Hartnup disorder is an inherited metabolic condition that involves the transport of certain amino acids (for example, tryptophan and histidine) in the small intestine and kidneys.

Hartnup disorder is possibly the most common metabolic condition involving amino acids. It is an inherited condition. A child must inherit the defective gene from both parents in order to be seriously affected.

The condition usually starts between ages 3 - 5 years.

Most people show no symptoms. If symptoms do occur, they usually appear in childhood and may include:

• Diarrhea
• Mood changes
• Nervous system (neurologic) problems, such as abnormal muscle tone
• Red, scaly skin rash, usually when skin is exposed to sunlight
• Sensitivity to light (photosensitivity)
• Short stature
• Uncoordinated movements

A urine test that checks for high levels of "neutral" amino acids and normal levels of other amino acids may be done for this disorder.

The gene for Hartnup has been found, but DNA testing is not yet available. Biochemical tests are available.

Treatments include:

• Avoiding sun exposure by wearing protective clothing and using a sunscreen of 15 factor or higher
• Eating a high-protein diet
• Taking supplements containing nicotinamide
• Undergoing psychiatric treatment, such as the use of antidepressants or mood stabilizers, if mood swings or psychiatric problems occur

Most people with this disorder can expect to live a normal life with no disability. Rarely, there have been reports of severe nervous system disease and even deaths in families with this disorder.

There are usually no complications. When they occur, complications may include:

• Changes in skin pigment that are permanent
• Psychiatric problems
• Rash
• Uncoordinated movements

Nervous system symptoms are usually reversible. However, in rare cases they can be severe or life-threatening.

Call for an appointment with your health care provider if you have symptoms of this condition, especially if you have a family history of Hartnup disorder. Genetic counseling is recommended if you have a family history of this condition and are planning a pregnancy.

Genetic counseling may help prevent some cases. Eating a high-protein diet may prevent amino acid deficiencies that cause symptoms.

Rezvani I. Defects in Metabolism of Amino Acids. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF, eds. Nelson Textbook of Pediatrics. 18th ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 85.