First, they'll place needles into each disc that will be tested. the doctors that use heavy sedation will do this while you are knocked out. When all the needles are in place, they will bring you around and ask you questions to make sure you are lucid.
Then, they will inject contrast into the first disc while watching on a flouroscope (real time xrays). They get multiple pieces of data from this. They can see where the contrast goes. This lets them know how serverely degenerated the disc nucleus is. Healthy disc material will absorb the contrast giving a 'cotton ball' appearance with the contrast forming a cloud in the middle of the disc. Severely degenerated nucleus will quickly find it's way out instead of being absorbed into the nucleus. You can see how tough and stringy the nucleus is. You can see the annular tears. You can see where the contrast runs out through the open tears. You can see the outline of structures in the canal area as the contrast outlines them (if there are open annular tears.)
As important, or even more important is the appearance of the contrast and how it moves through the system, is the provocation of pain. When they inject the contrast they pressurize the disc, recreating some higher pressure that you may feel when you sneeze, slip, etc... If pressurizing the disc provokes pain, they will ask you if it is YOUR pain... is this what you experience when you feel YOUR pain? That would be described as concordant pain that gives higher confidence in the result.
10/10 concordant pain from the degenerated discs, but 0/10 in a 'control disc' would indicate a positive result.
One problem with discography is that it is a 'double subjective' test. It relys on the doctor's subjective assessment of your subjective assessment of your pain. We have difficulty interpreting the results when you get ambiuous results... like 6/10 'maybe concordant' pain, or out of control pain responses when the needles are being placced or control discs are being pressurized.
Some doctors will use pressure manometry and will plot the pressures with the pain responses. Something like "opening pressure, 18 psi, pain began at 45 psi, the patient reported 10/10 concordant pain and the test was stopped at 85 psi with a firm endpoint.
The firm endpoint indicates that the doctor could not increase pressure and that it was solid. If there are big annular tears that allow the contrast to escape, it may be impossible to generate much pressure.
The test is somewhat controversial. Most of the doctors I know who do not embrace discography do so based on outdated information. The doctors I know who use discography do not view it as the ultimate decider and understand the limitations of the test. It is just another source of information. Well done, well documented tests with conclusive results are easy to have confidence in. Poorly done or poorly documented tests with ambiguous results don't carry much weight.
I hope this helps,
Mark
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