Chapter 131. Cutaneous Changes in Errors of Amino Acid Metabolism

Inborn errors of amino acid metabolism are inherited defects of enzymes that result in important disturbance in amino acid biology.1 Most of them are autosomal recessive disorders. Amino acids are the molecular units that make up proteins, and all proteins are various combinations of 20 specific naturally occurring amino acids.

Diseases of amino acid metabolism may lead to multiorgan disorders with signs and symptoms of mental retardation and dysfunction in numerous additional organs, including skin and hair.2 With more sophisticated diagnostic techniques, metabolic diseases have gained increasing importance, and numerous rare entities have been identified, including hyperphenylalaninemias, hypertyrosinemias, disorders of histidine metabolism, disorders of proline and hydroxyproline metabolism, hyperornithinemias, dysfunctions of urea cycle enzymes, disorders of lysine metabolism, disorders of branched-chain amino acid and keto acid metabolism, disorders of transsulfuration with homocystinuria and homocystinemia, sarcosinemia, and nonketotic hyperglycinemia.3

Several diseases have important cutaneous manifestations that might be a clue to diagnosis (Table 131-1). An example of such a key manifestation representing errors of amino acid metabolism is acrodermatitis acidemica. These patients have characteristic cutaneous lesions that are similar to those in acrodermatitis enteropathica (see Chapter 130). Psoriasiform dermatitis on the trunk and extremities may occur together with diffuse alopecia and brittle hair.4 In general, acral dermatitis with perioral erythema and desquamation is associated with poor feeding, lethargy, hypotonia, vomiting, and dehydration. Patients often have progressive neurologic symptoms. Patients with branched-chain aminoacidopathies may have methylmalonic acidemia, propionic acidemia, glutaric acidemia, biotinidase deficiency, multiple carboxylase deficiency, or maple syrup urine disease, all of which are sometimes accompanied by cutaneous problems.

In general, all patients with acrodermatitis acidemica have absolute missing enzyme activity of their autosomal recessive mutated gene. Diagnosis is confirmed by documentation of increased amounts of amino acids in the urine. Exact diagnosis is the first step for adequate treatment of these diseases.

Hartnup disease results from a deficiency of neutral amino acid transport within the kidneys and small intestine and leads to pellagra-like features in a photodistribution (see Chapter 130). Neurologic deficiencies and reduced mental skills are often associated. The treatment of choice is nicotinamide.

The genetic tyrosinemias are characterized by the accumulation of tyrosine in body fluids and tissue.5,6 Tyrosine is a semiessential amino acid, derived from the liberation of tyrosine from hydrolysis of dietary or tissue protein, or from the hydroxylation of the essential amino acid phenylalanine, and is the starting point for the synthesis of catecholamines, thyroid hormones, and melanogenesis. There are three types of tyrosinemia. The skin is not involved in tyrosinemia types I and III, but it is involved in tyrosinemia II, which is also called the oculocutaneous tyrosinemia and Richner–Hanhart syndrome [Online Mendelian Inheritance in Man (OMIM) #276600]. Tyrosinemia II is a rare distinctive clinical symptom complex involving the eyes, skin, and central nervous system and is potentially treatable. Tyrosine levels are elevated because of a deficiency of hepatic tyrosine aminotransferase (TAT).

Clinical Features

Approximately 100 patients with this clinical syndrome have been reported. Goldsmith and Reed were the first to correlate the oculocutaneous syndrome with an underlying tyrosinemia.7 All patients had tyrosinemia, phenolic aciduria, and inflammatory skin and eye lesions. The sexes are affected equally, and the disease is worldwide in distribution. Transmission of tyrosinemia II is autosomal recessive. Consanguinity has been documented, and occurrence in several siblings within a family is well known (pseudodominant pattern). Heterozygotes are unaffected clinically and demonstrate no biochemical alterations of tyrosine or its metabolites in blood or urine.

