Ammonia: what adult neurologists need to know

Risk factors and outcome of hyperammonaemia in people with epilepsy

BACKGROUND

Hyperammonaemia is a recognised complication of antiseizure treatment but risk factors leading to individual patient susceptibility and outcome remain unclear.

OBJECTIVE

To identify risk factors for hyperammonaemia and investigate the impact of its management on clinical outcomes.

METHODS

We carried out a retrospective observational study of adults with epilepsy who had ammonia tested over a 3-year period. Hyperammonaemia was defined as ammonia level > 35 μmol/L. Patients were classified into two groups: hyperammonaemic and non-hyperammonaemic. Association analyses and linear regression analysis were used to identify risk factors for hyperammonaemia.

RESULTS

We reviewed 1002 ammonia requests in total and identified 76 people with epilepsy who had ammonia concentration measured, including 26 with repeated measurements. 59/76 (78%) were found to have hyperammonaemia. There was borderline statistical significance of hyperammonaemia being less common in patients with an established monogenic/metabolic condition than in those with structural or cryptogenic epilepsy (P = 0.05). Drug resistance, exposure to stiripentol and oxcarbazepine were identified as risk factors for hyperammonaemia. We found a dose-dependent association between valproate and hyperammonaemia (P = 0.033). Clinical symptoms were reported in 22/59 (37%) of the hyperammonaemic group. Improved clinical outcomes with concurrent decrease in ammonia concentration were seen in 60% of patients following treatment adjustment.

CONCLUSIONS

Drug resistance and exposure to stiripentol, oxcarbazepine or high-dose valproate are associated with an increased risk of hyperammonaemia. Clinicians should consider symptoms related to hyperammonaemia in patients on high-dose valproate or multiple antiseizure treatments. Prompt identification of hyperammonaemia and subsequent treatment adjustments can lead to improved clinical outcomes.

Late-onset ornithine transcarbamylase deficiency mimicking a focal opercular syndrome

A previously healthy 27-year-old man was brought to hospital after been found late at night confused, agitated and talking incoherently. He represented 12 days later with focal seizures, progressing to anarthria and encephalopathy. MR scan of brain showed diffuse cerebral oedema and his plasma ammonia was >2000 µmol/L (12-55 µmol/L). He developed refractory status epilepticus and subsequently died. Genetic analysis identified an ornithine transcarbamylase (OTC) gene mutation on the X chromosome. We discuss this atypical presentation of OTC deficiency as a rare but treatable cause of hyperammonaemic encephalopathy.