Musculoskeletal manifestations in Alkaptonuria: A cross-sectional study : Medicine

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Research Article: Observational Study

Musculoskeletal manifestations in Alkaptonuria

A cross-sectional study

Khan, Aysha Habib MBBS, FCPS, FACPS, FRCP (Edin)a; Afroze, Bushra MBBS, DCH, MCPS, FCPSb; Majid, Hafsa MBBS, FCPSa,∗; Zaidi, Yusra MBBSa; Jamil, Azeema Master of Sciencea; Jafri, Lena MBBS, FCPSa

Editor(s): Saranathan., Maya

Author Information
Medicine 100(51):p e28241, December 23, 2021. | DOI: 10.1097/MD.0000000000028241


1 Introduction

Alkaptonuria is a rare autosomal recessive metabolic disorder caused by absolute or relative deficiency of the hepatic enzyme homogentisate 1, 2-dioxygenase (HGO).[1] The true incidence of alkaptonuria is unknown, and there is wide variability in the reported incidences. For example, in the National Registry of the UK, the incidence of albuminuria is 1 in 500,000, whereas it is 25 in 500,000 in Slovakia, the Dominican Republic, and the Middle East. This high incidence in Slovakia, the Dominican Republic, and the Middle East could be due to the high rate of intermarriages in these populations.[2–5]

The enzyme HGO forms an intermediate metabolite, homogentisic acid (HGA), in the tyrosine degradation pathway, which appears in urine when HGO is deficient, as shown in Figure 1.[6] The HGA is oxidized to black-colored benzoquinone acetic acid, which causes the urine of patients to turn black on standing and is a pathognomonic finding in patients with albuminuria.[7] Benzoquinone acetic acid is converted into a melanin-like pigment, which polymerizes with connective tissue, discoloring the tissues, and making them brittle, weak, and susceptible to rupture.[8,9] This pigment deposition leads to poor quality of life and significant alkaptonuria-induced multimorbidity.[10]

Figure 1:
The metabolic pathway of tyrosine metabolism, showing block in the pathway at the level of homogentisate 1, 2-dioxygenase leading to Alkaptonuria.

The present study aimed to determine the patient characteristics and clinical presentation of Alkaptonuria cases reported by the Biochemical Genetics Lab (BGL).

2 Materials and methods

An observational study was conducted at BGL in the Department of Chemical Pathology, Department of Pathology and Laboratory Medicine, Aga Khan University, Karachi, Pakistan. Approval from the institute's Ethical Review Committee (ERC) was obtained (5085-Pat-ERC-17) before the commencement of the study. All patients tested for urinary organic acid (UOA) analysis from January 2013 to December 2019 were reviewed, and only those who presented with alkaptonria, including those diagnosed with other inherited metabolic disorders, were excluded. Clinical details were collected by telephone interviews with patients or guardians after verbal informed consent. Details including demographics, birth, drug and family history, symptomatology, and clinical manifestations related to joint involvement were captured on a structured clinical history form.

Urine organic acid was analyzed by gas chromatography-mass spectrometry (GCMS) on an Agilent GC-MS system (Agilent Technologies, US) using ethyl acetate and derivatization by bis (trimethylsilyl) acetamide compound. The 3, 3 Dimethyl glutaric acid is used as an internal standard. Patients were diagnosed with Alkaptonuria based on the presence of an HGA peak on the UOA chromatogram, appearing at a retention time of 16 minutes (Fig. 2A). Ion spectrum matching of HGA was performed with the qualifying ion reference library (Fig. 2B).

Figure 2:
(A) Urine organic acid chromatogram performed by gas chromatography-mass spectrometry, showing a peak of homogentisic acid (HGA). The x-axis represents the retention/elution time, while the y-axis represents the abundance of molecules. HGA appears as a trimethylsilyl derivative at a retention time of 16 minutes, measured against an internal standard (3,3 dimethylglutarate). HGA is reported as a large, moderate, or small peak relative to the peak of the internal standard. (B) Qualifying ion spectrum of homogentisic acid. The x-axis shows the mass to charge ratio, while on the y-axis is the abundance of molecules, the above picture shows the qualifying ion spectrum of an Alkaptonuria patient compared to the mass to charge ratio of homogentisic acid from qualifying ions reference library shown in the lower figure. The qualifying ions of homogentisic acid were 341, 284, and 252.

