Equine Exertional Rhabdomyolysis: Classification, Diagnosis, and Return to Exercise

At a Glance

Equine exertional rhabdomyolysis (ER) is a syndrome of muscle pain, stiffness, and cramping associated with exercise. It is classified as sporadic or recurrent. Sporadic cases are often linked to management factors such as overexertion, electrolyte imbalances, or respiratory disease. Recurrent cases frequently have an underlying myopathy such as polysaccharide storage myopathy (PSSM) or recurrent exertional rhabdomyolysis (RER). Diagnosis requires clinical examination, serum muscle enzyme measurement, and may include muscle biopsy or genetic testing. Return to exercise depends on the underlying cause, resolution of muscle damage, and a structured reconditioning program.

Clinical Presentation and Initial Assessment

Recognizing an Acute Episode

A horse with exertional rhabdomyolysis presents with stiffness, shortened stride, reluctance to move, and firm, painful muscles over the back and hindquarters. The horse may sweat excessively, have an elevated heart rate and respiratory rate, and show signs of distress. In severe cases, the horse may refuse to move, lie down, or exhibit myoglobinuria (dark red or brown urine). The Merck Veterinary Manual describes tying-up as a syndrome of muscle cramping and damage that occurs during or after exercise [4].

The veterinarian must distinguish ER from other causes of acute lameness or colic. Observation of the horse at rest and during a brief walk can help localize the problem to the muscles. Palpation of the epaxial and gluteal muscles reveals firmness and pain. The horse may resist backing or turning.

Immediate First Response

When an acute episode is suspected, stop exercise immediately. Walk the horse slowly if it is willing and able. If the horse refuses to move, allow it to stand quietly in a safe, comfortable stall. Provide fresh water. Do not force the horse to move if it is painful or distressed.

The first-response goal is to reduce muscle damage and pain while awaiting veterinary assessment. The Merck Veterinary Manual advises that mild cases may resolve with rest alone, but veterinary evaluation is needed for moderate to severe episodes [4].

Urgent Veterinary Escalation Criteria

Call a veterinarian immediately if the horse shows any of the following:

• Complete refusal to move or inability to stand
• Severe distress, including uncontrolled sweating, high heart rate, or respiratory distress
• Dark red or brown urine (myoglobinuria)
• Signs of colic or dehydration
• History of recurrent episodes or known myopathy
• No improvement after 30 minutes of rest

The veterinarian will assess the horse, measure serum muscle enzymes, and provide appropriate medical therapy. Do not administer any medications without veterinary guidance.

Classification of Exertional Rhabdomyolysis

Sporadic Exertional Rhabdomyolysis

Sporadic ER occurs in horses without an underlying myopathy. It is triggered by a specific event or management factor. Common triggers include:

• Overexertion beyond the horse's fitness level
• Electrolyte imbalances from heavy sweating or inadequate salt intake
• Respiratory infections or other febrile illness
• Poor conditioning or abrupt increase in exercise intensity
• Hot, humid weather
• Dehydration

The Merck Veterinary Manual notes that sporadic tying-up is often associated with management errors and can be prevented by correcting these factors [4].

Diagnosis of sporadic ER relies on a thorough history, physical examination, and measurement of serum creatine kinase (CK) and aspartate aminotransferase (AST). CK rises within hours of muscle damage and peaks at 4 to 6 hours. AST rises more slowly and remains elevated longer. In sporadic cases, these enzymes return to normal within 7 to 14 days after the episode if the trigger is removed.

Recurrent Exertional Rhabdomyolysis

Recurrent ER describes horses that have repeated episodes of tying-up. These horses often have an underlying myopathy. The two most common recurrent myopathies are polysaccharide storage myopathy (PSSM) and recurrent exertional rhabdomyolysis (RER). The Veterinary Clinics of North America: Equine Practice published a review of muscle conditions affecting sport horses that discusses these conditions in detail [9].

Polysaccharide Storage Myopathy (PSSM)

PSSM is characterized by abnormal accumulation of glycogen and polysaccharide in muscle fibers. It is most common in Quarter Horses, draft breeds, and Warmbloods. A genetic mutation in the GYS1 gene causes type 1 PSSM. Type 2 PSSM has no known genetic cause and is diagnosed by muscle biopsy.

Horses with PSSM often show signs of muscle stiffness, poor performance, and recurrent tying-up. They may be calm and easy to handle but develop episodes after a day or two of rest followed by exercise. The Equine Veterinary Journal published a study on skeletal muscle metabolic response to exercise in horses with tying-up due to PSSM [10].

Diagnosis of PSSM involves:

• Genetic testing for the GYS1 mutation (type 1 PSSM)
• Muscle biopsy with periodic acid-Schiff (PAS) staining to detect abnormal glycogen storage
• Serum muscle enzyme measurement

The Pferdeheilkunde journal published a discussion of PSSM in four cases, highlighting the importance of biopsy and dietary management [12].

