Ochronosis is the bluish black discoloration of certain tissues, such as the ear cartilage and the ocular tissue, seen with alkaptonuria, a metabolic disorder. Additionally, ochronosis can occasionally occur from exposure to various substances such as phenol, trinitrophenol, resorcinol, mercury, picric acid, benzene, hydroquinone, and antimalarials.
Ochronosis was defined by Virchow who histologically described the connective tissue in alkaptonuria, given the cartilage's ochre, or yellow, hue under the microscope.
Alkaptonuria is a rare autosomal recessive metabolic disorder caused by deficiency of homogentisic acid oxidase, the only enzyme capable of catabolizing homogentisic acid (HGA). Alkaptonuria features a defect in the biochemical pathway by which phenylalanine and tyrosine are normally degraded into fumaric and acetoacetic acid. The genetic defect is autosomal recessive and is mapped to the HGO gene on arm 3q1, and 18 genetic missense mutations are known to cause homogentisic acid oxidase aberrations.[1] This deficiency results in accumulation and deposition of HGA in cartilage, causing the characteristic diffuse bluish black pigmentation. Interleukin 6 (IL-6) has demonstrated involvement in the pigmentation process of chondrocytes.[2] These affected connective tissues become weak and brittle with time, leading to chronic inflammation, degeneration, and osteoarthritis.
Exogenous ochronosis, in which bluish black pigmentation of cartilage is noted iatrogenically by exogenous agents, has been seen after exposure to antimalarials and noxious substances including phenol, trinitrophenol, benzene, hydroquinone, mercury, resorcinol, and picric acid.
Alkaptonuria is a rare autosomal recessive disease with a prevalence of 1 case per 1 million population.
International
Alkaptonuria occurs worldwide, with the highest frequency seen in Slovakia and Dominican Republic, in which the prevalence approaches 1 case per 19,000 inhabitants.[3]
Race
Alkaptonuria is seen in persons of all races.
Exogenous ochronosis is more typically seen in African and Afro-Caribbean populations due to the use of skin-bleaching products containing hydroquinone in attempts to lighten the appearance of the skin.[4]
Sex
The incidence of alkaptonuria is equal in both sexes.
Age
Alkaptonuria is present at birth and is often diagnosed by discoloration of the diapers. Up to 25% of patients with alkaptonuria do not have the characteristic dark urine staining, and many patients remain undiagnosed until adulthood.
Patients with alkaptonuria can expect a normal life span; nevertheless, the complications of debilitating arthritis, cardiovascular compromise, and ochronotic skin alterations will occur. Additionally, chronic kidney disease affects the natural history of the disease by accelerating the onset of major complications.[5]
Mortality/morbidity
With the absence of homogentisic acid (HGA) oxidase in liver and kidney cells, HGA accumulates. The black urine of patients with alkaptonuria results from renal excretion of HGA, while ochronotic pigment is a sequela of HGA accumulation in the connective tissues of individuals who are affected.
In alkaptonuria, the accumulation of HGA inhibits collagen cross-linking by affecting a crucial enzyme in collagen synthesis, leading to a diminution of structural collagen integrity. This results in ochronotic arthropathy, which occurs in men aged in their fourth and fifth decades; women develop similar complications in their sixth decade. The larger joints are most affected with early calcification, narrowing, and collapse of the intervertebral discs. In addition to joint disease, reports suggest an increased incidence of cardiovascular disease due to cartilaginous changes of vessel walls.
Homogentisic acid oxidase requires atmospheric oxygen, ferrous ion, and sulfhydryl groups for normal function, and the enzyme is inhibited by quinones. HGA is colorless in solution but darkens on exposure to air, especially in the presence of alkali. Individuals with acidic urine may not demonstrate the very dark-colored urine characteristic of this condition.
Patients with alkaptonuria should be informed that they will have a normal life span, despite pigmentary alterations and arthritis that materialize in mid life. Patients should also be aware that they will need cardiovascular follow-up care in their later years.
Alkaptonuria is often recognized at birth when parents note discoloration of the urine.
Nevertheless, many patients with this metabolic disorder are symptomless until ochronotic changes occur with bluish black pigmented patches in the sclera developing in patients aged 30-39 years.[6] These ocular discolorations are located between the corneal margin and the inner canthus.
The fourth decade often marks the onset of thickening and blue-black or gray-blue discoloration of the ear cartilage.
Other body locations that frequently display the alteration in skin hue are the eyelids, the forehead, the cheeks, the axillae, the genital region, the nail beds, the buccal mucosa, the larynx, the tympanic eardrum, and the tendons (most easily demonstrated by the patient making a fist).
Ochronotic arthropathy develops later with arthritic symptoms.[7]
Dark urine noted in diapers is usually the presenting manifestation of this entity.
For those cases of alkaptonuria not diagnosed in infancy, black cerumen and axillary pigmentation may be noted before age 10 years.
Gray-black scleral pigmentation in the configuration of small, dark rings is noted in the third decade.
Ear cartilage alterations in the fourth decade include discoloration with a grayish blue hue, followed by structural changes with stiffness, contour irregularities, and calcification. Note the images below.