Dermatologic Features

Patients have hyperkeratotic yellowish skin lesions limited to the palms and soles, which in 80% of cases occur on the peripheral pressure-bearing areas.5,7 Lesions usually begin during the first year of life. The skin lesions are painful, nonpruritic, and frequently associated with hyperhidrosis (Fig. 131-1). Diffuse plantar keratoderma and self-mutilation have been observed in individuals with tyrosinemia II.8 Leukokeratosis of the tongue has been reported. Bullous lesions may occur and progress rapidly to erosions; these then become crusted and hyperkeratotic.9 The fingertips and hypothenar eminences are commonly involved. Rare patients have hyperpigmentation and/or hyperkeratotic subungual lesions, and in older persons hyperkeratosis in the elbows, knees, and ankles may appear.2

Ophthalmologic Features

The most important manifestations of oculocutaneous tyrosinemia are those involving the eye, because they can lead to permanent visual impairment.10 Eye lesions occur weeks to months before the skin lesions. Ophthalmologic symptoms start as early as the first day of life and as late as 38 years of age. Tearing, redness, pain, and photophobia are early signs and symptoms; late findings include corneal clouding and central or paracentral opacities, which are initially intraepithelial and can progress to superficial or deep pseudodendritic keratitis (Fig. 131-2). Slit-lamp examination may reveal some degree of corneal ulceration, and occasionally birefringent crystals of tyrosine may be observed. Neovascularization is prominent. The eye disease occurs in 75% of affected patients and may lead to scarring, nystagmus, and exodeviation. Ocular symptoms may show spontaneous remission and recurrences and may occur independently of other manifestations. Topical therapy is ineffective, and herpes simplex virus and bacterial cultures consistently yield negative results.

Neurologic Features

Mental retardation of varying degrees is reported in half the cases, as is normal mental development. Hyperactivity has been observed in several affected children, as has abnormal language development. Untreated tyrosinemia II during pregnancy may be associated with mental retardation or seizures in the children resulting from that pregnancy.11

Laboratory Findings

Amino Acid Abnormalities

In establishing the diagnosis in a child with tyrosinemia II, laboratory verification of plasma amino acid and urine organic acid levels is mandatory. The blood and urine tyrosine levels of affected patients are markedly elevated. Levels of other amino acids are not increased. Urinary tyrosine metabolite levels are elevated; these include p-hydroxyphenylpyruvic acid, p-hydroxyphenyllactic acid, p-hydroxyphenylacetic acid, and N-acetyltyrosine (Fig. 131-3). All these metabolic effects are consequences of the deficiency of hepatic TAT. A fluorometric procedure exists as well as a tandem mass spectrometry, which is used for neonatal screening. Recently, an asymptomatic newborn was detected to have Richner–Hanhart syndrome on the third day of life as a result of the newborn screening. Interestingly the 8-year-old brother with persistent plantar hyperkeratotic plaques of the soles of yet unknown origin was subsequently identified to be also affected tyrosinemia type II.12

TAT is a pyridoxal phosphate-dependent cytoplasmic enzyme that transaminates tyrosine, forming p-hydroxyphenylpyruvate (PHPPA). The human TAT gene contains 12 exons and transcribes a 2.75-kb messenger RNA (mRNA) that codes for a 454-amino acid protein.5 The liver is the richest source of TAT; this specific TAT is not present in skin. In tyrosinemia II, the liver biopsy specimen shows little or no soluble TAT, although there is normal or slightly increased mitochondrial tyrosine (aspartate) transaminase activity.12,13 Mitochondrial aspartate aminotransferase using tyrosine as a substrate produces increased amounts of PHPPA from the increased amounts of tyrosine available in tyrosinemia II. Because mitochondria do not have PHPPA oxidase activity, PHPPA and its metabolic products increase and appear in the urine, which creates the unusual situation in which metabolites are increased both proximally and distally to the defective enzyme.

Histopathology

Routine histopathologic analysis of the skin is not diagnostic; it shows hyperkeratosis, parakeratosis, and acanthosis.9 In some cases, electron microscopy studies have shown lipid droplets in the cornified layer, increases in tonofibrils and keratohyalin, very tightly packed microtubular and microfibrillar masses, and minute tyrosine crystals in keratinocytes and Merkel cells.14

Genetics

Mutations occur in the human TAT gene, which results in high levels of tyrosine. TAT maps to the q22-q24 region of chromosome 16 and has been sequenced.5 Multiple point mutations scattered throughout the TAT gene have been associated with tyrosinemia II.15

Interestingly, in mink with the disease TAT unlinked gene regulatory mutation rather than TAT deficiency was thought to be the cause of tyrosinemia.16

PATHOPHYSIOLOGY OF THE SKIN AND EYE LESIONS IN TYROSINEMIA II

In vitro studies showed that tyrosine crystals can cause release of lysosomal enzymes and as yet uncharacterized heat-labile chemotactic factors.18,19 Crystal-induced inflammation has been studied in model systems using calcium pyrophosphate dihydrate and monosodium urate monohydrate.

