Post-Laser Twin Anemia Polycythemia Sequence: Diagnosis, Management, and Outcome in an International Cohort of 164 Cases

The aim of this study was to investigate the management and outcome in the post-laser twin anemia polycythemia sequence (TAPS). Data of the international TAPS Registry, collected between 2014 and 2019, were used for this study. The primary outcomes were perinatal mortality and severe neonatal morbidity. Secondary outcomes included a risk factor analysis for perinatal mortality and severe neonatal morbidity. A total of 164 post-laser TAPS pregnancies were included, of which 92% (151/164) were diagnosed antenatally and 8% (13/164) postnatally. The median number of days between laser for TTTS and detection of TAPS was 14 (IQR: 7–28, range: 1–119). Antenatal management included expectant management in 43% (62/151), intrauterine transfusion with or without partial exchange transfusion in 29% (44/151), repeated laser surgery in 15% (24/151), selective feticide in 7% (11/151), delivery in 6% (9/151), and termination of pregnancy in 1% (1/151). The median gestational age (GA) at birth was 31.7 weeks (IQR: 28.6–33.7; range: 19.0–41.3). The perinatal mortality rate was 25% (83/327) for the total group, 37% (61/164) for donors, and 14% (22/163) for recipients (p < 0.001). Severe neonatal morbidity was detected in 40% (105/263) of the cohort and was similar for donors (43%; 51/118) and recipients (37%; 54/145), p = 0.568. Independent risk factors for spontaneous perinatal mortality were antenatal TAPS Stage 4 (OR = 3.4, 95%CI 1.4-26.0, p = 0.015), TAPS donor status (OR = 4.2, 95%CI 2.1–8.3, p < 0.001), and GA at birth (OR = 0.8, 95%CI 0.7–0.9, p = 0.001). Severe neonatal morbidity was significantly associated with GA at birth (OR = 1.5, 95%CI 1.3–1.7, p < 0.001). In conclusion, post-laser TAPS most often occurs within one month after laser for TTTS, but may develop up to 17 weeks after initial surgery. Management is mostly expectant, but varies greatly, highlighting the lack of consensus on the optimal treatment and heterogeneity of the condition. Perinatal outcome is poor, particularly due to the high rate of perinatal mortality in donor twins.


Introduction
The twin anemia polycythemia sequence (TAPS) can arise from chronic unbalanced feto-fetal transfusion through minuscule placental anastomoses in monochorionic twins, leading to anemia in the TAPS donor and polycythemia in the TAPS recipient [1]. Unlike twin-twin transfusion syndrome (TTTS), TAPS develops in the absence of the twin oligohydramnios-polyhydramnios sequence. The iatrogenic form of TAPS, post-laser TAPS, can develop in 2-16% after laser surgery for TTTS due to the presence of minuscule residual anastomoses [2][3][4]. The rate of post-laser TAPS can be reduced by using the Solomon technique instead of the selective laser technique (3% vs. 16%, respectively) [2]. With this approach, the entire placental vascular equator is laser photocoagulated, thereby blocking all anastomoses, even the miniscule ones that may not be visualized. Although the rate of residual anastomoses has significantly dropped after the implementation of the Solomon technique, post-laser TAPS may still occur [2,5,6]. Options to manage post-laser TAPS include expectant management, preterm delivery, intrauterine transfusion (IUT) with or without a partial exchange transfusion (PET), fetoscopic laser surgery, and selective feticide [7]. The best treatment has not been established. The technical feasibility of a second intrauterine intervention may be limited due to complications that have arisen from the initial laser procedure for TTTS, such as amnion-chorion separation or preterm premature rupture of the membranes (PPROM). For informed decision making regarding the preferred intervention, it is crucial to have insight into perinatal outcome of post-laser TAPS twins. Due to the rarity of the complication, perinatal outcome is insufficiently investigated, and available information is based on small cohort studies. To expand our knowledge on TAPS, we set up the TAPS Registry, an international collaboration aimed at collecting data on diagnosis, treatment, and outcome in TAPS twins.
