Dear Editor,

This letter seeks to provide clarification to the publication of Vázquez-Costa et al. entitled “Validation of a Set of Instruments to Assess Patient- and Caregiver-Oriented Measurements in Spinal Muscular Atrophy: Results of the SMA-TOOL Study” [1]. Although the main conclusion of the authors in this paper remains correct, important updates and corrections are warranted, pertaining to the version of the Spinal Muscular Atrophy Independence Scale–Upper Limb Module (SMAIS–ULM) used. This article is based on previously conducted studies and does not contain any new studies with human participants or animals performed by any of the authors.

Description of the SMAIS and SMAIS−ULM Scales

The SMAIS–ULM is a patient- or observer-reported instrument for use in clinical studies that assesses the level of assistance required to complete different activities of daily living, such as washing and hygiene, dressing, eating and drinking, picking up and moving objects, writing, and using a computer among individuals with type 2 spinal muscular atrophy (SMA) and non-ambulant individuals with type 3 SMA. It was developed using qualitative and quantitative approaches by Trundell et al. [2].

During the qualitative phase, the SMAIS was developed following a review of qualitative literature and instruments that assessed everyday activities and independence. It was further refined through two rounds of qualitative interviews with patients and caregivers. The result was a 29-item SMAIS scale that was scored on a 5-point scale (cannot complete at all without help [0 points], requires a lot of help [1 point], requires a moderate amount of help [2 points], requires a bit of help [3 points], and no help required [4 points]). However, during the quantitative analysis phase, psychometric analyses revealed that several items from the 29-item SMAIS showed a statistical misfit, suggesting that these items were measuring different concepts. Further conceptual examination of the items revealed that seven items measured mobility/strength and chore items whereas the remaining 22 items were found to measure upper limb-related activities of daily living. Additionally, the 5-point response options showed poor discrimination. Consequently, the SMAIS scale was collapsed into a 3-point scale (does not need help [2 points], needs some help [1 point], cannot do this at all without help [0 points], and not applicable [also scored 0 points]) to provide a more targeted assessment of the level of independence [2, 3]. This resulted in the 29-item SMAIS, scored on a 3-point scale, where the 22 upper limb items formed the total score, and the seven items related to mobility/strength and chores were scored separately.

Further validation analyses were conducted on the 22-item SMAIS. These analyses found the scale to be reliable, valid, and disease specific, and able to detect meaningful change in the level of assistance required to complete activities of daily living in individuals with type 2 SMA and non-ambulant individuals with type 3 SMA [2]. Further, it was decided that the seven items related to mobility/strength and chores should be dropped from the final version of the scale due to their measurement of a different concept. The final, fully validated 22-item scale was renamed the SMAIS−ULM and exists as two versions: a caregiver-reported version (for caregivers of individuals with SMA aged ≥ 2 years) and a patient-reported version (for individuals with SMA aged ≥ 12 years). The SMAIS−ULM is available for use in research and development or clinical practice [2].

Figure 1 provides a high-level overview of the development of the SMAIS and the differences between the initial version included in the publication of Vázquez-Costa et al. [1] and the final validated version [2] now available for external use.

Fig. 1
figure 1

Schematic on the development of the validated SMAIS–ULM [2] and the SMAIS used in the Vázquez-Costa et al. publication [1]. SMAIS Spinal Muscular Atrophy Independence Scale, SMAIS−ULM SMAIS−Upper Limb Module

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Clarification of the SMAIS Scale in the SMA-TOOL Study

Prior to the final validation of the 22-item SMAIS−ULM scale, the 29-item version of the SMAIS, scored on a 3-point scale, was shared for inclusion in a specific research project, denoted the SMA-TOOL study. Once validation of the scale was completed, the 29-item version was retired from use. Therefore, the readers should be aware that the scale described in the SMA-TOOL study was the original 29-item SMAIS where the total score consisted of 22 items with seven items scored separately and the scoring completed using the 3-point response scale (Fig. 1). The 29-item version is no longer available for external use.

It should be noted that the majority of analysis in the SMA-TOOL study was done using the 22 items, in line with the final SMAIS–ULM. In some instances, the research found differing results to what was determined during validation of the scale. For instance, both the patient- and caregiver-reported SMAIS were used to compare the relationship with the Clinician Global Impression-Improvement scale when determining the sensitivity to change. The authors showed that only the caregiver-reported SMAIS total score (based on 22 items) changed in the predicted way in relation to the changes in the Clinician Global Impression-Improvement scale [1]. However, during validation of the SMAIS−ULM, Trundell et al. showed that both the patient and caregiver version of the 22-item SMAIS−ULM demonstrated similar responsiveness in the Clinical Global Impression-Change scale when determining sensitivity to change [2].

Secondly, the following statement appears in the Discussion section of the publication [1]: ‘However, in that study, only 22 of the 29 original items were considered. Our data suggest that if the original 29-item version were used, the caregiver version would be more reliable for assessing the degree of independence of SMA patients in daily living activities.’ We believe that this statement should be disregarded. The validation study conducted by Trundell et al. demonstrated validity of the 22 items on a 3-point scale, and this message is contradictory to the results of the original validation study [2, 3].

Finally, we would like to clarify that in the Methods section of the publication [1], the exploratory factorial analysis referencing ‘new items’ referring to a separate measure included in the set (assessing perceived fatigability, breathing and voice, sleep and rest, and vulnerability) were not the seven items that were scored separately from the 22 items included in the SMAIS−ULM.

Single Point of Clarification

We would like to clarify a singular update that should be addressed. In the Abstract, the mean caregiver SMAIS score should be reflected as 27.6, rather than 7.6.

Conclusion

The primary focus of this letter was to provide readers with the details of the SMAIS versions used in the original publication [1]. While the 29-item version was provided for use in this study, only the validated 22-item SMAIS–ULM scale is available externally. These clarifications do not change the results of this study; however, we believe it would be beneficial for clinicians and researchers working in SMA to understand which version of the scale was used since this is no longer available, and for future data comparisons [2].