Cytology · MU Sofia

Twins. Malformations.

Covers monozygotic and dizygotic twins and how each carries a different malformation risk, then goes through specific twin malformations such as conjoined twins, twin-to-twin transfusion syndrome, selective growth restriction, acardiac twins and parasitic twins. It also lists teratogenic, placental and mechanical environmental factors, the cellular mechanisms of embryogenesis that can be disrupted, and how malformations are managed before and after birth.

Inside: 6 sections · 8 labelled pictures · 16 flashcards · 5 min

Topic 93 of 97 in the MU Sofia syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 6 sections, about 664 words (5 min of reading), with 16 flashcards.

In this note

  1. Introduction
  2. Types of Twins and Malformations:
  3. Specific Malformations in Twins:
  4. Environmental Factors Influencing Malformations:
  5. Cellular Mechanisms and Their Disruption:
  6. Management of Twin Malformations:

Introduction

Environmental factors can significantly affect embryonic development, particularly in the context of twin pregnancies, and can result in various malformations.

Types of Twins and Malformations:

  • Monozygotic twins, arising from a single fertilised egg, share the same genetic material, but they have a higher risk of malformations due to shared placental structures and amniotic environments.
  • Dizygotic twins, from two separate eggs fertilised by different sperm, are genetically distinct and have a lower risk of malformations.
  • Malformations can involve any organ or system, and are more common in monozygotic twins due to errors during embryonic division.

Specific Malformations in Twins:

  • Conjoined twins occur when the division of a single fertilised egg is incomplete, resulting in physically connected twins who may share tissues, organs, or body parts. Common types include thoracopagus (joined at the chest), omphalopagus (joined at the abdomen), and craniopagus (joined at the skull).
  • Twin-to-Twin Transfusion Syndrome (TTTS) affects monochorionic-diamniotic twins (sharing one placenta but having separate amniotic sacs), causing unequal blood flow between the twins via placental connections. This can lead to growth restriction, organ malformations, or death in one or both twins.
  • Selective Intrauterine Growth Restriction (sIUGR) occurs when one twin's growth is significantly impaired, often due to unequal placental sharing, leading to asymmetry in development or malformations in the smaller twin.
  • Acardiac Twin (Twin Reversed Arterial Perfusion, TRAP) is a rare complication where one twin lacks a functioning heart and relies on the other twin for blood circulation. The acardiac twin is severely malformed with poorly developed organs.
  • Parasitic twins involve one twin that is underdeveloped and physically dependent on a more developed twin. The parasitic twin may present as an attached mass or extra limbs.

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