Dear friend, welcome to HCM.
normally the A-P diameter of the Lumbar canal which has the thecal sac
is greater than 13 mm.
8 mm is severe / absolute narrowing and hence the nerve
compression due to narrowing of the exit foramina (holes) and slipped disc
your spine surgeon
will be the best judge to guide your treatment.
I hope I have answered your query well.
take care and keep me posted on your progress.
Good luck !