What is xeroderma pigmentosum?

Katie Mahar suffers from XP and can't risk exposure to the sun. Her parents founded Camp Sundown.
Katie Mahar suffers from XP and can't risk exposure to the sun. Her parents founded Camp Sundown.
AP Photo/Jim McKnight

There are people who must spend their lives in the dark or risk terrible injuries and diseases. They avoid sunlight. Their windows are covered with protective film, blinds and curtains. They only venture outdoors at night. Even walking into a room lit by fluorescent bulbs can be risky. They suffer from a rare condition called xeroderma pigmentosum (XP). In the United States, the frequency of cases is about one in every 250,000 people [source: Diwan].

This genetic condition affects a person's ability to recover from ultraviolet radiation. Upon exposure to ultraviolet rays, a person with XP will experience severe damage almost instantly. The person's skin will begin to blister mere moments after exposure. These injuries can lead to even more serious consequences.

A pair of physicians named Moriz Kaposi and Ferdinand von Hebra first identified the disease in 1874. Kaprosi named the disease in 1882. The term xeroderma pigmentosum refers to the appearance and texture of affected skin -- it's leathery and has increased pigmentation.

Much later, physicians discovered that the disease was caused by a genetic disorder. The genes that carry XP are recessive -- both the mother and father must carry the gene to produce a child with XP. Because the genes are recessive, it's possible for a person to be a carrier without suffering from the symptoms of XP him- or herself.

The condition has three stages. The first stage begins around six months after birth. Up to that point, the child's skin typically appears to be normal and healthy. But around six months of age, children with XP tend to develop freckles and scaly skin upon exposure to the sun. The second stage includes more extensive damage and noticeable symptoms -- the skin might become rubbery or leathery upon exposure to ultraviolet radiation. The third stage involves the development of skin cancer.

While the skin's sensitivity to ultraviolet radiation is the most common symptom of XP, some children with the disease suffer other problems. Around 20 percent of children with XP also suffer neurological damage. Developmental disorders aren't uncommon and some children may lose the ability to speak, walk or perform other tasks.