Case Series of Creatine Deficiency Syndrome due to Guanidinoacetate Methyltransferase Deficiency.

Guanidinoacetate methyltransferase (GAMT) deficiency is the second most common defect in the creatine metabolism pathway resulting in cerebral creatine deficiency syndrome (CCDS). We report three patients from two unrelated families, diagnosed with GAMT deficiency on next-generation sequencing. All the probands had happy predisposition as a predominant manifestation in addition to the reported features of global developmental delay, seizures, and microcephaly. This further expands the phenotype of CCDS. The workup for creatine deficiency disorder should be included in the diagnostic algorithm for children with nonsyndromic intellectual disability, especially in those with a happy demeanor. These cases exemplify the utility of magnetic resonance spectroscopy of the brain in the workup of nonsyndromic intellectual disability to diagnose a potentially treatable disorder. In addition, documentation of low serum creatinine may be supportive. Early diagnosis and treatment is essential for better prognosis.