Midfielder Spring shrugged off two tackles midway through the first half before striking a 20-yard effort - which appeared to take a deflection off Matt Taylor - over the head of replacement goalkeeper John Sullivan.
Sullivan had been on the pitch since the game's defining moment in the sixth minute when first-choice stopper Ben Hamer was sent off having raced out of his area and handled a David Mooney lob as the striker raced on to a pass from Lee Cook. Referee Dean Whitestone had no hesitation in dismissing Hamer, with Scott Wagstaff the unfortunate player to be sacrificed for Sullivan's introduction.
The O's might have doubled their lead on the stroke of half-time but Jimmy Smith's header was contentiously ruled out for offside.
Mooney had three good chances to extend the home side's advantage in the second half before he was substituted, but it was the visitors who finished the stronger of the two sides with Orient 'keeper Lee Butcher punching clear from Yann Kermorgant.
Centre-back Ben Chorley then protected his side's slender lead when he cleared a shot off the line from Rhoys Wiggins with seven minutes remaining, ending a nine-match unbeaten run for Chris Powell's side.