Discussion:
Main findings:
The cumulative live birth rate is significantly higher with TLI
incubators when compared with SC incubators. This appears to be driven
by two factors. Firstly, there was an improvement in live birth rate per
fresh embryo transfer in the TLI arm with no significant change in the
number of embryos transferred or the multiple pregnancy rate. Secondly,
there was a significant increase in the number of embryos available for
cryopreservation in the TLI arm which translates into a trend towards a
higher proportion of those with unsuccessful fresh embryo transfers
having access to cryopreserved embryos in the TLI arm.
Strengths and limitations:
Strengths:
To our knowledge, this is the second study comparing cumulative live
birth rate per oocyte retrieval between TLI and SC incubators and the
first to show a significant difference. A criticism of TLI incubators in
the ESHRE guideline was the paucity of cumulative live birth reporting
in scientific literature which would help to assess the relative
importance of the ‘undisturbed culture’ versus ‘embryo selection’. The
data presented here aims to provide a better perspective on this
underexplored aspect of TLI incubators. Additional strengths of the
study include the single-centre nature which ensures that the laboratory
conditions were uniform throughout the study period and the large sample
size which provided statistical power. Furthermore, the baseline
denominator was per oocyte retrieval which reduces selection bias due to
failed embryo transfer.
Limitations:
A retrospective study does carry its limitations, primarily in terms of
confounding factors. We attempted to adjust for this by an analysis for
baseline characteristics and adjustment for factors such as age and
number of oocytes retrieved but there is still a potential for unknown
confounding factors that we have not adjusted for. As the findings are
from a single unit, generalisability of these findings will be limited
but the findings of this study may form the rational basis for approval
for a well-powered randomised controlled trial. Findings from other
centres are urgently needed to either confirm or refute these findings
to determine whether change in laboratory practice to incorporate TLI
would help in other fertility clinics.
Interpretation (in light of other
evidence):
Comparison with previous
literature:
The first studies comparing TLI and SC incubators were exploratory, had
a small sample size and primarily focussed on the safety and
comparability of TLI to SC incubators. The latest Cochrane review13 noted an absence of RCTs examining cumulative live
birth rate and the ESHRE recommendations on good practice of TLI
incubators 6 was able to identify only one study
(previously published from our unit) on cumulative live birth rate in
the available literature. In this previous study, we did note an
improvement in live birth per fresh embryo transfer but failed to note a
significant difference in cumulative live birth per oocyte retrieval4. However, in the period of the previous study,
sequential culture media was used and so the opening of TLI incubator
doors was required on day 3 for a change of the culture medium.
Biological plausibility of the
findings:
The availability of morphokinetic data from TLI incubators led to the
identification of ‘poor prognostic’ features which were developed into
factors for exclusion. The suggested exclusion factors included
unevenness at the two-cell stage, multinucleation at four-cell, direct
first cleavage and irregular or reverse cleavage 3.
Apart from true direct cleavage from one to three cells for which there
appears to be reasonable evidence for a poor prognosis, the rest of the
exclusion criteria do not appear to have a consistent prognostic effect
between studies 14,15. Additionally, embryos may be
capable of auto-correction, and an embryo which displays features of the
exclusion criteria, but nonetheless develops into a top-quality
blastocyst appears to have a better prognosis than embryos which arrest
during their development 2. An alternate approach has
been to not rely on exclusion criteria, but generate algorithms based on
embryo morphokinetics to select the best embryo. Such algorithms have
been generated through statistical analysis in combination with
observation and expert opinion of embryologists and more recently using
artificial intelligence for data analytics. Whilst these algorithms have
shown promise in initial studies, they do not appear to be translatable
across different embryology laboratories, which is a major limitation to
widespread adoption 1. There is ongoing research on
using artificial intelligence and deep learning to improve algorithms,
with early attempts being made into introducing such algorithms into
clinical practice 16,17.
The Cochrane review noted that use of algorithms did not appear to
confer an additional benefit to TLI with conventional morphological
assessment (OR for ongoing pregnancy rate 0.61, 95% CI 0.32 to 1.20, 1
RCT, N = 163) 13.
An increase in cumulative live birth rate with TLI with this group
suggests that the undisturbed culture may contribute to this uplift. We
have hypothesised in our previous paper on perinatal outcomes that the
undisturbed culture in TLI may play a role in the impact of TLI systems.
The choice of culture media may play a role, but successful embryo
development also relies on a stable embryo culture environment.
Laboratory factors can impact on media efficacy and embryo development
resulting in significantly different outcomes 18. A
change in temperature can destabilise the cellular cytoskeleton
(including the meiotic spindle) and might also affect embryo metabolism.
Differences in embryo aneuploidy rates amongst egg donor programmes
between different clinics 19 serves to illustrate the
crucial factor of laboratory conditions (including maintaining a stable
temperature) on the development of a competent embryo. A recent review
listed several patient, clinical and laboratory factors which might
affect the risk of embryo aneuploidy indicating that there is room for
further research to improve embryo competence in IVF clinics20.