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Alkaptonuria - Causes, Symptoms, and Management

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Alkaptonuria is a less common metabolic disease known as black urine disease. You can learn more about this condition by scrolling down this article.

Published At October 27, 2022
Reviewed AtJune 13, 2024

What Is Alkaptonuria?

Alkaptonuria is an inherited condition characterized by the accumulation of homogentisic acid in the body, causing urine to turn black when exposed to air. Homogentisic acid is a toxic substance. The accumulation of homogentisic acid occurs when the body cannot produce enough of an enzyme called homogentisic dioxygenase. This enzyme is used to break down the toxic substance. When the body cannot produce enough homogentisic dioxygenase (HGD), the homogentisic acid builds up, causing the bones and cartilage to get discolored and very brittle. This condition leads to osteoarthritis, and people also have their urine turning dark brown or black when exposed to the air. Alkaptonuria is black urine disease, AKU, or homogentisic acid oxidase deficiency.

What Are the Causes of Alkaptonuria?

Alkaptonuria is an inherited condition in an autosomal recessive pattern, meaning that both parents should carry the gene mutation to pass it on to the child. It is caused by a mutation in the homogentisate 1,2 dioxygenase (HGD) gene. This HGD gene offers instructions for the production of the homogentisic dioxygenase enzyme. This enzyme breaks down the amino acids phenylalanine and tyrosine, the essential building blocks of proteins.

The enzyme's role in the process is impaired when there is a mutation or defect in the gene. As a result, the toxic substance, homogentisic acid, accumulates in the body. Excessive homogentisic acid and other compounds get deposited in the connective tissues, causing the cartilage and the skin to darken. Over time, this accumulation of toxic substances in the joints can lead to arthritis. In addition, this homogentisic acid is excreted in the urine, making the urine change color when exposed to air.

What Are the Symptoms of Alkaptonuria?

Over time, homogentisic acid gradually builds up in the tissues throughout the body. It can accumulate in any body part, including the cartilage, tendons, bones, nails, ears, and heart. It typically stains the tissues and causes various problems. Early signs of alkaptonuria can be seen during childhood, such as dark black stains on the child’s diaper, and the symptoms can become very obvious over time.

  • Bones and Joints: People with alkaptonuria develop joint problems when they reach their 20s or 30s. They start to experience lower back pain and stiffness, followed by knee, hip, and shoulder pain. Later over age, the bones and cartilage become brittle and break, resulting in joint or spinal damage.

  • Ears: There will be thickening and bluish-black discoloration of the ear cartilage. This condition is referred to as ochronosis. This is one hallmark sign of alkaptonuria in adults. And even the ear wax can be black or reddish-brown.

  • Eyes: Most people with alkaptonuria develop brown or gray spots on the whites of their eyes.

  • Skin And Nails: There can be discoloration of the skin, particularly the areas exposed to the sun and where the sweat glands are located, such as the cheeks, forehead, and armpits. The sweat can also be discolored and stain the clothes. Nails can also turn bluish.

Other Symptoms Include:

  • Shortness of breath or difficulty breathing. When the bones and muscles present around the lungs stiffen, preventing the chest from expanding, it can lead to symptoms like this.

  • Deposits of homogentisic acid around the heart valves can harden them, turning them brittle and black. The blood vessels also stiffen and weaken over time.

  • Deposits of homogentisic acid can lead to the formation of kidney, bladder, and prostate stones.

How Do One Understand If Their Condition Is Alkaptonuria?

Doctors usually diagnose the condition based on the signs and symptoms the patient reports. One primary symptom that helps diagnose this condition is a change in urine color when exposed to air.

How Is Alkaptonuria Diagnosed?

Alkaptonuria is diagnosed through clinical evaluation, laboratory tests, and genetic analysis. Here are the primary methods used for diagnosis:

Clinical Evaluation: A thorough medical history and physical examination can reveal characteristic signs of alkaptonuria, such as darkened urine, ochronosis (bluish-black discoloration of connective tissues), and arthropathy (joint disease).

Urine Tests:

  • Urine Color: One of the earliest and most noticeable signs of alkaptonuria is the darkening of urine upon standing and exposure to air due to the oxidation of homogentisic acid.

  • Homogentisic Acid Quantification: The definitive laboratory test involves measuring the levels of homogentisic acid in the urine. Elevated levels of homogentisic acid are a hallmark of alkaptonuria. Methods such as gas chromatography-mass spectrometry (GC-MS) can be used.

