CMAP changes upon symptom onset and during treatment in spinal muscular atrophy patients: lessons learned from newborn screening



Early identification and treatment of spinal muscular atrophy (SMA) are crucial but difficult. In this study, we aimed to assess the significance of compound motor action potential (CMAP) amplitude in patients identified through a newborn screening program.


We initiated a large-scale population newborn screening program for SMA in Taiwan in 2014. Patients had access to treatment through clinical trials or expanded use programs. Symptomatic patients were evaluated regularly, including CMAP exams.


Among 364,000 screened newborns, 21 were diagnosed with SMA. The incidence of SMA was around 1 in 17,000 live births, and 70% developed SMA type 1. All infants with two SMN2 copies became symptomatic before the age of 1 month. CMAP amplitudes of 12 newborns were available, including 6 who were subsequently treated with nusinersen. We found that a rapid decrease of CMAP amplitude was an early predictor of symptom onset. Pretreatment CMAP and rapid increment of post-treatment CMAP could predict better treatment outcomes.


This study prospectively demonstrated the incidence of SMA and its types. Our results imply the importance of pretreatment CMAP amplitude and rapid reversal of post-treatment CMAP amplitude with regard to disease presentation and also treatment outcomes.


Spinal muscular atrophy (SMA) is caused by the deletions or pathogenic variants of the survival motor neuron 1 gene (SMN1) (MIM 600354), leading to motor neuron degeneration and resulting in progressive muscle weakness and atrophy. Patients affected by the most common form of SMA, SMA type 1 (SMA1), usually experience symptom onset within the first few weeks of life. A few specific treatments are currently available for SMA, but early, or preferably presymptomatic, initiation of treatment seems to be critical.1,2,3,4,5 We initiated a large-scale population newborn screening program (NBS) for SMA in Taiwan in 2014, and have successfully identified newborns with SMN1 deletion.6 In this report, we assess the impact of the early detection of SMA through this program, and the association between compound motor action potential (CMAP) amplitude and symptom onset and treatment outcomes.


The SMA NBS has been described previously, with modification of the primers (sequence upon request) since 2018 to avoid the previously described false positives.6 SMN2 copies were confirmed both by a droplet digital polymerase chain reaction (ddPCR) assay using the original screening dried blood spot (DBS) sample and also a multiplex ligation-dependent probe amplification (MLPA) assay using DNA extracted from whole-blood samples of the same patients, as described in our previous publication.6 SMN2 c.859G>C modifier was also checked in genetically diagnosed SMA patients.7 Patients had access to treatment through clinical trials or expanded use programs (NCT02386553, NCT02865109, NCT03837184, NCT03505099, NCT04042025). Infants with two copies of SMN2 were treated immediately after the diagnosis, while those with three copies were followed monthly for symptom onset before starting treatment, and those with four copies were evaluated in outpatient clinics or followed by phone every 4 to 6 months. The evaluations included neurological examinations, developmental milestones, and CMAP, of which the maximal CMAP amplitude of the ulnar nerve was recorded by experienced pediatric neurologists. A minimum of four G1 placements over the hypothenar eminence were used to ensure the measurement of maximum CMAP, as described in previous studies.8 The parents were taught to identify the symptoms of SMA. The phenotype of SMA was classified depending on the age of symptom onset.9 Symptomatic patients also received multidisciplinary standard of care at the specific neuromuscular joint clinic in our hospital.

Ethics statement

This study was approved by the Institutional Review Board of National Taiwan University Hospital (NTUH) (201308058RIN and 201702057RINB), and the parents of all participants signed informed consent forms.


From November 2014 to December 2019, 364,000 newborns were screened for SMA. Among them, 21 were confirmed to have SMN1 dysfunction, including 20 with an SMN1 homozygous deletion (95%) and one who had a false negative screening test with an SMN1 heterozygous deletion (5%). This patient presented with hypotonia since 5 months of age. When she was diagnosed at 12 months of age, she was profoundly hypotonic, could not sit without support, and had difficulty swallowing. A physical examination revealed tongue fasciculation, an absence of deep tendon reflexes. One copy of her SMN1 gene was deleted. She also harbored a c.373G>A (V125M) variant, a novel variant predicting to be damaging, on her SMN gene. The incidence of SMA in our cohort was ~1 in 17,000 (95% CI: 11 350–26,530) newborns. SMN2 copy-number analysis showed two SMN2 copies in nine infants (43%), three copies in six infants (29%), and four copies in six infants (29%) (Table 1). None of these 21 patients had the SMN2 c.859G>C modifier.

