Should SDMT substitute for PASAT in MSFC? A 5-year longitudinal study

The multiple sclerosis functional composite (MSFC) includes the Paced Auditory Serial Addition test (PASAT) as a measure of cognition.

We compared the MSFC incorporating the Symbol Digit Modalities test (SDMT) (MSFC [sdmt]) to the usually applied MSFC (MSFC [pasat]) in a sample of 46 ptients with relapsing-remitting MS who were followed over a five-year period. Magnetic resonance imaging was performed at baseline.

The Expanded Disability Status scale (EDSS) deteriorated significantly over 5 years (P < 0.01), but MSFC scores remained stable. MSFC [sdmt] correlated with EDSS at all time points of evaluation, but MSFC [pasat] correlated with EDSS only at baseline, 1, and 5 years. The 5-year EDSS correlated significantly with baseline MSFC [sdmt] and MSFC [pasat] but did not correlate after adjustment for baseline EDSS. No significant correlation was found at baseline between MSFC and imaging parameters (lesion load, brain parenchymal fraction [BPF], ventricular fraction, mean magnetization transfer ratio of lesions and normal-appearing brain tissue), but baseline BPF correlated significantly with changes of SDMT z score (P = 0.0003), MSFC [pasat] (P = 0.006), and MSFC [sdmt] (P = 0.0002) over 5 years.

We propose not to substitute PASAT by SDMT in the MSFC but to consider SDMT as a complementary useful approach to evaluate overall MS disease.