MRI vs. Myelogram
Benefits and Drawbacks of Diagnostic Tests for Back Pain
Question: I recently had an MRI. I went to another doctor for a second opinion, and he wants to do a myelogram in addition to an MRI. What's the benefit of this? Are there any complications associated with myelograms that I should know about?
— Oakland, CA
Answer: There are several advantages to a myelogram. First of all, MRI quality varies markedly. For example, some have very low-strength magnets or poor rendering software. Open MRIs tend to have poor image quality, and all MRI scans are susceptible to artifact. Artifact is interference with the magnetic waves that create the image (just like how a magnet by your TV screen warps the picture). We call all unintended imaging changes artifact. A common example of an MRI artifact is motion of the patient from anxiety or even breathing. Metal from dental fillings or hardware instrumentation can also create significant misrepresentation of the imaging results.
Myelograms are usually accompanied by CT/CAT scans, which often have higher resolution than MRI. That means the images show more detail, much like a higher resolution digital camera produces sharper pictures. Consequently, more subtle signs of disease may be identified. CT scans also show bony spur growth, calcification, and general bone anatomy much better. This is important in evaluating bone density and facet joint anatomy, as well as planning surgical procedures.
Since a myelogram uses dye to fill the actual nerve root sleeves, the surgeon can better see and assess the degree of nerve root compression. In other words, there may be compression of the fluid around the nerves, but plenty of room for the nerves themselves.
Despite the many benefits of myelography, there are some complications you should be aware of. The risk of any of these is typically cited at 5%.
Complications of Myelograms
The most common risk is a headache caused by changes in spinal fluid pressure. When this occurs, patients get a very bad headache, which is treated with bed rest and rarely, injection of blood at the injection site.
Another possible complication is allergic reaction. This typically occurs because the contrast dye contains iodine. Patients sensitive to eating shell fish are particularly susceptible to this side effect. Likewise, the dye can increase the risk of seizures.
Women who are pregnant should not get a myelogram since it exposes the baby to radiation. MRIs do not use radiation, so they are safe for use in pregnancy.
Other rare complications of myelogram include injuring the nerve root with the needle and bleeding. This is rare, but could lead to numbness or weakness. For that reason, most physicians have patients stop all blood thinners prior to the injection.
Infection is a rare complication from any invasive procedure. The risk is typically much less than 1%. Diabetic or immunocompromised patients are at higher risk. A very rare complication from the needle insertion is the formation of an epidermoid tumor. This is where skin cells from the surface are tracked into the patients with the needle. This may cause an abnormal growth of a benign tumor called an epidermoid. The risk of this is one in several thousand.
Finally, the injection and dye may cause inflammation of the arachnoid lining (a thin, delicate layer overlying the nerves). This condition, called arachnoiditis, can lead to numbness and/or sensory loss in the extremities.
Overall, myelography is a very safe procedure that is well tolerated in most people. While it has fallen out of favor because of the ease of MRI, myelography still remains a valuable tool in assessing patients with back pain.