Prognosis
Mortality has declined to 20% as a result of improvement in early diagnosis and intensive care management. Death is primarily attributable to neurologic manifestations of the disease (i.e., cerebral edema and consequences of increased intracranial pressure). In earlier studies, patients have been reported to have died from cardiovascular collapse, myocardial dysfunction, respiratory failure, gastrointestinal bleeding, and secondary bacterial sepsis. If the neurologic manifestations of disease are mild or resolve rapidly, visceral function also recovers completely with no residua.
Patients with a higher incidence of neurologic complications include: those <2 years old; those with ammonia levels >45 micrograms/dL; those that show rapid progression from stage 1 to stage 3; those that present with stage 4 or 5; and those with both liver and muscle involvement.[22]
The best predictor for neurologic sequelae is the ammonia level. Only 3% of patients with ammonia levels <45 micrograms/dL will exhibit permanent neurologic sequelae, whereas 11% have neurologic deficits if levels are >45 micrograms/dL.[22] Levels >350 micrograms/dL are associated with increased mortality.[20]
Significance of age on outcome
Published studies have suggested young age as a poor prognostic factor. In the UK, limited data suggest that 70% of survivors over 5 years old will have no residua, whereas disease in younger children seems to have a poorer prognosis.[14]
Neurologic deficits are more likely to become apparent later in life, when developmental capabilities are easier to assess. One small study, which included 18 patients treated from 1979 to 1986 attending normal schools, compared their performance with that of the sibling nearest in age. Ability was significantly reduced if disease occurred during the first 12 months of life compared with presentation at an older age.[28]
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