Postmortem diagnosis of idiopathic hyperammonemia in a horse.

The research discusses a case in which a Quarter Horse stallion was diagnosed with idiopathic hyperammonemia after death. The condition, originating from unknown cause, occurred following abdominal pain after breeding activity and resulted in severe neurologic symptoms requiring the horse to be euthanized. Following death, tests revealed significantly high concentration levels of ammonia in the horse’s plasma, cerebrospinal fluid and aqueous humor, indicating a diagnosis of hyperammonemia and encephalopathy.

Background and Clinical Presentation

• The research revolves around a 6-year-old Quarter Horse stallion that was referred to the Oklahoma State University Veterinary Medical Teaching Hospital due to abdominal pain following breeding activity.
• The horse showed further signs of illness, including diarrhea and worsening neurologic symptoms such as circling, ataxia, and head pressing.
• The horse’s condition rapidly deteriorated, leading to severe self-destructive behaviour and ultimately euthanization during the evaluation.

Investigations and Findings

• Antemortem biochemical and hematologic tests that were undertaken showed hypocalcemia – a deficiency of calcium in the bloodstream but did not show any signs of hepatic disease.
• However, due to the horse’s signs and symptoms, idiopathic hyperammonemia and encephalopathy were suspected.
• 10 hours after euthanization, samples of the horse’s cerebrospinal fluid (CSF) and aqueous humor – the liquid found within the eye – were collected for ammonia analysis using a colorimetric assay.
• The results from the horse in question were compared to those from six other horses that had also been euthanized due to various other diseases and were not suspected of encephalopathy.

Result Analysis and Conclusion

• The ammonia concentrations in the plasma, CSF and aqueous humor samples from the horse were significantly higher than the normal range found in the six other euthanized horses.
• These findings indicated hyperammonemia, which was the suspected cause of the encephalopathy leading to the neurological symptoms and the eventual death of the horse.
• Due to the acute nature of hyperammonemic encephalopathy, there is often insufficient time to make an antemortem diagnosis, and therefore postmortem analysis of CSF and aqueous humor ammonia concentrations have been suggested as a useful alternative for diagnosis of hyperammonemia in horses.