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Ochronosis revealing a perioral bluish black macular eruption after hydroquinone use.
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Periorbital discoloration due to excessive use of topical bleaching preparations containing hydroquinone.
Ochronotic discoloration can also be detected on the nasal tip, the costochondral junctions, the extensor tendons of the hands, the cheeks, the fingernails, and the buccal mucosa.
Arthritic signs of ochronotic arthropathy develop in the third and fourth decades. Narrowing of the joint spaces and disk calcifications occur.[14] Lower back, hip, and knee pain and stiffness are not uncommon. Inflammation, calcification, and rupture of tendons and ligaments may also occur.[15]
The clinical discoloration of the skin is a result of the formation of ochronotic pigment granules in the dermis and in the apocrine glands. Thus, the ochronotic changes of cartilaginous and tendinous structures are best appreciated in regions in which the overlying skin is of minimal thickness, such as the ears, the nasal tip, and the extensor tendons of the hands.
Accumulation of this pigment in cardiac tissue can lead to an inflammatory reaction and progressive heart valve dysfunction, with aortic stenosis being the most common, but in rare cases aortic regurgitation may also be observed.[16, 17]
Alkaptonuria is a genetic disorder related to a deficiency of homogentisic acid oxidase.[8]
Ochronotic pigmentation can develop from medications. Similar skin and cartilage alterations can be induced by quinacrine administration and at sites of quinine injections.[9] Quinines directly inhibit homogentisic acid oxidase. Carbolic acid topical applications to cutaneous ulcers have also induced ochronotic skin alterations.
Exogenous ochronosis has been reported with topical applications of phenol and hydroquinones to the skin.[10, 11, 12, 13] In the case of hydroquinone, it is reported that 35% of black Africans exhibit ochronotic skin changes when using a 6-8% hydroquinone preparation over a prolonged period. Indeed, the prevalence among users of these skin lighteners has been stated to be 69% in a South African study. In African Americans, this cutaneous adverse effect of hydroquinones has also been reported, even when using 2% hydroquinone products. With exogenous ochronosis, the arthropathy seen with alkaptonuria does not occur.
Most laboratory testing for alkaptonuria detects the alterations in the urine. Increased urinary levels of homogentisic acid (HGA) are characteristic of this metabolic disorder.
Elevated levels of HGA in the urine, blood, and other tissues can be determined by specific enzymatic and colorimetric tests, direct spectrophotometric methods, high-performance liquid chromatographic testing, and molecular techniques.
Other simple urinary studies include darkening of urine with the addition of sodium hydroxide, black reaction with FeCl3, and blackening of photographic emulsion paper with alkali added to urine.
In patients with ochronotic arthropathy, radiography and MRI help identify characteristic and diagnostic features, including articular space narrowing up to osseous ankylosis, calcifications, osteophytosis, and reactive sclerosis of the articular surfaces.[13, 19, 20] Bone scintigraphy can be useful in evaluation, correlation with the clinical course, and follow-up of such patients.[21]
Synovial fluid examination of affected joints shows characteristic frequent pigmented fibrillar connective tissue, which are golden-brown with microscopy, while being black on gross examination.[22]
Arthroscopy can be used in diagnosing cases of ochronotic arthropathy.[23]
Dermoscopy has proved useful in exogenous ochronosis. In addition to melasma findings, dermoscopy reveals amorphous, densely pigmented structures obliterating some follicular openings and multiple thin, short, arciform structures.[24, 25]
Skin biopsy samples with hematoxylin and eosin staining reveal yellowish brown – pigmented bodies in the dermis that represent altered widened elastic fibers, as well as in macrophages, endothelial cells, apocrine glands, and epidermal basement membranes. The deposits do not lose their pigmentation after 3 days in 10% hydrogen peroxide. Furthermore, the ochronotic pigment reacts with all routine stains for melanin. Such deposits can also be seen in cartilage and elastic tissue.
Exogenous ochronosis reveals ochronotic collagen fibers leading to the formation of ochronotic colloid milium.[26] The dermal cell infiltrate is variable but often granulomatous. Transfollicular elimination of these ochronotic fibers has been reported.
Experimental evidence from 2016 has shown the presence of serum amyloid A (SAA) in several alkaptonuric chondrocytes, classifying alkaptonuria as a secondary amyloidosis. SAA in alkaptonuric chondrocytes has been shown to localize to actin, vimentin, and β-tubulin cytoskeletal proteins.[27]
Note the images below.
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Upon microscopic examination, amber-colored, oval structures are detected in the mid-to-upper dermal tissues (hematoxylin and eosin, original magnific....
View Image
Upon higher magnification (of Image 3), ochronosis reveals homogenization and swelling of collagen bundles (hematoxylin and eosin, original magnificat....