Differential Diagnosis

(See Box 131-1). Corneodermatoosseous syndrome (OMIM #122440), an autosomal dominant symptom complex of volar keratosis and keratitis that is clinically similar to tyrosinemia II, has been described in one family.20 The lesions were more extensive than those in the classic Richner–Hanhart syndrome. In addition, brachydactyly with short distal phalanges, short stature, and soft teeth may occur.

Treatment

With consumption of a low-tyrosine, low-phenylalanine diet (Mead Johnson, Tyromex-Ross), there is a rapid decrease in tyrosine to normal levels (30–90 μM). Skin and eye lesions cleared within days and weeks in all individuals treated with the diet7,12 (see Fig. 131-2C; Fig. 131-4; Box 131-2).

Some patients have responded objectively to etretinate, although plasma tyrosine levels remained unchanged.21 In none of the patients studied has there been a response to cortisone acetate, ascorbic acid, pyridoxine, or folic acid, which are cofactors or known inducers of TAT and PHPPA oxidase. Surgical treatment is recommended for palmar lesions but not for the plantar form.

Because the consequences of tyrosinemia II are serious and a safe treatment is available, a patient presenting with any atypical bullous or hyperkeratotic disease on the palms and soles in the first months of life should be screened for tyrosine and its metabolites; screening tests are available in most hospital laboratories.22 Amino acid analysis is necessary to confirm the diagnosis and follow the response to diet therapy. Dietary control in pregnancy should be recommended. A patient with tyrosinemia II was reported who had undergone two untreated pregnancies. During the pregnancies, plasma tyrosine level was raised, and the infants were retarded. On the other hand, several normal children of mothers with the disease have been observed.23

Phenylketonuria (phenylpyruvic oligophrenia, PKU) is an autosomal recessive disorder of aromatic amino acid metabolism (OMIM #261600) in which phenylalanine cannot be converted to tyrosine. Its discovery over 70 years ago has established the link between metabolic disease and intellectual impairment, and led to the development of neonatal screening programs across the globe. In addition, it demonstrated how effective treatment could lead to a near normal outcome for affected individuals. PKU is caused by a deficiency of hepatic phenylalanine hydroxylase or cofactors for phenylalanine hydroxylase. PAH deficiency seems to be a protein-misfolding disease in which global conformational changes hinder molecular motions essential for physiological enzyme function.24 There are additional causes that may lead to hyperphenylalaninemias, and eight subtypes have been described. Maternal PKU may lead to hyperphenylalaninemia. Unless the mother follows a special diet, the child is at risk to develop the typical signs and symptoms of PKU. Defects of biopterin metabolism, prematurity, liver disease, use of medications such as methotrexate, and high-protein diets may produce findings similar to those in PKU. Genotype-based prediction and classification of the biochemical phenotype is now possible in the majority of newborns with hyperphenylalaninemia.24

Epidemiology

The prevalence of PKU varies markedly in various countries. The highest incidence of PKU is found in Ireland and Scotland (1 in 4,000), whereas in Finland only 1 in 40,000 births are affected. The overall frequency among northern Europeans was calculated to be approximately 1 in 10,000 and in the United States classic PKU was estimated to occur in approximately 1 in 11,000 births in the white population. Among blacks the frequency was estimated to be only approximately 1 in 50,000.25

Clinical Features

The clinical hallmarks of PKU are mental retardation, diffuse hypopigmentation, seizures, eczematous dermatitis, and photosensitivity.26 Newborns appear to be normal and develop manifestations of psychomotor alterations between 4 and 24 months of age. Early symptoms include heavy vomiting. The most important defects are marked mental retardation with irritability, hyperactivity, self-destructive behavior, hypertonicity, and seizures. Peculiar gait and increased deep tendon reflexes are noted. Microcephaly, brain calcification, and cataracts are observed quite commonly. Typically, a mousy odor induced by phenylacetic acid is noted in urine and sweat.

