Paris, France (AFP) Stiff-person syndrome, which has forced Canadian superstar singer Celine Dion to postpone her European tour, is a very rare neurological condition that causes progressive muscle stiffness as well as muscle spasms.   Around one in a million people are estimated to have the condition, according to the US National Organization for Rare Disorders (NORD). Symptoms develop over a matter of months or years, normally between the ages of 30 to 60, and […]