Two cases of atypical twinning: Phenotypically discordant monozygotic and conjoined twins

On the origin of zygosity and chorionicity in twinning: evidence from human in vitro fertilization

Assisted reproduction is presumed to increase monozygotic twin rates, with the possible contribution of laboratory and medical interventions. Monozygotic dichorionic gestations are supposed to originate from the splitting of an embryo during the first four days of development, before blastocyst formation. Single embryo transfers could result in dichorionic pregnancies, currently explained by embryo splitting as described in the worldwide used medical textbooks, or concomitant conception. However, such splitting has never been observed in human in vitro fertilization, and downregulated frozen cycles could also produce multiple gestations. Several models of the possible origins of dichorionicity have been suggested. However, some possible underlying mechanisms observed from assisted reproduction seem to have been overlooked. In this review, we aimed to document the current knowledge, criticize the accepted dogma, and propose new insights into the origin of zygosity and chorionicity.

Characterizing the coalescence area of conjoined twins to elucidate congenital disorders in singletons

Shared anomalies, always located close to the area of coalescence and observable in virtually every type of conjoined twinning, are currently seen as separate anomalies caused by mostly unknown and seemingly unrelated pathways rather than being connected to the twinning mechanism itself. Therefore, most (case) reports about conjoined twins are mere descriptions of (external) dysmorphologies lacking reflections on the possible origin of their concomitant anomalies. As we will demonstrate in this article, shared anomalies are influenced, and in some cases solely and sequentially explained, by interaction aplasia and neo‐axial orientation; two embryological mechanisms to which each set of conjoined twins is subjected and are responsible for their ultimate phenotypical fate. In this review, we consider how the ventral, lateral and caudal conjunction types and their intermediates determine the phenotypic presentation of the twins, including patterns of shared malformations and anomalies, which in themselves can be indistinguishable from those encountered in singleton cases. Hence, it can be hypothesized that certain anomalies in singletons originate in a fashion similar to that in conjoined twins.

Conjoined twins in 2020 - state of the art and future directions

PURPOSE OF REVIEW

A number of high profile conjoined twin separations have been extensively covered by the world media. Anaesthesia for conjoined twins is a procedure rarely experienced by paediatric anaesthetists. The increased survival of the twins has prompted discussion as to the most appropriate selection of patients, teams and hospitals to provide exceptional anaesthetic care.

RECENT FINDINGS

The number of conjoined twins presenting for surgery remains low with many infants not surviving foetal or early neonatal life. Anaesthetic management of less common conjoined infants such as craniopagus twins has highlighted the benefit of careful patient selection, extensive preoperative investigations and meticulous multidisciplinary team planning. The role of simulation of possible adverse perioperative events has been highlighted. Three dimensional anatomical models and virtual reality systems have permitted surgical planning in advance of actual intervention. A number of legal and ethical concerns have been reported especially in the setting of emergency separation where surgery is likely to contribute to death of one of the twins.

SUMMARY

There appears to be an expanding role for international teams with extensive separation experience becoming involved in international teleconferencing to improve patient management in low-resource countries. Whether the perioperative outcome is better when the conjoined twins are transferred to major centres for surgery or teams operate in the twin's country of origin remains to be seen.