Any blood glucose value below 40mg/dl is considered hypoglycemia.

Clinical features of hypoglycemia:
The clinical features arise due to

Autonomic nervous system activation (epinephrine release) -
Sweating, tremulousness, palpitations (tachycardia), anxiety, hunger, nausea, vomiting and weakness.

Decreased cerebral glucose level- headache, confusion, visual disturbances, lethargy, restlessness, inability to concentrate, somnolence, personality changes, seizures, and even coma.

Symptoms of hypoglycemia in infants are more subtle and include cyanosis, apnea, hypothermia, hypotonia, lethargy, jitteriness and seizures.Hypoglycemia should always be suspected in a case of convulsion, coma, or sudden deterioration in neurobehavioral functioning.