In the current study, the data from this TAPS Registry were used to (1) characterize diagnosis, treatment modalities, and outcome in post-laser TAPS twins, (2) to compare perinatal outcome between TAPS donors and recipients, and (3) to investigate potential risk factors for adverse perinatal outcome.

Experimental Section
The TAPS Registry, established in 2014, was a web-based registry for anonymous data collection (www.tapsregistry.org). Fetal therapy centers across the world were invited to participate. Participating centers were provided with personal credentials to enter the data of their TAPS cases into the online registry. Between 2014 and 2019, seventeen specialized fetal therapy centers contributed to data collection (see Appendix A).
All monochorionic twin pregnancies diagnosed with post-laser TAPS were considered eligible for this study. Pregnancies with spontaneous TAPS were excluded and were described in a separate study [8].
Antenatal diagnosis of TAPS was based on discordant middle cerebral artery peak systolic velocity (MCA-PSV) measures, with an increased MCA-PSV (>1.5 multiples of the median (MoM)) in the TAPS donor, indicative of fetal anemia, combined with a decreased MCA-PSV measure (<1.0 MoM) in the TAPS recipient, indicative of fetal polycythemia [9]. Postnatal diagnosis was reached by the presence of an inter-twin hemoglobin difference > 8.0 g/dL combined with at least one of the following: a reticulocyte count ratio > 1.7 or the presence of only minuscule vascular anastomoses (diameter < 1 mm) detected through color dye injection of the placenta [10,11]. Cases that were diagnosed with TAPS within one week after laser surgery for TTTS were excluded from the study, unless TAPS persisted. We did this as a large MCA-PSV discrepancy shortly after laser is likely to be a result of hemodynamic re-equilibration and is not based on TAPS [12].
The following information was obtained from local medical records: gravidity, parity, location of the placenta, moment of diagnosis (antenatal or postnatal), gestational age (GA) at diagnosis, TAPS stage at diagnosis, and the presence of additional ultrasound findings including "starry-sky liver" in the recipient and/or a difference in placental echogenicity. The severity of antenatal TAPS was determined according to the previously published staging system by Slaghekke et al. [13]. For post-laser TAPS specifically, the following data regarding preceding TTTS were collected: Quintero stage, GA at laser, laser technique, operator's opinion on completeness of the laser procedure, and TTTS-donor-recipient role. In addition, the antenatal management for TAPS was recorded, including expectant management, delivery (defined as a delivery within 7 days after diagnosis), IUT (± PET), fetoscopic laser surgery, selective feticide, and termination of pregnancy (TOP). Furthermore, information on placental color dye injection was collected, including classification of the type (arterio-venous (AV), veno-arterial (VA), arterio-arterial (AA), veno-venous (VV)), and the number and size of anastomoses. Perinatal outcome measures included: TAPS donor/recipient status, birth weight, hemoglobin and reticulocyte values, treatment with blood transfusion or partial exchange transfusion on Day 1, the presence of severe neonatal morbidity and/or severe cerebral injury, and the occurrence of perinatal mortality.
Primary outcomes were perinatal mortality and severe neonatal morbidity. Perinatal mortality was defined as fetal demise or neonatal death within 28 days after birth. In the context of selective feticide or TOP, a distinction was made between spontaneous fetal demise and intended fetal demise. Severe neonatal morbidity was defined as the presence of at least one of the following, diagnosed within 28 days after birth or prior to discharge: respiratory distress syndrome requiring mechanical ventilation and surfactant, patent ductus arteriosus requiring treatment, necrotizing enterocolitis ≥ Stage 2 [14], retinopathy of prematurity ≥ Stage 3 [15], amniotic band syndrome, ischemic limb injury, or severe cerebral injury. Severe cerebral injury was diagnosed in case of one of the following abnormalities was identified on cerebral imaging: intraventricular hemorrhage ≥ Stage 3 [16], ventricular dilatation (including post-hemorrhagic ventricular dilatation) [17], cystic periventricular leukomalacia ≥ Grade 2 [18], porencephalic or parenchymal cysts, arterial infarction, or other severe cerebral lesions associated with an adverse outcome.
