However, bilirubin concentrations are normal. Hypoglycemia may be present early in the course of the encephalopathy, particularly in children under the age of 4 years. Infants under one year of age may have a different clinical presentation, with little or no vomiting, but with sudden onset of encephalopathy following a viral infection. Respiratory difficulties, including hyperventilation and sudden apnea, are prominent. Seizures are frequently encountered in the younger age group.
Laboratory studies are similar in infants and older children.
Pathologic changes associated with Reye Syndrome are most marked in the liver. Diffuse, microvesicular fatty accumulation is seen within the cytoplasm. Mitochondrial swelling and pleiomorphism are observed on ultrastructural examination, as well as depletion of glycogen stores and swelling of smooth endoplasmic reticulum. Histochemical stains reveal a reduction in succinic acid dehydrogenase activity. Fatty changes may also be found in other organs, such as heart, skeletal muscle and kidney, although these are usually not as striking as the abnormalities found in liver.