Diprosopus (Greek διπρόσωπος, "two-faced", from δι-, di-, "two" and πρόσωπον, prósopon [neuter], "face", "person"; with Latin ending), also known as craniofacial duplication (cranio- from Greek κρανίον, "skull", the other parts Latin), is an extremely rare congenital disorder whereby parts (accessories) or all of the face is duplicated on the head.
Although classically considered conjoined twinning (which it resembles), this anomaly is not normally due to the fusion or incomplete separation of two embryos. It is the result of a protein called sonic hedgehog homolog (SHH). (The name of this protein was inspired by the Sonic the Hedgehog videogame character and is part of an idiosyncratic naming tradition in molecular biology research that some have criticized as frivolous.)
The SHH protein and its corresponding gene have been found to play an important role in signaling craniofacial patterning during embryonic development. Among other things, the SHH protein governs the width of facial features. In excess it leads to widening of facial features and to duplication of facial structures. The greater the widening, the more of the structures are duplicated, often in a mirror image form. This has been demonstrated in the laboratory by introducing pellets of the SHH protein into chicken embryos, resulting in chickens with duplicate beaks. Insufficient amounts of that protein lead to opposite conditions such as cyclopia where facial features are insufficiently developed.
Healthy brain development is also dependent on the signaling function of the SHH protein. During embryonic development, the SHH protein directs embryonic cells to organize in specific areas that later become specialized neural tissues, thus controlling the size and shape of brain structures.
Diprosopus often occurs in combination with other congenital disorders, particularly anencephaly, neural tube defect and cardiac malformations. When present, the brain may show abnormalities ranging from partial to complete duplication of brain structures, and/or underdevelopment of brain tissues.
Few two-faced animals have survived due to associated internal organ abnormalities and brain abnormalities. One of the most famous was Ditto the pig. Ditto was raised to adulthood, but died of pneumonia caused by food inhalation when breathing through one muzzle while eating with the other.
Cats with the condition are known as 'Janus cats', after the Roman god. In July 2006, a 6-year-old male Janus cat called "Frank and Louie" from Millbury, Massachusetts USA received publicity. In his case, only one esophagus (and possibly only one trachea) was functional and aided survival. In September 2011, when Frank and Louie was 12 years old, it was announced that he will appear in the 2012 Guinness Book of World Records as the longest-surviving Janus cat on record.
Most human infants with diprosopus are stillborn. Known instances of humans with diprosopus surviving for longer than minutes to hours past birth are very rare; only a few are recorded. In 2002 and 2003, two living male infants with partial diprosopus were described in the medical literature in separate case reports. One infant was born with duplication of the nose and the cerebral frontal lobes, two widely spaced eyes, a small, underdeveloped central eye socket, and a large, asymmetric mouth. The other infant was born with duplication of the upper and lower jaw, two tongues ending in the same base, cleft palate, a slightly divided tip of the nose, and two widely spaced eyes, as well as absence of the corpus callosum, duplication of the pituitary gland and stalk, and abnormalities in the midbrain. Because they were born with a milder, partial form of diprosopus, both infants were considered candidates for surgical correction of their abnormal facial features.
Lali Singh 
In 2008, a baby girl born in India, Lali Singh, became the most recently known person to have the condition diprosopus. She was born March 10, 2008 in Sanai Sampūra village near Delhi; the birth was delayed by dystocia caused by her large head, and she was born in a hospital with an episiotomy. She was one of the very few infants with diprosopus to survive well past birth. She may have been the only known living individual with complete facial duplication. Her facial features included two pairs of eyes, two noses, and two mouths (but only one pair of ears). Lali Singh, daughter of Sushma and Vinod Singh, lived in the Indian village of Saini Sunpura. There, she was seen as the reincarnation of the goddess Durga, who has three eyes. It was also thought that Lali was an incarnation of the Hindu god Ganesh.
As of April 2008, Sushma and Vinod Singh had declined an offer from local doctors to evaluate their daughter through CT or MRI scanning. Without diagnostic imaging, it was not possible to know the full extent to which the child's condition might have affected her brain and other vital structures in her head and neck. Thus, any estimation of her ability to survive or even thrive could only be speculative, though Lali's family described her as functioning normally. It is also unknown whether neurosurgeons or craniofacial surgeons, if consulted, would have had feasible solutions to offer with respect to corrective surgery. A local doctor told reporters that the baby should be considered a healthy child who currently was living a normal life, a previously unknown occurrence among sufferers of the disorder.
Lali's two middle eyes suffered from corneal opacity due to abnormal anatomy of the facial muscles, which prevented her from properly closing those eyes. (Before, it was wrongly blamed on camera flashes.)
Cleft palate caused difficulty feeding her under village conditions. A poor diet of bottle-fed sugar solution and diluted milk, allowed to drip down her throat as she could not suck properly because of the cleft palate, weakened her condition, and vomiting and infection started. Admission to hospital was delayed by discussion (including taking her back home from hospital) among her extended family and her village's headman. Finally, her parents alarmed at her illness and dehydration, defied her other relatives and took her back to hospital, where under proper medical treatment including antibiotic and a saline drip she started to improve, stopped vomiting, started drinking milk and defecating normally; but 6 hours later, at two months old to the day, she died of a heart attack. She was buried in her village, as is usual in Hinduism with children who die very young. Later a temple was built at the village in her memory.
See also 
- Conjoined twins
- Janus, a Roman god with two faces, after whom Janus cats are named
- Durga, a three-eyed Hindu goddess
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