Table 1 Clinical and electrophysiological findings, treatment and follow-up of 21 patients diagnosed by newborn screening between November 2014 and May 2020.

A total of 17 infants were followed, of whom 12 (70%) developed SMA1 (Figure S1). Nine (75%) of these SMA1 infants (five with two SMN2 copies and four with three SMN2 copies) were treated either with nusinersen or AVXS-101, and the other three (all with two SMN2 copies) died without treatment (Table 1). All eight infants with two SMN2 copies followed became symptomatic before the age of 1 month, while five (63%) showed symptoms at birth. Some of the subtle symptoms at birth improved transiently, followed by rapid deterioration. Five of the six infants with three SMN2 copies became symptomatic before the age of 1 year, of whom four developed SMA1 (80%) defined by age of onset, and one developed SMA type 2a. The other infant (patient 11) was treated presymptomatically, and so the clinical type was undefined. Only one of the four followed infants with four SMN2 copies became symptomatic at the age of 37 months. The age of symptom onset was significantly different among the infants with different SMN2 copies (p < 0.001, Figure S2).

CMAP amplitude values of the ulnar nerve of 12 infants were available, including six who were subsequently treated with nusinersen, four who were not treated, and two before entering an AXVS-101 trial. The initial CMAP amplitudes were much lower in the infants with two SMN2 copies (1.39 ± 0.79 mV) than in those with three copies (3.70 ± 0.59 mV) (Fig. 1). CMAP amplitude decreased rapidly in all SMA1 infants. In the infants with three SMN2 copies (patients 13–15), the ulnar CMAP amplitudes dropped significantly when the muscle power of their upper extremities was still normal. In the infants with type 2 and 3, a decline in CMAP amplitude was also correlated with symptom onset, although the speed of decline was slower than in those with SMA1.

Fig. 1: Changes in compound motor action potential (CMAP) amplitude (amp) of the ulnar nerve against age in 12 infants with spinal muscular atrophy (SMA) identified through a newborn screening program.

The infants are grouped into those with two (red), three (blue), and four (green) copies of SMN2. Changes in the pretreatment periods are connected by dashed lines, and changes after nusinersen treatment are connected by solid lines. The upper right insert is an enlargement of the infants with two copies of SMN2 at age 0–6 months, to better show the rapid decline in CMAP amplitude in the first month of age.

CMAP data from six infants who were treated with nusinersen were available. The pretreatment CMAP amplitudes were 0.89 ± 0.76 mV and 2.36 ± 1.71 mV in the patients with two and three SMN2 copies, respectively, compared with 1.33 ± 0.22 mV and 3.69 ± 2.84 mV 6 months after treatment. In those with a pretreatment CMAP amplitude ≥2 mV (patients 14 and 15), CMAP increased significantly after treatment, accompanied by normal motor development. In those with a pretreatment CMAP amplitude <2 mV (patients 3, 4, and 13), the increase in CMAP after treatment was small and slow, although still accompanied by functional improvements. The only outlier was patient 7 (with two SMN2 copies) whose pretreatment CMAP amplitude was <1 mV and who showed significant improvements in CMAP after treatment. He had mild motor developmental delay, while he could sit independently at the age of 9 months and walk holding on at the age of 15 months.


The incidence of SMA in our population was around 1 in 17,000, which is lower than expected because of carrier screening and prenatal diagnosis.10,11 Very recently, a study of population-based newborn screening showed that the SMA incidence in New York State, due to the uptake of carrier screening, is also low with 1 in 28,137.12 In this study we prospectively observed the onset of symptoms in SMA patients identified by newborn screening. The high percentage (70%) of SMA1 in our cohort suggests that compared with historical experience with type 1, fewer patients died early and that the symptoms of type 1c were probably detected earlier than others.13 Studies of SMA newborn screening from Australian and German groups also revealed that all patients with two SMN2 copies had early symptom onset at the first visit or before the age of 3 months if presymptomatic therapies were not applied.14,15 All patients with two or three SMN2 copies in our cohort, who have not received treatment or before treatment, developed type 1 (n = 13) or 2a (n = 1); thus, our results further support that newborn babies with two or three SMN2 copies should be treated immediately.16 With regard to newborns with four copies, immediate presymptomatic treatment has not been used in Taiwan or in Australia. Therefore, those cases with four copies were not reported as screening positives in one Australia research pilot study.14

