Prognosis
Mortality has declined to 20% as a result of improvement in early diagnosis and intensive care management. Death is primarily attributable to neurological manifestations of the disease (i.e., cerebral oedema 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 neurological manifestations of disease are mild or resolve rapidly, visceral function also recovers completely with no residua.
Patients with a higher incidence of neurological complications include: those <2 years old; those with ammonia levels >32 micromol/L (>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.[21]
The best predictor for neurological sequelae is the ammonia level. Only 3% of patients with ammonia levels <32 micromol/L (<45 micrograms/dL) will exhibit permanent neurological sequelae, whereas 11% have neurological deficits if levels are >32 micromol/L (>45 micrograms/dL).[21] Levels >250 micromol/L (>350 micrograms/dL) are associated with increased mortality.[19]
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]
Neurological 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.[27]
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