Early infantile eczema, indistinguishable from atopic dermatitis, or seborrheic dermatitis may be one of the first signs of PKU and occurs in 20%–50% of affected infants during the first year of life. Later on, the incidence may be even higher. Dramatic clearing of eczematous skin lesions with a low-phenylalanine diet has been reported in some patients, and a potential role for altered biotin recycling is suggested.27 A further skin clue in PKU is pigment dilution, with impressive pale pigmentation, blond hair, and blue eyes seen in 90% of patients. The pigment dilution and hair loss is reversible with phenylalanine restriction.28 The color changes may be especially striking in the rare black or Japanese patient with PKU, whose eye or hair color may be very different from that of other members of the ethnic group. The increased levels of phenylalanine and its oxidation products (phenylpyruvic acid, o-hydroxyphenylacetic acid, phenylacetic acid) inhibit the enzyme tyrosinase and therefore reduce melanization.28

Patients develop moderate photosensitivity. Edematous scleroderma-like induration of the skin of the extremities, which spares the hands and feet, is characteristic.29 Plaque, guttate or generalized morphea, and atrophoderma of Pasini and Pierini (see Chapter 64) as well as lichen sclerosus et atrophicus (see Chapter 65) have been observed in PKU.30

Diagnosis

Diagnosis of PKU is made by documenting elevated levels of phenylalanine in the serum (20 mg/dL or higher, 10–50 times the normal level). Plasma tyrosine levels may be normal or elevated. In addition, elevated urinary phenylpyruvic acid levels can be documented. Additional laboratory abnormalities include phenylalanine hydroxylase deficiency, phenylpyruvic acidemia, and increased urinary levels of o-hydroxyphenylacetic acid and phenylacetylglutamine. Increased urinary levels of phenylpyruvic acid can be detected by a typical green coloration of urine after addition of 10% ferric chloride. In numerous countries, PKU screening is performed in the neonatal screening program either by the semiquantitative bacterial inhibition assay method in which several drops of capillary blood may show an elevated level of phenylalanine in a microbiologic procedure (Guthrie test) or by a quantitative chemical reaction method (Quantase test).31 In affected babies, serum phenylalanine levels begin to rise on the third or fourth day of life. Prenatal diagnosis is possible by performing amniocentesis or chorionic villus sampling with identification of the gene. In addition, preimplantation genetic diagnosis for PKU made possible the birth of PKU-free children.32

Pathophysiology and Histology

The classic type of PKU is induced by a deficiency of phenylalanine hydroxylase or its cofactor tetrahydrobiopterin and a consequent accumulation of the precursor phenylalanine. The normal phenylalanine–tyrosine ratio is 1:1, whereas in PKU it is more than 3:1. It is widely believed that the pigment dilution is a result of the inhibitory effect of phenylalanine on tyrosinase. Biopsy specimens show increased fibroblasts and histiocytes and atrophy of skin appendages. The elastic fibers are scanty and fragmented and are thus different from those found in true scleroderma. Histologic examination shows a normal number of melanocytes but a decrease in mature melanosomes.

Differential Diagnosis

Atopic dermatitis and scleroderma as primary disorders must be distinguished from PKU (Box 131-3). Premature infants and individuals with liver disease may also show elevated plasma levels of phenylalanine.

Treatment

With a low-phenylalanine diet, which should probably be followed for life, the skin color, photosensitivity, odor, and eczema are reversible. In addition, early diet can dramatically reduce mental retardation, and if the blood phenylalanine levels are maintained at reasonable levels in early childhood, normal to low-normal intelligence can be expected33 (Box 131-4). However, rigorous control of diet can lead to protein deficiency and an eczematous dermatitis. Recently, sapropterin has been shown to increase phenylalanine tolerance while maintaining blood phenylalanine control.34–37

Definition and Clinical Features

Argininosuccinic aciduria (ASA; OMIM #207900) is an autosomal recessive disorder of the urea cycle. Its incidence is estimated to be 1 in 70,000. ASA is characterized by hepatomegaly, mental retardation, seizures, episodic lethargy and ataxia, and friable, brittle hair with the morphology of trichorrhexis nodosa (see Chapter 88). The deficiency of an essential cytosolic enzyme in the urea cycle, argininosuccinase (argininosuccinate lyase), causes citrullinemia and an increase in blood ammonia. Argininosuccinate is increased in the blood and spinal fluid and is excreted in large amounts (2–9 g/day) in the urine.

