There is a significant difference between diagnostic and therapeutic injections. These injections occasionally can be one and the same but generally, there is a difference.
Most non-diagnostic interventionalist’s goals are to try to give patients pain relief. These physicians are not there to accurately diagnose the exact cause of the pain as that is not their assigned task. They are not surgeons, they are pain relievers. These doctors use therapeutic injections in which a large volume of anesthetic and steroid is utilized to cover as many levels (or pain generators) as possible. Comfort of the patient is also important so IV sedation is used liberally to keep the patient at ease. These sedation medications of course take time to wear off so accurate thought is lost during the “golden diagnostic window” (the first three hours after the injection).
Injectionists who focus on diagnosis have to think differently. Diagnostic injections rely on only one structure to be anesthetized per injection. Obviously if two or three structures are numbed, there would be no way of ascertaining which structure could be the pain generator (assuming there is only one). Using a small volume of anesthetic agent on a specific structure is a very good way to determine if this structure is causing pain. If only a portion of pain is relieved with this one injection, then the deduction is that this structure is causing only this portion of the pain and other structures yet to be tested could be causing the other component(s) of pain. Further injections can then identify other pain structures to complete the picture.
Diagnostic injections depend upon an awake, minimally or non-sedated patient to accurately assess the pain levels after the injection. Essentially, the nerve, disc, facet, pars or canal is “numbed” with an anesthetic (Lidocaine, Marcaine or Novocain) and the patient has to evaluate if this numbed structure yields relief of the original pain. If the patient is too sedated, the patient will not be able to accurately measure the amount of pain relief (using a 0-10 visual analog scale) for the first three hours (the critical diagnostic window used to determine pain relief).
The diagnostic window is the period of time that the anesthetic agent is effective. This is similar to the injection a dentist gives you to numb your jaw. The total time your jaw is numb (about 2-3 hours) is generally about the same time this spinal injection will work to numb the area injected, be it a nerve, disc, spinal canal or facet. If the patient is too sedated during this period of time, the diagnostic window will be lost and with that, any information.
Pain medication also has to be limited prior to the injection. While some of these medications last only a short time (Fentanyl), some medications can last over 4 hours and will throw off the results of this test as pain relief from the narcotic can be misinterpreted as pain relief from the block. Patients should not take their regular or extra dose of pain medication prior to the injection.
Some patients have very active degradation enzymes and numbing agents do not work effectively on these individuals. This means that a diagnostic injection will not generally work. The best screening question is whether a dentist office injection is effective or if the dentist has to inject multiple times to gain some anesthesia effect. If multiple injections are the case, a high concentration anesthetic injection (2-4%) might work diagnostically but will cause a motor block of the nerve. This means that the patient will not be able to use that myotome (muscles connected to that nerve root) for 8-12 hours. This can be effective for the lumbar spine as patients can deal with a partially weak leg but should never be used in the cervical spine due to potential block of the spinal cord and temporary respiratory depression.
The patient has to aggravate the pain prior to the injection. If the pain level is a “2”prior to the injection and drops to a “1”, this yields little information. However, if the pain prior to injection is a “7” and drops to a “2”, this is quite valuable and demonstrates a direct correlation between the injected area and the pain relief.
In addition, some patients have to perform an activity to produce the pain (walk or lift for example). If the patient is too sedated, they will not be able to perform the activity necessary to aggravate pain after the injection (during this 2-3 hour diagnostic window) and the diagnostic portion of this injection will be lost. A good example is a patient with spinal stenosis who only aggravates pain with standing and walking. If they lie on the post-op gurney for an hour after the injection and then are driven home for another hour, the diagnostic window will close and this patient will have no idea if they gained relief for the first two hours.
One common misconception that many interventionists make is to assess the patient days after an injection. This is the period when the steroid (also injected at the same time as the anesthetic) becomes active. Steroids take between 24-36 hours before they become active. The steroid will distribute throughout the body and reduce irritation wherever inflammation is active. Feeling better outside the period of the three hour diagnostic window is not helpful to understand the pain generators. I could inject steroid into a knee and in three days, the back will feel better.
A pain diary needs to be filled out immediately in real time to make sure the results are memorialized. Memory is not always accurate. Having a patient fill the diary out in the office days to weeks later can be erroneous and can lead to false findings and conclusions.