Reviewer's report
Title:Glucose Intolerance and Gestational Diabetes Risk in Relation to Sleep
Duration and Snoring During Pregnancy: A Pilot Study
Version: 1Date:19 February 2010
Reviewer: Edwina Yeung
Reviewer's report:
Summary
The authors conducted a prospective study of the association between sleep
factors in early pregnancy (snoring and sleep duration) and the risk of gestational
diabetes (GDM). They also investigated the association between continuous
measures of glucose during the 50-g GDM screening that all women received
and sleep factors. This study was nested in a cohort of 1290 women attending
two prenatal clinics in Seattle, WA. The study found that snoring and very low
sleep duration (#4 hours) were each significantly associated with increased mean
maternal 1-hour glucose during the screening test. Due to the small number of
women who developed GDM (n=68), they found relative risks suggestive of
associations between sleep factors and GDM but none reached statistical
significance. They further tested for interaction between overweight and sleep
factors and also found that overweight snorers had higher risk than overweight
non-snorers for GDM but the interaction did not prove to be significant.
Major Compulsory Revisions
-There needs to be a clearer rationale for why the interaction between overweight
and snoring was tested. (i.e. p.7 under the hypothesis statement) According to
the beginning of the introduction, the authors stated (with good evidence) that
lack of or poor sleep quality has been shown in other studies to be associated
with obesity. Thus, the causal diagram between sleep and GDM might have been
through the mechanism of overweight not modified by it? Regarding results to
table 3, these are essentially repeated analyses of the right side of table 2 using
a singular reference group. However, it is unclear why the authors stated
"insufficient data" in both tables as there is data (albeit very imprecise due to only
1 GDM case who was lean and snored). If there isn't sufficient data, the
interaction with overweight should not have been investigated at all.
- If snoring/sleep deprivation leads to obesity as the author had reviewed for
evidence of such in the introduction, then the interpretation of the increased risk
with snorers among overweight women is probably due to residual confounding
by BMI (i.e. overweight women who snore tend to have larger BMI than
overweight women who do not). An analysis adjusting for BMI may be more
informative than stratifying and testing for interaction. If the association is
independent of BMI, it may show better evidence of an independent pathway.
-Further adjusting for BMI in the continuous analysis with glucose values from the
GDM screening test would be informative too for the same reasons. One reason
the authors may not have included BMI is that they used a statistical definition of
confounding rather than using the a priori knowledge that they summarized on
the causal relationships between these factors. The authors in their statistical
methods mentioned that all covariates were included if they altered the RR by
10% or more. Previous research has shown that this is not an optimal method of
determining confounders (Hernan et al Am J Epidemiol 2002;155:176–84).
Multiple models, one showing age and race adjusted and another additionally
adjusting for BMI may be more informative.
-For GDM and sleep duration - power of detection may have also been
influenced by the use of 9 hours as the reference when there was no clear
indication that this was beneficial over 5-8 hours. Even in continuous association
(p.9) the authors stated that the difference in glucose was 2.3mg/dl but with
range overlapping 0 (p=0.27). It was not well justified therefore why the authors
decided to group the women in this way.
Minor Essential Revisions
-Readers may want to know the sample size of women for which the GDM
diagnostic testing was required but was unable to locate that information. (i.e. for
figure 1b)
-The analysis performed in the bottom of page 10 for the risk per hour of nightly
sleep may have been unnecessary as it does not give more information due to
imprecise estimates. The assumption of a linear trend in risk again does not
seem justified as Figure 1a shows that the confidence interval for GDM risk
overlap greatly for sleep in the 5-9 hours range.
-Bottom of page 7 "glucose intolerance and GDM risk" - as "glucose intolerance"
was never tested in the sense that there are clinical definitions (IGT/IFG), the
authors may want to use another term ("glycemia"?). It was also used in the
abstract.
-Minor error on p.13 sentence should read "OR=7.0; 95%" not "OR=7.0; 955"
-The phrase "Data not shown" on page 9 can be removed as data was shown
(the beta and 95% confidence interval...)
Discretionary Revisions
-Abstract: not need to state RR=6.91 for last sentence of results section as the
sentence already states "6.9-fold..."
-The sentence on P. 8 stating "All reported confidence intervals were calculated
at the 95% level" can be removed as this was already mentioned on p.7.
-p.12 on the discussion of the association with greater sleep duration with
T2D/GDM- maybe mention how that association could be due to subclinical
disease?
-p.14 - grammatical error - sleep duration and snoring "were" rather than "was"
obtained from self-report?
-p.14 was unclear what was meant by "attendant" in the sentence with
"imprecision of relative risk estimates..." Word can be removed.
Level of interest: An article whose findings are important to those with closely
related research interests
Quality of written English: Acceptable
Statistical review: No, the manuscript does not need to be seen by a
statistician.
Declaration of competing interests:
I declare that I have no competing interests