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The hepatic lesion in Reye's syndrome (acute encephalopathy with fatty degeneration of viscera) was studied by light microscopy of sequential biopsy specimens obtained in 49 children. The hepatic lesion is a morphologically characteristic, rapidly evolving, and reversible toxic hepatitis. In specimens obtained with 48 hr of onset of neurological deterioration, the severity of the diffuse microvesicular steatosis is best appreciated in frozen sections stained for lipid content. Variation in severity of hepatocyte glycogen depletion in early biopsies correlates with other histological measures of severity, and with the occurrence of hypoglycemia, severity of the encephalopathy at the time of admission, and mortality rate. Histochemical studies suggest that the hepatic lesion is attributable to mitochondrial injury and other evidence that supports this hypothesis is briefly reviewed. The etiology of the syndrome and its relationship to the viral disease which usually precedes it are unknown.

The treatment of 66 children with Reye syndrome proved by hepatic biopsy or autopsy is described. Prior to the utilization of exchange transfusion early in the course of the disease, our case fatality rate was 100% of nine patients. With early diagnosis and early exchange transfusion, the case fatality rate was reduced to 27% of 44 patients. During the 1974 epidemic of Reye syndrome, 26 children were treated. In 18 children the diagnosis was established by hepatic biopsy; 16 received one or more exchange transfusions. There were no deaths among these 26 patients. In the 1974 epidemic, the national case fatality rate was estimated to be 40%. Exchange transfusion appears to have been an important factor in the reduction of the case fatality rate among our patients.

Despite greater than 23 years of study, an incomplete understanding of the etiology, epidemiology and pathogenesis of Reye's syndrome persists. Better understanding of the disease has been hampered by the lack of a good animal model on which hypotheses of its pathogenesis could be tested. Human studies indicate that a primary mitochondrial injury may lead to complex metabolic disturbances that produce the observed pathophysiology. Specific directions regarding avenues for future research should pursue two lines: a good animal model still needs to be developed in which the biochemical and morphologic alterations identified in Reye's syndrome are duplicated. This model should include an antecedent viral illness but may not require aspirin exposure as an essential ingredient. With the identification of a satisfactory model, specific questions about the roles of environmental toxins or medications may be answered. Study of noncomatose cases of Reye's syndrome should continue. The specific emphasis should be to delineate what factors (NH3, free fatty acids and dicarboxylic acids) may be implicated in the pathogenesis of the CNS disease with the hopes of devising strategies for more effective treatment of encephalopathy and its attendant morbidity and mortality.

Behavioral measurement of brain function was conducted in 40 children, one or more years after their recovery of Reye syndrome. Test measures included standard indices of intelligence, school achievement, visual-motor coordination and social maturity, plus the Halstead-Reitan Neuropsychological Batteries. There was a strong correlation between the degree of impaired neuropsychologic function and clinical grade at admission, the duration of impaired consciousness, and the number of exchange transfusions required. Patients with milder disease had normal brain function and fewer school problems. Language and perceptual-motor performance significantly improved with increasing years in recovery, suggesting that some of the disturbances of brain functioning are transient. The statistical analysis indicated that there are lasting, often subtle disturbances of higher cognitive function as a result of Reye syndrome. These deficits, not always apparent on clinical examination, are clearly correlated with the extent of neurologic involvement. This quantitative assessment of neuropsychologic function is a basis for determining the "quality of survival" in Reye syndrome, and such measurements should be included in the comparative evaluation of Reye syndrome treatment programs.