Blood Tests:

  • Homogentisic Acid Levels: Elevated levels of homogentisic acid can also be detected in the blood, though urine testing is more common and more reliable for diagnosis.

  • Radiological Examination:

    • X-rays and MRI: Imaging studies may show characteristic signs of ochronotic arthropathy, such as calcification of intervertebral discs and joint spaces, particularly in the spine and larger joints.

Other Laboratory Tests:

  • Synovial Fluid Analysis: In cases of joint involvement, analysis of the synovial fluid from affected joints can sometimes reveal the presence of ochronotic pigments.

What Is the Outlook for Alkaptonuria?

Alkaptonuria is a rare inherited disorder and the prognosis for those who have the condition depends on how the symptoms and related complications are dealt with in the long run. Alkaptonuria is an inherited chronic disease, for which there is no cure, but if properly treated, most of the patients can live a normal life.

Here are some key aspects of the outlook for alkaptonuria:

Life Expectancy: Children with alkaptonuria do not have significantly reduced life expectancy and usually live normal lives. Yet the worsening of the disease and its impact on the joints and spinal column may lead to changes in the quality of life.

Symptom Progression:

  • Childhood: Signs may begin with dark-colored urine and pass unrecognized or without further evaluation.

  • Early Adulthood: As part of the pigment changes, the ear cartilage and sclera may also darken. Ochronosis, the bluish-black discoloration of connective tissues, will begin to become noticeable.

  • Middle Age: Musculoskeletal problems, especially in the joints and spine, become more prevalent as one advances in age. Chronic pain and stiffness are common due to severe arthritis in patients’ spines, hips, knees, and other joints.


  • Joint and Spine Issues: The overview of progressive arthritis may cause severe disability in the affected patient. Musculoskeletal complaints, such as kyphosis (an increase in the forward curvature of the spine), are frequently seen.

  • Cardiovascular Complications: The accumulation of homogentisic acid results in valvular heart disease, as the substance forms deposits in the heart's valves. Another risk is the calcification of coronary artery disease.

  • Kidney and Prostate Stones: Homogentisic acid may cause renal and prostatic stones in the case of its accumulation.

  • Psychosocial Impact: Many patients with chronic pain and restricted mobility will face psychological problems, and they should be offered psychological counseling and support to address the long-term consequences of the disease.

How Is Alkaptonuria Managed?

Alkaptonuria is a long-term and chronic condition, and there is no particular treatment for the condition. However, certain medications and treatment choices help to cope with the symptoms.

  • Diet.

  • Exercise.

  • Medications.

  • Surgery.

  • Emotional help.

Diet: If alkaptonuria is diagnosed early in childhood, it is possible to slow down its progression by restricting protein intake. Reduced protein intake can reduce tyrosine and phenylalanine levels in the body. This reduced protein intake can also lower the side effects of the medications taken during treatment.

Exercise: Exercises help improve some of the symptoms of alkaptonuria, such as joint stiffness and pain. Regular and routine exercises help build muscles and strengthen the joints. But you have to ensure that the exercises do not strain your joints. If you feel there is additional strain, avoid such exercises that stress the joints. Besides this, exercises also help lower stress, decrease weight, and improve posture.


  • Nitisinone: Nitisinone is not approved for alkaptonuria, but currently, Nitisinone is recommended to treat this condition as it is known to reduce the levels of homogentisic acid in the body. At present, this is just an experimental treatment; the evidence of its effectiveness is still underway, and so far, there have been promising results from this treatment.

  • Painkillers: Doctors may also recommend a few painkillers to manage pain.

Surgery: In some instances of alkaptonuria, surgery can be suggested when the joints are damaged and require replacement or when heart valves or blood vessels are hardened. Some of the commonly preferred surgical procedures include:

Emotional Help: Being diagnosed with chronic conditions like alkaptonuria can be emotionally stressful, but anxiety and depression can worsen the symptoms. So getting a doctor’s help is very important for mental health.


Certain symptoms can considerably affect the quality of life. However, people with alkaptonuria can lead a normal life, and life expectancy is fairly normal. In addition, taking appropriate treatment for their symptoms can help delay the complications of the conditions.

Frequently Asked Questions


Which Is the Most Prevalent Region for Alkaptonuria?

