Alkaptonuria is a rare inherited autosomal recessive metabolic disorder caused by deficiency of homogentisic acid oxidase (homogentisic acid 1, 2 dioxygenase) enzyme.1 The enzyme homogentisic acid oxidase is absent in the liver and kidneys. Consequently, homogentisic acid is excreted in urine and the urine turns dark brown or black on oxygenation and alkalinization.2 This condition is characterized by a triad of manifestations, including homogentisic acid in the urine (alkaptonuria), ochronosis characterized by deposition of homogentisic acid in connective tissue and cartilage, and spondylo-arthropathy involving the axial and appendicular skeleton (ie, arthritis of the spine and large joints).1 Less common manifestations include renal, urethral, and prostatic calculi; cardiovascular abnormalities; and valvular disease.3–5
Alkaptonuria was one of the first conditions in which the law of mendelian recessive inheritance was proposed as well as 1 of the conditions in the charter of the group of inborn errors of metabolism. The term ochronosis was first coined by Virchow6 in 1866, when he found pigmentation of tissues that appeared ochre, meaning yellow, when examined microscopically.7 This disorder is rare, with an incidence of 1:125,000 to 1:1 million worldwide.1,4,8,9 However, in certain areas of eastern Europe, chiefly Slovakia, and the Dominican Republic, the incidence is as high as 1:19,000.10
The authors report 9 cases of patients who had alkaptonuria with ochronosis. Most previous studies of alkaptonuria were limited to patients from a single family tree or a few case reports. This study highlights nonfamilial, sporadic presentation and atypical manifestations of alkaptonuria that were not reported before. Patients were informed that data about their cases would be submitted for publication, and they consented.
Alkaptonuria, along with its sequela, ochronosis, is a rare disease of aromatic amino acid metabolism. The earliest case was described in an Egyptian mummy, Harwa dating from 1500 b.c.11,12 The term alkapton was first used by Boedeker13 in 1859 to describe urine discoloration caused by a reducing compound that was identified as homogentisic acid by Wolkow and Baumann14 1891. In his Croonian lectures of 1908, Garrod15 coined the term inborn error of metabolism and proposed that alkaptonuria results from a deficiency of an enzyme that normally splits the aromatic ring of homogentisic acid. The deficient enzyme was identified by La Du16 in 1958. Pollak et al17 located the alkaptonuric gene to 3q2 in a 16-cM region. Joint disease in alkaptonuria was initially reported by Albrecht,18 and later Osler19 clinically diagnosed ochronosis for the first time in 2 brothers with alkaptonuria. A worldwide review showed that approximately 600 cases have been reported since 1962.20
Accumulation of homogentisic acid in various tissues leads to polymerization to an oxidized product, benzoquinone, that causes tissue injury that leads to melanin-like polymers.21 Several theories exist as to how alkaptonuria results in ochronosis, including acting as a chemical irritant that causes inflammation and rapid degeneration of the joints and physically binding to connective tissue and altering the structure and interactions of the macromolecules.3
In most pediatric patients, darkening of the urine is the only feature that suggests alkaptonuria. This reaction is pH-dependent and never occurs in acidic urine. In the current series, 8 of 9 patients had a history of urine discoloration in childhood. One patient who was 47 years old had minimal urine discoloration, although urine pH was normal on examination. The diagnosis in this patient was made intraoperatively during total hip replacement for fracture nonunion of the neck of the femur when the femoral head was found to have black discoloration along with bluish-black discoloration of the soft tissue around the hip. Patients are usually asymptomatic until the third or fourth decade and usually do not seek medical attention at an early age. The probable reason cited is the decrease in renal clearance with age.22 In the current series, 6 of 9 previously asymptomatic patients presented with low backache in the third to fourth decade. They did not seek medical attention for skin or urine discoloration, and all had a history of urine discoloration since childhood. Results of renal function tests and creatinine clearance were normal.
Ochronosis presents as bluish-black or grayish-blue pigmentation of the pinna and outer ocular tissues such as the sclera, cornea, and conjunctiva. Scleral pigmentation (Osler’s sign) starts around the third decade. This sign was found in 2 patients. The effects are most noticeable in areas where the body is exposed to the sun and where sweat glands are located and are best appreciated in regions with thin overlying skin. All patients in the series had bluish-black pigmentation of the pinna. In addition, patients may have widespread dusky discoloration of the cheeks, forehead, axilla, and genitals. Nails may be stained brown.5 Brown nail color was found in only 2 patients in the authors’ series.
