Alkaptonuria (ochronosis) is an inherited aminoacidopathy of the phenylalanine/tyrosinemetabolism (Figure 1). Phenylalanineis an essential amino acid that is irreversibly hydroxylated to tyrosine byhomogentisic acid (HGA) oxidase, which is found in the liver and kidneys.In alkaptonuria, the enzyme is absent, and HGA accumulates in collagenoustissues such as cartilage and tendon, especially in the ear, nose, cheeks,conjunctiva, cornea, and sclera. Although conjunctival involvement in ochronosisis rare, it should be considered in the differential diagnosis of pigmentedlesions and deposits of the ocular surface. Often, ocular pigmentation isthe initial manifestation of the disease.
Graphic representation of themetabolic cascade of phenylalanine in normal and alkaptonuric conditions.The deficiency of homogentisic acid oxidase causes accumulation of homogentisicacid in the involved tissues.
We report a case of bilateral conjunctival pigmentation as the initialmanifestation of alkaptonuria and review the literature on ocular ochronosis.
A 49-year-old white man on initial examination reported red eye andforeign body sensation for the last 3 to 6 months in both eyes, but more prominentlyin the left eye. Ophthalmologic examination revealed corrected visual acuityof 20/20 OU. The intraocular pressure was within normal limits in both eyes,and the fundi were unremarkable. Bilateral conjunctival pigmentation was present(Figure 2). The pigmentation wasdescribed as yellow-tan to dark brown, with a powdery appearance involvingthe interpalpebral bulbar conjunctiva. Pigmentation was more prominent nasallyin the left eye. Bilateral lesions consistent with pinguecula were present.The pigmentation was seen extending beyond the elastotic changes of the pinguecula.A biopsy sample of the lesion from the left eye was obtained to exclude primaryacquired melanosis (Figure 3). Histopathologicexamination of the conjunctival tissue showed elastotic degeneration of thecollagen fibers admixed with yellowish waxy globules and fiber-like depositsthat were slightly refractile, as seen in hematoxylin-eosin–stainedslides (Figure 4). Deposits werefound under the epithelium and in the superficial stroma. Melanin stain (Fontana-Masson)and special stains for elastic fibers disclosed that the deposits were negativefor melanin and strongly positive for elastotic material (Figure 5). After more clinical history was obtained, we learnedthat the patient had a history of early-onset osteoarthritis. We suggestedmeasuring urine levels of HGA. The results showed an elevated level of morethan 100 mmol of HGA per millimole of creatinine. The collective findingswere those of conjunctival ochronosis associated with pinguecula (Figure 6).
Clinical photograph of the bulbarconjunctiva of the right eye, with light brown pigmentation of the temporalportion (left arrow, temporally located) and changes consistent with pinguecula(right arrow, by the limbus).
Clinical photograph of the bulbarconjunctiva of the left eye after biopsy, with pigmentation (arrow) and erythema(asterisk) at the site of biopsy.
Histologic section of the conjunctivalbiopsy sample shows slightly acanthotic epithelium. The underlying stromacontains wavy yellowish waxy deposits (hematoxylin-eosin, original magnification×64).
At high magnification, the yellowishhomogeneous ochronotic pigment (on the right) differs from the elastotic degenerationof the stroma (on the left) (hematoxylin-eosin, original magnification ×100).
The large masses of ochronoticpigment (under the epithelium) and the marked actinic elastosis of the stromastain black with the stain for elastic fibers (Movat pentachrome, originalmagnification ×64).
