Motor fatigue in MS: transcranial magnetic stimulation

Epub ahead of print
Scheidegger et al. Corticospinal output during muscular fatigue differs in multiple sclerosis patients compared to healthy controls. Mult Scler. 2012 Feb 21. 

Background: In MS, fatigue is a common and often a disabling symptom. Fatigue has multiple causes with central motor fatigue playing an important role.

Objective: The objective of this study was to analyse the central motor conduction changes (speed and size of the electrical impulse) in relation to muscle contraction force (power) during muscle fatigue and recovery in MS’ers compared to healthy controls.
Methods: A total of 23 MS’ers with fatigue and 13 healthy subjects were assessed during 2 minutes of fatiguing exercise of the abductor digiti minimi muscle of the hand (muscle that makes the little finger move outwards) and the subsequent 7 minutes of recovery. Central motor conduction was quantified by transcranial magnetic stimulation using the triple stimulation protocol and calculating a central conduction index (CCI).
Transcranial magnetic stimulation is a method that uses a magnetic stimulus to activate the nerve cells on the surface of the brain. It is painless. 

Results: Force declined to 36% of the pre-exercise level (SD 16%; p < 0.01) in MS’ers and to 44% (SD 9%, p < 0.01) in healthy subjects (group differences, not statistically significant). The decline of the CCI was significantly less marked in patients (-20%, SD 26%, p < 0.05) than in healthy subjects (-57%, SD 15%, p < 0.05; group differences, p < 0.05). The decline of force and CCI were not correlated in either group.
Conclusions: During a fatiguing exercise, the decline in central motor conduction is significantly less pronounced in MS’ers than healthy subjects, although the reduction of force is similar.
“These results are counter-intuitive; I would have expected MS’ers to have declined more than healthy controls. Maybe MS’ers compensate more and therefore have less spare capacity. As with all science the result of this experiment may be incorrect and will need to be reproduced by other groups, preferably using a larger sample size.” 

2 thoughts on “Motor fatigue in MS: transcranial magnetic stimulation”

  1. Is motor fatigue what causes me to peter out when I'm walking or doing some other physical activity? Is there a practical way to measure motor fatigue? My neuro seems to think that the 25 foot timed walk covers walking overall, but there's a pretty profound disconnect between my times on that (which have been fairly stable in the 5-6 second range for 4 years) and my ability to walk any distance (which has declined dramatically)

  2. Re: "Is motor fatigue what causes me to peter out when I'm walking or doing some other physical activity?…"Yes, this is related to activity-dependent conduction block in axons that are either demyelinated or inadequately remyelinated. This causes a reduced safety factor of conduction that fails with exercise or a rise in body temperature. Yes, we can measure this using evoked potentials or central motor conduction times. I agree with the disconnect between the 25-ft walk and free motor activity. This is why we are proposing to test several different outcomes when we assess response to Fampridine. I often ask my patients to do a timed-walk, every 6 or 12 months, to assess disease progression. For example, walking to the local shop and back. I prefer real-life activities rather than artificial ones in the clinic.

Leave a Reply

%d bloggers like this: