Makindo Medical Notes.com

In patients with acute liver failure, a blood ammonia level may have a special prognostic value. In hyperammonaemia states that subsequently lead to elevated ammonia in the brain, astrocytes convert ammonia to glutamine. Glutamine is not toxic, but it is osmotically active, and as it accumulates, it leads to astrocyte swelling and brain oedema.

Causes

• Hepatic encephalopathy: brought on by constipation, infections, GI bleeding, electrolyte problems, or certain medications
• Portosystemic shunt, Sodium Valproate, Salicylates, total parenteral nutrition, gastrointestinal haemorrhage, and steroid use
• Sampling: Prolonged application of a tourniquet or fist-clenching can result in a falsely elevated blood ammonia level
• Ornithine transcarbamylase deficiency, Reyes syndrome: a rare but serious illness that can affect the liver and was due to Aspirin use in kids, which has declined since the 1980s.
• Urea cycle disorder. This affects how your body gets rid of waste made from breaking down protein. In newborns, it presents vomiting, lack of energy, irritability, or seizures.

Aetiology

• Ammonia is a compound of nitrogen and hydrogen with the formula NH3
• The body breaks down foods containing protein into amino acids and ammonia
• Then converting the ammonia into urea which is excreted.

Clinical

• Forgetfulness, mild confusion, and irritability
• Progressive coma, seizures, cerebral oedema
• Impaired memory, shortened attention span
• sleep-wake inversions, brain oedema, intracranial hypertension
• Seizures, ataxia and coma

Investigations: Ammonia sent in EDTA tubes and delivered to the laboratory on ice.

• Arterial ammonia levels higher than 200 microgrammes/dL are strongly associated with cerebral herniation in patients with acute liver failure.
• Venous blood, if appropriately collected, transported in ice, and handled quickly for analysis, has been shown to be as useful as arterial blood in ammonia measurement
• FBC, U&E, LFTs, Prothrombin time, albumin, liver USS, Liver tests
• EEG can help, CT head, MRI head as needed

Management

• ABC and manage the underlying cause
• Ammonia is sent in EDTA tubes that must be delivered to the laboratory on ice.

References