Valproate-induced hyperammonemic encephalopathy versus asymptomatic hyperammonemia in adults with epilepsy: a cross-sectional study
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Background and Objective
To compare clinical features of valproate (VPA)-induced hyperammonemic encephalopathy and asymptomatic hyperammonemia to inform monitoring strategies.
Methods
This cross-sectional study at Affiliated Hospital of Jiangsu University included 25 adults with epilepsy hospitalized between January 2024 and March 2026 with serum ammonia >33 μmol/L during VPA therapy. Nine met predefined VPA-induced hyperammonemic encephalopathy (VHE) criteria; 16 had asymptomatic hyperammonemia. Clinical, laboratory, electroencephalography (EEG), and neuroimaging parameters were compared.
Results
VHE patients had higher ammonia (median 76.0 [IQR 51.2–155.7] vs 42.5 [33.5–66.0] μmol/L; P<0.001) and shorter VPA duration (median 0 [0–60] vs 30 [0–120] months; P=0.005) than asymptomatic patients. Crucially, VPA trough concentrations and liver enzymes did not differ between groups. Diffuse EEG slowing occurred exclusively in VHE patients(66.7% vs 0%; P<0.001); prior cerebral infarction was more frequent in VHE (88.9% vs 43.8%; P=0.040). All VHE patients improved clinically within 1–3 days and achieved ammonia normalization within 3–10 days after VPA withdrawal without specific ammonia-lowering therapy.
Conclusions
VHE typically emerges early in VPA therapy, is unpredictable by routine therapeutic drug monitoring or liver function tests, and resolves rapidly upon discontinuation. Clinicians should maintain a low threshold for ammonia testing in patients with new neurological symptoms during early VPA treatment, particularly those with prior cerebrovascular disease or diffuse EEG slowing. These findings support integrating ammonia screening into VPA safety protocols beyond standard therapeutic drug monitoring.
Key Points
Elevated blood ammonia levels during valproate therapy do not reliably predict the presence of brain-related symptoms, as some patients remain entirely asymptomatic despite significant laboratory abnormalities.
Clinical decisions should be guided by observable changes in mental status and behavior rather than ammonia test results alone, to avoid both unnecessary treatment and delayed recognition of true drug-induced encephalopathy.
Routine monitoring for adults on valproate should pair laboratory testing with regular assessment of cognitive function, ensuring that symptomatic cases are promptly identified while avoiding over-intervention in stable patients.
