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Xeroderma pigmentosum (XP) is a rare, inherited disease involving extreme sensitivity to sunlight (photosensitivity), specifically the sun’s ultraviolet (UV) rays. UV radiation can cause extreme damage to the skin and eyes of patients with this disease.

Patients with XP typically experience severe, skin-blistering sunburns, even after minimal sun exposure. These acute sunburns usually last longer than normal, sometimes for several weeks. As the disease progresses, the skin becomes excessively dry and rough, with signs of premature aging similar to that of elderly people who have spent many years in the sun. However, these changes in the skin usually begin in infancy and may include:

• Skin atrophy (thinning)
• Discolorations (hypopigmentation)
• Irregular dark spots (hyperpigmentation)  
• Growths or tumors

Frequently, the eyes of XP patients may also become irritated, bloodshot and clouded following exposure to UV radiation.

XP is extremely rare. According to the Xeroderma Pigmentosum Society, an estimated one out of every million people has XP in the United States. The condition affects both sexes and people of all ethnic groups but certain populations tend to have a higher incidence of the disorder. In Japan, the number of cases is estimated at one per every 100,000 people. Incidence is also increased in North Africa and the Middle East, especially in communities with greater consanguinity (intermarriage of blood relatives).

XP – also called Kaposi disease – was first described in 1870 by dermatologists Ferdinand von Hebra and Moriz Kaposi. Originally called xeroderma, Kaposi later added the pigmentosum to describe the characteristic changes in the skin pigment of people with the disorder. Kaposi first recognized that only the areas of skin exposed to sun were affected and that there was a hereditary component to the disorder. The medical community identified the genes (basic units of heredity for all living organisms) that trigger the skin disease several decades later.

There is no cure for XP and the skin deterioration that occurs with the disease is cumulative and often irreversible. People with this condition often require total, lifelong protection from sunlight or UV radiation-emitting agents (e.g., mercury-vapor lamps, tanning booths). XP has severe, life-threatening complications that may include:

• Permanent skin damage and scarring
  
  
• All forms of skin cancer (basal cell carcinoma, squamous cell carcinoma, malignant melanoma)
  
  
• Cancerous tumors in the eyes and mouth area

Sometimes patients with XP may experience progressive neurological problems, including:

• Developmental (learning) disabilities
• Mental retardation
• Dementia
• Hearing loss that may lead to deafness
• Loss of motor skills

In addition, although rare, XP may co-exist with other extremely rare hereditary disorders, such as:

• Cockayne syndrome (CS). A rare disorder characterized by a failure to grow, impaired development of the nervous system, abnormal sensitivity to sunlight (photosensitivity) and premature aging. Hearing loss, eye abnormalities and severe tooth decay are other common features. Problems with internal organs are also possible in people with CS. In contrast to XP, though, skin cancer is rare in CS.
  
  
• De Sanctis-Cacchione syndrome. This condition is characterized by all the skin and eye problems of XP along with neurological abnormalities, including mental retardation, unusually short stature (dwarfism), and underdevelopment of the sexual reproductive glands (hypogonadism).