Hypomagnesemia, seizure, and intellectual disability (HSMR) syndrome is a rare condition that is characterised by decreased magnesium levels in the blood, seizures, and mild to moderate intellectual disability. In severe cases, structural brain malformations have been found by MRI. This congenital disease is caused by genetic defects in the CNNM2 gene. This gene has been found to be of importance in the kidney for maintaining magnesium levels in the body. How this genetic defect results in intellectual disability and causes epilepsy is still unknown.

Some patients have a family history, meaning at least one parent has the genetic defect in CNNM2, while others have de novo mutations (no family history).