Abstract Details Title An Adult Case of Non-Ketotic Hyperglycinemia Due to Novel Compound Heterozygous Variants in GLDC Topic Child Neurology and Developmental Neurology Presentation(s) Child Neurology and Developmental Neurology Posters (7:00 AM-5:00 PM) Poster/Presentation Number 050 Objective Review the diagnosis of non-ketotic hyperglycinemia (NKH) through a clinical case report. Implications of rapid exome sequencing are also highlighted. Background NKH is among the family of disorders of impaired glycine metabolism leading to accumulation in tissues throughout the body. These result in encephalopathy and a wide range of focal neurologic deficits, including movement disorders. Design/Methods The clinical course, medical history, family history, biochemical studies, and whole exome sequencing results from a patient with acute-onset encephalopathy and choreiform movement disorder are presented. Results A previously healthy 38 year-old right-handed Caucasian female presented with encephalopathy and choreiform movements associated with an upper respiratory infection. Her condition rapidly deteriorated, with hypoxic respiratory failure, severe encephalopathy, and constant choreiform movements occurring two days following presentation. Extensive serum and CSF studies, as well as FDG-PET imaging, were unremarkable. MRI revealed mild septo-optic dysplasia. She was trialed on cyproheptadine, valproate, clonazepam, levodopa/carbidopa, levetiracetam, clobazam, risperidone, and dantrolene for her choreiform movements without effect. Serum amino acid profile showed markedly elevated glycine. Rapid exome sequencing with concurrent maternal and sibling samples revealed that the patient was a compound heterozygote for two likely pathogenic variants both previously unreported in GLDC; namely c.1759C>G (p.Q587E) and c.2777T>C (p.I928T). A variant of uncertain significance was also identified in RANBP2 c.527G>A (p.R176H) (associated with infection-induced encephalopathy). A known pathogenic variant in PKP2 c.1132C>T (p.Q378X) for arrhythmogenic right ventricular cardiomyopathy was also identified. Ultimately, the patient was started on sodium benzoate and dextromethorphan, glycine levels normalized, and she gradually recovered. Conclusions NKH must be considered in patients presenting with acute-onset encephalopathy and movement disorder associated with abnormal amino acid profile. Rapid exome sequencing should be sent early to confirm the diagnosis expeditiously and assess for secondary findings. Authors/Disclosures Andrew Dhawan, MD (Cleveland Clinic) PRESENTER Dr. Dhawan has nothing to disclose.