Usually nothing. With modern techniques and the tiny needles, headaches are very rare. You really should not expect any side effects.
No worries . A myelogram is a common diagnostic study that most patients tolerate well. The physician will place you on a flouroscopy table, prep your lower back, inject a local anesthetic, place a needle in your back, inject dye, remove the needle, place a bandaid and summon the radiology tech to take x-rays. Then you will have a ct. You will be observed for several hours and be discharged.
Usually nothing but. Occasionally some soreness at the needle site and infrequently a headache that usually gets better with rest and fluids, usually caffeinated.