Secondary outcomes included diagnosis-and therapy-related characteristics, hematological and placental characteristics, and a risk factor analysis for spontaneous perinatal mortality and severe neonatal morbidity. Cases with intended fetal demise in the context of selective feticide or termination of pregnancy were excluded for the risk factor analysis for spontaneous perinatal mortality. Since TAPS cases may be managed according to different strategies in one pregnancy, management-group assignment was based on the first treatment that was performed. The following parameters were investigated in the univariate risk analysis: GA at diagnosis of TAPS, GA at laser for TTTS, days between laser for TTTS and development of TAPS, Quintero stage, antenatal TAPS stage, persistence of TTTS-TAPS donor-recipient status, type of antenatal management, and GA at birth (in weeks). For antenatal TAPS stage, the highest recorded antenatal TAPS stage was selected. In case of TAPS Stage 5, the highest TAPS stage before Stage 5 was used. For the risk factor analysis for severe neonatal morbidity, two more parameters were added: severe growth restriction defined as birth weight < 3rd centile and the presence of postnatal TAPS.
The following additional outcomes were determined: inter-twin hemoglobin difference (highest hemoglobin value-lowest hemoglobin value), reticulocyte count ratio (highest reticulocyte value (‰)/lowest reticulocyte value (‰)), the presence of severe growth restriction (defined as a birth weight < 3rd centile according to Hoftiezer [19]), postnatal TAPS stage (according to Slaghekke [7]), and the configuration of anastomosis type per TAPS placenta.
Statistical analyses were carried out using SPSS Version 25.0 (IBM, Armonk, NY, USA). Data are presented as the mean ± standard deviation (SD) or as median and interquartile range (IQR) and/or range (minimum-maximum), as appropriate. A p-value < 0.05 was considered statistically significant. Differences between donors and recipients were calculated using the paired t-test for normally distributed continuous outcomes. To account for the fact that observations between co-twins are not independent, the Generalized Estimated Equation module was executed for analyses per fetus or neonate. Potential risk factors were checked for correlation using Spearman's rank test (R). A correlation coefficient R > (−) 0.7 was considered to indicate a strong relationship between the factors. Potential risk factors for perinatal mortality and severe neonatal morbidity were assessed in a univariate logistic regression model. A multivariate logistic regression model was applied to the variables that showed significant association in the univariate analysis. Results are expressed as odds ratios (OR) with 95% confidence intervals (CI).

Results
Of the 422 TAPS cases entered into the TAPS Registry, two-hundred forty-nine (59%) were spontaneous TAPS and were excluded from the study. The remaining 173 (41%) were post-laser TAPS and eligible for the study. In eight post-laser TAPS cases, TAPS was diagnosed within one week after laser for TTTS. As TAPS did not persist one week after laser surgery for TTTS, these cases were excluded. One case was excluded based on TAPS being diagnosed at Stage 5. A total of 164 post-laser TAPS cases were included in the analysis for the current study.
In cases that were diagnosed with TAPS at birth (72%; 76/106), the median inter-twin hemoglobin difference was 12. , and GA at birth (OR = 0.8, 95%CI 0.7-0.9, p = 0.001) were identified as independent risk factors for spontaneous perinatal mortality. Univariate risk factor analysis showed that severe neonatal morbidity was significantly associated with GA at birth (OR = 1.5, 95%CI 1.3-1.8, p < 0.001) and Quintero Stage 2 (OR = 3.2 95%CI 1.1-9.7, p = 0.038). Both parameters were included in multivariate risk factor analysis as no correlation was found between the two (R = −0.022, p = 0.701). Multivariate risk factor analysis revealed that only GA at birth was an independent risk factor for severe neonatal morbidity (OR = 1.5, 95%CI 1.3-1.7, p < 0.001). More details on the risk analyses for perinatal mortality and severe neonatal morbidity are presented in Tables A2 and A3 of Appendix B.

