Makindo Medical Notes

In patients with acute liver failure, elevated blood ammonia levels may have significant prognostic value. In hyperammonaemia, ammonia accumulates in the brain, where astrocytes convert it to glutamine. Although glutamine itself is not toxic, it is osmotically active, and as it accumulates, it causes astrocyte swelling and brain oedema.

Aetiology

• Ammonia (NH₃) is a compound of nitrogen and hydrogen.
• The body breaks down proteins into amino acids and ammonia.
• The liver converts ammonia into urea, which is excreted by the kidneys.

Causes

• Hepatic encephalopathy: Triggered by constipation, infections, gastrointestinal bleeding, electrolyte imbalances, or medications.
• Portosystemic shunt: When blood bypasses the liver, contributing to elevated ammonia levels.
• Medications: Sodium Valproate, Salicylates, total parenteral nutrition (TPN), and steroids can elevate ammonia levels.
• Sampling errors: Prolonged tourniquet application or fist-clenching during blood sampling can falsely elevate ammonia levels.
• Ornithine transcarbamylase deficiency: A genetic disorder affecting the urea cycle, leading to ammonia buildup.
• Reye’s syndrome: A rare condition associated with aspirin use in children, leading to liver damage and hyperammonaemia.
• Urea cycle disorders: These affect the body’s ability to eliminate ammonia, presenting with vomiting, irritability, or seizures in newborns.

Clinical Features

• Forgetfulness, mild confusion, irritability.
• Progression to coma, seizures, cerebral oedema.
• Impaired memory, shortened attention span.
• Sleep-wake disturbances, brain oedema, intracranial hypertension.
• Seizures, ataxia, coma in advanced stages.

Investigations

• Arterial ammonia levels above 200 micrograms/dL are strongly associated with cerebral herniation in acute liver failure.
• Venous blood, if collected and handled properly (in EDTA tubes, transported on ice), can be as reliable as arterial blood for measuring ammonia levels.
• Additional tests: FBC, U&E, LFTs, Prothrombin time, albumin, liver ultrasound, and liver function tests.
• Imaging: EEG for brain function assessment, CT or MRI if needed.

Management

• Provide supportive care: Manage airway, breathing, and circulation (ABC).
• Treat the underlying cause of hyperammonaemia.
• Ensure ammonia samples are sent in EDTA tubes and delivered to the lab on ice.