Definitions of the clinical types of SMA may need to be revised when prospective symptom monitoring and disease-modifying treatments both become common.17 For example, the symptom onset time described in our prospective study was defined by neurologists, which should be earlier than that detected by parents. In addition, because of the effective treatment, we could only use the disease onset time for classification but not the maximal motor function achieved. For example, our patient 14 was type 1c but can now walk, suggesting that interventions may modify the phenotypes of patients. Moreover, interventions may lead to a new phenotype instead of changing from type 1 to 2 or 3, which should further remind clinicians to carefully describe the phenotypes in treated patients. Taken together, these findings suggest that the scenario of complication management and standard of care in SMA will need to be changed in the future.18

In this report we provide evidence that CMAP amplitude can serve as a sensitive method to earlier detect the onset of disease from presymptomatic to symptomatic. Our results revealed that the CMAP values were lower and decreased faster in the infants with two SMN2 copies than in those with three or four copies, similar to the findings of Swoboda et al. with regard to the natural history of denervation in SMA.8 We also demonstrated the rapid decrease of CMAP amplitude in the very beginning of clinical or subclinical symptoms and signs in our type 1 patients, both two and three copies, suggesting the use of rapid decline in CMAP amplitude to earlier detect infantile-onset SMA for prompt treatment.17

We also correlated CMAP to treatment. The NURTURE study demonstrated a trend regarding higher baseline CMAP values, post-treatment D64 CMAP values, and better prognosis.3 In our patients with two copies, although patient 7 had even lower pretreatment CMAP values, he still demonstrated a late increase in CMAP values 9 months after treatment. Interestingly, patients with three copies of SMN2 (patients 14 and 15) exhibited a rapid decline in CMAP upon symptom onset, but had a rapid recovery of CMAP after the initiation of treatment. It is likely that, under the current follow-up protocol, motor neurons can still be rescued even after the occurrence of symptoms. Our results further clearly indicated that pretreatment CMAP and a rapid reversal or increase in CMAP after treatment were closely related to the developmental outcomes. Therefore, CMAP appears to have great potential in clinical application and in the evaluation of therapeutics for infantile-onset SMA patients. Still, the application of CMAP in evaluating therapeutics for later-onset SMA patients with four SMN2 copies needs further investigations.

In conclusion, this report demonstrates the incidence of SMA and its types, and the importance of pretreatment CMAP amplitude as well as the change of post-treatment CMAP with regard to disease presentation and also treatment outcomes. CMAP may have great potential in clinical application and in the evaluation of therapeutics in SMA patients. However, there are several limitations to this study, including the small sample size of treated patients and a lack of detailed clinical and CMAP information of the patients with four SMN2 copies. Considering the latest treatment recommendations,19 further studies regarding the natural history and application of CMAP amplitude in treatment, especially in patients with four SMN2 copies, are needed.


  1. 1.

    Finkel RS, Mercuri E, Darras BT, et al. Nusinersen versus sham control in infantile-onset spinal muscular atrophy. N Engl J Med. 2017;377:1723–1732.

    CAS  Article  Google Scholar 

  2. 2.

    Poirier A, Weetall M, Heinig K, et al. Risdiplam distributes and increases SMN protein in both the central nervous system and peripheral organs. Pharmacol Res Perspect. 2018;6:e00447.

    Article  Google Scholar 

  3. 3.

    De Vivo DC, Bertini E, Swoboda KJ, et al. Nusinersen initiated in infants during the presymptomatic stage of spinal muscular atrophy: Interim efficacy and safety results from the Phase 2 NURTURE study. Neuromuscul Disord. 2019;29:842–856.

    Article  Google Scholar 

  4. 4.

    Al-Zaidy SA, Kolb SJ, Lowes L, et al. AVXS-101 (onasemnogene abeparvovec) for SMA1: comparative study with a prospective natural history cohort. J Neuromuscul Dis. 2019;6:307–317.

    Article  Google Scholar 

  5. 5.

    Ramdas S, Servais L. New treatments in spinal muscular atrophy: an overview of currently available data. Expert Opin Pharmacother. 2020;21:307–315.

    CAS  Article  Google Scholar 

  6. 6.

    Chien YH, Chiang SC, Weng WC, et al. Presymptomatic diagnosis of spinal muscular atrophy through newborn screening. J Pediatr. 2017;190:124–.e121.