Discussion
This was the first large international study investigating management and outcome in post-laser TAPS twins. Our study showed that post-laser TAPS generally developed within one month after laser for TTTS, but could be detected up to 17 weeks after laser intervention. Management for post-laser TAPS was mostly expectant, but varied considerably, highlighting the lack of consensus for optimal treatment. In this cohort, perinatal outcome was poor, particularly due to high perinatal mortality rates in TAPS donors. This study provided important information for clinicians involved in the care for TTTS twins treated with laser surgery and might contribute to a better understanding of post-laser TAPS. This is the first study that gives a clear overview of the time of onset of post-laser TAPS and and shows that there is a wide range in timing of presentation. This variation might be attributed to two factors. First, reversal of the donor-recipient role could result in a slower development of post-laser TAPS. Possibly, former TTTS recipients that became TAPS donors may be protected against anemia for a longer period of time due to the excess of blood they received during TTTS. In contrast, former TTTS donors that became TAPS donors might suffer sooner from anemia due to their relatively hypovolemic state. Alternatively, TAPS might develop later in cases with compensating blood supply, allowed by VA, AA, or VV anastomoses. Our results showed that half of the post-laser TAPS cases presented within two weeks after laser surgery for TTTS. Although all cases in this study had signs of ongoing TAPS after the first week, spontaneous normalization of a large MCA-PSV discordancy after laser surgery has also been reported in previous literature [12,20]. Importantly, a large MCA-PSV difference shortly after laser might in some cases be the result of fetal hemodynamic re-equilibration after intervention, rather than the onset of post-laser TAPS due to the presence of a patent anastomosis. Consequently, intervening directly within 1-2 weeks after laser surgery in these cases could lead to unnecessary treatment since there is no ongoing transfusion. Therefore, close follow-up ultrasound examination to identify persistence or progression of a MCA-PSV discrepancy after the first weeks after laser surgery is recommended to confirm the diagnosis of post-laser TAPS and prevent unnecessary intervention and exposure to iatrogenic risks.
Our study demonstrated that 81% of the surgeons initially thought that the laser for TTTS was complete. This "low index of suspicion" causes TAPS to often occur unexpectedly and shows that operator-reported completeness cannot be relied upon. Interestingly, approximately a third of post-laser TAPS twins were treated for TTTS with the Solomon technique. This illustrated that, although the Solomon technique has been proven to decrease the incidence of post-laser TAPS [2], clinicians should remain vigilant for the development of this complication even after a complete Solomon line was thought to be achieved. In agreement with the current recommendations of the twin guideline of the International Society of Ultrasound in Obstetrics and Gynecology [21], we strongly underline the importance of strict routine MCA-PSV Doppler follow-up examination in TTTS twins treated with laser surgery during the entire pregnancy to check for the presence of post-laser TAPS.
We found high rates of perinatal mortality, particularly in the TAPS donor, reflecting the detrimental impact of fetal anemia on perinatal survival. Remarkably, TAPS donors only showed increased risk for perinatal mortality. After birth, donors and recipients had similar rates of severe neonatal morbidity, suggesting that neonatal health is more strongly related to the degree of prematurity than TAPS donor-recipient status. Importantly, this study demonstrated that GA at birth is a strong risk factor for both perinatal mortality and severe neonatal morbidity in post-laser TAPS twins. Compared to post-laser TAPS survivors that were previously investigated in the Solomon trial [22], we reported similar rates of severe neonatal morbidity (38% vs. 39%) and higher rates of perinatal mortality (18% vs. 26%). Of note, long-term outcome was not investigated in this study, and therefore, a difference in long-term neurodevelopmental impairment between donors and recipients cannot be precluded.
