A Psychometric Evaluation of Maximum Phonation Time and S/Z Ratio as Pragmatic Outcome Measures of Bulbar Function in Adults With Spinal Muscular Atrophy.

INTRODUCTION/AIMS: A pragmatic evaluation of bulbar function among adults with spinal muscular atrophy (awSMA) is needed, requiring the validation of a low-cost, feasible outcome measure (OM). Maximum phonation time (MPT) and S/Z ratio (S/Z) are potential low-cost OMs for bulbar function. This study aimed to evaluate the psychometric properties of MPT and S/Z among awSMA.

METHODS

This single-site prospective observational study followed awSMA over 12 months. Each participant completed MPT, S/Z, and a battery of routinely used OMs at baseline and 12 months. The psychometric properties of intra-rater reliability (IRR), test-retest reliability (TRT), concurrent validity (CV), predictive validity (PV), and sensitivity to change were evaluated.

RESULTS

Fifteen awSMA completed the study, with a mean age of 35.5 (SD: 16.7) and 47% male participants. MPT correlated moderately with forced vital capacity (liters) and peak cough flow, and demonstrated high IRR (0.95, 0.94) and TRT over 12 months (0.80). MPT exhibited poor sensitivity to change over 12 months (0.08, 95% CI: -0.56 to 0.71). The S/Z did not exhibit significant CV, and demonstrated only modest TRT (0.55, 95% CI: 0.06-0.83), and low sensitivity to change (0.25, 95% CI: -0.32 to 0.83).

DISCUSSION: MPT is a low-cost pragmatic tool to measure bulbar function and a surrogate for respiratory function OMs among awSMA. MPT may be helpful for patients with limited access to alternative bulbar or respiratory measures or for telehealth clinical care settings. MPT's low sensitivity to change limits its clinical utility over a 12-month interval. Larger studies are necessary.