Alkaptonuria is a very rare inherited condition occurring when the body cannot completely breakdown the two proteins known as tyrosine and phenylalanine. This happens with the insufficient amount of certain enzymes essential in the breakdown of homogentistic acid. As a result, the homogentistic acid accumulates in the body, causing dark colored urine that may turn black when exposed to air. This change may not be apparent for several hours and may often go unnoticed. Individuals with this condition have a normal life expectancy but may experience severe forms of symptoms ranging from pain and problems with their joint movements affecting the quality of life.
Symptoms
The initial symptom of alkaptonuria is the dark colored urine. In its absence, these individuals may continue into adulthood until other apparent symptoms develop. The homogentisic acid may accumulate over several years in different parts of the body that include nails, ears, heart and tendons. The affected tissues may become stained causing complications. The additional symptoms include brittle cartilage, lower back pain, discolored sweat causing black patches of skin, breathing difficulties, hardening of heart values, weakened blood vessels and the development of kidney or bladder stones. The arthritic symptoms of the back, hips and knees usually start around age 30.
Causes
Alkaptonuria is inherited in an autosomal recessive pattern where both the copies of the gene, one from each parent, contain the mutated gene. The parents may be the carrier of the mutated gene and not exhibit any symptoms associated with this condition. The mutation in the gene HGD causes this disorder which is essential in the production of an enzyme known as homogentisate oxidase. This enzyme breaks down the amino acids phenylalanine and tyrosine. The defective gene causes the malfunction of the enzymes resulting in the accumulation of homogentistic acid in the body. As a result, the symptoms associated with this condition develop.
Diagnosis
The diagnosis is based on the symptoms, evaluation of the patient’s medical history and consists of a variety of tests. Commonly, the identification of homogentisic acid in the urine can be indicative of this condition although not all cases are presented with dark urine. The genetic testing can detect the mutation in the gene responsible for alkaptonuria. People affected with this condition over the age of 40 may be recommended to have echocardiography to identify abnormalities of the heart. A CT scan can detect complications such as coronary artery calcification.
Treatment
The treatment focuses on the specific symptoms that affect each individual. Anti-inflammatory medications can relieve the joint pain while others may require physical therapy to strengthen the muscles and joints. If the condition occurs in children, the progression can be slowed with the restriction of protein intake. Most of these individuals may require surgical intervention to replace the hip, knee or shoulder joints by the age of 50-60. Surgery may also be necessary for individuals with kidney or prostrate stones. In general, individuals with this condition should avoid strenuous physical exercise that stresses the spine and joints.
References
http://www.nhs.uk/Conditions/alkaptonuria/Pages/Introduction.aspx
https://rarediseases.org/rare-diseases/alkaptonuria/
https://ghr.nlm.nih.gov/condition/alkaptonuria#diagnosis