    Article  Google Scholar 

  7. 7.

    Prior TW, Krainer AR, Hua Y, et al. A positive modifier of spinal muscular atrophy in the SMN2 gene. Am J Hum Genet. 2009;85:408–413.

    CAS  Article  Google Scholar 

  8. 8.

    Swoboda KJ, Prior TW, Scott CB, et al. Natural history of denervation in SMA: relation to age, SMN2 copy number, and function. Ann Neurol. 2005;57:704–712.

    CAS  Article  Google Scholar 

  9. 9.

    Munsat TL, Davies KE. International SMA Consortium meeting (26–28 June 1992, Bonn, Germany). Neuromuscul Disord. 1992;2:423–428.

    CAS  Article  Google Scholar 

  10. 10.

    Su YN, Hung CC, Lin SY, et al. Carrier screening for spinal muscular atrophy (SMA) in 107,611 pregnant women during the period 2005-2009: a prospective population-based cohort study. PLoS One. 2011;6:e17067.

    CAS  Article  Google Scholar 

  11. 11.

    Verhaart IEC, Robertson A, Wilson IJ, et al. Prevalence, incidence and carrier frequency of 5q-linked spinal muscular atrophy—a literature review. Orphanet J Rare Dis. 2017;12:124.

    Article  Google Scholar 

  12. 12.

    Kay DM, Stevens CF, Parker A, et al. Implementation of population-based newborn screening reveals low incidence of spinal muscular atrophy. Genet Med. 2020;22:1296–1302.

    CAS  Article  Google Scholar 

  13. 13.

    Calucho M, Bernal S, Alias L, et al. Correlation between SMA type and SMN2 copy number revisited: an analysis of 625 unrelated Spanish patients and a compilation of 2834 reported cases. Neuromuscul Disord. 2018;28:208–215.

    Article  Google Scholar 

  14. 14.

    Kariyawasam DST, Russell JS, Wiley V, Alexander IE, Farrar MA. The implementation of newborn screening for spinal muscular atrophy: the Australian experience. Genet Med. 2020;22:557–565.

    Article  Google Scholar 

  15. 15.

    Vill K, Kolbel H, Schwartz O, et al. One year of newborn screening for SMA—results of a German pilot project. J Neuromuscul Dis. 2019;6:503–515.

    Article  Google Scholar 

  16. 16.

    Glascock J, Sampson J, Haidet-Phillips A, et al. Treatment algorithm for infants diagnosed with spinal muscular atrophy through newborn screening. J Neuromuscul Dis. 2018;5:145–158.

    Article  Google Scholar 

  17. 17.

    Tizzano EF, Zafeiriou D. Prenatal aspects in spinal muscular atrophy: from early detection to early presymptomatic intervention. Eur J Paediatr Neurol. 2018;22:944–950.

    Article  Google Scholar 

  18. 18.

    Tizzano EF, Finkel RS. Spinal muscular atrophy: a changing phenotype beyond the clinical trials. Neuromuscul Disord. 2017;27:883–889.

    Article  Google Scholar 

  19. 19.

    Glascock J, Sampson J, Connolly AM, et al. Revised recommendations for the treatment of infants diagnosed with spinal muscular atrophy via newborn screening who have 4 copies of SMN2. J Neuromuscul Dis. 2020;7:97–100.

    Article  Google Scholar 

Download references


Biogen partially sponsored the study of newborn screening and long-term follow-up. The study sponsor had involvement in the study design, but the authors made the final decision. The study sponsor had no involvement in the collection, analysis, and interpretation of data; the writing of the report; or the decision to submit the manuscript for publication.

Author information



Corresponding author

Correspondence to Yin-Hsiu Chien MD, PhD.

Ethics declarations


W.C.W. and W.L.H.: consultant on advisory boards, research and/or speaker honoraria from for Biogen. Y.H.C.: consultant on advisory boards, research and/or speaker honoraria from/for Biogen, Avexis/Novartis. Y.H.C.: consultant on advisory boards, research and/or speaker honoraria from/for Biogen, Avexis/Novartis.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Weng, W., Hsu, Y., Chang, F. et al. CMAP changes upon symptom onset and during treatment in spinal muscular atrophy patients: lessons learned from newborn screening. Genet Med (2020).

Download citation


  • SMA type
  • incidence
  • nusinersen
  • compound motor action potential (CMAP)
  • outcome