Initial screening can be done with a simple biochemical test on a urine sample. With this testing, urine color changes to black with the addition of sodium hydroxide and a transient green color appears with the addition of ferric chloride. Other tests include the ammoniacal silver nitrate test, Benedict’s test, and the N-butanol test.23
The diagnosis is usually confirmed by detecting and measuring the amount of homogentisic acid in urine with gas liquid chromatography, thin-layer chromatography, or enzymatic spectrophotometry. In the current series, all patients were initially screened with 2 of the previously mentioned biochemical tests, mainly the addition of sodium hydroxide and ferric chloride. In all patients, the diagnosis was confirmed by thin-layer chromatography. Magnetic resonance imaging was used to establish the diagnosis in the patient with caries of the spine.
In ochronosis, radiographs of the lumbar spine show gross calcification, vacuum phenomenon, and loss of height of the intervertebral disks accompanied by fusion (pseudo-block) vertebrae formation, predominantly in the dorsolumbar region.24 This typical presentation of reduced intervertebral disk space, vacuum phenomenon, and wafer calcification of the intervertebral disks on radiographs of the dorsolumbar region was seen in all patients. Although calcification has been seen in the menisci, symphysis pubis, pinnae, nasal cartilage, tendons, kidneys, heart walls, and great vessels,25 1 patient in the authors’ series had unusual intramedullary calcification of the femur.
Grossly, the involved tissue usually shows pigmentation that varies from brown to black. Microscopically, the hyaline cartilage shows deposition of ochre-colored granules that is more marked in the deeper layers.4 Brownish-black pigmentation of the femur head, known as black hip, along with degenerate pigmented articular cartilage was found in 2 of the patients during hip replacement (Figure 9).
Intraoperative photographs showing inferior (A) and superior (B) views of black pigmentation of the femur head.
Ochronotic arthropathy is a particularly troublesome feature. Premature arthritis in the large joints develops after the third decade and usually affects the weight-bearing joints, mainly the spine, hips, knees, and later the shoulders. Musculoskeletal manifestations are first noted in the spine. Typically, involvement of the large peripheral joints occurs several years after spinal involvement.20 Usually, involvement of these joints is severe and often requires joint replacement.4,24,26,27 In the authors’ series, 1 patient had severe arthropathy of 1 hip and underwent hip joint replacement. After 4 years of follow-up, he had a good Harris Hip Score of 86 and could perform normal routine activities. In contrast to rheumatoid arthritis, the smaller joints of the hand and feet are unaffected. The sacroiliac joint is also spared, thereby distinguishing this condition from ankylosing spondylitis. In the authors’ series, 7 of 9 patients had low backache primarily as the initial presentation. Associated involvement of the hips and knees occurred in 2 patients in the fifth decade, and the rest of the patients were in the third and fourth decades at the time of the study.
Heart problems often start after 50 years of age.2 Patients may have signs of aortic or mitral valvulitis. Pigment deposits that lead to the formation of atherosclerotic plaques and calcification of the coronary arteries may be seen.28,29 Fisher et al20 reported a 69-year-old (seventh decade) patient with cardiovascular involvement who had severe aortic stenosis, calcified valves, and gross aortic pigmentation. The mean age of patients in the authors’ series was 39.7 years. None of the patients had cardiovascular involvement, possibly because they were relatively young. However, all of the current patients underwent electrocardiography and echocardiography to rule out cardiovascular complications.
The differential diagnosis includes osteoarthritis, ankylosing spondylitis, rheumatoid arthritis, and calcium pyrophosphate arthropathy. Disk calcification in the spine can also mimic hyperparathyroidism, hemochromatosis, amyloidosis, diffuse idiopathic skeletal hyperostosis, or surgical spinal fusion. All patients in this study had normal serum calcium, phosphate, and parathyroid hormone levels; therefore, hyperparathyroidism was excluded from the differential diagnosis. Similarly, in the absence of symmetrical polyarthritis, ruled out by relevant tests and investigations, there was no history of early morning stiffness, the rheumatoid factor finding was negative in all patients, and the erythrocyte sedimentation rate was normal in 7 patients except 1 who had caries of the spine. Rheumatoid arthritis was also ruled out because none of the patients had symmetrical polyarthritis or morning stiffness; rheumatoid factor was negative in all 9 patients; and the erythrocyte sedimentation rate was raised only in 1 patient with caries of the spine and normal in the other 8 patients. The HLA-B27 finding was negative in all patients, excluding ankylosing spondylitis. Serum ferritin levels were normal in all patients; this finding, associated with normal liver and renal profiles in all patients, excluded hemochromatosis.
