Less frequent findings are echogenic shadowing foci in worm centers due to calcification, and small, nearly anechoic regions within nodules due to fluid. The findings are so characteristic that onchocercomata are reliably distinguished from lipomata, fibromata, foreign body granulomata, and other masses (Fig. 26.29).

There are two other important applications of filarial ultrasonography. Ultrasonography can be used in epidemiological surveys and will more accurately define the prevalence of infection; compared with physical examination, ultrasonography will identify more nodules in patients than can be palpated. This is particularly useful in those with Sowda, where no nodules are palpable.

Ultrasonography is also useful in clinical research to assess the effectiveness of antimicrofilarial drugs. Studies in West Africa have described characteristic changes within onchocercomata during treatment with suramin (Figs. 26.29, 26.30). The sonographic findings correlate well with the histology of nodulectomy specimens, showing first an increase in echogenicity and then an increase in hypoechoic foci within nodules. Over a 1 year period there is loss of the lateral acoustic shadows, increasingly poor distinction between the capsule and adjacent host tissue, decreased volume of the nodule, and, in some cases, the development of tiny echogenic shadowing foci. Such increasing echogenicity in worm centers is the result of nematode fragmentation. Later, there is vacuolation and involution, and necrosis is visualized as hypoechoic foci within nodules. The surrounding fibrous capsule is gradually resorbed, becoming similar to host tissue; calcification may occur centrally.

Fig. 26.28. Longitudinal and transverse sonograms through an enlarged lymph node in a patient with onchocerciasis. Note the more uniform echogenic core and the absence of a lateral acoustic shadow. (Courtesy of Drs. K. Darge and M. Leichsenring).

Fig. 26.29A,B. Sonograms of onchocercal nodules in a West African patient. A A skin nodule before suramin therapy shows typical features. B After 3 months of therapy, there is increased echogenicity in the worm centers within the nodules (arrow) due to fragmentation of the worms. Longitudinal (left) and transverse (right) sections. (Courtesy of Drs. K. Darge and M. Leichsenring).

Fig. 26.30A,B. The ultrasonography of onchocerciasis. Longitudinal (left) and transverse (right) images of a nodule before A and 6 months after B suramin therapy. Note that the lateral acoustic shadows seen in A (arrow) have disappeared in B, and the worm center is no longer identified. (Courtesy of Drs. K. Darge and M. Leichsenring).