Ochronosis & Arthritis
Ochronosis describes the abnormal accumulation of dark pigment in the skin and in connective tissues. Ochronosis occurs as a side effect of a rare disorder called alkaptonuria, which also causes arthritis in early adulthood. The Genetics Home Reference website reports that alkaptonuria affects only 1 out of every 250,000 to 1 million people worldwide.
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Causes
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Alkaptonuria (also called alkaptonuria ochronosis) occurs due to a genetic mutation on the HGD gene, according to the National Library of Medicine. The HGD gene interferes with the body's ability to break down tyrosine and phenylalanine, two amino acids. This leads to a buildup of homogentisic acid in body tissues. People inherit alkaptonuria ochronosis from their parents. The HGD mutation is recessive, meaning both parents must carry copies of the mutated gene for their child to develop the disease.
Symptoms
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The body attempts to flush excess levels of homogentisic acid out with urine, resulting in dark, sometimes black urine, reports the National Library of Medicine. Dark urine may occur from birth, but some people remain unaware that they have alkaptonuria until ochronosis develops in early adulthood. Pigmentation changes due to ochronosis may include darkening around the ears and the presence of dark spots on the eyeball. Ochronosis of the skin presents as blue-black areas of skin that generally develop after age 30.
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Diagnosis
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The presence of ochronosis and related symptoms suggests a person suffers from alkaptonuria ochronosis, indicates a need for further testing. Alkaptonuria tests center on a simple urine test. Urine affected by alkaptonuria ochronosis has a characteristic dark color in most cases. Left exposed to oxygen for several hours, the urine steadily darkens, eventually turning almost black. Urine tests for alkaptonuria mix ferrous chloride into the urine, which also turns the urine black. People of Slovakian and Dominican descent are more likely to develop alkaptonuria, according to the Genetics Home Reference website.
Complications
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Excess amounts of homogentisic acid in connective tissue cause joint inflammation that results in arthritis. The National Library of Medicine reports that approximately 50 percent of adults with alkaptonuria ochronosis develop some degree of arthritis. Arthritis caused by alkaptonuria occurs most often in the spine and large joints. People living with alkaptonuria develop kidney and prostate stones more often than the general population. Most seriously, perhaps, alkaptonuria increases the risk of coronary heart disease and accumulations of homogentisic acid on the heart valves.
Treatment
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Arthritis caused by alkaptonuria may be treated with analgesics for pain management and anti-inflammatory medication, both of which are standard treatments for arthritis symptoms. The National Library of Medicine reports some cases of alkaptonuria respond well to high dose vitamin C treatment. Vitamin C treatment appears to slow the accumulation of pigmentation in joint cartilage, which in turn slows the progression of arthritis. If homogentisic acid builds up on the heart valves, surgical replacement of the valves may become necessary.
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References
Resources
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