Quinlan has announced his retirement with immediate effect after reviewing the results of a recent MRI scan with a neurosurgeon.
Quinlan said: "It was my firm intention to see out the remaining year of my contract.
"And it is with great regret that I have had to end my career in this way. Rugby has been a massive part of my life for many years, and I am going to miss it greatly.
"It's been a wonderful couple of years at Franklin's Gardens. I have loved playing for the Saints with some great guys and under some great coaches.
"I hope to contribute what I can for the remainder of the season to ensure there will be Premiership rugby at the Gardens next season and European success this year."
Quinlan joined Northampton from Leinster in 2005, going on to make 35 appearances.
Saints head coach Paul Grayson said: "We are disappointed to lose a player of his quality.
"It is a cruel way for his career to finish this way. He put his body on the line time and again for the club.
"We are losing a leader as well as a well-liked member of the squad."