What symptoms may suggest phenylacetate toxicity during glycerol phenylbutyrate treatment when ammonia levels are normal or low?

During glycerol phenylbutyrate treatment, unexplained somnolence, confusion, nausea and lethargy with normal or low ammonia may suggest elevated phenylacetate, particularly in children younger than 2 months; vomiting, headache or sleepiness without high ammonia or another intercurrent illness also warrant assessment.