The Symbol Digit Modalities Test (SDMT) is a sensitive and validated test of cognition in MS. It measures processing speed, attention and working memory. How to use and interpret the SDMT in clinical practise remains unclear. Some authors have suggested that a change of 4 or more points should be considered clinically significant

This is a single center, 5 year, observational study looking at cognition in natalizumab treated MS patients. The SDMT was administered every 4 weeks just prior to their infusion. We rotated through 5 different versions of the SDMT.

63 patients were recruited and 45 completed the study.

The mean SDMT score for the group continued to improve throughout the study.

Patients with baseline SDMT scores in the impaired range also improved but remained impaired relative to the non-impaired group.

The within-subject standard deviation (WSD) was 5.58 and it remained unchanged independent of the period sampled or whether the patients were impaired or not at baseline.

A raw change of 8 in SDMT score corresponded to a Reliable Change Index (RCI) of 1.01.

Using an RCI of 1.01 (raw SDMT change of 8.0) and comparing month 60 to baseline, only 1 patient experienced a decline, 23 remained stable and 21 improved. This was accompanied by a concomitant mean change in EDSS of 0.5 increase, 0.13 reduction and 0.15 reduction respectively.

The percentage of patients that improved their SDMT scores by 8.0 or more relative to the previous month  increased over time and plateaued after 4 years.

In everyday practise a change in SDMT score of 8 or more should be considered clinically significant especially if sustained.

Natalizumab treated MS patients over a 5 year period showed a sustained learning effect which in itself suggest preservation of cognitive reserve.