The neuropsychological profile of SSADH deficiency, a neurotransmitter disorder of GABA metabolism

Background and objectives: Succinic semialdehyde dehydrogenase deficiency (SSADHD) is an inherited metabolic disorder resulting in hyper-physiologic concentrations of the neurotransmitter γ-aminobutyrate (GABA). This study aims to provide the most comprehensive description, to date, of the neuropsychological profile of individuals with SSADHD and assess whether neuroimaging, neurophysiologic, and biochemical indices of cortical inhibition correlate with those of standardized behavioral tests. Methods: Participants enrolled in the SSADHD Natural History Study underwent medical and neurological examinations, magnetic resonance imaging (MRI) and spectroscopy (MRS), biochemical tests of GABA and its related metabolites, transcranial magnetic stimulation (TMS), and gene expression quantification, as well as complete neuropsychological assessment including standardized measures for cognition, adaptive skills, motor function, receptive and expressive language, autism spectrum disorder, and behavior problems. Results: The neuropsychological profile of the study's 65 enrollees [54 % females, median (interquartile range) age 9.6 (5.4-14.7)] consisted almost universally of intellectual disability, delays in adaptive skills, and deficits in expressive more than receptive language. Autism Spectrum Disorder was noted in ∼50 %, and behavioral problems in ∼70 %, predominated by obsessive-compulsive behaviors and attention problems but also including affective problems, anxiety, and, rarely, aggression and possible psychosis. Correlation analyses showed that increased internalizing, externalizing, and overall psychiatric morbidity significantly correlated with increasing age (R = 0.391, p = 0.033), as well as age-independent indices representing decreased cortical inhibition such as lower MRS-derived GABA (R = -0.530, p = 0.029) and TMS-derived resting motor threshold (R = -0.418, p = 0.053). Discussion: The natural history study of SSADHD indicates that intellectual disability, delayed adaptive skills, and expressive>receptive language deficits are nearly universal, with behavior problems in the vast majority. Increased psychiatric morbidity in SSADHD with age-independent decreased cortical inhibition may serve as the basis for establishing disorder-specific biomarkers for behavioral and psychiatric outcomes in SSADHD and other non-syndromic psychiatric disorders.