Central motor conduction examined with the triple stimulation technique correlates with pyramidal tract dysfunction and disability in patients with multiple sclerosis

Inflammatory lesions and axonal degeneration of central motor conduction pathways result in gait disturbance and result in increasing disability in patients with multiple sclerosis (MS). Conventional transcranial magnetic stimulation can be used to estimate central conduction times, but is a less sensitive method to detect axonal damage or conduction block and correlates weakly with clinical disability. The triple stimulation technique (TST) examines the corticospinal tract by magnetic stimulation of cortical motor areas followed by sequential distal and proximal peripheral nerve electric stimulation. The TST potential is a result of the collision of the sequential stimuli. The TST potential is compared with sequential stimulation of peripheral nerves alone, and central conduction failure can be quantified with the resulting amplitude ratio.

Methods: Upper limb (to abductor digiti minimi muscle) and lower limb (to abductor hallucis muscle) TST was applied to 50 healthy volunteers to establish normal values and 60 patients with relapsing remitting, secondary progressive and primary progressive MS. Patients also underwent neurological examination including the Expanded Disability Status Score (EDSS) according to Kurtzke and in the Multiple Sclerosis Functional Composite Score (MSFC) to quantify clinical upper und lower limb function.

Results: The EDSS of patients varied between 0 to 8.5. Central conduction failure quantified with TST correlated significantly with the EDSS (r=0.45 for the TST to upper limb, r=0.6 for TST to lower limb) and with the upper and lower limb motor function scores of the MSCF.

Conclusion: TST is a useful method for examining central conduction failure in different stages of disability in patients with MS and correlates well with clinical deficits and disability scales reflecting motor involvement. TST could serve as a surrogate marker for axonal degeneration of motor pathways in MS.