There are two ways to accomplish this block:
1. Standard differential block. This is done by intrathecally injecting several solutions. Injections are spaced 5 to 10 minutes apart and the patient is evaluated after each injection.
A. The first solution is 5 mL of preservative-free normal saline. If the patient gets pain relief from the saline injection, this might be attributed to:
1. Placebo effect (reported in up to 30% of patients), which is usually short-lived and the pain usually comes back in a few minutes.
2. Psychological pain, in which case the patient gets an extended pain relief, which may last for days or even permanently.
B. If no relief is achieved, the second solution will be 4 to 5 mL of 0.5% procaine,with 5 mL of normal saline. This will block the sympathetic fibers without sensory or motor effects. If the patient gets pain relief with this injection, the pain is probably sympathetically mediated and the patient will benefit from a sympathetic block.
C. The third solution is 1 mL of 5% procaine added to 9mL of normal saline - somatic blockade. If the patient gets pain relief, the pain is somatic in origin and treatment should be focused on this direction.
D. The fourth solution is 2 mL of 5% procaine added to 2mL of saline. This would cause a complete motor block. If the patient continues to have the pain after complete motor and sensory spinal block, the patient's pain is considered central. This might be caused by a true organic lesion above the level of the spinal block (that is why spinal differential block is done above the suspected pain level), encephaliza-tion of the pain because of the intensity and direction, psychologically mediated pain, or the patient may be malingering.
2. Retrograde differential spinal block (more frequently used). This is done by using two solutions.
A. The first solution is 2mL of normal saline, same interpretations as in the standard differential spinal block.
B. The second solution is 1mL of 10% procaine and 1 mL of cerebral spinal fluid. This will give complete motor and sensory spinal blockade. If the patient's pain continues, it is central. If the patient's pain resolves, pain assessment should be performed every 10 minutes until there is a return of motor and then sensory function. If the pain returns with the return of the sensory function, the pain is somatic in origin. If the pain returns a few hours after the return of the sensory function, it is sympathetically mediated pain.
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