Task-dependent deterioration of balance underpinning cognitive-postural interference in MS.

OBJECTIVE: To investigate which concurrent cognitive task (if any) had the most detrimental effect on balance control of patients with multiple sclerosis (MS).

METHODS: In a dual-task experiment, we evaluated the reciprocal effect of simultaneously performing a postural and a cognitive task on balance and cognition in 52 patients and 26 sex- and age-matched controls. Balance was assessed by static posturography, while cognition was scored as number of correct items at 3 different neuropsychological tests, i.e., the Symbol Digit Modalities Test (SDMT), word list generation (WLG), and Stroop Color-Word Test (SCWT).

RESULTS: In both single and dual-task conditions, the patients had larger postural sway and worse scores at SDMT, WLG, and SCWT than the controls (p < 0.05). Test-retest reliability was excellent for all dual-task metrics (85%-94%). By means of 2-way analyses of the variance, we found significant main effects of dual task on balance, regardless of the concurrent cognitive task (p < 0.001). There was no main effect of dual task on cognitive performance across all the 3 task conditions (p ≥ 0.1). We observed a significant condition-by-group interaction effect on balance only when the SCWT was administered as concurrent task (p = 0.01), indicating a greater dual-task cost of balance for the patients than controls (53% vs 28%, p = 0.04).

CONCLUSIONS: We suggest that tasks exploring executive functions involved in discriminating conflicting stimuli may be the most suitable to unmask the cognitive-postural interference phenomenon in patients with MS. This may support the hypothesis that MS-related damage constrains brain networks to subserve both postural control and executive functions.