A Case of Congenital Disorder of Glycosylation Type 1b Presenting as Hyperinsulinemic Hypoglycemia and Failure to Thrive.

We describe initial manifestations, approach to diagnosis, and treatment of a patient with congenital disorder of glycosylation type 1b (CDG 1b), previously managed as acetylcarnitine deficiency. A 9-year-old girl initially diagnosed with and treated for acetylcarnitine deficiency at an outside hospital presented with recurrent hypoglycemia, failure to thrive, poor weight gain, and short stature. She had discontinued levocarnitine therapy because of lack of response, and testing with us demonstrated a normal carnitine and acyl carnitine panel and hyperinsulinemic hypoglycemia during a diagnostic fast. Oral diazoxide and hydrochlorothiazide were initiated with resolution of hypoglycemia. She had iron deficiency anemia, but an upper gastrointestinal evaluation was normal. Genetic testing confirmed a diagnosis of CDG 1b caused by deficiency of mannose phosphate isomerase. Oral mannose was started with gradual reduction in and eventual discontinuation of the diazoxide dose. Hypoglycemia in the pediatric age group needs a systematic approach. It is important to raise awareness of CDG 1b, which can present as persistent hyperinsulinemic hypoglycemia. Mannose supplementation can ameliorate clinical symptoms and biochemical abnormalities.