3 Results

Twenty-one cases of alkaptonuria were reported over a 7-year period. The mean and standard deviation of the patients was mean age 19.4 ± 24.5 years (range 0.2–66 years) and male to female ratio of 2:1 (Table 1). Nine patients presented after the second decade of life, whereas 8 cases presented within the infantile period. Parental consanguinity was present in 15 of 21 patients with alkaptonuria. The pathognomonic feature of urine darkening on standing or dark-colored staining of underclothing was observed in all patients (21/21). Most patients had symptoms for 12.2 ± 6.6 years (range 3–25 years).

Table 1 - Clinical presentation of patients with alkaptonuria reported by BGL.
Common clinical features
No. Year of UOA testing/publication City of residence Sex Presenting age in years Duration of symptoms in years Ochronotic pigmentations Joint pain Urine darkening on standing
Biochemical genetics laboratory data, n = 12 (2013–2019)
1. 2019 Nawab Shah M 66 25 Yes Yes Yes
2. 2019 Rawalkot F 47 15 Yes Yes Yes
3. 2019 Karachi F 0.8 No No Yes
4. 2019 Peshawar M 1 No No Yes
5. 2019 Rawalkot M 36 7 No Yes Yes
6. 2018 Lahore M 0.5 No No Yes
7. 2018 Hyderabad M 2.2 No No Yes
8. 2018 Karachi M 0.2 No No Yes
9. 2018 Karachi F 1.5 No No Yes
10. 2018 Karachi M 1.7 No No Yes
11. 2017 Lahore M 28 10 No Yes Yes
12. 2017 Rawalpindi F 35 8 Yes Yes Yes
13. 2017 Karachi M 1.8 No No Yes
14. 2017 Karachi F 0.75 No No Yes
15. 2016 Karachi F 0.9 No No Yes
16. 2016 Lahore M 55 20 Yes Yes Yes
17. 2016 Hyderabad M 0.1 No No Yes
18. 2016 Lahore M 57 15 Yes Yes Yes
19. 2015 Lahore M 0.3 No No Yes
20. 2015 Lahore M 40 7 No Yes Yes
21. 2014 Rawalpindi F 33 3 Yes Yes Yes
F = female, M = male, UOA = urine organic acid.

Musculoskeletal involvement, low back pain, and stiffness followed by chronic knee joint pain were observed in adult patients only; joint pain in 9 of 21 patients (Supplemental figure 1, shows the knee radiographs of an AKU patient). Ochronotic pigmentation on the face/hands and ear cartilage was seen in 4 and 2 patients, respectively. Apart from 1 patient with mixed renal stones, none of the patients developed any cardiac or renal complications associated with albuminuria. All patients were symptomatically treated with ascorbic acid, NSAIDS, prescribed a low-protein diet, and physiotherapy, but none were on Nitosone therapy.

4 Discussion

Alkaptonuria, a rare disease caused by subtle signs and symptoms, is frequently under-diagnosed. Only 12 cases have been reported from Pakistan until 2019, with the first report published in 2005. Patients are often evaluated for porphyria, which also has the same feature of darkening of urine while standing as Alkaptonuria. These patients are often diagnosed late, which leads to poor quality of life. In infants, the staining of diapers is an essential early diagnostic clue for albuminuria, which parents and physicians often miss. In addition, fewer laboratories are performing sensitive and specific tests for the diagnosis of alkaptonuria, which have led to the loss of patients. Moreover, this became evident after the availability of the right diagnostic tool (UOA evaluation by GCMS), 21 cases were diagnosed in only 7 years, whereas literature reports only 12 cases from Pakistan in >70 years.

