Electromyography in patients
with peripheral nerve lesions does not show denervation potentials in
muscles of similar myotomes innervated through a different nerve. The
distribution of arm weakness due to a peripheral nerve lesion is usually
very different than that produced by an upper motor neuron lesion. The
differentiation between an upper motor neuron lesion and peripheral nerve
injury can usually be made on clinical grounds. Electromyography and nerve
conduction velocities are abnormal in peripheral nerve lesions.