The detailed clinical and biochemical features of the disease and the principles of therapy are reviewed by Nyhan, Barshop, and Ozand.2 The hair abnormality is more characteristic of the late-onset disease, but in the early cases this peculiar type of alopecia brought the first patients to clinical attention.38 Weaning and a change to a diet with a higher protein content can be precipitating factors in clinical disease. Essential to current therapeutic regimens are diets providing adequate energy, protein, and arginine.

The molecular pathology of the nucleic acids in this disease has been clarified, but the detailed pathophysiology of the hair defect remains to be determined.39–41

Hair Defects

Clinical and Morphologic Features.42

About half the reported patients have had abnormally friable hair with visible trichorrhexis nodosa (see Chapter 88); the other half have had grossly normal hair. Brushing and combing accentuate breaking (i.e., brittle hair). No definite correlation can be made between liver argininosuccinase level, argininosuccinic acid levels, arginine levels, and the degree of hair abnormalities; some patients with severe ASA have had seemingly normal hair. Some patients’ hair has improved with diet or without specific therapy. Hair shafts may show pseudotrichothiodystrophy under polarizing microscopy with the typical dark and light banding.45 Diet reverses grooving and abnormal polarization of hair shafts in argininosuccinase deficiency.44 Eyelashes, eyebrows, and general body hair, as well as the nails, may be involved.

Molecular Nature of Hair Defects

The basic nature of the hair defects is not known.45,46

Increasing arginine in the diet aids hair growth and structure in this disease, which suggests that the decrease in arginine may be related to the defective hair. Because the urea cycle is depressed in ASA, the arginine required for protein synthesis would come predominantly from dietary sources. Hair protein is rich in arginine (up to 10% of the amino acid residues are arginine); furthermore, the citrulline present in certain specialized proteins in the medulla and internal root sheath is derived from arginine.

Other Disorders of the Urea Cycle30

In citrullinemia (OMIM #215700), a rare recessive disease caused by the absence of argininosuccinic acid synthetase, there is somatic and mental retardation and increased levels of citrulline in blood, urine, and cerebral spinal fluid; low to normal values of plasma arginine; and increased blood ammonia levels.48 Hair in such patients may be brittle, breaking easily. Microscopically, plucked hairs may be flattened, may be twisted through 180° on their own axes, and may show breakage at the point of twisting.47 Irregular areas of dystrophy and interruption of the cuticle were observed.

Deficiency of the cytosolic enzyme argininosuccinic acid synthetase is associated with trichorrhexis nodosa (see Chapter 88). Its overall clinical presentation is almost identical to that of ASA, although the plasma citrulline levels are usually higher (more than 1 mM) in the former disease. Enzymatic and genetic analyses confirm the diagnosis.

Diagnosis

Retardation, ammonia intolerance, and abnormal hair will suggest these syndromes. The diagnosis is confirmed by high-voltage electrophoresis or ion-exchange chromatography of the urine, blood, or spinal fluid.48 Only a tiny percentage of the cases of trichorrhexis nodosa and brittle hair are due to ASA. Prenatal diagnosis is possible after chorionic villus sampling or amniocentesis to ensure the birth of unaffected infants.40

Alkaptonuria (OMIM #203500), or homogentisic acid oxidase (HGO) deficiency, is a rare metabolic disorder. Excessive homogentisic acid (HGA) is excreted in the urine, which often turns dark, and HGA accumulates in connective tissues, including the dermis49 (ochronosis).

