How do i find out if some of the anesthetic in an epidural steroid injection went into the subdural space?

Not Enough Detail: ..To answer your question precisely. The inadvertent "central" spread of epidural anesthesia can cause apnea (cessation of breathing), profound drop in bp, profound muscle weakness and loss of consciousness. Lesser degrees of dural "spread" might be evidenced by the unusually rapid onset, or spread upward, of anesthesia.

Hard to say: Some of the anesthetic probably gets into the subdural space just by diffusion through the dura. The difference is in how much anesthetic got there and what the original concentration of the local anesthetic was. If you weren't supposed to have a motor block(weakness of muscles) and did it might have been due to subdural migration.. But that can't be stated with any certainty, either.

See below: If it did you will have loss of motor function in your legs or a persistent numbness in your legs.

Why?: What does it matter? If some anesthetic got into the subdural space you will have better relief. However if you have a headache, or increased pain it might be something to evaluate. typically headaches will resolve majority of the time on their own. If not you might need an epidural blood patch.

It's very difficult: at this point. The only way to tell for sure where the needle is located is to inject some contrast media and check with x-ray. Medication spread in the subdural space is spotty, frequently one-sided and excessive. You will not get a headache though.

Hard to tell: If you don't have a dural puncture headache probably not. In any event there is nothing to do about this if it did.