Quantification of Uhthoffʹs phenomenon in multiple sclerosis: a magnetic stimulation study

Objective: To quantify temperature induced changes (=Uhthoff phenomenon) in central motor conduction and their relation to clinical motor deficits in 20 multiple sclerosis (MS) patients. Methods: Self-assessment of vulnerability to temperature and clinical examination were performed. We used motor evoked potentials to measure central motor conduction time (CMCT) and applied the triple stimulation technique (TST) to assess conduction failure. The TST allows an accurate quantification of the proportion of conducting central motor neurons, expressed by the TST amplitude ratio (TST-AR). Results: Temperature induced changes of TST-AR were significantly more marked in patients with prolonged CMCT (P=0.037). There was a significant linear correlation between changes of TST-AR and walking velocity (P=0.0002). Relationships were found between pronounced subjective vulnerability to temperature and (i) abnormal CMCT (P=0.02), (ii) temperature induced changes in TST-AR (P=0.04) and (iii) temperature induced changes in walking velocity (P=0.04). CMCT remained virtually unchanged by temperature modification. Conclusions: Uhthoff phenomena in the motor system are due to varying degrees of conduction block and associated with prolonged CMCT. In contrast to conduction block, CMCT is not importantly affected by temperature. Significance: This is the first study quantifying the Uhthoff phenomenon in the pyramidal tract of MS patients. The results suggest that patients with central conduction slowing are particularly vulnerable to develop temperature-dependent central motor conduction blocks.