fluid collects as the inflammatory process evolves. Parasitic cysts may reach very large size, but fortunately they are not too common in this modern age.
Special features of cysts peculiar to certain organs are discussed in the respective chapter. (“Cyst-like” lesions which develop by unusual collection of fluid in a normal body cavity are not classed as cysts; a bursitis, a hydrocele, and a ganglion are not true cysts; therefore, these are discussed in a later chapter.) A true cyst must have an abnormal limiting membrane as the cyst wall. Cysts usually require operative treatment.