FDS: Whose dance is it anyway?

A key ultrasound finding on filaria has been challenged by researchers from the Christian Medical College (CMC), Vellore. The researchers contend that the dancing particles described by the filaria dance sign (FDS) are not caused by the adult filarial worm, but by microfilaria.

First described in 1994, the relation between FDS and the adult filarial worm has since formed the basis for many other filarial studies, including epidemiological studies (The American Journal of Tropical Medicine and Hygiene, June 1994, Vol. 50:6, pp. 753-757).

However, Dr. Shyamkumar N.K and his colleagues from CMC contend that real-time ultrasound and wet smear video microscopy of a 35-year-old patient suggest the dancing forms of FDS are in fact mircofilaria and not the adult worm. Their findings have been published as a case report in The Internet Journal of Urology.

In an interview with AuntMinnieIndia.com, Shyamkumar said the findings could impact the disease management of filaria. Found mostly in tropical regions, filaria is common in coastal areas of India. Patients start becoming symptomatic when the microfilaria, larvae of the adult filarial worm, are released into the blood.

"We feel that these small dancing particles are microfilaria and not the adult worms in view of their size (<1 mm) and their location within minute lymphatic spaces (2 mm)," stated the authors of the paper published in The Internet Journal of Urology.

The paper describes the case of a 35-year-old man with FDS in the epididymal region. Aspiration of fluid from an epididymal cystic lesion yielded motile microfilaria. "There was remarkable similarity in the movement of the microfilaria on wet smear microscopy and ultrasound," wrote the authors.

Although microfilaria are theoretically beyond the resolution of high-frequency ultrasound tranducers, they can be eminently demonstrated on ultrasound examination when aleatory movements are seen in scrotal lymphatics, concluded the authors.

"Our hypothesis is to say that the sign is due to numerous microfilaria present in the dilated lymphatic sacs. Here we can just appreciate the ‘presence’ (not morphology) of these microscopic structures due to a combination of fluid background, high mobility of the particles, and possibly reflective nature of the microfilaria," Shyamkumar elaborated in a written response to AuntMinnieIndia.com.

Based on this hypothesis, the paper goes a step further and suggests the term FDS should be replaced with microfilarial dance sign (MDS).

Whether or not this will happen, only time and further studies can tell. But clearly, the study makes an important point of a curious finding on ultrasound and throws up many interesting possibilities. The biggest of which is the use of affordable, low-cost ultrasound to detect microfilaria.

And as Shyamkumar himself states, "Our description is limited to explaining the mechanism of the sign and interpretation. Its impact on the management is beyond the scope of the published article. We look forward to experts' comments."

By N. Shivapriya
AuntMinnieIndia.com staff writer
August 12, 2003

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