Genetically, alkaptonuria is inherited as an autosomal recessive trait. All of the close and remote relatives of all patients in the current study underwent screening, and there was no familial presentation in 7 patients. Only 2 patients, who were brother and sister, were affected, but their parents and grandparents had no symptoms.
Compression of the cervical cord as a result of alkaptonuric arthropathy of the atlantoaxial joint has been reported.30 In the current series, 8 of 9 patients had no neurologic deficit or gait disturbance. Findings on cervical spine examination were normal in all 9 patients. One patient with caries of the spine had exaggerated deep tendon reflexes of the lower limb along with an extensor Babinski response that resolved after 3 months of antitubercular therapy.
Osteoporosis associated with alkaptonuria was reported in previous studies.31,32 Fisher et al20 reported low-energy trauma of the distal radius and femoral fracture despite 2 years of alendronate therapy. In the authors’ patients, although none presented with a fracture, 1 patient showed evidence of osteoporosis on radiographs of the lumbosacral spine. This finding was confirmed by bone mineral density analysis (T-score, 1.3). This patient had no risk factors for osteoporosis, such as malnutrition, immobility, smoking, medication (corticosteroids, anticonvulsants), or family history of osteoporotic fractures, although she had thyrotoxicosis.
Tendon ruptures, chiefly of the Achilles tendon, have occurred and have been treated successfully with primary repair. One patient in this series also had subcutaneous nodules around the knees and Achilles tendons that were mobile and were not fixed to underlying structures. He also had patchy alopecia over the scalp. This presentation of ochronosis has not been reported before.
Pseudo-ochronosis has been described as a result of argyria,33 which is a localized collection of silver granules in the glands, in the walls of blood vessels, and among elastic fibers. Pseudo-ochronosis is also found after long-term use of L-dopa, methyldopa,34 antimalarial agents, or products containing hydroxychloroquine, phenol, resorcinol, and mercury or picric acid.35 No patient in this study had a history of use of these drugs.
It is necessary to obtain a history of drug intake in all patients because minocycline-induced pigmentation36 or intramuscular injection of quinine, leading to bluish-black pigmentation in the buttocks,37 resembles exogenous alkaptonuria. The authors found no such history among their patients.
Currently, only symptomatic treatment of complications of alkaptonuria is available, and there is no specific or effective treatment of alkaptonuria. There are anecdotal reports that a diet low in protein, especially containing tyrosine and phenylalanine, can help to delay or partially reverse late manifestations.38 Vitamin C partially ameliorates the condition by impairing the polymerization of homogentisic acid21 and preventing homogentisic acid oxidase from binding to connective tissue. Wolff et al39 treated 2 adults with a high dose of ascorbic acid. They found that its derivative, benzoquinone, completely disappeared from urine. In the authors’ study, all patients were given vitamin C 500 mg 3 times a day. Six showed some clinical improvement, as assessed by a subjective pain rating score, on subsequent follow-up. Although nitisinone has been approved by the US Food and Drug Administration for the treatment of tyrosinemia type 115, its safety profile and outcome have not been established in alkaptonuria.4
Because alkaptonuria is rare, there is a high likelihood that it will be unnoticed, especially in areas where the incidence is low. Therefore, clinicians need a high index of suspicion and awareness to make the diagnosis of ochronosis. Low backache is common in the general population and is also the presenting symptom in the third to fourth decade in patients with ochronosis. There is a high probability that ochronosis in a patient who presents with low backache will be diagnosed as a simple case of dorsolumbar spondylosis unless the typical clinical features of urine discoloration and specific radiologic findings are correlated with backache. In developing and tropical countries, a high incidence of bone and joint tuberculosis exists, but its association in a patient with ochronosis confounded the diagnosis.