Serum salicylate concentration was measured at admission in 130 children with liver-biopsy-confirmed Reye's disease. Mean serum salicylate was 12.3 mg/dl and mean salicylate concentrations by neurological grade (Lovejoy) were: stage I, 12, stage II, 13, stage III, 11, stage IV, 13, and stage V, 13 mg/dl. However, mean serum salicylate (15 mg/dl) at admission in 21 patients who died or had serious neurological deficits was significantly higher than that in 103 patients who survived without neurological sequelae (10 mg/dl). Serum salicylate in a group of 27 age-matched, community-matched control children collected consecutively over the period 1978-80 was less than 2 mg/dl, and children with varicella or influenza had salicylate concentrations indistinguishable from apparently well classmates or siblings. It is impossible to determine from this data whether salicylates are involved in the aetiology of or in determining the outcome of Reye's disease. Increased concentrations of salicylates at admission could be the result of excessive dosage because of a greater severity of the prodromal illness, or to diminished excretion because of impaired hepatic metabolism. It seems likely that serum salicylate concentrations entered the toxic range in many patients with Reye's disease before they presented for treatment. Most had been vomiting and had diminished oral intake for 33-55 h before hospital admission. Since the average number of hours from the beginning of vomiting to admission was no different in non-comatose and comatose cases, the time at which salicylate concentration was measured in relation to the last dose was probably similar in the two groups and therefore does not account for the higher levels in children with poor outcome. Salicylates are mitochondrial toxins and mitochondria are known to be significantly injured in Reye's disease; therefore, it seems wise to avoid the use of aspirin in children during outbreaks of Reye's disease.

Influenza virus (A/Ohio/7/76) was demonstrated by haemagglutination and by direct electron microscopy in chick embryo fluids inoculated with liver, muscle, cerebrospinal fluid, and nasotracheal secretions obtained from a boy with Reye's syndrome, 8 days after the onset of the upper-respiratory-tract symptoms and at the height of neurological and hepatic involvement. This finding accords with the hypothesis that Reye's syndrome may be an epiphenomenon of influenza-virus infection. It is postulated that the pathogenesis of Reye's syndrome may be related to the other influenzal eipiphenomena including influenzal pneumonia in previously healthy persons and influenza-associated myopathy, both of which, like Reye's syndrome, become clinically evident about 7 days after the first clinical signs of influenza.

Fifty-eight children with Reye's syndrome (RS) confirmed by liver biopsy were treated at the University of Cincinnati between 1963 and 1974. Cases were clustered in the winter and spring with the peak in February and March. These coincided with the occurrence of influenza and, numerically, were associated more closely with influenza B than with influenza A. Six of the 58 cases were associated with chickenpox. Twenty-six children with RS were seen from 1963 to 1971, before the beginning of a systematic epidemiologic and virologic study. Viral infection was documented in either the patient or a contact in only 19% of those studied. Between 1971 and 1973, after the study was initiated, 16 cases of RS were diagnosed and viral infection was confirmed in 56% of these. In 1974, an epidemic of RS occurred during an influenza B epidemic and viral infection was found in either the patient or a contact in 81% of 16 cases. This study demonstrates that an association with viral infection can be proven in the majority of cases of RS when an intensive investigation is undertaken. During this study no significant environmental toxic exposures could be idenified. Most children had taken aspirin and other medications, and seven children had a history of excessive aspirin ingestion.

Acute and recovery biopsies of three patients with Reye's Disease are described. Pleomorphic changes of neuronal mitochondria were identified in all of the acute biopsies, similar in appearance to the characteristic alterations of hepatic mitochondria. Distinctive myelin bleb formation may be directly attributable to the mitochondrial injury. The mitochondrial lesion is reversible. There is morphologic evidence for regeneration and repair of myelin; but the presence of myelin ovoids at long intervals after recovery indicates a loss of some myelinated fibers. The neuronal mitochondrial changes, pleomorphism with matrix expansion, and myelin bleb formation, reflect a specific biochemical injury be attributable to ischemic injury secondary to brain edema.