Epidemiology

Alkaptonuria is inherited as an autosomal recessive trait. Pedigrees suggestive of a dominant mode of transmission contain a high degree of consanguinity and, when subjected to careful scrutiny, are actually found to show pseudodominance. In general populations, alkaptonuria is rare (1 in 250,000), but clusters of high incidence are found in certain groups with inbreeding (e.g., in Slovakia and Santo Domingo). Among Slovakian newborns the incidence is 1 in 19,000. Disease distribution is worldwide, and there is an approximately equal incidence in both sexes.50

Etiology and Pathogenesis

The HGO gene maps to the q21-q23.60 region of chromosome 3. Multiple mutations have been found and the mutation spectrum was reviewed recently.50,51–53 The mutations change residues, which are conserved evolutionarily between Aspergillus HGO and human HGO, which suggests that they are fundamental to the activity of HGO. Both compound heterozygotes and homozygotes have been detected.54

The biochemical pathway by which phenylalanine and tyrosine normally undergo oxidative degradation to acetoacetic acid is shown in Fig. 131-3. HGA (2,5-dihydroxyphenylacetic acid), the last molecule in the sequence to contain an intact aromatic ring, is cleaved to maleylacetoacetic acid (Fig. 131-5). The enzyme catalyzing ring cleavage, HGO, is normally present in the soluble fraction of liver and kidney cells. It is highly specific for HGA. HGO mRNA is found in liver, kidney, and prostate.54 Atmospheric oxygen, ferrous ion, and sulfhydryl groups are required for enzymatic function. Quinones inhibit the enzyme. HGO activity is totally absent in both liver and kidney tissue from alkaptonuric subjects. In patients with alkaptonuria, HGA undergoes renal excretion or is transformed to ochronotic pigment within connective tissue. HGA may not be present in the first days of life due to the absence of enzymatic activity of other enzymes in the pathway of tyrosine catabolism.

The renal clearance of HGA is extremely high (up to 400–500 mL/minute) in both normal and alkaptonuric subjects, which indicates active tubular secretion of HGA. This explains how with relatively low fasting plasma concentrations of HGA (in the range of 3 mg/dL) excretion may be up to 4–8 g/day in HGO deficiency. Compounds inhibiting this secretion may be an important factor in ochronosis. Once excreted, HGA (which is itself colorless in solution) gradually oxidizes to dark products. Oxidation occurs by degrees when the urine is exposed to air, but it can be hastened markedly by alkalinization. Urinary pH is the major variable causing the darkening of the urine, and some patients with acidic urine may never have spontaneously black urine. A diet high in protein or tyrosine increases the amount of HGA excreted in disease.

The precise manner by which HGA accumulation in tissues leads to ochronosis is only partially understood. A presumed HGA polymer has never been characterized.55

Alkaptonuric mice (aku) do not have black urine or deposition of pigment, which is possibly related to the mouse's ability to synthesize ascorbic acid.56

Experimental ochronosis induced by prolonged feeding of high-tyrosine diets to rats may delineate the precise interaction between HGA and its products and connective tissue.57 HGO also has been produced in experimental animals by l-phenylalanine feeding and diets deficient in sulfur amino acids or tryptophan and the iron chelator α,α′-dipyridyl.

HGA inhibits lysyl hydroxylase, an enzyme crucial for collagen cross-linking, in chick embryo calvaria, which suggests that a reduction in the structural integrity of collagen consequent to deficient hydroxylysine-derived cross-linkages may be responsible for cartilaginous degeneration in alkaptonuria.58

Clinical Features

Dark urine is not always the initial manifestation of HGO deficiency. The urine is most apt to discolor rapidly when pH is above 7.0 and when reducing substances, such as ascorbic acid, which normally protect HGA from oxidation, are not present in sufficient quantity. An early diagnosis of HGO deficiency is frequently made when (1) it is specifically sought because of family history, (2) discoloration of diapers occurs after cleansing in (alkaline) soap, (3) the urinary pH favors the oxidation of HGA, or (4) testing for urinary glucose with Benedict's solution yields an orange precipitate (which indicates a reducing substance) accompanied by a dark supernatant. A positive finding for Benedict's reaction and a negative result on glucose analysis with a glucose oxidase test reagent strongly suggests the diagnosis.

Although the diagnosis of alkaptonuria may be made during childhood, in rare cases the disorder is not detected until the individual develops pathologically significant changes in connective tissue in the third or fourth decade.59 If coincidental renal disease prevents effective HGA excretion, the development of ochronosis may be accelerated, and diffuse hyperpigmentation may result.

