When I had sciatica back a few years ago, I remember one of the tests they always did was to raise my leg off the table while I was lying down and see if that caused my pain. This time when I went in to see the doctor, they also did the test with me sitting up. There was no pain in the sitting position, but it did hurt lying down. What does that mean?
You have just described the supine straight leg raise test and the sitting straight leg raise test. Both put tension on the sciatic nerve and test to see if the nerve is being compressed or pinched as a cause of your pain. The sitting straight leg raise test is also known asthe flip test because some people back when tension on the nerve starts to cause pain.
Changing position by leaning back, bracing the hands on the table behind you, or tipping the pelvis under also takes pressure off the nerve. Although the original test was described as positive for sciatic nerve compression when the person flipped back, more recent tests show that the response may be more sublte as just described. In one study of patients with known disc herniation compressing the sciatic nerve, the sitting leg raise test was negative more often than positive.
As a result of that study, it's been recommended that the sitting leg raise test should just be used to confirm the results of the supine straight leg raise.It's only really valid when the supine straight leg raise test is positive in the first 45 degrees of leg lift. That's when the sitting leg raise is the most reliable test to confirm the presence of sciatic nerve tension.
The fact that your sitting leg raise test was negative may have occurred because you shifted your trunk or pelvis to offset the tension. The examiner may not have seen that or recognized those compensatory movements as a positive response. If you had remained upright in the starting position throughout the test, the pain might have been reproduced (thus giving you a positive test).