Imaging Diagnosis

Filamentous calcification within nodules may be seen radiographically, particularly on soft tissue extremity films or on mammograms. The pattern is, however, nonspecific; if the region is biopsied, the diagnosis may still be elusive because microcalcification occurs late during the inflammatory process, long after the worms are dead and fragmented. Nodules may be recognized in skin on various plain films of the skull, extremities, chest, and abdomen. Unless calcification has occurred, parasitic disease may not be suspected, and such entities as neurofibromatosis, Kaposi's sarcoma, and other metastatic disease will all look similar. Both CT and magnetic resonance imaging (MRI) will demonstrate onchocercal nodules, but there have been no studies to investigate their value in diagnostic or other aspects of disease. This is largely due to the high cost of equipment and poor availability in the developing countries where onchocerciasis is a problem.

Ultrasonography is becoming more widely available in the developing world and studies have demonstrated its value in onchocerciasis. The appearance of skin nodules on ultrasonography was first described in Sudan over 10 years ago. More recently, with improvements in two-dimensional gray-scale imaging, and especially the use of higher frequency transducers, onchocercomata have been further characterized and differentiated from other nodules and lymph nodes. In Liberia, nodules have been imaged and excised, allowing direct comparison of sonograms and pathological sections. A typical pattern comprises: (1) a lateral acoustic (refractive) shadow, (2) a hypoechoic rim or layer, and (3) a central zone of intermediate echogenicity in which numerous tiny, highly echogenic foci are seen (this is referred to as a worm center) (Figs. 26.26, 26.27). Solitary and conglomerate nodules can be distinguished, with the latter having multiple worm centers (Fig.26.27).

Fig. 26.26. The characteristic sonographic appearance of a solitary onchocercal nodule. Note the worm center (small arrows) with small dot and rod-like echogenic foci. The lateral acoustic shadows (larger arrows) and echo-poor capsule (arrowhead) are also visible. (Courtesy of Drs. K. Darge and M. Leichsenring).

Fig. 26.27. The sonogram on a conglomerate onchocercal nodule. The worm centers are indicated by large arrows. The lateral acoustic shadows (arrowheads) and capsule (small arrows) are also seen. (Courtesy of Drs. K. Darge and M. Leichsenring).