Dark brown or black cerumen may be present in the first decade, even in those younger than 5 years of age. Axillary skin pigmentation (greenish blue, blue, greenish yellow, or brown) in the pattern of glandular orifices may be present late in the first decade. This may be accompanied by staining of underwear (“sweat that stains”). A patient with pigmentary changes confined to sun-exposed areas has been described.60 Caviar-like ochronotic papules have been reported recently in a patient with ochronosis.51

A grayish blue tinge overlying ear cartilage is common in adulthood but is rarely seen before 20 years of age.61 Ochronotic discoloration is rarely seen in childhood but is common in adulthood and can affect the sclera, cornea, conjunctiva, tarsus, and eyelid skin. Scleral involvement is noted in most patients62,63 (Fig. 131-6). Scleral discoloration generally is restricted to that portion of the globe exposed by the palpebral fissure. The scleral pigmentation is usually triangular, with the base of the triangle facing the cornea. Tiny “oil droplets” of ochronotic pigment appear at the inner and outer poles of the corneas in advanced ochronosis. Later in the disease, structural changes result in loss of transillumination, stiffening, irregular contours, and eventually, in the third decade, calcification of the pinnae. The tympanic membrane may be blue. Tinnitus and variable degrees of deafness have been ascribed to ochronotic degeneration of the tympanic membrane and underlying ossicles. Laryngeal and tracheal cartilage becomes heavily pigmented but is asymptomatic.

The visible changes that occur with the passage of time are primarily due to the formation of ochronotic pigment granules in the dermis and sweat follicles and, most important, to the transmission of ochronotic discoloration through thin areas of skin overlying pigmented cartilage and tendon. The latter pigmentation, which is fairly uniform in ochronosis, is most apparent at the nose tip, ear (Fig. 131-7), costochondral junctions, and extensor tendons of the hands. Pigmented colloid milium on the dorsa of the hands have been reported.64 Pigmentation of the skin is less prominent but may occur in a butterfly pattern on the nose and cheeks and even the palms as a presenting sign.65 Rarely, bluish gray fingernails and intensely dark nevi have been reported.66 Insidious progression of ochronotic arthropathy, which generally begins in the third and fourth decades in men and approximately 10 years later in women, is the most disabling manifestation of alkaptonuria.67 The disease is more severe in males. Bouts of acute inflammation may occur. Hip, knee, and shoulder limitation is an early sign. Lumbar pain, lordosis, kyphosis, and sciatica are common. Radiographs show a characteristic appearance of early calcification of the intervertebral disk and later narrowing of the intervertebral spaces, with eventual disk collapse and progressive loss of height (Fig. 131-8). The hands and feet generally are spared. Pseudogout may coexist with ochronosis. It is of note that exogenous ochronosis and striae atrophicae may follow the use of bleaching creams in individuals who do not have alkaptonuria.66

There is some suggestion of an increased incidence of cardiovascular disease in patients with ochronosis, but accelerated arteriosclerosis has not been clearly documented. At postmortem examination, pigmentation is commonly observed in the heart valves and annuli and in arteriosclerotic plaques.68 Prostatic symptoms in older men frequently are due to the formation of soft pigmented calculi in the alkaline secretions of the ducts and sinuses of that gland. Porous black renal stones containing calcium, phosphate, and oxalate also have been reported. The prostate has high levels of HGO mRNA, which may explain the black prostate calculi that are occasionally present in the disease.41

Laboratory Findings

Aside from the excretion of HGA, alkaptonuric patients show no abnormalities on routine clinical laboratory tests. Normal individuals do not excrete HGA; therefore, darkening of the urine on the addition of sodium hydroxide is presumptive evidence of alkaptonuria.

Other tests based on the reducing properties of HGA include the black reaction after treatment with FeCl3 and blackening of photographic emulsion paper on application of a drop of alkaptonuric urine followed by a drop of sodium hydroxide. Specific identification and quantification of urinary (as well as blood) HGA can be achieved by the use of a direct spectrophotometric method using HGO or with high-performance liquid chromatography or stable isotopes.54

With the development of ochronotic arthropathy, radiographs of the spine show characteristic disk calcification, which occurs rarely in other forms of spondylitis (see Fig. 131-8). Periostitis, ligament calcification, and sacroiliac sclerosis are not features of ochronotic spondylitis.

