| Craniopagus twins | |
| Field: | Neurosurgery, neuroimaging, medical genetics |
| Onset: | In utero |
| Diagnosis: | Obstetric ultrasonography |
| Frequency: | 1 in 2.5 million live births |
Craniopagus twins are conjoined twins who are fused at the cranium.[1] The union may occur on any portion of the cranium, but does not primarily involve either the face or the foramen magnum; the two brains are usually separate, but they may share some brain tissue. Conjoined twins are genetically identical and always share the same sex. The thorax and abdomen are separate and each twin has their own umbilicus and umbilical cord.
The condition is extremely rare, with an incidence of approximately 1 in 2.5 million live births. An estimated 50 craniopagus twins are born around the world every year, with only 15 twins surviving beyond the first 30 days of life.[2] Relatively few craniopagus twins survive the perinatal period; approximately 40% of conjoined twins are stillborn and an additional 33% die within the immediate perinatal period, usually from organ abnormalities and failure.[3]
However, 25% of craniopagus twins survive and may be considered for a surgical separation; several such attempts occur annually worldwide.[4] Advances in neuroimaging, neuroanesthesia, and neurosurgery have demonstrated that a successful outcome is possible. Among all conjoined twins, craniopagus twins account for a mere 2% to 6%.[5]
The first classification system was developed by O'Connell in 1976, where craniopagus twins were classified as total and partial. In the former, twins shared an "extensive surface area with widely connected cranial cavities", while in the latter, only a "limited, superficial surface area" was connected.
In 2006, Stone and Goodrich came up with a more nuanced classification system, which is the "most commonly used" system.[5] Partial CPT was defined as "lacking substantial shared dural venous sinuses", whereas total CPT "share a large portion of their dural venous sinuses and present with pronounced brain compression, which leads to distortion within the cranium".
Total craniopagus twins share a large portion of dural venous sinuses and present with pronounced brain compression, leading to distortion within the cranium. Stone and Goodrich also defined two main subtypes, based on whether the long-axis angle between the twins was angular or vertical. The angular subtype has a higher rate of comorbidities than the vertical subtype.[5]
Vertical craniopagus calvaria is continuous and is further subdivided on the basis of intertwin axial facial rotation:
Another classification system divides total craniopagus twins into four categories:
Having this kind of juncture means that there would be one common continuous cranium housing four cerebral hemispheres. An incomplete dural septum typically separates the flattened cerebral hemispheres. In total vertical craniopagus, the major cerebral arterial supply is usually confined to each respective twin and in some cases conjoined brain tissue may contain a larger artery. Within this category there are three smaller subdivisions that basically outline the different rotational symmetry of the junction.[7]
Stone and Goodrich define partial craniopagus twins as lacking substantial shared dural venous sinuses, with limited surface area involvement, with either intact crania or cranial defects. It is less common than total craniopagus twins.
In partial craniopagus twins, the unions are usually frontal and less commonly occipital and vertical. Angular frontal junctions occur when the two twins are joined at any part of the forehead. Occipital twins are joined at the occipital lobe in the back of the head and vertical are joined on the top of the head and usually face opposite directions. The junctional diameter is often smaller in partial forms and occasionally an incomplete layer of bone may be present between the twins. Each child maintains independent calvarial convexities except at the common area of skull junction. The dura of both children may be intact or deficient and cortical gyri may interdigitate.[8] Additionally, shared dural venous sinuses is usually absent, or, if it is present, negligible. These twins usually undergo successful separation and both twins may live to lead normal lives.
The exact nature of how conjoined twins develop in utero remains unclear. Embryologists have traditionally attributed identical twinning as "splitting or fission" of either the inner cell mass of pleuripotent cells or early embryonic disc at 13–14 days of gestation just before the primitive streak. Some theorists suggested that conjoined twins develop as a result of the failed division (fission) of a single fertilized ovum. However a new hypothesis suggests that cranial fusion occurs between two separate embryos before the end of the 4th week of gestation. This can happen because the cranial neuropore is still open, which is responsible for the ultimate fusion and formation of the brain stem and central nervous system. Furthermore, this secondary fusion of embryonic discs could implicate that intact skin will not fuse to other intact skin, including the ectoderm of the embryo. This means that two embryonic discs could only unite in locations where the ectoderm is absent. Moreover, the fusion occurs from neural folds of two separate, dorsally oriented embryonic discs, and the union can occur only after the ectoderm is disrupted to allow the neural and surface ectodermal layers to separate from each other.
