Diagnosis

Vertebral Hydatid Cyst

Discussion

Hydatid cyst is produced by the larval stage of Echinococcus granulosus and is more prevalent in sheep- and cattle-raising areas. In human, E. granulosus is contracted by ingestion of the eggs, which are contained in the feces of the dog. After ingestion, the embryos escate from the eggs, traverse the intestinal mucosa, and are disseminated via venous and lymphatic channels. Cysts may develop in various viscera, particularly the lier and the lungs. Bone lesions are reported in 1 to 2 percent of cases of echinococcosis. Osseous involvement almost invariably is related to primary infection and is not the result of extension from a neighboring soft tissue lesion. Although hematogenous seeding of the skeleton in echinococcosis conceivably can occur in any site, one bone, a few adjacent bones, or one skeletal region usually is affected. When several adjacent bones are involved, skeletal contamination generally has resulted from direct invasion of one or more bones from another skeletal site. The spine is involved in about 50 percent of cases, and such involvement may lead to paraplegia or nerve root compression. Rib and costochondral abnormalities can occur as an isolated phenomenon, in association with vertebral lesions, or as a result of erosion from adjacent pleural cysts. Intraosseous foci of hydatid cyst predominate in the spongiosa and consist of minute, separate thin wall cysts. As the cysts enlarge, cortical thinning and expansion, pathologic fracture, and soft tissue extension can ensue. Periosteal bone formation is unusual. Soft tissue cysts proliferate, become delineated by a think fibrous membrane, and may contain seropurulent fluid.

Findings

The histologic and gross pathologic findings in the bone differ somewhat from changes in other organ systems. Cysts developing in the viscera are characterized by a well-developed outer host adventitial layer; those occurring in osseous tissue lack this outer layer. Hence the enlarging cystic lesions extend within the medullary canal. Secondary cystic lesions, termed daughter cysts, can be identified, leading to a multivesicular appearance. Radiographs can reveal single or multiple expensile cystic osteolytic lesions containing trabeculae. These may be associated with cortical violation and soft tissue mass formation, with calcification. The radiographic characteristics in the spine include, lack of osteoporosis and sclerosis in the host bone, absence of damage to the intervertebral discs, presence of posterior elements and rib involvement and occasionally, intralesional calcification, and subperiosteal and subligamentous extension of disease. CT features of echinococcosis include a soft tissue mass adjacent to sites of bone involvement. The center of the mass contains fluid with low attenuation values that does not enhance after intravenous administration of contrast material. Cystic lesions also are identified with MR imaging within the bone nad adjacent soft tissue. The signal characteristics of the cyst are variable and may not be diagnostic. The presence of numerous cystic lesions appears characteristic. Accurate diagnosis may be aided in some patients by eosinophilia (25 to 35 percent), and positive results on complement fixation test, intradermal injection of hydatid fluid, and indirect hemaglutination test. Needle biopsy of the lesions may lead to further dissemination of infection.

Complication of osseous involvement in echinocosccosis include pathologic fracture, secondary infection, especially with staphylococci, rupture into spinal canal with neural problems, including paraphegia, transarticular extension with osseous collapse and deformity, and intrapelvic extension with compression of adjacent structures.

References

1. Resnick D.: Echinococcosis, Diagnosis of Bone and Joint Disorders, 3rd edition 2539, 1995.

-F. Saremi, M.D.