This prospective observational study was conducted in the Department of Pediatrics of a Medical college hospital.

Children aged 3 mo to 2 y, admitted in the Department of Pediatrics between March 2021 and September 2022, with unexplained GDD (one or more of the following: mild encephalopathy with or without infrequent seizures, hypotonia/hypertonia, prolonged feeding time, failure to thrive, unexplained mild respiratory distress or infrequent vomiting) were enrolled in this study. Children with history of transient hypoglycemia in newborn period or at presentation were also included. The study was time bound and included all the children fulfilling the inclusion criteria in the study period. Such children were admitted for a day or two for further evaluation, evaluation of comorbidity particularly hearing and vision assessment and to initiate specific therapy if needed. These children are mostly from distant places. During the admission, parents were taught physiotherapy and or occupational therapy as needed. Children with identified causes of GDD such as perinatal asphyxia/hypoxic ischemic encephalopathy (HIE) stage 2 or 3, TORCH infections, post-meningitis sequelae, chromosomal disorder or malformations, hypothyroidism, bilirubin encephalopathy, neuroregression and children who were already on megadoses of vitamin supplements were excluded. The study did not include children with acute presentation of encephalopathy, severe acidotic breathing, persistent frequent vomiting or acute unresponsive state.

Institutional Ethics Committee clearance (IEC- 859/2020) was obtained and CTRI registration (CTRI/2021/04/032521) was done. Informed consents were obtained from the parents of the study children.

In each case, history and examination findings were recorded in the predesigned study proforma. All children underwent initial metabolic studies including blood glucose, arterial blood sample analysis, renal function tests, uric acid, serum electrolytes, liver function tests (LFTs), plasma ammonia, arterial blood lactate and pyruvate, and urine ketone/ reducing substances. Tandem Mass Spectroscopy (TMS) and Gas Chromatography and Mass Spectrometry (GC-MS) tests when indicated were obtained and analysed. Electroencephalography (EEG), neuroimaging, ophthalmological and hearing evaluation, and thyroid function tests were carried out based on clinical indications.

Initial metabolic tests were analyzed as follows: Hypoglycemia: Serum glucose less than 60 mg/dL, elevated lactate: more than 22 mg/dL, hyperammonemia: serum ammonia more than 150 mcg/dL, low urea: urea less than 5 mg/dL, elevated lactate by pyruvate ratio: more than 50, metabolic acidosis: pH <7.35 with bicarbonate <18 mEq/L, high anion gap (AG): >16 and elevated Creatine phosphokinase (CPK): >260 U/L. Total bilirubin >2 mg/dL, direct bilirubin >1 mg/dL with elevated transaminases thrice the normal (normal AST and ALT up to 40 IU/L) were considered suggestive of hepatic involvement, urine ketones +/- were considered for ketotic and non-ketotic hypoglycemia and urine reducing substances- presence/absence was considered.

From the abnormalities in the initial metabolic workup, following probable metabolic diagnosis were considered for the purpose of analysis: (1) Urea cycle disorders- hyperammonemia, low urea, with or without liver function test abnormalities, (2) Fatty acid oxidation defect disorders- metabolic acidosis, hypoglycemia, absent urinary ketones, (3) Organic academia- acidosis, increased AG, increased ammonia >150 mcg/dL, hypoglycemia, (4) Galactosemia-hypoglycemia with urinary reducing substances, (5) Mitochondrial disorders- increased lactate or increased L/P ratio >50, abnormalities in LFT, increased CPK and (6) Glycogen storage disorders- hypoglycemia, abnormal liver function tests, raised CPK and ketonuria.

Specific abnormalities found in TMS, GC-MS were categorized as normal or as one of the above disorders.

Statistical analysis was done using SPSS software version 24. Descriptive analysis including frequency distribution and percentages for categorical data were used for analysis. There were no drop outs during the study.