["Journal of Intellectual Disability Research, EarlyView. ", "\nABSTRACT\n\nBackground\nChronic hyponatraemia is highly prevalent in older adults and is associated with various adverse health outcomes; however, its prevalence among individuals with severe motor and intellectual disability (SMID) remains unclear. This study aimed to determine the prevalence of hyponatraemia in hospitalized patients with SMID and to identify potential contributing factors.\n\n\nMethods\nWe retrospectively analysed serum sodium (sNa) levels in patients with SMID admitted to our hospital. Hyponatraemia was defined as an sNa level < 135 mEq/L. Demographic and clinical data, including age, sex, body weight, nutritional management and medication status, were collected. Serum concentrations of antiseizure medications (ASMs) were evaluated in patients receiving these treatments.\n\n\nResults\nA total of 122 patients were included. The primary diagnoses were cerebral palsy (57.3%), sequelae of intracranial infection (11.4%), hypoxic encephalopathy (10.7%), drug‐resistant epilepsy (9.0%), sequelae of intracranial haemorrhage (4.9%) and congenital disorders including chromosomal abnormalities (6.5%). Hyponatraemia was identified in 22 patients (18.0%). Epilepsy was present in 112 patients, and 93 were receiving ASM therapy. No significant association was observed between hyponatraemia and primary diagnosis. However, univariate analyses revealed that the use of multiple ASMs was significantly associated with hyponatraemia (p = 0.028). Multiple regression analysis demonstrated that higher serum levels of carbamazepine (p < 0.0001) and phenytoin (p = 0.0026) were significantly associated with lower sNa levels.\n\n\nConclusions\nIn our hospital, 18% of patients with SMID exhibited hyponatraemia. Careful monitoring of serum sodium levels is warranted in patients with SMID and epilepsy treated with carbamazepine or phenytoin.\n\n"]