Hydatid thrill: an explanation of the phenomenon.

The hydatid thrill is a rare but pathognomic sign of hydatid disease. The physical basis of the thrill was studied in two patients with the disease. The thrill occurs when a mother cyst packed with unruptured daughter cysts is gently percussed. The consistency of the wall of the daughter cyst and the tension of the fluid inside it are responsible for making the cysts vibrate and produce the sensation of a thrill. Free fluid from ruptured daughter cysts dampens the vibrations. It is extremely rare for a mother cyst to have all the daughter cysts in an intact condition.