Ochronosis and Alkaptonuria

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.[10] 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.[11] 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. Note that localized argyria may be mistaken for exogenous ochronosis.[12]

Alkaptonuria is a rare autosomal recessive disease with a prevalence of 1 case per 1 million population. Alkaptonuria occurs worldwide, with the highest frequency seen in Slovakia and Dominican Republic, in which the prevalence approaches 1 case per 19,000 inhabitants.[13]   While alkaptonuria is seen in persons of all races, exogenous ochronosis is more typically seen in African and Afro-Caribbean populations because of the use of skin-bleaching products containing hydroquinone in attempts to lighten the appearance of the skin.[14]

The incidence of alkaptonuria is equal in males and females. 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.[15]

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 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.

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.[16] These ocular discolorations are located between the corneal margin and the inner canthus. One case involved a 64-year-old male patient first diagnosed with ochronosis from a knee arthroscopy 30 years prior. He was asymptomatic except for brown-colored deposits in the sclera, episclera, cornea, conjunctiva, and limbus. Fundoscopy was normal, and no treatment was required.[17]

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).

AbdulKarim et al present the interesting case of a 66-year-old woman with known alkaptonuria who presented with ochronosis of the bilateral breasts and chest wall. The patient had concomitant tender lumps and dense breast tissue with coarse calcifications.[18]

Ochronotic arthropathy develops later with arthritic symptoms.[19]

Lee et al relate an interesting case of exogenous ochronosis in a 66-year-old woman with discolored patches in the perioral area and bilateral sclerae. Over-the-counter skin bleaching cream and unspecified supplements were the suspected culprits.[20]

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.

(See the images below.)

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.[21] Lower back, hip, and knee pain and stiffness are not uncommon. Inflammation, calcification, and rupture of tendons and ligaments may also occur.[22]

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.[23, 24]  There is also a single reported case of high-grade internal carotid artery stenosis secondary to ochronosis. The carotid plaque found in this case was noted to be heavily brown-black pigmented.[25]

Alkaptonuria is a genetic disorder related to a deficiency of homogentisic acid oxidase.[26]

Ochronotic pigmentation can develop from medications. Similar skin and cartilage alterations can be induced by quinacrine administration and at sites of quinine injections.[27] 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.[28, 29, 30, 31] 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.[31, 33, 34] Bone scintigraphy can be useful in evaluation, correlation with the clinical course, and follow-up of such patients.[35]

Chu et al point out that ankylosing spondylitis can present similarly to ochronosis on lumbar spine radiography. Both conditions may cause loss of lumbar lordosis and decreased intervertebral disc space. However, ochronosis can cause vacuum disc phenomenon with broad syndesmophytes and wafer-like disk calcification. Also, ochronosis does not typically affect the sacroiliac joint.[36]

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.[37] The dermal cell infiltrate is variable but often granulomatous. Transfollicular elimination of these ochronotic fibers has been reported.

Experimental evidence 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.[38]

(See the images below.)

In a literature review including 11 exogenous ochronosis cases, solar elastosis was found in all cases. Histopathologic analysis using light microscopy revealed elastotic fibers transitioning to ochronotic bodies in all cases. Additionally, all ochronotic bodies were Verhoeff-van Gieson-positive.[39]  

Although both the Dogliotti and Phillips classification systems are used to grade and stage exogenous ochronosis lesions, Ramia de Cap et al assert that these systems, while useful clinically, are not suitable for histopathologic grading. This assertion is due to the lack of consistent clinical-histopathologic concordance in exogenous ochronosis.[40]  

Ranganath et al have shown that nitisinone 2 mg daily and 10 mg daily are beneficial in patients with alkaptonuria, with 10 mg displaying a greater decrease in HGA. The authors note that a diet lower in protein would likely improve safety with the higher dose. Nitisinone 10 mg was approved by the European Medicines Agency for adults with alkaptonuria in 2020.[8]  Nitisinone is approved in the United States for the treatment of children with tyrosinemia type I. It acts by inhibiting the enzyme 4-hydroxyphenyl pyruvic acid and therefore can decrease formation of HGA.[9]

Vitamin E and N-acetyl cysteine have been examined as novel potential therapies to prevent damage to articular cartilage.[4] Ochronotic arthropathy is treated with physiotherapy, analgesia, rest, and prosthetic joint replacement when necessary.

Exogenous ochronosis caused by topical hydroquinone use has been treated successfully with picosecond laser.[41]  

With exogenous cutaneous ochronosis induced by topical hydroquinones, carbon dioxide lasers and dermabrasion have been reported to be helpful.[5, 6] Reports have described effective therapy with the Q-switched alexandrite 755-nm laser.[7]

Arthroscopy has been found to be effective in alleviating joint pain and improving mobility in cases of ochronotic arthropathy.[4] 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.[42]

Surgical replacement of cardiac valves may be necessary in cases of ochronosis. Selvakumar et al relate the case of a 72-year-old man with ochronosis and aortic stenosis necessitating aortic valve replacement with a bioprosthetic valve. The authors assert that the stenosis was not secondary to typical, idiopathic calcification but rather to calcification resulting from an inflammation response to the pigments.[43]   Aortic valve replacement in another onchronosis case was performed using a mechanical bileaflet valve.[44]

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.

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.[45] 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.[46]