Creatine Transporter Deficiency Presenting as Failure to Thrive: A Case Report of a Novel SLC6A8 Variant Causing a Treatable but Likely Underdiagnosed Genetic Disorder

Cerebral creatine deficiency syndromes (CCDS) are a rare group of inherited metabolic disorders (IMDs) that often present with nonspecific findings including global developmental delay (GDD), intellectual disability (ID), seizures, hypotonia, and behavioral differences. Creatine transporter (CRTR) deficiency is the most common CCDS, exhibiting X-linked inheritance and an estimated prevalence as high as 2.6% in individuals with neurodevelopmental disorders. Here, we present a 20-month-old boy with worsening failure to thrive (FTT) and GDD admitted for evaluation. He was found to have persistently low serum creatinine levels and a family history notable for a mother with learning disabilities and a maternal male cousin with GDD. Urine analyses revealed a marked elevation of creatine and elevated creatine:creatinine ratio suggestive of CRTR deficiency. Molecular genetic testing of SLC6A8 identified a maternally inherited hemizygous variant and brain magnetic resonance spectroscopy (MRS) showed diffusely diminished creatine peaks, further supporting the diagnosis of CRTR deficiency. The proband was started on creatine, arginine, and glycine supplementation and has demonstrated improved development. This case highlights that CRTR deficiency should be considered in all patients presenting with FTT and abnormal neurodevelopmental features, particularly if creatinine levels are low on serum chemistry studies. The nonspecific presentation of this condition in males and females likely has resulted in CRTR deficiency being underdiagnosed. There are existing therapies for individuals affected with CRTR deficiency and other CCDS, highlighting the importance of early diagnosis and intervention for affected individuals.