Magnetically evoked motor potentials in demyelinating and axonal polyneuropathy: a comparative study

We investigated the value of magnetically evoked motor potentials (MEPs) fo r the differentiation of demyelinating and axonal polyneuropathies. The stu dy population comprised 107 patients, with polyneuropathy verified by elect romyography (EMG) and nerve conduction study (NCS), who had also been exami ned by means of MEP. MEPs were evoked by magnetic stimulation of the cortex and the spinal roots and were recorded from three upper limb muscles and t wo lower limb muscles bilaterally. From the EMG/NCS results 53 patients wer e characterized as having primary demyelination (demyelinating patients) an d 54 as having axonal involvement (axonal patients). Demyelinating patients were classified as acute (acute inflammatory demyelinating polyradiculoneu ropathy: AIDP) or chronic (chronic inflammatory demyelinating polyradiculon europathy: CIDP) according to the duration of illness. A series of indices were calculated from MEP results. One demyelinating patient and two axonal patients had normal MEPs. The MEPs of the demyelinating patients showed sig nificantly longer peripheral conduction times, larger interside differences and lower amplitudes than the axonal patients. The central conduction inde x and the amplitudes upon cortical stimulation were significantly higher in patients with CIDP than in those with AIDP. Peripheral conduction time pro longed by more than 85% in at least one of the 10 muscles studied or a peri pheral conduction index of above 9.4 were pathognomonic for demyelination. By combining all criteria 75% of the patients could be categorized as CIDP vs. AIDP in accordance with the EMG/NCS diagnosis. Likewise, 83% were categ orized correctly as demyelinating versus axonal according to the EMG/NCS da ta.