Alkaptonuria affects one in 250,000 to a million people globally. However, the condition is reported to be most prevalent in certain regions of Slovakia, where the incidence is higher (about one in 19,000 individuals), and in the Dominican Republic.


Which Enzyme Deficiency Causes Alkaptonuria?

Alkaptonuria is caused due to a genetic mutation in the HGD gene, which contains the makeup of the homogentisate oxidase enzyme, which is responsible for breaking phenylalanine and tyrosine. Due to enzyme dysfunction, homogentisic acid and associated compounds get deposited in the connective tissue.


How Is Alkaptonuria Different From Phenylketonuria?

Both alkaptonuria and phenylketonuria are inherent metabolic disorders. The difference between the both is that in the former, the body cannot metabolize phenylalanine and tyrosine, while in the latter, the body fails to metabolize phenylalanine. The former results in homogentisic acid accumulation, and phenylpyruvate accumulates in the latter. Phenylketonuria is more common than alkaptonuria.


What Is the Inheritance Pattern of Alkaptonuria?

Alkaptonuria is an autosomal recessive disorder. The offspring must acquire two copies of the mutated HGD gene (one from each parent) to be prone to this disease. Carriers or ones who have a single copy of the mutated HGD gene are usually asymptomatic.


What Is the Role of Vitamin C in Alkaptonuria?

High doses of vitamin C or ascorbic acid supplementation have been used to treat alkaptonuria, as vitamin C hinders the accumulation of homogentisic acid by preventing the oxidation of homogentisic acid to benzoquinone acetate. However, it has been found that vitamin C further increases the risk of renal calculi. Hence, vitamin C is not a suitable treatment for young infants.


How Is Alkaptonuria Tested?

A urine test is an ideal method to confirm an alkaptonuria diagnosis. Adding ferric chloride to the patient’s urine turns the urine black. This is due to the presence of homogentisic acid in the urine. The urine also tends to turn brownish-black when exposed to air.


Can Alkaptonuria Spread From Person to Person?

Alkaptonuria is a genetic, inheritable disease with an autosomal recessive pattern. Hence, it can only be transmitted across the generation. It is not a communicable disease, and those in close contact have no chance of getting it.


Which Is the Most Susceptible Age Group for Alkaptonuria?

The earliest age of recognition is when the parent notices dark-stained nappies. However, if this is missed, the disorder may remain untouched till adulthood until the late 20s or early 30s. Usually, the blue-black pigmentation within the connective tissues (noticeable in the skin) occurs after age 30.


How to Cure Alkaptonuria?

Alkaptonuria is managed with dietary modification and exercise. Analgesics can take an edge off the pain, and the patient might require surgery to replace damaged joints. Nitisinone, an unlicensed, under-study drug, is prescribed at National Alkaptonuria Center in Royal Liverpool University Hospital (UK), which helps reduce homogentisic acid levels. The drug has shown promising results.


What Is the Color of Healthy Pee?

Healthy pee is usually pale yellow in color. Darkening of the urine can indicate a deviation from normal bodily function. Darker yellow urine indicates dehydration, orange for liver problems, and dark brown may be due to certain foods, medications, and metabolic pathology.


What Is the Color of the Urine if the Kidneys Fail?

Failure of the kidney may lead to the accumulation of certain substances in the urine. This may turn the urine brown, red, or purple. These changes may be attributed to protein, sugar, red blood cells, white blood cells, or cellular casts in urine.


How to Self-Test for Kidney Health at Home?

At-home kits are available that require only a sample from a finger prick or urine to evaluate kidney health. These blood kits measure creatinine, eGFR (estimated glomerular filtration rate), and blood urea nitrogen (BUN). Urine kits are also, available as dip-sticks, and a smartphone app analyzes the color obtained on the stick to show the results.


What Does Proteinuria Look Like?

The patients may have symptoms like generalized illness, nausea, vomiting, fatigue, and swelling. Urinary signs of foamy or bubbly pee may indicate high protein excretion levels in the urine. The individual must visit a urologist for the best care.


How Serious Is Proteinuria?

With proper diagnosis, care, and treatment, the outlook seems promising for proteinuria patients. Lifestyle modification and medications must be indulged diligently. Studies have reported shorter than normal life expectancies, and untreated or ignored proteinuria conditions may even result in death.
Dr. Samer Sameer Juma Ali Altawil
Dr. Samer Sameer Juma Ali Altawil



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