Table 2.
Baseline characteristics and measurements of lung function
| Patients | ||||||
| SMA type |
Type 1c (n = 6) |
Type 2a (n = 48) |
Type 2b (n = 34) |
Type 3a (n = 43) |
Type 3b (n = 35) |
Type 4 (n = 4) |
| M: F | 3: 3 | 19: 29 | 12: 22 | 18: 25 | 18: 17 | 4: 0 |
| SMN2 copies | ||||||
| 2 | 1 | 1 | 1 | 1 | 1 | – |
| 3 | 5 | 44 | 27 | 21 | 5 | – |
| 4 | – | 3 | 5 | 21 | 25 | 4 |
| 5 | – | – | – | – | 3 | – |
| n/a | – | – | 1 | – | 1 | – |
| Mechanical ventilation: n (% of total) | 5 (83.3%) | 23 (47.9%) | 3 (8.8%) | 5 (11.6%)b | 1 (2.9%)b | 0 |
| Median age at start of mechanical ventilation (IQR) | 14.6a (13.1–25.9) | 12.3b (8.2–16.9) | 16.8 (12.7–20.8) | 39.9c (35.9–48.3) | 40.0c | n/a |
| Assessments | ||||||
| Lung function test | Patients,n (%) | No. of patient assessments | Median no. of assessments per patient (range) | |||
| FEV1 | 163 (95.9) | 784 | 5 (1–40) | |||
| FVC | 167 (98.2) | 668 | 4 (1–32) | |||
| VC | 80 (47.1) | 646 | 6 (1–38) | |||
Legend: SMA spinal muscular atrophy; n number of patients or assessments; M males, F females, SMN2 survival motor neuron 2 gene, IQR interquartile range; n/a: not available, FEV1 forced expiratory volume in 1 s, FVC forced vital capacity, VC vital capacity
a: the high median age at which mechanical ventilation was initiated in patients with SMA type 1c is explained by the fact that in The Netherlands it was uncommon to initiate mechanical ventilation for infants with SMA type 1 until recent years, as it was considered not ethical to prolong life without any realistic outlook for further improvements of motor function at a later time. This has changed in the past years, following the introduction of SMN protein augmenting drugs and current clinical drug trials. b: the exact age at which mechanical ventilation was started is unknown for one patient; c: excluded are two patients with SMA type 3a and one patient with type 3b using either bi-level or continuous positive airway pressure for obstructive sleep apnoea syndrome. Ages are shown in years