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Terminology and causes of equine rhabdomyolysis
The terminology used in descriptions of muscle disease has become confusing with a variety of terms still in common usage. Clinical or descriptive terms such as 'Monday morning disease', 'tying up', 'set fast' and 'azoturia' have become replaced by terms that reflect both the pathology found in muscle and our current understanding of molecular disorders leading to that pathology. The terminology is likely to adapt further as other specific disorders become known.
The term myopathy is a generic term that simply refers to muscle disease. Rhabdomyolysis more specifically refers to the breakdown of muscle. However, both terms are often used interchangeably. A useful classification scheme for rhabdomyolysis in adult horses is shown in Box 1 . Some of the disorders included may have limited histopathology consistent with muscle cell lysis, while others show spectacular damage.
Box 1: Classification of rhabdomyolysis in adult horses
Exertional rhabdomyolyses are the most common type seen. Sporadic exertional rhabdomyolysis represents the classic acute case of 'tying up', while chronic exertional rhabdomyolysis should be suspected if patients present with repeated episodes.
The most common and clinically important rhabdomyolyses are described below, while others, including some that are encountered in foals, are listed in Table 1 .
Disorder | Key features |
Glycogen branching enzyme disease | Affects young quarterhorse foals. Is 100 per cent fatal |
Hyperkalaemic periodic paralysis | Is an inherited skeletal muscle defect of the sodium channel. Only affects quarterhorse breeds. Is episodic. Animals will have high potassium levels during an episode |
Immune-mediated myositis | History of Streptococcus equi infection affecting either the individual or the farm. Animals will have concurrent signs of vasculitis |
Ionophore toxicity | History. Affected horses also have cardiovascular signs |
Bacterial myositis | Is focal. Affected animals may have a history of recent intramuscular injection |
Viral myositis | Poorly described in horses. Is possibly due to influenza virus or herpesvirus |
Post-anaesthetic/recumbency myopathy | Affected animals have a history of recent anaesthesia with or without hypotension |
White muscle disease | Is due to vitamin E deficiency. Affects foals and young horses. Is rare |
Myotonia congenita/dystrophy | Affects foals and young horses. Is very rare |
Sporadic exertional rhabdomyolysis
Sporadic exertional rhabdomyolysis is generally associated with over-training or over-exertion above the animal's level of fitness. Alternatively, the condition may be seen in...