Incidence, subtypes and severity of cerebral palsy in infants born extremely preterm in Switzerland: A retrospective study comparing two time periods.

Background
In 2011, the Swiss Society of Neonatology issued new guidelines for the care of preterm infants at the limit of viability leading to higher survival of infants born <28 weeks of gestation. It is unclear whether and how these recommendations affected the prevalence and severity of cerebral palsy (CP).
Objective
To investigate whether the prevalence, severity, and subtypes of CP in extremely preterm infants differ in two successive birth cohorts.
Methods
Retrospective, population-based analysis of prospectively collected data on infants born <28 weeks of gestation. CP prevalence, subtypes (Surveillance of cerebral palsy in Europe - SCPE classification), and severity (Gross Motor Function Classification System - GMFCS) assessed at 2 years corrected were compared between the two birth cohorts (2006-2011 and 2012-2017).
Results
Of 3244 registered infants, 2090 survived, of whom 1764 were followed up (84 %). Mortality was 38 % for the first period and 33 % for the second (p = 0.003) and 112 were diagnosed with CP. CP prevalence was 34.5 per 1000 live births (37.0 for 2006-2011 and 32.4 for birth-years 2012-2017, p = 0.476). A trend towards more bilateral spastic CP (2006-2011: 32 % and 2012-2017: 50 %, p = 0.055) and more severe cases (2006-2011: 14.3 % and 2012-2017: 24.9 %, p = 0.154) was observed in the second period. CP severity was associated with cystic periventricular leukomalacia (PVL) (OR 3.4, 95 %-CI 1.1-10.3, p = 0.033) and necrotizing enterocolitis (NEC) (OR 5.5, 95 % CI 1.2-25.1, p = 0.028) but not with other neonatal morbidities.
Conclusion
These results suggest that the greater number of bilateral forms and severe cases of CP could be due to the higher number of surviving infants in the 2011-2017 cohort. PVL and NEC are the factors mostly associated with severe cases of CP in Switzerland.