Testicular torsion occurs when the spermatic cord (from which the testicle is suspended) twists, cutting off the testicle's blood supply, a condition called ischemia. The principal symptom is rapid onset of testicular pain. The most common underlying cause is a congenital malformation known as a "bell-clapper deformity" wherein the testis is inadequately affixed to the scrotum allowing it to move freely on its axis and susceptible to induced twisting of the cord and its vessels.