Mutations in the KCNJ11 gene, which encodes the Kir6.2 subunit of the 
ATP-sensitive potassium channel, often result in neonatal diabetes. Patients 
with this mutation have been successfully transitioned from insulin to 
sulfonylurea (SU) therapy without compromise in their glycemic control. Among 
patients with neonatal diabetes due to KCNJ11 mutations, approximately 25% have 
neurological findings including developmental delay, motor dysfunction, and 
epilepsy, known as DEND syndrome. There have been rare cases of juvenile 
patients with intermediate DEND syndrome (iDEND) reporting variable improvement 
in neurological function following transition from insulin to SU treatment. We 
describe the response to glyburide in a 15-yr-old boy with severe global 
developmental delays resulting from the KCNJ11 mutation V59M. The patient was 
discovered to have diabetes mellitus at 11.5 months of age, making this the 
oldest age at diagnosis of a KCNJ11 mutation-related case of neonatal diabetes. 
Because consensus has been to screen patients for this mutation only if younger 
than 6 months at the time of diagnosis, we suggest that all patients under the 
age of 12 months at diagnosis should receive genetic testing for monogenic 
causes of diabetes.