Complications of Cystic Hydatid Disease (continued)

Infection

Infection occurs only after communicating or direct rupture (usually the former) because bacteria-transporting blood vessels do not penetrate the intact pericyst. Infection associated with communicating rupture and biliary obstruction can result in an abscess within the pericyst cavity or in cholangitis, and probably always kills the parasite. Because of the danger of infection, biliary obstruction must be relieved promptly. Infection leads to loss of clarity of interior detail of the cyst, and fluid elements become echogenic by ultrasound and denser by CT. The endocyst is usually seen to have detached from the pericyst, and occasionally gas is produced in the cyst.

Premature death

This is defined as death of the parasite that is not the result of starvation of a hypermature Type II lesion; it may occur after rupture (Fig. 3.32), infection or medical treatment. When a Type I lesion dies prematurely after rupture it may develop directly into a type III lesion. However, a prematurely dead Type I lesion followed for 30 months did not calcify (Lewall), so it is possible that such lesions are sometimes eventually resorbed. Unless infection produces gas, premature death and infection have identical appearances on CT and ultrasound although they have very different clinical presentations. In both instances the fluid component becomes echogenic on ultrasound and denser on CT (Fig. 3.32), and there is a loss of clarity of the internal detail of the lesion. With both methods of imaging the endocyst is usually seen to be detached from the pericyst. Premature death apparently does not produce a diagnostic change in the MR signal. Infection and premature death are indistinguishable with current imaging methods.

The proposed classification of the natural history and complications of hydatids (Fig. 3.26) is modified from two earlier classifications (Lewall).

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