5 Characteristics of Alkaptonuria patients reported from Pakistan

A review of published literature from 1947 to 2019 identified 9 case reports and 1 original study reporting alkaptonuria in 3 patients from Pakistan (Table 2). In all the reports except one, the presence of HGA in urine was confirmed by colorimetry; 1 study utilized paper chromatography to identify HGA, whereas none utilized GCMS for UOA analysis.[6–8,11–13] The medical specialties where patients presented initially included medicine (n = 3), rehabilitation medicine (n = 2), dermatology (n = 3), laboratory (n = 3), and radiology (n = 1). A male predominance was observed in these case reports. Only 1 report mentioned the consanguinity of the patient's parents. The most common age of presentation was in the fourth or fifth decade of life, with 1 case presented in their 20s and infancy. Clinical details were available for only 9 patients. Adult patients had symptoms for 10 to 15 years. A family history of alkaptonuria was provided in 4 of 9 case reports, and 1 to 2 siblings of each patient were affected. Musculoskeletal involvement was seen in all cases from the literature; all cases had joint pain, 5 of 9 also reported restriction movements, whereas 1 patient each had a height loss of 15 cm and flexion contractures. Except for 1 patient who developed renal stones, none had cardiac or renal complications. Arthritis, notably in the spine and large joints (Ochronotic musculoskeletal manifestations), was a common presentation observed in both patients reported from BGL and in a literature review from Pakistan.

Table 2 - Review of case reports published from Pakistan on Alkaptonuria (1996–2016).
S# Publication year Specialty of presentation Sex Age at presenting. y Duration of symptoms Family history of AKU Clinical presentation
1 1996 Medicine (Peshawar) M 45 11 A brother and niece have AKU Arthritis (wrists, elbows, shoulders, knees, ankles)Restricted spinal movements.Ochronotic pigmentation of ear cartilageBrownish discoloration of scleraRenal stones which turn black on exposure to airStaining of vests and clothes by his sweat.
2 2008 Pediatrics (Lahore) F 1 month Not recorded Elder sister (6 y) has AKU Urine darkens on standing
3 2009 Dermatology (Karachi) M 48 10 Not asked Recurrent joint pains. Swollen and tenderleft knee joint. Difficulty walking. Ochronotic pigmentation of hands. Bluish yellow hyperkeratotic papules and plaques along the margins of palms, sides of fingers and thumbs suggestive of Keratoelastoidosis marginalis. Similar pigmentation was also prominent on his cheeks, forehead, ear lobes and axillae.Nails were markedly discolored. Eyes showed dark brown to black pigment deposition
4 2010 Radiology/pathology )Lahore) M 38 2 Not present in family Pain in joints (knees, shoulders, elbows, lower back)Progressive stiffness & restricted movement of spineOchronotic pigmentation of ear cartilage
5 2005 Dermatology (Quetta) M 26 3 Not present in family Pain in joints (lower back, knees)Ochronotic pigmentation of ear cartilageUrine darkens on standingBlack cerumen in ears
6 2013 Medicine (Azad Kashmir) M 55 15 Brother has it Pain in joints (lower back)Difficulty in walking, uses 2 sticks to support himself. Presence of kyphosis.Over the years, Macular and Hyperkeratotic papular ochronotic discolouraton of skin over face and hands.Osler spots on sclerae.
7 2016 Rehabilitation medicine (Lahore) M 48 15 Father has it Chronic pain in lower back and both knees. Loss of height from 165 cm to 150 cm.Ochronotic papules on bilateral extensor surface of second finger.Gray pigmentation of both sclerae.
8 2016 Rehabilitation medicine (Lahore) F 60 20 Not present in family Chronic aching pain in both knees and dorsal spine. Flexion contractures of both knees. Bilaterally knee crepitus present.Ochronotic patches over her nose, proximal palms, index fingers, thumbs, dorsal foot.Macules in sclera and over face, ears, and knees.

