A probabilistic simulation model to assist the localization of nerve lesions

I present a formal, mathematical specification of a probabilistic expert system to assist the localization of nerve lesions. The program is based on an anatomical model of the peripheral nervous system of the human upper limb. The simulation model defines a joint probability distribution over the states of nerves and clinical manifestations. A simple, general-purpose heuristic algorithm is used to approximate conditional probabilities of interest. It is shown how an upper bound on the expected approximation error can be measured experimentally; this upper bound is 0.05 for the system described here, although the bound can be made arbitrarily small by expending more computational effort. The expert system is compared with the nearest-neighbour statistical classification rule on two databases of 26 and 25 cases respectively. The expert system makes fewer errors, although the observed difference does not reach statistical significance. Possible future refinements to the model are explored, and the advantages of specifying expert systems formally are discussed.