Polymelia is a birth defect in which an affected individual has more than the usual number of limbs. It is a type of dysmelia. In humans and most land-dwelling vertebrates, this means having five or more limbs. The extra limb is most commonly shrunken and/or deformed. The term is from Greek πολυ- "many", μέλεα "limbs". Sometimes an embryo started as conjoined twins, but one twin degenerated completely except for one or more limbs, which end up attached to the other twin. Sometimes small extra legs between the normal legs are caused by the body axis forking in the dipygus condition. Notomelia (from Greek for "back-limb-condition") is polymelia where the extra limb is rooted along or near the midline of the back. Notomelia has been reported in Angus cattle often enough to be of concern to farmers.Cephalomelia (from Greek for "head-limb-condition") is polymelia where the extra limb is rooted on the head. Origin
Tetrapod legs evolved in the Devonian or Carboniferous geological period from the pectoral fins and pelvic fins of their crossopterygian fish ancestors. Fish fins develop along a "fin line", which runs from the back of the head along the midline of the back, round the end of the tail, and forwards along the underside of the tail, and at the cloaca splits into left and right fin lines which run forwards to the gills.
Hydroa vacciniforme (HV) is a very rare, chronic photodermatitis-type skin condition with usual onset in childhood. It was first described in 1862 by Bazin. It is sometimes called "Bazins hydroa vacciniforme". A study published in Scotland in 2000 reviewed the cases of 17 patients and estimated a prevalence of 0.34 cases per 100,000 population. In this study they reported an average age of onset of 7.9 years. Frequently the rash first appeared in the spring or summer months and involved sun-exposed skin. The rash starts as a vesicular eruption, later becoming umbilicated, and resulted in vacciniform scarring. It is most frequently found on the nose, cheeks, ears, dorsum of the hand, and arms (places that are most exposed to light). Causes
Hydroa vacciniforme is commonly associated with reactivation of a latent Epstein-Barr virus (EBV) formerly acquired by an asymptomatic or infectious mononucleosis-causing infection. It is therefore classified as one of the Epstein-Barr virus-associated lymphoproliferative diseases and termed EBV+ HV. These cases of EBV+ HV may progress to another EBV+ lymphoproliferative disease such as T cell lymphoma, T cell leukemia, B cell lymphoma, and B cell leukemia. Natural History
In many children hydroa vacciniforme (HV) regresses spontaneously by early adulthood. In the 29 patients followed by Iwatuski et al., 11 of the 18 with definite or probable HV were available for follow-up and all were alive without progression of their symptoms. Some had recurrent eruptions of HV.