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An unusual palmoplantar pigmentation

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Q1: What is your diagnosis?

The diagnosis is alkaptonuria. The patient had scleral pigmentation with an unusual palmoplantar pigmentation and joint involvement.

Q2: What are the bedside tests by which you can confirm your diagnosis?

The following are the simple bedside tests which can be useful in diagnosis:

Alkali test: addition of few drops of alkali such as sodium hydroxide or potassium hydroxide darkens the urine.

Photographic paper test: application of a drop of alkaptonuric urine over photographic paper followed by the addition of few drops of sodium hydroxide leads to a black spot on the photographic paper.

Black” Benedict's test: when urine is added to Benedict's reagent, it produces a black coloured ring.

Ferric chloride test: addition of ferric chloride to the patient's urine produces black colour.

Q3: What are the complications of this disease?

Complications are listed in box 1.

Box 1: Complications of alkaptonuria

Musculoskeletal
  • Arthritis.

  • Kyphosis.

  • Sciatica.

  • Disc calcification and collapse.

  • Ochronotic arthropathy.

  • Pseudogout.

Cardiovascular
  • Atherosclerosis.

  • Valvular stenosis.

Calculi
  • Prostatic calculi.

  • Pigmented renal calculi.

Others
  • Deafness.

Discussion

Alkaptonuria is the first human disease shown to be inherited as an autosomal recessive trait and is due to the absence of the enzyme homogentisic acid oxidase (HGAO) in the liver and kidneys.1 Affected homozygotes occur with a frequency of around one in 200 000.2 HGAO helps in the degradation of phenylalanine and tyrosine. Homogentisic acid (HGA) is an intermediate of this pathway and blockade in conversion of HGA to maleyl acetoacetic acid results in accumulation of excessive amounts of HGA in the body. This in return is deposited in various connective tissues (ochronosis) and subsequently undergoes oxidation and polymerisation.3

Alkaptonuria is asymptomatic in childhood and the only sign present in some instances is the dark staining of nappies (diapers). However, the later external manifestations include greyish blue discoloration of the sclera and the cartilage of the external ear. The cutaneous manifestation in the form of fine speckled blue black discoloration occurs mainly in the sun exposed areas while greenish blue pigmentation occurs in regions of high sweat gland density. The most disturbing and disabling manifestation of alkaptonuria is the insidious gradually progressive ochronotic arthropathy, which generally begins in the third and fourth decade in males and about 10 years later in females. Pigmentation of the heart valves, aorta, genitourinary tract, laryngeal, tracheal, and bronchial cartilages has also been described.3 4

The diagnosis is usually made from the triad of degenerative arthritis, ochronotic pigmentation, and urine that turns black upon alkalinisation.2 Histopathological examination of the skin lesion showing homogenised thickened ochronotic pigment in the upper dermal collagen further confirms the diagnosis of alkaptonuria.3

Treatment is mostly symptomatic; ascorbic acid may be helpful.5

The unique feature of this case is the palmoplantar pigmentation, which is exceedingly rare.

Final diagnosis

Alkaptonuria.

References

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