Theoretical Aspects of Pain

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Abstract
The perplexing observation that occasionally pain still may be perceived in an extremity that to all appearances is completely analgesic following solid spinal block, is discussed. Such pain may occur almost immediately following spinal block in patients suffering from causal-gia or phantom limb. It may occur after a remarkably constant time interval of tourniquet compression. And finally it may be noted only when a large nerve trunk, such as the sciatic nerve is manipulated. Although of seemingly different origin, the pain pattern is remarkably similar under all these circumstances. It is a deep, burning, poorly localized, diffuse and uncomfortable ache. This response characterizes the pain as being transmitted by small, slow, amyelinated C-fibers. With an adequae cutaneous level of spinal analgesia, complete interruption of all pain-carrying fibers below this level might be expected. However, since pain under certain uncommon circumstances still can be perceived, it seems most probable that some pain fibers enter the cord above the level of block. Laboratory and clinical evidence strongly points to the presence of peripheral afferent amyelinated pain-carrying fibers from the extremities in the sympathetic trunks. Such fibers use the trunks (the "paraspinal pathways") to gain access to the spinal cord at a more rostral segmental level via white communicating rami. These fibers are ordinarily blocked by impulses from faster fiber groups and are therefore not consciously perceived. Only when the faster fibers are blocked, as by tourniquet compression, or when all slow fibers are stimulated simultaneously, as by direct manipulation of the sciatic nerve, is their message able to gain access to the central nervous system. Under these circumstances a lumbar sympathetic block or a raised spinal segmental level may provide complete relief of pain.

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