[Conduction bloc in peripheral nerves. Facts and hypotheses].

In the terminological dictionary of the AAEE conduction block is defined as inability of impulse to pass through a certain point in nerve fibre, although conduction is preserved below that point. Conduction block is manifested as a quantitatively corresponding drop of the amplitude and area of motor evoked potential (MEP) in the distal part in relation to the values of these parameters proximal to the block. Excessive desynchronization of conduction in nerve fibres and certain technical factors should be excluded. Multifocal conduction block is found in many neuropathies, particularly purely motor ones. The probable mechanisms responsible for the difference between motor and sensory nerve fibres with respect to their susceptibility to block development have been discussed. The understanding of conduction block could have important clinical implications, especially in the differential diagnosis of chronic demyelinative neuropathies, in the selection of immunological research methods, in the choice of treatment and prognostication of possible improvement (e.g. in the Guillain-Barre syndrome). Purely motor neuropathy with multifocal conduction block may simulate amyotrophic lateral sclerosis. The development of block in neuropathies is associated frequently with increased level of antibodies against GM1 and the interrelation between these phenomena is not known; the development of block may be due to deposition of GM1 antibodies in internodes or Ranvier's nodes with production of demyelination or with damage to ion channels. Although the concept of conduction block has been introduced in electroneurographic diagnosis there are still many doubts as to the correctness of diagnostic criteria, indispensable technical conditions, and also to the basic problems, such as e.g. 1) is conduction block really limited to acquired neuropathies (its possible occurrence in congenital neuropathies is discussed), 2) why is conduction block not reflecting clinical improvement, 3) in what degree is block reflecting the loss of motor units. These questions are discussed by the author in the light of literature reports and own experience.