How are subarachnoid blocks administered for pain management?

Updated: Jun 19, 2018
  • Author: Anthony H Wheeler, MD; Chief Editor: Meda Raghavendra (Raghu), MD  more...
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Differential subarachnoid block can be used as a diagnostic procedure in differentiating pain caused by somatic nociceptive sensory nerves, sympathetic hyperactivity, and pain from a primarily central source, including that of psychogenic etiology. Classically, this is performed by an anesthesiologist who inserts a microcatheter into the subarachnoid space. Bonica described a technique using a 32-gauge polyamide catheter, 91 cm long, which can be inserted through a 25-gauge or 26-spinal gauge spinal needle. During the procedure, cardiorespiratory monitoring, as well as sympathetic, sensory, and motor neural assessment, should be ongoing. After insertion of the catheter, 8-10 mL of saline solution are infused as control. Some anesthesiologists have advocated aspiration of 8 mL of CSF and then CSF re-injection because of the controversial belief that isotonic saline solution may induce a change in sensation.

The operator then injects 8-10 mL of 0.25% procaine, which should produce a sympathetic neural blockade; sympathetic neural functions are monitored, as well as any reported changes in the patient's pain. Subsequently, 8-10 mL of 0.5% procaine is injected to produce a sensory block, which can be assessed by pinprick, touch, and pinch. Finally 8-10 mL of 1% procaine is injected to produce a motor blockade. During each stage of the procedure, the patient's pain intensity, spinal level of the sensory block, and neurophysiological and behavioral changes, as well as the quality of the analgesic effect, are monitored.

Pain that responds to isotonic saline or "placebo" is presumed to have a non-nociceptive origin; therefore, possible contributing psychogenic factors should be evaluated. If a sympathetic blockade accompanied by objective evidence of sympathetic interruption alleviates the pain, sympathetic hyperactivity may account for a component of the pain. Elimination of the pain with 0.5-1% procaine should indicate that the pain has a somatic origin. Failure of any solution to block the pain also implies a central or psychogenic etiology.

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