Although no present medical treatment is available for alkaptonuria, genetic advances offer hope that corrective measures are forthcoming. Some have advocated diets low in tyrosine and phenylalanine, thereby reducing the toxic byproduct homogentisic acid (HGA). Additionally, a diet high in vitamin C might prevent oxidation of homogentisic acid. Dietary changes have been advocated by some authors.[28]
One possible hope is that nitisinone proves effective.[29] The US Food and Drug Administration (FDA) has approved this drug for the treatment of tyrosinemia type 1. It significantly lowers the urinary excretion of HGA by inhibiting 4-hydrophenylpyruvate dioxygenase and, theoretically, would reduce HGA accumulation. Testing presently is assessing safety and long-term results.
Vitamin E and N-acetyl cysteine have been examined as novel potential therapies to prevent damage to articular cartilage.[23] Ochronotic arthropathy is treated with physiotherapy, analgesia, rest, and prosthetic joint replacement when necessary.
With exogenous cutaneous ochronosis induced by topical hydroquinones, carbon dioxide lasers and dermabrasion have been reported to be helpful.[30, 31] Reports have described effective therapy with the Q-switched alexandrite 755-nm laser.[32]
Arthroscopy has been found to be effective in alleviating joint pain and improving mobility in cases of ochronotic arthropathy.[23] Knee, hip, shoulder, and other joint replacements may be needed if ochronotic arthropathy progresses substantially. Surgical intervention for prostate and renal stones may also be necessary. Aortic valve replacement is indicated in the case of aortic calcification and stenosis.[33]
Consultations with a rheumatologist for the arthropathies that develop should be considered.
An expert in medical genetics may be of assistance in screening families for this autosomal recessive metabolic disorder.
Cardiac assessments, including echocardiography to detect aortic or mitral valve calcification and stenosis, are needed upon entering the fourth decade of life.
Activities are restricted in adult life because of arthritic complaints. Physical or occupational therapy may be indicated in the individual management of joint pain and in order to help with muscle strength and flexibility.
At the present state of knowledge, no medical therapy for ochronosis and alkaptonuria is available.
Nitisinone, a potent inhibitor of an enzyme in the tyrosine catabolic pathway, has been studied in this condition. The FDA has approved this drug for the treatment of tyrosinemia type 1. It significantly lowers the urinary excretion of homogentisic acid by inhibiting 4-hydrophenylpyruvate dioxygenase and, theoretically, would reduce homogentisic acid accumulation. In recent studies, biochemical parameters improved and the safety level was high, but clinical parameters were not affected.[34] Use of this drug should be investigated in younger individuals to perhaps help prevent the development of debilitating arthritis.
Additionally, a pilot study has looked at the use of antioxidants to counteract the production of melaninlike pigmentation and oxidative stress related to homogentisic acid and its metabolites.[35]
Paul N Skiba, University of North Carolina at Chapel Hill School of Medicine
Disclosure: Nothing to disclose.
Coauthor(s)
Christopher Sayed, MD, Clinical Assistant Professor of Dermatology, Clinician Educator, Director of Medical Student Education, Director of Hidradenitis Suppurativa Clinic, University of North Carolina at Chapel Hill School of Medicine
Disclosure: Serve(d) as a speaker or a member of a speakers bureau for: Abbvie, Inc.
Craig N Burkhart, MD, MSBS, Assistant Professor, Department of Dermatology, University of North Carolina at Chapel Hill School of Medicine
Disclosure: Nothing to disclose.
Specialty Editors
Richard P Vinson, MD, Assistant Clinical Professor, Department of Dermatology, Texas Tech University Health Sciences Center, Paul L Foster School of Medicine; Consulting Staff, Mountain View Dermatology, PA
Disclosure: Nothing to disclose.
Warren R Heymann, MD, Head, Division of Dermatology, Professor, Department of Internal Medicine, Rutgers New Jersey Medical School
Disclosure: Nothing to disclose.
Chief Editor
Dirk M Elston, MD, Professor and Chairman, Department of Dermatology and Dermatologic Surgery, Medical University of South Carolina College of Medicine
Disclosure: Nothing to disclose.
Additional Contributors
Catharine Lisa Kauffman, MD, FACP, Georgetown Dermatology and Georgetown Dermpath
Disclosure: Nothing to disclose.
Craig G Burkhart, MD, MPH, Clinical Professor, Department of Medicine, Medical College of Ohio; Clinical Assistant Professor, Department of Medicine, Ohio University College of Osteopathic Medicine
Ochronosis revealing a perioral bluish black macular eruption after hydroquinone use.
Periorbital discoloration due to excessive use of topical bleaching preparations containing hydroquinone.
Upon microscopic examination, amber-colored, oval structures are detected in the mid-to-upper dermal tissues (hematoxylin and eosin, original magnification X40).
Upon higher magnification (of Image 3), ochronosis reveals homogenization and swelling of collagen bundles (hematoxylin and eosin, original magnification X100).
Ochronosis revealing a perioral bluish black macular eruption after hydroquinone use.
Periorbital discoloration due to excessive use of topical bleaching preparations containing hydroquinone.
Upon microscopic examination, amber-colored, oval structures are detected in the mid-to-upper dermal tissues (hematoxylin and eosin, original magnification X40).
Upon higher magnification (of Image 3), ochronosis reveals homogenization and swelling of collagen bundles (hematoxylin and eosin, original magnification X100).
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