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Validity of a meta‐analysis of risk factors for neonatal brachial plexus palsies

EDITOR–We read with interest the recent systematic review and meta‐analysis by Van der Looven et al. A meta‐ analysis is a quantitative statistical analysis of similar studies which mostly tend to include randomized controlled trials (RCTs), providing the highest validity and the least bias, followed by prospective cohort studies, case–control/retrospective cohort studies, and case series with increasing bias. Multiple strategies and review methodologies are available to search databases and identify studies including the Preferred Reporting Items for Systematic Reviews and Meta‐ Analyses (PRISMA) statement, Meta‐analyses Of Observational Studies in Epidemiology (MOOSE) guidelines, and Grant’s review as used by Van der Looven et al. All have a level of subjectivity since search filters are defined by the researchers, producing varying results for the same database registry search. Van der Looven et al.’s meta‐analysis mixed three different study types: mainly retrospective cohort and case– control studies (3rd tier), and no RCTs. The chosen studies lack similarity regarding reported risk factors, data collection years, and place (four continents, 11 countries), and included hugely varying counts of live births with and without neonatal brachial plexus palsies (NBPP). The database registries collected data for either billing or outcome purposes and varied from single hospital to multiple state and national registries which either used continuous counts or discontinuous extrapolated counts to obtain population estimates. Gutman et al. identified reduced fidelity for a database using data collected for billing and not outcome purposes resulting in underreporting of surgical complications. The huge variation in data collection is reflected in the considerable heterogeneity for the five risk factors of up to 98% and raises doubts about the quality assessment of each study using the Newcastle‐Ottawa Scale, which has been shown to have low reliability between reviewers. Van der Looven et al. reduced the heterogeneity by removing between 17% and 43% of studies from the already decimated number of studies used for the individual risk factor meta‐analyses. This is equivalent to manipulating the eligibility criteria and reduces the validity and increases the bias in interpreting the findings. The challenge in collating studies within systematic reviews which have not used the same methodologies results in difficulties calculating the total number of live births, especially if the studies have utilized the same database. While ideally all studies should have data for all five risk factors, this not being the case here raises issues not only on calculations of total live births but questions how to interpret findings from five separate meta‐analyses as performed by Van der Looven et al. A further issue relates to how population estimates are calculated. Abzug et al. determined findings from extrapolated data from the Kids Inpatient Database (KID) for the 1997 to 2012 period. However the database only yields sample data from hospital bills for 5 564 628 births which are 8.5% of live births born in the United States during the study period. No real data are available for the additional extrapolated 18 594 798 live births. No data were collected for every 2‐year block between the collection years. This limitation needs to be recognized as the missing years of the KID could alter the reported negative trend (https://www.hcup-us.ahrq. gov/db/nation/kid/kid_statelevelestimates.jsp). All included case–control studies used an arbitrary sized control group and random or systematic sampling of their controls without assessing if they were representative for their population. This can lead to unreliable biased results. Abzug et al. reported a decrease of the NBPP rate but the used Swedish registry data showed an increase from 0.17% in 1987 to 0.27% in 1997. All other studies do not provide trends. We do not think that it is valid to conclude that the included studies show that the NBPP rate is decreasing and that the overall incidence was 0.174%, because of the huge variability between the study data which also reduces the reliability of the risk factor data.