Hartnup disease, also referred to as Hartnup disorder, is a hereditary metabolic disorder that makes it difficult for your body to absorb certain amino acids from the intestine and to reabsorb these amino acids from the kidney. Amino acids are essential building blocks that are responsible for creating protein in the body. This inherited condition is an autosomal recessive trait. This means that people who are born with the condition have inherited a mutated gene from both parents.
The disease was named for the Hartnup family of England, who were looked at in a 1956 study of the disease (Sekulovic, 2011). Four of eight family members were found to have excessive amounts of amino acids in their urine, skin rash, and ataxia (lack of coordination of voluntary muscle movements). These signs and symptoms are characteristic of Hartnup disease, which affects the skin and the brain.
Hartnup disease is a relatively rare disorder, occurring in one per 24,000 births in North America and Australia (Sekulovic, 2011). Rare as it is, Hartnup disease may be the most common amino acid disorder in humans.
The condition normally begins to show symptoms in infancy or in the first few years of a child’s life. It causes “attacks” of symptoms. The attacks last for about two weeks and the frequency of attacks decreases as the child grows older.
The disease is caused by a mutation of the gene that controls the processes of amino acid absorption and reabsorption. Scientists are not sure why the mutation occurs. If both parents carry the mutated gene, the children will likely have Hartnup disease.
In healthy individuals, the body absorbs specific amino acids into the intestines and then reabsorbs them in the kidneys. Individuals with Hartnup disease, on the other hand, cannot properly absorb certain amino acids from the small intestine. They also cannot reabsorb these amino acids from the kidney. As a result, an excessive amount of amino acids exits the body through urination, leaving the body with insufficient amount of these amino acids.
One of the amino acids affected by this condition is tryptophan, which is a building block for proteins and vitamins. Without enough tryptophan, the body cannot produce enough of the B-complex vitamin niacin. A niacin deficiency is responsible for the sun-sensitive rash (called a “pellagra” rash) seen in patients with Hartnup disease, and contributes to the dementia as well.
Symptoms of the disease particularly affect your brain and skin. The brain and skin require B-complex vitamins to remain healthy and to function properly. Since patients with Hartnup disease are lacking in crucial amino acids, the condition impedes protein production and can trigger specific mental and physical symptoms, including:
- short statue
- sensitivity to light
- an unsteady gait: described as a wide-based gait in which the patient walks with legs more spread apart than is normally seen (it may resemble when some people who are drunk walk with a wide-based gait to keep their balance)
- rapid mood swings
- intention tremor
- speech difficulties
- abnormalities in muscle tone: either muscles can become more tight (with increased resistance to stretch which is spasticity or they can become hypotonic decreased tone)
A skin rash, usually resulting from exposure to sunlight, is a common symptom. The rash is an intermittent red, scaly rash over the face, neck, hands and legs. Initially the rash is red but over time can progress to an excematous-like rash. The skin changes can become permanent with changes in pigmentation of your skin and with prolonged sun exposure
These symptoms may be triggered by sunlight, poor nutrition, the use of sulfonamide drugs, and emotional or physical stress. Although symptoms usually appear in infancy or early childhood, they may appear later, in early adulthood. Acute symptom attacks generally become less frequent as you get older.
A urinalysis test (urine sample test) can measure the amount of amino acids excreted through your urine. If there are high levels of “neutral” amino acids, this may be a sign of Hartnup disease. However, this test alone isn’t enough to diagnose Hartnup disease, so your doctor will also go over your personal and family medical history. This includes discussing your symptoms, how frequently you have them, and when they first began. A blood test can also be used to check your levels of B vitamins, including niacin, to help confirm the diagnosis.
Treatment consists of a change in diet, avoidance of sunlight, and prescribing sulfonamide drugs (Adams & Victor, 2012)
Consuming foods that contain the B-complex vitamin niacin can significantly reduce your symptoms. Good sources of niacin include:
- fortified and whole grains
- peanut butter
Meat, poultry, fish, and peanuts are also excellent sources of protein. Choose lean meat and skinless poultry; the fat and skin are high in saturated fat, which can cause high cholesterol.
Your doctor may also suggest complementing your diet with B-complex or niacin vitamin supplements (such as nicatonic acid). The supplement dosage will depend on the severity of your niacin deficiency.
Additionally, it can be helpful to avoid direct exposure to the sun through the use of sunscreen and protective clothing.
Complications are rare. However, it is possible to undergo changes in skin pigmentation, have trouble with coordination of physical movements, or develop psychiatric problems as a result of this condition.
In most cases, people with Hartnup disease can live normal, healthy lives. However, in rare cases, people have developed diseases of the nervous system. Nervous system conditions can be life threatening, but in most cases they can be treated effectively.