The union in craniopagus twins may happen at any portion of the calvarium.[9] The juncture can involve either the entire diameter of the head or any portion of the head and can be positioned at a multitude of rotational angles. In fact, craniopagus twins are rarely found in a symmetrical union. Apart from this, the vertebral axes may have a straight line. Despite this, the angle of the vertebrae is the ultimate dictator in how the individuals heads actually face. The majority of twins face either the same way or the exact opposite direction.
Conjoined twins, including craniopagus twins, can be diagnosed using standard ultrasound procedures during a pregnancy. In part because treatment of conjoined twins varies largely, many parents make the decision to terminate pregnancy due to the prognosis and quality-of -life issues. If the parents choose to continue the pregnancy, mother and babies will be closely monitored. In almost all cases, a C-section delivery is planned, often two to three weeks before the due date.[10]
After the twins are born, parents and doctors decide whether or not separation surgery is possible. The doctors also must consider the possibility of reconstructive surgery and the social and learning issues the twins may have to face after they are separated.
Due to advances in neuroimaging, neuroanesthesia, and neurosurgical techniques, successful separation operations have become more common. Physical traits like joined brain tissue, shared arteries and veins, as well as defects in the skull and dura mater complicate a separation operation.[5]
Neuroimaging especially plays an important role because it is imperative in surgical planning to understand the shared vascular anatomy—including the brain parenchyma, calvaria, and dura mater—because separating shared vessels can lead to thrombosis, air embolism, cerebral infarction, and hemorrhage. Technologies such as CT scans, MRIs, and angiography are used to map the shared vascular structures.[5]
In 2011, The New York Times Magazine covered a story of two craniopagus twin girls who share a brain and seem to show all different kinds of physiological and emotional responses due to their condition.[29] Imagery revealed an attenuated line stretching between the two brains and forming a "thalamic bridge", a bridge connecting the two thalami. Knowing that the thalamus acts as a major control panel within the body, it is believed that the girls share part of this 'control panel' and so when one girl drinks the other one feels it. A more recent report on the twins was released in November 2017.[30]
Although there is not an overwhelming amount of research surrounding how the union between craniopagus twins leads to different personality, cognitive and motor traits, there have been some studies exploring what it actually means to share a brain. In the case of Tatiana and Krista, it is possible that the twins shared some conscious thought. Studies of the thalamus' role in the brain provide neurological data that help explain these behavioral observations that these two twins experience.
Thalamo-cortico-thalamic circuits are the looped neural pathways that connect the thalamus to the cerebral cortex, and then the cerebral cortex back to the thalamus. Because the thalamus is mainly responsible for relaying sensory messages from the body to the brain, it is possible that there is a lot of overlap between the twins' sensory reception and the actual response it creates within the brain. One study examines this by studying the thalamus when it is at a persistent vegetative state that is when the patient is awake but not conscious. This study proved that the cortical activity on its own is not conscious and that all the activity between the loops of the thalamus, the cerebral cortex and the thalamus itself are all conscious actions. Another study of the thalamus reaffirms that the thalamus does not answer yes/no questions but instead acts as a mediator between different parts of the cerebral cortex and systemic sensory reception.[31]
These loops actually may account for the relationship between Tatiana and Krista. At the neuronal level, communication is dense network of neurons linked between themselves and the coordinator (in this case the thalamus) that finally sends a message to the cortex. On top of this, there are links between the cortex that send messages back through the coordinator and finally to the rest of the body. The brain's ability to function through loops and circuits is a good model to explain why Tatiana consciously feels what Krista is physically experiencing. Additionally there is some level of synchronization between the two twins. Another study found that for craniopagus twins, their connection to each other is comparable to our normal appendages and that their bodies have obvious overlap physically and psychologically.[32] Because most cases of craniopagus twins are unique, the research outlining general connections between craniopagus twins is limited. However, this example provides insight into the effects of a union between twins who essentially share the same sensory relay system in the thalamus.
Conjoined twinning is one of the oldest known birth defects in history and examples with human's fascination with twins is extremely evident throughout historical literature. Although there are cases of conjoined twins dating back to as early as the 10th century, it was not until 1491 that the first case was documented.
Apart from that, Sebastian Münster’s Cosmographia universalis provides the true first account of craniopagus twins who happened to live for ten years, exceeding many expectations during that time.[33] He describes the set of twins as being a unique malformation and a punishment from their mother's mistake. In French barber surgeon Ambroise Paré’s 16th-century book, On Monsters and Marvels, various types of "supernatural" twinning are illustrated and described as "monstrous and marvelous creatures that proceed from the judgment of God", suggesting that conjoined twins and specifically craniopagus twins were viewed as literal monsters in the 16th century.[34]