Three inborn errors affecting creatine metabolism are known in humans: two disorders of creatine synthesis including arginine:glycine amidinotransferase (AGAT) deficiency (Item et al. 2001) and guanidinoacetate methyltransferase (GAMT) deficiency (Stoeckler et al. 1996b); and one disorder of cellular creatine transport, namely the X-linked creatine transporter (CRTR, SLC6A8) deficiency (Salomons et al. 2001). Cerebral creatine deficiency is the common biochemical feature, and therefore these disorders are also described as creatine deficiency syndromes (CDS) in the literature. So far, only four patients with AGAT deficiency and about 30 patientswithGAMTand 100 patients with CRTR deficiency have been diagnosed. Therefore, studies in a sufficient number of patients are widely lacking and recommendations on treatment and follow-up remain preliminary. Principles of treatment and critical discussion of the experience obtained so far are given in this Introduction. In the tables, guidelines for the practical management of a patient are given that might allow comparison of data obtained fromsingle patients and finally find out evidence for the most effective treatment strategies. Clinical and Biochemical Phenotype.
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