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TYROSINAEMIA

An Introduction to Tyrosinaemia

There are three types of Tyrosinaemia caused by disorders of tyrosine metabolism:

Tyrosinaemia Type l

Tyrosinaemia type I is caused by a deficiency of fumarylacetoacetate hydroxylase resulting in severe liver and kidney failure and eventually death. The aim of dietary management is to prevent the accumulation of phenylalanine, tyrosine and sometimes methionine by means of a low protein diet. The protein requirements are met by supplementing the diet with a tyrosine, phenylalanine and/or methionine free amino acid mixture. If NTBC is prescribed then a phenylalanine and tyrosine free protein substitute is normally used. A phenylalanine, tyrosine and methionine free protein substitute is used only when the patient is unresponsive to NTBC or it is unavailable. Whilst dietary management is important in Tyrosinaemia, NTBC has significantly improved management and survival in Tyrosinaemia type I.

Tyrosinaemia Type II

Tyrosinaemia type II is caused by a deficiency of tyrosine aminotransferase leading to eye lesions, skin lesions and neurological complications. The aim of dietary management is to prevent the accumulation of phenylalanine and tyrosine by means of a low protein diet. The protein requirements are met by supplementing the diet with a tyrosine and phenylalanine free amino acid mixture.

Tyrosinaemia Type lll

Tyrosinaemia type III is a very rare form of tyrosinaemia resulting in convulsions, ataxia and mental retardation. The aim of dietary management is to prevent the accumulation of phenylalanine and tyrosine by means of a low protein diet. The protein requirements are met by supplementing the diet with a tyrosine and phenylalanine free amino acid mixture. Guidance on amino acid intakes have been extrapolated from MRC guidelines on PKU management.

Tyrosinaemia Type I is more common than Type II and Type III is very rare. The incidence of type I is about 1 in 100,000 births in the Western World. 

To listen to a dietitian explain what Tyrosinaemia is click here.