To assess reliability and the added clinical value of static posturography as a disability marker for progressive multiple sclerosis (PMS), in comparison with the MS Functional Composite (MSFC).

There is an urgent unmet need for disability markers for PMS. Balance impairment is highly prevalent in PMS. Static posturography using a force platform is a quantitative measure of postural control not routinely used in clinical practice.

People with PMS (age 18-65 years, EDSS score 3.5-6.0) were recruited from the outpatient clinic. Participants performed a posturography protocol and a battery of walking, balance and cognitive tests. Path Length and Mediolateral (ML) Displacement were calculated from posturography data.

Seventy-three people with MS participated in the study, mean age 52.4 years (SD 8.5), mean disease duration 13.8 years (SD 10.3). Thirty-one (42.5%) required unilateral assistance to walk. Eyes Open Path Length and ML Displacement showed good-to-excellent consecutive test-retest reliability (intraclass correlation coefficient, ICC 0.924 and 0.818 respectively; average-measures, absolute-agreement, two-way mixed effects model; n=54) but only moderate two-week test-retest reliability (ICC 0.584 and 0.560 respectively; n=15).

The addition of ML Displacement to the MSFC (n=73) created a four-component z-score (“MLDIS-MSFC”) that increased the proportion of variance in a linear regression model (effect size, adjusted R²) accounting for physical quality of life (QOL), overall QOL, walking ability and bladder dysfunction by 37% (18.9% vs 13.8%), 114% (15% vs 6.3%), 39% (5.3% vs 3.8%) and 36% (6.4% vs 4.7%) respectively. The addition of Path Length to the MSFC (“PL-MSFC”) increased the proportion of variance accounting for fear of falling by 42% (9.1% vs 6.4%). Path Length alone predicted upper extremity function (adjusted R²=16%, p<0.0001).

Static posturography enhances the clinical validity of the MSFC as a disability marker for PMS. Evaluation of test re-test reliability over a longer time interval, and minimal clinically important difference, are warranted.