Pathology

Yellow to light-brown (ochre) pigment granules, which led to the original designation of ochronosis, are present as free bodies in dermal macrophages.69 Irregular masses may be over 100 μm in diameter. The pigment, in contrast to melanin, is not bleached by 10% H2O2 after 72 hours.69 Routine special stains for melanin react with the ochronotic pigment.

Electron microscopic studies show smaller-sized homogeneous bodies fusing to form larger nonmembrane-bound structures.69 Although the original pigment is brown, Tyndall scattering of light makes involved skin appear blue.

The tendency of connective tissue, cartilage in particular, to darken gradually over the years constitutes the cardinal pathologic finding in alkaptonuria. Intervertebral disks are pigmented (jet black) and darken when examined. Articular cartilage, when heavily pigmented, displays the degenerative changes of fibrillation, fissuring, fragmentation, and erosion to bare bone. Phagocytosis of collagen fibrils is found in synovial macrophages.70

Diagnosis and Differential Diagnosis

The diagnosis of alkaptonuria may be made on the basis of typical urinary discoloration or may await the onset of ochronosis in adulthood. Inasmuch as the disease behaves in a quite stereotypical manner with few confusing variants in its mode of presentation, it has been concluded that the diagnosis need only be thought of to be made. Other causes of dark urine—melaninuria, porphyria, myoglobinuria, bilirubinuria, and hematuria—should not to be confused with alkaptonuria. An ochronosis-like pigmentation of skin and cartilage has been produced iatrogenically by quinacrine administration over a period of months and at the site of quinine injections.63 Pigmentation due to antimalarial treatment is usually much more pronounced on mucosal surfaces and will fluoresce with a Wood's lamp. A slate gray to blue pigmentation in sun-exposed sites occurs after amiodarone treatment, and similar discoloration also involving the mucous membranes is seen in argyria and chrysiasis (Box 131-5).

It is possible that the reversible ochronotic pigmentation caused by prolonged carbolic acid treatment is due to polymerization of the carbolic acid by HGA polyphenol oxidase to an HGA polymer-like substance that differs from the polymer found in the genetic disease by the reversibility of the polymerization.

Exogenous ochronosis and pigmented colloid milia have been reported in a number of South Africans who used hydroquinone bleaching creams at strengths greater than 2% for a prolonged period.66,71,74 Forty-two percent of women and 15% of men who used hydroquinones developed exogenous ochronosis.72 A similar condition has been seen in American blacks.73–75

Treatment

The course of alkaptonuria is generally slow but irreversible. Treatment is primarily supportive, with close observation for the development of arthropathy, cardiac disease, and urinary tract disease (Box 131-6). Appropriate management includes genetic counseling, pain management with nonsteroidal anti-inflammatory agents, physical therapy to increase range of motion, and regular follow-up visits.20 Avoidance of high-protein, high-phenylalanine, and high-tyrosine diets has been reported to be important. A well-balanced normal diet seems the best approach during childhood.76–78 It has been postulated that large amounts of dietary vitamin C may be helpful, because vitamin C protects HGA against oxidation and thus may prevent the deposition of ochronotic pigment; however, no long-term clinical studies have been undertaken to verify the effectiveness of this approach.76 In recent years nitisinone has been used in ochronotic patients in an investigational basis and excretion of homogentisic acid could be decreased.49,78

It is disappointing that despite advances in our biochemical understanding of alkaptonuria and the disposition of accumulated HGA to form ochronotic pigment in connective tissue, this information has yet to be translated into a successful therapeutic program for managing the disease. Exogenous cutaneous ochronosis induced by hydroquinone has been treated with the carbon dioxide laser.79

Course and Prognosis

The ultimate course in adults with alkaptonuria is that of increasing pigmentation and skeletal incapacity. Little can be done to interrupt this progression; however, the disease is not incompatible with a normal life span, and the oldest patient on record lived to 99 years of age.

Table 131-1 lists some of the abnormalities of amino and organic acid metabolism and their associated skin manifestations.