Post-laser TAPS twins showed an overall more detrimental outcome than spontaneous TAPS twins [8]. The exact cause of this difference in outcome is not entirely clear, but it is likely to be multifactorial. The first and most obvious explanation is that post-laser TAPS twins have previously experienced TTTS, a severe condition itself [23]. Therefore, their fetal condition might already be compromised when they start developing TAPS. Given the fact that half of the post-laser TAPS cases occur within the first two weeks after laser, twins have had only limited time to recover, making them more prone to decompensation. A second explanation could be found in the angioarchitecture of placentas of post-laser TAPS twins. This study represents the biggest cohort of injected post-laser TAPS placentas and confirms previous findings that post-laser TAPS placentas often show only one or a few placental anastomoses [24]. Additionally, we found that most cases only had unidirectional AV anastomoses, without compensating flow from VA, AA, or VV anastomoses in the opposite direction. This might lead to an accelerated deterioration of TAPS, resulting more rapidly in abnormal Doppler blood flows, hydrops, and fetal death. Interestingly, a minority of post-laser TAPS placentas did not show residual anastomoses, in spite of the presence of confirmed postnatal TAPS. This could be explained by suboptimal color dye or by the presence of deep, hidden anastomoses [4,25]. Third, the choice of antenatal management might have also influenced the condition of post-laser TAPS twins. Our results showed that treatment for post-laser TAPS was diverse, but that the majority of the group was managed expectantly or received IUT (± PET); two treatment strategies that are not definitive in nature and allow the condition to progress. Possibly, laser was considered more challenging or not feasible in cases that already underwent laser, due to expected reduced visibility, membrane separation, iatrogenic monoamnionicity, PPROM, or because of the same reasons that caused the laser to be incomplete in the first place, such as the position of the placenta. Illustratively, we found a high rate (40%) of residual anastomoses in twins treated with laser reintervention. A detailed evaluation of differences in perinatal outcome between the various management strategies will be presented in a separate study [26].
As with all registries, this study was fully dependent on local registrations of post-laser TAPS cases. In many countries, TTTS cases are sent back to the referring hospital after laser procedure, leaving care in less experienced hands. As the diagnosis of post-laser TAPS is only reached by adequate MCA-PSV Doppler examination, hemoglobin and reticulocyte measures, and placental-injection studies, it is likely that some post-laser TAPS cases have been missed. Nonetheless, this study represented the largest cohort of post-laser TAPS twins to date and was able to provide valuable insights into management and outcome in post-laser TAPS.

Conclusions
To conclude, post-laser TAPS could occur at any time after laser for TTTS, is managed heterogeneously, and is associated with poor outcome, particularly in donor twins. Our findings necessitate further research into the best treatment option for TAPS. To investigate the best treatment for TAPS adequately, an international randomized controlled trial is needed.   Values are the odds ratios (OR) (95% confidence intervals (CI)), the standard error (SE), and the p-value. * Set to zero as the reference. † Twenty-three of 327 cases were excluded based on selective feticide or termination of pregnancy. ‡ Five cases had a missing Quintero stage value. § In 3 cases, persistence of TTS-TAPS donor-role was unknown. ¶ SPSS was not able to calculate OR for groups in which the event (spontaneous mortality) did not occur; therefore, a correction for non-occurring events was applied, and with this correction, an unaffected child was changed into an affected child, for all groups. TAPS, twin anemia polycythemia sequence; GA, gestational age; TTTS, twin-twin transfusion syndrome; IUT, intrauterine transfusion; PET, partial exchange transfusion. Values are the odds ratios (OR) (95% confidence intervals (CI)), the standard error (SE), and the p-value. * Set to zero as the reference. † Nine missing values (6 cases with unknown neonatal outcome and 3 neonates that died shortly after birth). SNM, severe neonatal morbidity; TAPS, twin anemia polycythemia sequence; GA, gestational age; IUT, intrauterine transfusion; PET, partial exchange transfusion.