Phantom body pain in paraplegics: Evidence for a central “pattern generating mechanism” for pain

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Abstract
Chronic pain may be caused by a loss of sensory input. Cases in the earlier literature were reviewed and new cases were presented of chronic pain patients who sustained total spinal cord sections. The completeness of the lesion was verified visually during surgical removal of injured tissue or as a result of segmental cordectomy to prevent nerve impulses produced by injured tissues from reaching the brain. Nevertheless, pain returned immediately or as long as 11 yr later. The pain was usually felt in definite parts of the body phantom and was often described as burning, crushing or cramping. The sympathetic ganglia, the only other possible route for nerve impulses from the legs, were also blocked in several patients without effect on the pain. There is convincing physiological evidence to show that deafferentation produces abnormal physiological activity in spinal and brain cells deprived of input. The cells fire spontaneously in high frequency bursts and may be triggered by inputs from adjacent structures; the abnormal firing may persist for hours after a single, brief triggering stimulus, and abnormal activity persisted for months. A model was presented, consistent with the gate control theory of pain, which proposes that synaptic areas along the transmission routes of the major sensory projection systems, from the dorsal horns to the somatosensory projection areas in the thalamus and cortex, may become pattern generating mechanisms. Not all pain is due to noxious input projected via A.delta. and C fibers from the periphery. Some pains are due to loss of sensory input and secondary changes in the firing patterns of neurons lying rostral to the primary sensory synaptic relays.