This retrospective cohort study was designed to determine the yield of genetic tests in hypotonic infants and develop a diagnostic algorithm. Out of 496 patients identified by International Classification of Diseases (ICD) 9/10 coding, 324 patients met the inclusion criteria. Diagnostic yields were 32% for karyotype, 19% for microarray, 30% for targeted genetic tests, 38% for gene panels, and 31% for exome sequencing. In addition, we considered the diagnostic contribution of ancillary tests, including neuroimaging, metabolic tests, and so forth. The combination of microarray and exome sequencing gave the highest diagnostic yield. None of the other tests added significant value in arriving at a diagnosis. Based on these results we propose that the vast majority of infants with congenital hypotonia should start with a microarray and proceed with exome sequencing, with the notable exception of infants with clearly syndromic features in whom karyotyping or targeted testing may be more appropriate.

Glycoprotein sialylation represents a critical posttranslational modification with diverse biological roles in animals. This review explores its multifaceted functions in the nervous system, with particular emphasis on neurophysiology, homeostasis, and associated neurological disorders. The sialylation pathway modulates key neural processes through effects on glycoprotein stability, localization, activity, and molecular interactions. Examples include its crucial role in the regulation of neuronal excitability by modulating the functions of voltage-gated ion channels. Recent studies have uncovered remarkably rapid, activity-dependent changes in synaptic sialylation, suggesting dynamic sialylation-mediated regulation of neural transmission and highlighting the importance of neuraminidases in these processes. Beyond synaptic function, sialylation mediates neuron-glia interactions through multiple mechanisms. It modulates immune functions regulated by siglecs and complement pathways while controlling microglial activation and neuroinflammation. The critical importance of proper sialylation is underscored by severe neurological manifestations associated with genetic defects in the sialylation pathway, including cognitive impairment, ataxia, and epilepsy. Furthermore, aberrant sialylation of glycoproteins and gangliosides has been implicated in neurodegenerative diseases (Alzheimer's and Parkinson's), brain cancers, and psychiatric disorders including schizophrenia and autism. Preclinical research has identified promising therapeutic strategies targeting sialylation. Studies demonstrate that polysialic acid administration reduces neurodegeneration, while siglec modulation alleviates age-related cognitive decline. Recent discoveries, including sialylated glycoRNA and insights from Drosophila models revealing unique sialylation-mediated glia-neuron crosstalk, have significantly expanded our understanding of this important regulatory system. These advances position sialylation as a promising therapeutic target for neurological disorders.