Long-standing arthritis in patients with albuminuria can be severe and disabling.[14,15] Typically, OCM begins in the third decade of life, which is earlier than autoimmune musculoskeletal manifestations and progresses more rapidly in males than in females.[13,14] Ochronotic pigmentation was observed in the ear cartilage and hands/face in 2 and 6 cases, respectively. Ochronotic pigmentation was frequently reported in the literature, whereas in the BGL data, only 6 of 21 patients had it. Clinical data obtained from BGL also identified arthritis as the most common presentation in patients. Most of these patients had symptoms for 3 to 25 years before being diagnosed with albuminuria, which may be due to the slow progression and limited awareness regarding this disease. Physicians involved in managing arthritis must specifically consider albuminuria in their differentials and inquire about other symptoms, notably the darkening of urine while standing.[15]

All patients with kaptonuria reported from the BGL and in the published literature were symptomatically treated and were mainly on analgesics. Few of the patients were on vitamin C supplementation; however, none were on dietary restriction, low-protein diet, physiotherapy, or nitisinone therapy. Few patients did not know which specialist to contact and had seen several general practitioners for their complaints.

The standard treatment for alkaptonuria aims to target specific symptoms of each individual. Pain management is tailored according to a person's symptoms, and narcotics may also be prescribed if needed. Dietary protein restrictions are usually ineffective in patients with albuminuria and can lead to further complications.[10] Vitamin C supplementation is advised, as it prevents the accumulation and deposition of HGA. However, studies are lacking regarding its effectiveness in long-term usage. Nitisinone, an FDA-approved drug for tyrosinemia, is presently under clinical trials for its potential use in Alkaptonuria.[16] Research has shown that it reduces HGA formation by diverting the metabolism toward the formation of 4-hydroxyphenyl lactic acid.[17] There is a limited global experience of long-term outcomes of nitisinone treatment and its effectiveness in young children. More research is needed to evaluate its effect on preventing the deposition of quinone derivatives of HGA in connective tissues and its safety or effectiveness in long-term usage in all age groups. Approximately half of the patients with albuminuria require surgery of their hip, knee, or shoulder joints, usually by the fifth or sixth decade of life.[18] Physiotherapy and occupational therapy are also advised to patients, as they help improve joint mobility and maintain the strength and flexibility of muscles and joints.[19]

To the best of our knowledge, this is the first national-level data from Pakistan reporting the clinical spectrum of albuminuria in our population. However, this study has several limitations, including a cross-sectional study design, a small number of Alkaptonuria cases, and the nonavailability of a confirmatory enzyme or mutational analysis. In addition, detailed clinical examination of the joints could not be performed in patients to determine the cause of musculoskeletal manifestations.

6 Conclusion

The present study suggests that the burden of Alkaptonuria goes unnoticed and not documented until chronic symptoms and signs of HGA deposition in tissue develop. The prevalence of albuminuria in Pakistan may be higher than that reported in the literature. Alkaptonuria is a rare autosomal recessive disorder of metabolism that is likely to be missed by physicians as a cause of musculoskeletal manifestations unless individually examined in such cases. There is a need to raise awareness among health practitioners to look for albuminuria in unexplained musculoskeletal manifestations.

Author contributions

AHK was included in conceiving ideas, reviewing chromatograms, and critically reviewing the manuscript for intellectual content. BA reviewed the manuscript for intellectual content. HM was involved in the data analysis and writing manuscripts. YZ was involved in data collection, literature search, and manuscript writing. AJ was involved in the UOA analysis and chromatogram review. LJ was included in reviewing chromatograms, literature searches, and critically reviewed the manuscript for intellectual content.

Conceptualization: Aysha Habib Khan.

Data curation: Hafsa Majid, Yusra Zaidi.

Formal analysis: Hafsa Majid.

Investigation: Azeema Jamil.

Methodology: Yusra Zaidi.

Project administration: Hafsa Majid.

Supervision: Aysha Habib Khan.

Validation: Bushra Afroze, Hafsa Majid, Lena Jafri.

Writing – original draft: Hafsa Majid.

Writing – review & editing: Aysha Habib Khan, Bushra Afroze, Yusra Zaidi, Azeema Jamil, Lena Jafri.


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alkaptonuria; homogentisic acid; musculoskeletal; Pakistan

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