Alkaptonuria has played a paradigmatic role in the history of humanand biochemical genetics. It was this rare autosomal recessive disorder thatled Garrod to demonstrate the applicability of the rediscovered mendelianlaws to Homo sapiens in 19021 andto formulate the fundamental concept of "inborn errors of metabolism" in 1908.2 Half a century later, La Du et al3 presentedthe first experimental evidence for a specific enzyme defect in humans: thedeficiency of HGA 1,2-dioxygenase activity in the liver of a patient withalkaptonuria. Homogentisic acid oxidase is excreted in the urine and sweat.Urine levels above the normal 100 mmol of HGA per millimole of creatinineare considered pathologic. When sodium hydroxide is added to freshly collectedurine, the HGA in the urine oxidizes and turns black within minutes. The sameeffect can be achieved by exposing the urine to room air for more than 12hours.4,5
The most serious consequences of ochronosis stem from deposits of thepigment in the articular cartilages of joints, nose, ear, and cardiac valves.The deposits of pigment cause the cartilage to lose its normal resiliencyand become brittle and fibrillated, especially in the intervertebral discs,knees, shoulders, and hips. Pigment polymer deposits located between and withincells form discrete granules or homogeneous laminated structures with a yellow-tanto dark brown color. The ochronosis deposits are more refractile than melanin.Ultrastructurally, the pigment appears more like melanin, but histochemicallyresembles elastin.5
Although the metabolic defect is present from birth, pigment depositsand degenerative arthropathy develop slowly and are usually clinically evidentby the fourth decade of life. Although alkaptonuria is not life threatening,it may be a crippling disease because severe osteoarthritis in ochronosisoccurs at a younger age than degenerative osteoarthritis.
Ocular ochronosis more frequently involves sclera and episclera nearthe insertion of the recti muscle (interpalpebral areas) than in the corneaand conjunctiva. Corneal pigmentation is usually bilateral and present inthe peripheral stroma as discrete pinhead-sized deposits of light brown toblack color. Histopathologic examination shows globules or curled, light yellow,curvilinear structures of varying size in the superficial stroma and surroundingtissues. Melanin stains usually do not distinguish these deposits from thoseof melanin. However, the ochronotic pigment appears somewhat more refractilethan melanin and is more variable in color, ranging from yellow-tan to darkbrown. Special stains for elastic tissue stain positive. Some authors havenoted that areas of intense scleral pigmentation are devoid of cells, suggestinga probable toxic effect of the pigment.5 Othershave found necrosis of fibrocytes in the most heavily pigmented areas.6 Kampik et al5 proposedthat the localization of the pigment, as seen by electron microscopy, mightbe interpreted as different stages in the development of the intensity ofthe coloration of the collagenous tissues of the eye. These authors proposea sequence in which deposition of HGA polymers occurs in a fine granular formaround collagen fibrils, altering and obscuring this structure. The granuleslater coalesce to form plaques, globules, and fiber-like structures, followedby necrosis of the fibrocytes.5
Recent research describes the existence of up to 18 known homogentisate1,2-dioxygenase (HGO) gene mutations.7 Thealkaptonuria (AKU) gene locus was mapped to humanchromosome 3q21-q23,8 and an animal modelfor alcaptonuria, the aku mouse, was described.9 Subsequently, the first gene encoding an HGO enzymewas cloned from the fungus Aspergillus nidulans.10 In 1996 and 1997, the human HGO gene was cloned andcharacterized. Two missense mutations cosegregatingwith alkaptonuria in 2 Spanish pedigrees and a third missense and a frameshiftmutation in Slovakian families established HGO asthe defective gene in alkaptonuria.8,11 Concurrently,13 additional mutations were found in unrelated subjects with alkaptonuriafrom 6 European countries, Algeria, Turkey, and Japan.12,13 Thelatest published study7 in 1999 describesthe identification of 2 homozygous missense mutations in 2 unrelated Germanpatients who were first diagnosed with this congenital disorder after theirreferral to ophthalmologists. The importance of recognizing this entity, whichenters in the differential diagnosis of pigmentations and deposits of theconjunctiva, is emphasized in our report, in which the recognition of thissystemic disease was the initial ocular manifestation of the disease.
The authors have no relevant financial interest in this article.
This study was supported in part by grants from the Retina ResearchFoundation, Houston, Tex, and Research to Prevent Blindness, Inc, New York,NY. Dr Font is recipient of a Senior Investigator Award from Research to PreventBlindness, Inc.
Arun Nayer, MD, provided the clinical photograph and additional patienthistory.
Correspondence: Dr Chévez Barrios, Department of Pathology,Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030 (firstname.lastname@example.org).
Barrios PC, Font RL. Pigmented Conjunctival Lesions as Initial Manifestation of Ochronosis. Arch Ophthalmol. 2004;122(7):1060-1063. doi:10.1001/archopht.122.7.1060