Concerns About Clinical Effectiveness of Newborn Screening for Spinal Muscular Atrophy—Reply

Abstract

COMMENT & RESPONSEIn Reply We appreciate the comments from Cheng and Hsu regarding our study 1 on the clinical effectiveness of newborn screening (NBS) for spinal muscular atrophy (SMA). Their observations bring valuable perspectives that merit further discussion.Our primary end point was the development of motor milestones, as these are clinically significant for patients. Motor milestones according to World Health Organization criteria 2 represent a robust outcome that can be reliably used in realworld settings. They are widely used and do not require specific training, resulting in an almost complete dataset even in the setting of a noninterventional study. While the Hammersmith Infant Neurological Examination-2 (HINE-2) score and compound muscle action potential data are collected within the SMARTCARE registry, their use is not mandatory and they are not available for all patients. Furthermore, Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP INTEND) has been validated for SMA and shows a positive correlation with HINE-2 scores. 3 Thus, focusing on motor milestones and CHOP INTEND scores sufficiently illustrates the effectiveness of newborn screening without redundancy.Second, Cheng and Hsu suggest adding the Hammersmith Neonatal Neurological Examination (HNNE) and the floppy infant module to differentiate presymptomatic from mildly symptomatic patients. Although this differentiation is important, there are limitations to the widespread application of these tools. The study by Pane et al 4 involved only 17 patients, indicating limited experience and lack of testing in a larger cohort of patients with SMA. Additionally, the HNNE includes 34 items, which would add considerable complexity to the already comprehensive SMARTCARE examination recommendations.Finally, Cheng and Hsu commented on the proportion of symptomatic infants with 2 SMN2 copies within the NBS cohort at the initiation of treatment. Our data showed that 11 of 31 patients with 2 SMN2 copies were symptomatic at the start of treatment. 1 This reflects the real-world experience with NBS for SMA and corresponds well to a systematic review by Aragon-Gawinska et al, 5 which reported that approximately 50% of patients with 2 SMN2 copies identified by NBS were symptomatic at treatment initiation. To what extent this reflects a delay in treatment or the inherent biology of SMA remains to be determined. However, it is well known from clinical practice that infants with 2 SMN2 copies can become symptomatic within days or weeks. Therefore, it makes sense to reduce the time between positive screening results and initiation of treatment for these infants as much as possible. The phrase "time is motor neuron" 6 nicely illustrates the therapeutic window for infantile SMA and can best be accomplished by a combination of newborn screening followed by immediate availability of disease-modifying treatments.