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The Validity of Neurodynamic Tests in the Lower Limb Part 1 Femoral Slump

July 17, 2017 by physicalrevolution No Comments »

From my page on Specificity and Sensitivity¬†we can draw upon a meaning of Validity of a test and use the following paper as a real life example of the explanations on that page. So I perform in my clinic let’s say the femoral slump test to work out whether the pain that my patient is experiencing in their anterior thigh or knee for example is emanating from Nerve, Muscle or Joint. Neurodynamic tests are designed to implicate the nerve as a source (Not the sole source) of the patient’s pain. They let us know whether there is some mechanosensitivity in the nerve.

Reading this paper: “Specificity of the femoral slump for assessment of experimentally induced Anterior Knee Pain” by Lai et al 2012 offers some information as to whether or not the test is itself capable of differentiating between pain stemming from the muscle as source or the nerve. They took 12 asymptomatic people and injected saline into their fat pads in the knee. The people then rated the intensity of their pain before performing the femoral slump test by nodding the neck into flexion. They placed the patient just short of the sensation of stretch because the sensation of stretch may get confused with the sensation of pain (See article on “Stretching”). They were then placed into one of 3 groups – 1. Group whose pain decreased with the test 2. Group whose pain increased with the test 3. No change group. They found that the neck nodding did not significantly alter the the anterior knee pain.

So what is this test looking at in terms of validity. Well Validity is a combination of sensitivity and specificity. This test is looking at the number of TRUE NEGATIVES since we know that their pain is not stemming from the nerve because the saline was NOT injected into a NERVE. So we would therefore presume that the femoral slump test which seeks to implicate the nerve as a source, would test negative if it were a valid test of nerve as a source. So this paper is looking at the SPECIFICITY of the Femoral slump test. In this case it was found to be quite SPECIFIC because 75% of the people tested ( ie 9 people) did NOT report a change in their pain. BUT 3 did. So the SPECIFICITY is 75% as determined by this paper. SO what can we take away from this paper ?

Well it does not mean that EVERY patient on whom I perform this test who tests POSITIVE (Remember SPPIN Mnemonic) is guaranteed to have nerve sensitivity as the source of their pain. This is because the test was 75% SPECIFIC – so anyone quoting this paper and telling me that because the test is 75% specific it means their patient has nerve as a source of symptoms is not correct. We have to take other factors into consideration (absence or presence of Paraesthesia etc). All this paper tells us is that 12 people with saline injected into their fat pad on the most part tested negative for nerve mechanosensitivity in a test thought to implicate the femoral nerve more as a source of symptoms. The questions I would have to ask myself are: does the injection of saline into the fat pad replicate the same pain mechanism as the patient who comes into my clinic with pain in the front of their knee ? Also this was only performed on 12 individuals.

Also to really test the validity of the Femoral slump test – I would also want to test it’s SENSITIVITY – i.e it’s ability to identify TRUE POSITIVES – that would involve injecting the femoral nerve with saline so that we know that the femoral nerve has been sensitised and then carrying out the test to see how many POSITIVES i got. The problem with this is that it is a little more unethical and unpleasant to spike a nerve with saline !

Also just because my neurodynamic test is negative DOES not rule out the nerve as source because we may not be able to winde the nerve up sufficiently enough in this test to be able to rule it out as a source. BUT this is where you would have to correlate the extent to which you are winding the nerve up to elicit symptoms and the SIN factor of the patient’s presentation perhaps.

 

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