Trunk sway analysis to quantify the warm-up phenomenon in myotonia congenita patients

Patients with autosomal recessive myotonia congenita display myotonia and transient paresis that diminish with repetitive muscle contractions (warm-up phenomenon). A new approach is presented to quantify this warm-up phenomenon under clinically relevant gait and balance tasks.

Ten patients with DNA proven autosomal recessive myotonia congenita and 14 age-matched controls participated. Subjects performed six everyday gait and balance tasks. Balance control during these tasks was monitored using two angular velocity transducers that measured trunk movements in anterior-posterior (pitch) and medio-lateral (roll) directions at the level of the lumbar vertebral column. Tasks were performed under two conditions in randomised order: after a 10-minute seated rest period ("rested") and after having consecutively repeated the task five times ("warm-up"). Controls were also tested twice.

"Rested" patients showed the greatest abnormalities (increased sway in pitch and roll) for tandem walking and walking stairs. Balance impairment was also evident for all other tasks. After "warm-up," balance was markedly improved in patients, as reflected by decreased trunk sway (especially during tandem walking) and reduced task duration for all tasks. These results were not only evident at the group level but also clearly present in individual patients.

The results show that trunk sway analysis detects postural instability in myotonia congenita patients during everyday gait and balance tasks. Moreover, this technique provides a useful tool to quantify the warm-up phenomenon, suggesting a potential use as clinical endpoint in future clinical trials.