Studies in autotomy: its pathophysiology and usefulness as a model of chronic pain

An interesting behavioral syndrome results in animals from the same or similar types of lesions that lead to deafferentation pain in humans; many neurectomized animals begin to scratch, bite, or self-mutilate their denervated limb, a phenomenon termed autotomy. The proposition that this behavior in animals is a response to the chronic pain of peripheral nerve injury has met with considerable controversy. If this issue were resolved, then a better understanding of the neurophysiology of autotomy might help elucidate the mechanisms of the human conditions. To determine the association between deafferentation and the autotomy behavior, we developed a pharmacologically induced functional deafferentation preparation using chronic perineural lidocaine infusion of the sciatic nerve. This 'chronic lidocaine' model's behavior was compared with that of the neurectomy model. While autotomy was noted in 80% of the latter group, no animal undergoing a chronic perineural infusion of lidocaine autotomized. We thus conclude that autotomy is not a response to non-painful sensory deafferentation, but rather that this behavior is a response to pain. We also studied the development of autotomy in a variety of other focal denervation preparations. On the basis of these data, we conclude that autotomy is not due to loss of sensory input on a functional basis nor to an action potential-mediated process. Rather, nerve damage which coincidentally involves sensory loss is necessary and sufficient for the development of this behavior. We suggest that interruption of a humoral feedback process homeostatically operating within the first order sensory neuron with its effect exerted post-synaptically leads to autotomy. The evidence supports the existence of a loss of a transportable, humoral autotomy inhibitory factor.