Recurrent Exertional Rhabdomyolysis
Learn what the condition is, how it may be detected early, how it is treated or managed, and which breeds or species are linked to it.
Condition snapshot
A concise guide to the condition’s pattern, detection, management and urgency.
This snapshot is a general guide, not a diagnosis or treatment plan. New, severe or worsening signs require veterinary assessment.
About the condition
Recurrent exertional rhabdomyolysis, or RER, is a disorder of abnormal muscle contraction that produces repeated tying-up episodes in otherwise fit horses. It is most recognised in Thoroughbreds, Standardbreds and some Arabians, with a likely heritable component in Thoroughbreds. Disturbed regulation of calcium within muscle cells makes contraction unstable, especially during excitement and exercise.
Episodes commonly occur early in work or after the horse has been held back at a slower pace. Affected horses develop a shortened stride, firm painful hindquarter muscles, sweating, anxiety and reluctance to continue. Heart and respiratory rates rise from pain, and severe episodes produce myoglobinuria. Many horses appear normal between events and are not weak at rest.
Triggers include high nervous arousal, interruption of a regular training schedule, stall confinement, high-grain rations, lameness and exercising a fit horse below its accustomed level while excited. Fillies are overrepresented in some racing populations. RER differs from PSSM1: it is not caused by the GYS1 mutation and adding a high-fat diet without controlling calories and excitement does not solve every case.
Sporadic tying-up after excessive work, electrolyte disturbance or illness must be distinguished from a recurrent syndrome. Prognosis for athletic use is often good when training, feeding and temperament triggers are managed, but repeated muscle injury and unsafe handling during episodes can end a career. Each recurrence should prompt review of the programme rather than indefinite pain medication before exercise.
RER reflects abnormal regulation of muscle contraction, especially in fit, nervous Thoroughbreds and related lines. Episodes often occur after excitement or when a horse is held back at exercise, rather than simply after excessive carbohydrate storage. Painful hard muscles, sweating and elevated creatine kinase follow exertion, but the horse may be entirely normal between events.
Screening & Early Detection
Diagnosis combines a repeated compatible history with objective evidence of muscle injury. Creatine kinase is measured soon after an episode and AST is interpreted over subsequent days. Urine is checked for myoglobin, and electrolytes plus kidney values assess severity. Normal enzymes between episodes are expected and do not disprove prior rhabdomyolysis.
A standardised submaximal exercise test with CK before and several hours after work can identify continuing susceptibility and monitor management when performed after recovery. Muscle biopsy may support RER by excluding abnormal polysaccharide storage and other pathology, but findings are interpreted by an experienced laboratory. There is no established single commercial DNA test that diagnoses all RER.
The history should capture pace, restraint, rest days, turnout, feeding, oestrous cycle, excitement, lameness and medications. PSSM1 testing is appropriate in relevant breeds. Cardiac, respiratory and orthopaedic causes of poor performance require separate evaluation. A horse actively tying up is not forced through an exercise challenge; blood and urine are collected while it rests safely.
Treatment & Management
An acute episode is managed by stopping exercise, reducing stimulation and treating pain. Sedation can relieve anxiety and muscle contraction. Fluids are given when dehydration, extensive enzyme release or myoglobinuria threatens the kidneys; diuretics are not used without adequate circulating volume. The horse should not be walked repeatedly or transported while severely painful and stiff.
Prevention centres on calm, consistent daily exercise with minimal stall confinement. Susceptible horses often do best when worked before the yard becomes busy, allowed compatible turnout and not held back while highly excited. Training interruptions are followed by a deliberately gradual return. Underlying lameness is treated so altered movement does not trigger muscle stress.
Diet supplies adequate but not excessive calories, reduces starch and sugar from large grain meals, and uses forage plus fat or fermentable fibre when more energy is required. Electrolytes replace measured sweat losses but are not a cure for RER. Dantrolene or low-dose tranquillisation may be used strategically in selected horses under veterinary and competition rules. Serial CK and episode frequency show whether the plan works. Many racehorses continue performing when environment, schedule and ration are controlled together.
