Identification of motor conduction block despite desynchronisation. A method.

Identification of incomplete motor conduction block (CB) by the usual single stimulation method is problematic when impaired conduction velocity of the axons spared by the block exists at the site of the presumed CB, leading to desynchronisation. Indeed, the area of the compound muscle action potential (MAP) evoked by stimulation applied above this site may be reduced by the phase cancellation related to desynchronisation. A triple stimulation method with double collisions allows virtual displacement of the CB distally to the site where desynchronisation occurs. A first stimulus applied on the nerve above the site of the CB is followed--after a delay--by a stimulation applied distally on the nerve where a collision occurs. This collision, however, does not concern the antidromic wave of the blocked axons which travels towards the block. A third stimulus is then applied--after a second delay--below the site of the CB, where a second collision occurs in the blocked axons. Thus, the third stimulus evokes a MAP that concerns only the unblocked axons, as would the stimulus applied above the site of the CB. In this manner, desynchronisation occurring at the CB site is avoided. The method was developed on simulated CBs of the ulnar nerve at the elbow of 33 normal subjects and compared with the usual single stimulation method. Subsequently, it was evaluated on 8 pathological CBs of the ulnar nerve at the elbow and of the peroneal nerve at the fibular head. The method detects CBs of 5% or more despite the occurrence of desynchronisation.