The Canadian superstar had been rumoured to be singing a duet with Lady Gaga, but instead went solo on the Eiffel Tower to bring the four-hour event to a stirring climax. It was Dion’s first live performance in four years, and came a year and a half after she revealed a diagnosis of Stiff Person Syndrome (SPS). SPS is a rare neurological disorder that causes muscles to spasm and can be debilitating. It also affected her distinctive, forceful voice.