Understanding Hartnup Disease: Symptoms and Treatment

Hartnup disease is a rare genetic disorder that affects the absorption of specific amino acids in the body, leading to a variety of health issues. Named after the family in which it was first identified, Hartnup disease is characterized by a deficiency in the transport of neutral amino acids, particularly in the kidneys and intestines. This disorder can manifest in various ways, ranging from skin rashes to neurological problems.

What is Hartnup Disease?

Hartnup disease is primarily a metabolic disorder caused by a defect in the SLC6A19 gene. This gene is responsible for encoding a transporter protein that facilitates the absorption of neutral amino acids. When this transporter is dysfunctional, it leads to an amino acid imbalance in the body, particularly affecting tryptophan, which is crucial for the synthesis of niacin and serotonin.

Hartnup Disease Genetics

Hartnup disease is inherited in an autosomal recessive pattern, meaning that an individual must inherit two defective copies of the gene, one from each parent, to exhibit symptoms. Carriers who possess only one copy of the mutated gene typically do not show symptoms but can pass the gene to their offspring.

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Symptoms of Hartnup Disease

The symptoms of Hartnup disease can vary widely among individuals, with some experiencing mild symptoms and others facing more severe complications. The most common symptoms include:

Dermatological Symptoms

Photosensitivity: Patients often develop a red, scaly rash after exposure to sunlight, particularly on areas of the skin that are not usually covered by clothing.
Erythema: This refers to the reddening of the skin, which can be temporary or persistent, often exacerbated by sun exposure.

Neurological Symptoms

Ataxia: This is a lack of voluntary coordination of muscle movements, which can affect gait and balance.
Tremors and Unsteady Gait: Some patients may experience tremors or an unsteady walk, often attributed to the central nervous system's involvement.

Gastrointestinal Symptoms

Diarrhoea: Due to amino acid malabsorption, gastrointestinal disturbances like diarrhoea can occur.

Psychiatric Symptoms

Mood Swings and Anxiety: Changes in mood and increased anxiety levels are noted in some patients, likely due to serotonin and niacin deficiencies.

Hartnup Disease and Amino Acids

The inability to properly transport and absorb neutral amino acids, such as tryptophan, leads to their increased excretion in urine. Since tryptophan is a precursor for the synthesis of nicotinamide (a form of niacin), its deficiency can mimic symptoms of pellagra, a condition caused by niacin deficiency.

Diagnosis of Hartnup Disease

Diagnosing Hartnup disease involves a combination of clinical evaluation, family history, and laboratory tests. The hallmark diagnostic feature is the presence of elevated levels of neutral amino acids in the urine, detected through a urine amino acid chromatography test.

Genetic Testing

Genetic testing can confirm the diagnosis by identifying mutations in the SLC6A19 gene. This is particularly useful for family planning and understanding the genetic implications for future offspring.

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Hartnup Disease Treatment

Treatment for Hartnup disease is generally symptomatic and supportive. The primary goals are to manage symptoms and prevent complications.

Dietary Management

High-Protein Diet: Since amino acids are the building blocks of proteins, a high-protein diet can help mitigate the effects of amino acid malabsorption.
Niacin or Nicotinamide Supplements: Supplementation helps to prevent pellagra-like symptoms by compensating for the reduced synthesis of niacin from tryptophan.

Sun Protection

Use of Sunscreen: Regular application of broad-spectrum sunscreen can help prevent skin rashes and other photosensitivity-related issues.
Protective Clothing: Wearing long sleeves and hats can further reduce sun exposure and to avoid skin manifestations.

Symptomatic Treatment

Anticonvulsants and Psychiatric Medications: In cases of severe neurological or psychiatric symptoms, medications may be prescribed to manage these conditions effectively.

Regular Monitoring

Patients with Hartnup disease require regular follow-up appointments to monitor their nutritional status and adjust treatment plans as needed. Blood tests to assess amino acid levels and liver function can help guide therapy adjustments.

Prognosis and Management of Hartnup Disease

The prognosis for individuals with Hartnup disease is generally favourable, especially with early diagnosis and appropriate management. Most patients lead everyday lives with minimal impact from the disorder, provided they adhere to dietary recommendations and protective measures against sun exposure.

Long-term Management

Lifestyle Adjustments: Avoiding triggers such as excessive sun exposure and maintaining a balanced diet are crucial for long-term management.
Family Counseling: Genetic counselling is recommended for families affected by Hartnup disease to understand inheritance patterns and the risks for future generations.

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