A parasitic twin, also known as an asymmetrical or unequal conjoined twin, is the result of the processes that also produce vanishing twins and conjoined twins, and may represent a continuum between the two. Parasitic twins occur when a twin embryo begins developing in utero, but the pair does not fully separate, and one embryo maintains dominant development at the expense of its twin. Unlike conjoined twins, one ceases development during gestation and is vestigial to a mostly fully formed, otherwise healthy individual twin. The undeveloped twin is defined as parasitic, rather than conjoined, because it is incompletely formed or wholly dependent on the body functions of the complete fetus. The independent twin is called the autosite. Variants
Conjoined parasitic twins joined at the head are described as craniopagus or cephalopagus, and occipitalis if joined in the occipital region or parietalis if joined in the parietal region. Craniopagus parasiticus is a general term for a parasitic head attached to the head of a more fully developed fetus or infant. Fetus in fetu sometimes is interpreted as a special case of parasitic twin, but may be a distinct entity. The twin reversed arterial perfusion, or T.R.A.P. sequence, results in an acardiac twin, a parasitic twin that fails to develop a head, arms and a heart. The parasitic twin, little more than a torso with or without legs, receives its blood supply from the host twin by means of an umbilical cord-like structure, much like a fetus in fetu, except the acardiac twin is outside the autosites body.
The primary treatments of choice are:
Itraconazole, an antifungal azole, is given orally, with or without flucytosine. Alternatively, cryosurgery with liquid nitrogen has also been shown to be effective.Other treatment options are the antifungal drug terbinafine, another antifungal azole posaconazole, and heat therapy. Antibiotics may be used to treat bacterial superinfections.Amphotericin B has also been used.Photodynamic therapy is a newer type of therapy used to treat Chromblastomycosis. Prognosis
The prognosis for chromoblastomycosis is very good for small lesions. Severe cases are difficult to cure, although the prognosis is still quite good. The primary complications are ulceration, lymphedema, and secondary bacterial infection. A few cases of malignant transformation to squamous cell carcinoma have been reported. Chromoblastomycosis is very rarely fatal. Epidemiology
Chromoblastomycosis occurs around the world, most commonly in rural areas in tropical or subtropical climates.It is most common in rural areas between approximately 30°N and 30°S latitude. Over two-thirds of patients are male, and usually between the ages of 30 and 50. A correlation with HLA-A29 suggests genetic factors may play a role, as well. Social and cultural
Chromoblastomycosis is considered a neglected tropical disease, affects mainly people living in poverty, and causes considerable morbidity, stigma and discrimination. See also
List of cutaneous conditions

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