Hartnup disease is a rare genetic disorder that affects the metabolism of important vitamins in the body. Most people with the disease have only minor vitamin deficiencies and do not experience significant health problems. Severe cases may result in frequent skin rashes, muscle coordination problems, and mental illness. There is no cure for Hartnup disease, but most patients can find relief from their symptoms by following specialized diets and taking vitamin supplements to correct their deficiencies.
The intestines normally break down amino acids from foods and transport them to the liver and kidneys. Those organs, in turn, metabolize the chemicals into essential proteins and vitamins. In the case of Hartnup disease, a genetic defect impairs the ability of the intestines and kidneys to metabolize dietary amino acids. They are expelled as waste in the urine instead of being absorbed into the bloodstream. Hartnup disease is an autosomal recessive disorder that is only inherited if both parents carry a mutated copy of a particular gene, called SLC6A19.
The most significant deficiency that results from Hartnup disease is a shortage of nicotinamide, a type of B vitamin. A significant lack of vitamin B can cause skin problems and neurological symptoms. Some people develop chronic dermatitis that causes redness and itching all over their bodies. Symptoms tend to be the worst when the skin is exposed to direct sunlight.
People with major vitamin B deficiencies can also experience frequent episodes of muscle weakness, tremors, and poor coordination. Dementia is an uncommon but potentially debilitating consequence of Hartnup disease. A severe and untreated case of Hartnup disease that first appears in infancy may result in failure to thrive and irreversible mental retardation.
Doctors can usually diagnose and treat Hartnup disease before it causes major problems. Urine samples are tested for unusually high levels of amino acids, and blood samples are screened to check for vitamin deficiencies. Other potential causes of symptoms can be ruled out with brain imaging scans and complex genetic testing procedures.
Patients with relatively mild symptoms are instructed to maintain high-protein diets and take daily vitamin B supplements. Most neurological problems correct themselves when vitamin levels are returned to their normal ranges. Since sun exposure is a major trigger for dermatitis, patients may be told to wear protective clothing and sunscreen whenever going outdoors. Topical creams may be prescribed to ease skin symptoms if severe rashes continue to develop.