All childhood liver biopsy specimens from The Cincinnati Children's Hospital Research Foundation which had been prepared for light and electron microscopy were reviewed to identify biopsies from children with salicylate intoxication. Only two cases of primary salicylate intoxication were identified. The histopathology and ultrastructural pathology were compared to that in two cases of Reye's syndrome which were selected because they had been treated with salicylates and had comparable serum salicylate concentrations at the time of liver biopsy. Liver biopsy specimens from the cases of salicylate intoxication were nearly normal by light microscopy. Lipid accumulation was minimal, and the content of glycogen and succinic acid dehydrogenase activity was normal. Mitochondria and peroxisomes appeared normal. Light and electron microscopy of liver specimens from the two cases of Reye's syndrome revealed swollen hepatocytes with microvesicular fat and central nuclei. Glycogen content and succinic acid dehydrogenase activity were diminished. All hepatocyte mitochondria were enlarged, pleomorphic, and had an expanded matrix and no mitochondrial dense bodies. The histopathology and ultrastructural pathology of liver biopsy specimens in salicylate intoxication were different from those in Reye's syndrome. In children in whom the diagnosis of Reye's syndrome is obscure, liver biopsy with electron microscopic examination is necessary for definitive diagnosis.

We studied 83 biopsy-proved cases of Grade I Reye's syndrome to determine the outcome, possible clinical or laboratory predictors of progression to deeper coma grades, and hepatic ultrastructural findings. Seventy-eight patients had no change in coma grade during hospitalization, whereas five (6 per cent) had progression to deeper coma grades. All the patients survived without sequelae except one who sustained severe brain damage. The mean (+/- S.E.) level of serum ammonia on admission was significantly higher (P = 0.005) in patients whose disease progressed to deeper neurologic grades (291 +/- 42 micrograms per deciliter) than in those whose disease did not so progress (53 +/- 5 micrograms per deciliter), and the corrected prothrombin time was significantly more prolonged (P = 0.005) in patients with progressing coma (3.9 +/- 0.5 seconds) than in those whose coma grade did not change (1.6 +/- 0.2 seconds). The combination of a prothrombin time 3 seconds or longer than that of the control and a serum level of ammonia on admission of 100 micrograms per deciliter or more correctly predicted progression in 71.5 per cent of the cases (sensitivity, 100 per cent; specificity, 97.6 per cent). Our findings suggest that the prognosis is excellent for survival without sequelae in Grade I Reye's syndrome (98.8 per cent) when management includes hospital surveillance and intravenous glucose and electrolyte infusion.

Since Reye's syndrome is associated with hyperammonemia, we measured the urea-cycle enzymes in hepatic tissue of 13 patients. Expressed as nanomoles of citrulline per milligram of hepatic protein per minute, mean activity of carbamyl phosphate synthetase (6.27 +/- 2.45 S.D.) and ornithine transcarbamylase (136.19 +/- 41.83) in Reye's syndrome was reduced significantly (P less than 0.005) when compared with that of 25 "normal" controls (11.54 +/- 4.24 and 307.49 +/- 94.15, respectively). Activity was maximally reduced during the first days of clinical symptoms; it returned toward normal during the following week regardless of whether the disease ended in death or recovery. The activity of the two enzymes was normal in patients with salicylate intoxication or heritable argininosuccinic acid synthetase deficiency. The apparent Km of hepatic ornithine transcarbamylase for ornithine was in the normal range in patients with Reye's syndrome (mean 0.24 mM). These observations indicate that Reye's syndrome is associated with acquired and transient dysfunction of hepatic mitochondrial urea-cycle enzymes.

Cerebral biopsies were obtained for electron microscopy 48 and 72 hours after the onset of encephalopathy from a child with severe Reye's syndrome. Gravely ill at the time of craniectomy to relieve cerebral hypertension, the child survived and recovered good brain function; therefore, the biopsy findings appear to reflect the organelle pathology of the brain at a severe yet reversible stage in the disease process. The cardinal ultrastructural changes in the brain in Reye's syndrome are astrocyte swelling and partial deglycogenation, myelin bleb formation and universal injury of neuron mitochondria. The mitochondrial injury consists of matrix disruption with moderate but not massive swelling. Dilatation of rough endoplasmic reticulum and nuclear changes occurred only in neurons with severely altered mitochondria. The organelle pathology of the brain in this case did not resemble the organelle pathology of the brain in human "hepatic encephalopathy" or in experimental ammonia intoxication in primates. The mitochondrial ultrastructure of the cerebral neurons resembled the unique mitochondrial ultrastructural changes seen in the liver parenchyma in Reye's syndrome.