Hyperammonemia in 2 endurance horses

Wow this is the first I have heard of what is a pretty common problem in hard working horses: becoming depressed during maximal exertion. A number of blood dyscrasias come to mind (dehydration, electrolyte abnormalities, lowered blood ph) but this was not one. The blindness would be an unusual and alarming finding. Fortunately all these problems are treated the same even without a firm laboratory diagnosis: balanced IV fluids and supportive care.
DrO

Resolution of neurologic signs presumed to be associated with hyperammonemia in 2 endurance horses
Can Vet J. 2020 Aug;61(8):860-864.
Authors
C Langdon Fielding 1 , K Gary Magdesian 1 , Jennifer R Mayer 1
Affiliation
1 Loomis Basin Equine Medical Center, 2973 Penryn Road, Penryn, California 95663, USA (Fielding, Mayer); Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, 1 Garrod Drive, Davis, California 95616, USA (Magdesian).

This case report describes 2 endurance horses with non-hepatic hyperammonemia. The animals were competing in a 160-km endurance competition in extreme heat conditions and were presented for obtundation. One of the horses also had evidence of blindness. The blood ammonia concentration was elevated (196 µmol/L and 249 µmol/L) and both horses improved following treatment with intravenous fluids and supportive care. These are the first documented cases of clinical signs presumed to be associated with hyperammonemia in endurance horses. Despite the severity of the clinical presentation, both horses made a full recovery. Key clinical message: Non-hepatic hyperammonemia should be considered as a potential cause of blindness and obtundation in competing endurance horses. Horses appear to respond well to treatment with intravenous fluids.