P-153 Oocytes with narrow perivitelline space have poor fertilization and developmental potentials after ICSI
Authors: Shioya M , Kobayashi T , Sugiura T , Fujita M , Takahashi K 
Published: June 2022

Study question

Do oocytes with narrow perivitelline space have poor clinical outcomes after ICSI?

Summary answer

After ICSI, oocytes with narrow perivitelline space have an increased degeneration rate and decreased rates of 2PN and embryo development.

What is known already

Several studies reported that oocytes with dysmorphologies, such as zona pellucida (ZP) abnormalities and cytoplasmic inclusions (vacuoles, smooth endoplasmic reticulum cluster, refractile bodies), have poor developmental potential in IVF/ICSI. In research on perivitelline space (PVS), many studies have focused on the PVS with fragmentation as well as large PVS. On the other hand, oocytes with narrow perivitelline space (narrow PVS oocytes) are considered to have an immature cytoplasm, but there are few reports on the relationship between narrow PVS oocytes and clinical outcomes. In this study, we retrospectively analyzed the effect of narrow PVS oocytes on ICSI outcomes.

Study design, size, duration

This retrospective single-center study analyzed 11149 MII oocytes that underwent ICSI between January 2018 and October 2021. We observed the PVS of MII oocytes during ICSI, and oocytes with sufficient PVS between the ZP and cytoplasm were determined to be non-narrow PVS oocytes. Oocytes without sufficient PVS from any angle (PVS was observed only around the first polar body) were defined as narrow PVS oocytes.

Participants/materials, setting, methods

After ICSI, oocytes were cultured in ONESTEP medium (NakaMedical, Tokyo, Japan). Embryos that developed into blastocysts were used for single vitrified-warmed blastocyst transfer (SVBT). We compared the rates of degeneration, 2PN, cleavage, blastocyst formation, good-grade (Gardner’s criteria ≥BB) blastocyst, top-grade blastocyst (Gardner’s criteria=AA), and clinical pregnancy (presence of a gestational sac) between oocytes with narrow and non-narrow PVS. Logistic regression analysis with consideration of patient age, BMI, and basal AMH was performed for each outcome.

Main results and the role of chance

Of the 11149 MII oocytes, 570 and 10579 were determined to be narrow and non-narrow PVS oocytes, respectively. Narrow PVS oocytes showed significantly higher degeneration rates (aOR: 1.52, 95% CI: 1.12–2.06, p&lt;0.01) and lower 2PN rates (aOR: 0.77, 95% CI: 0.64–0.93, p&lt;0.01) after ICSI compared to non-narrow PVS oocytes. Furthermore, rates of cleavage (aOR: 0.52, 95% CI: 0.31–0.87, p&lt;0.05), blastocyst formation (aOR: 0.56, 95% CI: 0.45–0.70, p&lt;0.01), good-grade blastocyst formation (aOR: 0.59, 95% CI: 0.46–0.76, p&lt;0.01), and top-grade blastocyst formation (aOR:0.625, 95% CI:0.45–0.86, p&lt;0.01), were significantly lower in the narrow PVS oocytes.

Of the blastocysts developed, 32 and 1439 blastocysts from narrow PVS oocytes and non-narrow PVS oocytes, respectively, were used for SVBT. The clinical pregnancy rate was not significantly different between blastocysts developed from narrow (aOR: 0.52, 95% CI, 0.22–1.22, p = 0.131) and non-narrow PVS oocytes. However, in blastocysts developed from narrow PVS oocytes, clinical pregnancy was confirmed only in top-grade blastocysts (58.8% [10/17]), and blastocysts of other grades did not result in pregnancy (0% [0/15]).

Limitations, reasons for caution

The limitation of this study is that it was a retrospective analysis conducted at a single IVF center. It is necessary to confirm the reproducibility at other facilities because the evaluation of PVS differs among embryologists and IVF centers. Therefore, a prospective multicenter study is needed.

Wider implications of the findings

We found that the narrow PVS oocytes showed poor outcomes after ICSI. While a good pregnancy rate could be expected if a top-grade blastocyst from such oocytes was obtained and transferred, the embryonic development rate of narrow PVS oocytes is low.

Trial registration number

Not applicable