Originally published by Harvard Health.
What is the test?
A myelogram is an x-ray test in which dye is injected directly into your spinal canal to help show places where the vertebrae in your back may be pinching the spinal cord. It is sometimes used to help diagnose back or leg pain problems, especially if surgery is being planned.
How do I prepare for the test?
Tell your doctor ahead of time if you have ever had an allergic reaction to lidocaine or the numbing medicine used at the dentist’s office, or to x-ray dyes. You should also tell your doctor if you might be pregnant, since x-rays can harm the developing baby.
What happens when the test is performed?
Patients usually wear a hospital gown. Typically, you lie on your side with your knees curled up against your chest. In some cases, the doctor asks you to sit on the bed or a table instead, leaning forward against some pillows.
The doctor feels your back to locate your lower vertebrae and feels the bones in the back of your pelvis. An area on your lower back is cleaned with soap. Medicine is injected through a small needle to numb the skin and the tissue underneath the skin in the area. This causes some very brief stinging.
A different needle is then placed in the same area and moved forward until fluid can be injected through it into the spinal canal. This fluid is a type of dye that shows up on x-rays; this allows your doctors to get a clear picture of the fluid space around your spinal cord and to see places where the space is narrowed by bones around it. Because the needle must be placed through a small opening between two bones, the doctor must sometimes move the needle in and out several times to locate the opening. Because of the numbing medicine used in this area, most patients experience only a feeling of pressure from this movement. Occasionally some patients do get a sharp feeling in the back or (rarely) in the leg. Let your doctor know if you feel any pain.
Once the dye has been injected, the needle is removed and several x-ray pictures are taken of your back. Sometimes a CT (computed tomography) scan picture is taken instead.
What risks are there from the test?
Risks from this test are minimal. A few people have an allergic reaction to the dye used in the test. Some people have a headache or back soreness for a short time.
As with x-rays, there is a small exposure to radiation. In large amounts, exposure to radiation can cause cancers or (in pregnant women) birth defects. The amount of radiation from a CT scan is larger than from regular x-rays, but still very small—too small to be likely to cause any harm.
Must I do anything special after the test is over?
No. Usually a Band-Aid is the only dressing necessary for your back.
How long is it before the result of the test is known?
It takes about an hour to have your x-rays or CT scan developed and some time for the films to be reviewed by a radiologist. Usually your doctor can get the results within a day.