Recurrent Exertional Rhabdomyolysis (RER)

RER is a myopathy seen primarily in Thoroughbreds, Standardbreds, and Arabians. It is characterized by abnormal calcium regulation in muscle cells, leading to excessive muscle contraction and damage during exercise. RER is often triggered by excitement, high-grain diets, and intense training.

The Journal of Animal Breeding and Genetics published an estimation of heritability for tying-up syndrome in Thoroughbred racehorses, suggesting a genetic component [11].

Diagnosis of RER is based on:

• Breed and signalment
• History of recurrent episodes triggered by excitement or intense exercise
• Normal muscle enzyme levels between episodes
• Muscle biopsy showing characteristic histopathology (increased glycogen, fiber type changes)
• Exclusion of PSSM through genetic testing or biopsy

Nonexertional Rhabdomyolysis

Not all rhabdomyolysis is exercise-related. The Veterinary Clinics of North America: Equine Practice published a review of nonexertional rhabdomyolysis, which includes causes such as nutritional myopathy (vitamin E or selenium deficiency), toxic myopathy, and infectious myopathy [6]. The veterinarian should consider these differentials when the history does not fit exertional triggers.

Diagnostic Approach

History and Physical Examination

A detailed history is essential for classification. Key questions include:

• How many episodes have occurred?
• What was the horse doing before each episode?
• Is there a pattern related to rest days, diet changes, or weather?
• What is the horse's diet, including grain, hay, and supplements?
• What is the exercise program, including frequency, intensity, and duration?
• Has the horse had any recent illness or injury?
• Is there a family history of tying-up?

Physical examination should include:

• Palpation of epaxial and gluteal muscles for firmness and pain
• Assessment of gait at walk and trot
• Evaluation of hydration status
• Observation of urine color
• Rectal temperature, heart rate, and respiratory rate

Serum Muscle Enzyme Measurement

Serum CK and AST are the primary laboratory tests for diagnosing and monitoring ER. The Merck Veterinary Manual states that CK is the most specific indicator of acute muscle damage [4].

• CK: Rises within 2 to 4 hours of muscle damage, peaks at 4 to 6 hours, and returns to normal within 24 to 48 hours if no further damage occurs.
• AST: Rises within 6 to 12 hours, peaks at 24 to 48 hours, and returns to normal over 7 to 14 days.

Serial measurements help determine if muscle damage is ongoing or resolving. A rising CK after an episode indicates continued muscle damage or a new episode.

Normal resting CK values are typically below 300 U/L, but reference ranges vary by laboratory. Mildly elevated CK (300 to 1,000 U/L) may indicate subclinical muscle damage or recent exercise. Marked elevations (above 10,000 U/L) indicate significant rhabdomyolysis.

Muscle Biopsy

Muscle biopsy is indicated for horses with recurrent ER, persistent enzyme elevation, or suspected myopathy. The biopsy is taken from the gluteal or semimembranosus muscle and processed for histopathology, including PAS staining for glycogen.

The Veterinary Clinics of North America: Equine Practice review on muscle conditions affecting sport horses discusses the role of biopsy in diagnosing PSSM and RER [9].

Biopsy findings in PSSM include:

• Excessive glycogen storage
• PAS-positive polysaccharide inclusions
• Fiber type changes

Biopsy findings in RER include:

• Increased glycogen content
• Fiber type predominance (type 2B fibers)
• No polysaccharide inclusions

Genetic Testing

Genetic testing for the GYS1 mutation is available for type 1 PSSM. This test can be performed on blood or hair samples. A positive result confirms the diagnosis. A negative result does not rule out PSSM type 2, which requires biopsy for diagnosis.

The Merck Veterinary Manual notes that genetic testing is useful for breeding decisions and early diagnosis in at-risk breeds [4].

Additional Diagnostic Tests

Other tests may be indicated based on history and examination:

• Serum electrolyte panel (sodium, potassium, chloride, calcium, magnesium)
• Renal function tests (BUN, creatinine) to assess for myoglobinuric nephropathy
• Urinalysis to detect myoglobin
• Thyroid function tests if metabolic disease is suspected
• Vitamin E and selenium levels if nutritional myopathy is considered

Medical Management of Acute Episodes

Veterinary Medical Therapy

The veterinarian will determine the appropriate medical therapy based on the severity of the episode. Common treatments include:

• Non-steroidal anti-inflammatory drugs (NSAIDs) for pain and inflammation
• Fluid therapy to maintain hydration and support renal function
• Muscle relaxants such as methocarbamol
• Sedatives such as acepromazine or detomidine for severe pain and anxiety
• Electrolyte supplementation if imbalances are present

Do not administer any medications without veterinary guidance. Drug doses and withdrawal periods must be determined by the attending veterinarian.

Stall Rest and Monitoring

After an acute episode, the horse should be confined to a stall with deep bedding. Provide fresh water and hay. Monitor the horse for:

• Willingness to move and eat
• Urine color and output
• Muscle firmness and pain
• Heart rate and respiratory rate

The duration of stall rest depends on the severity of the episode and the rate of enzyme decline. Mild cases may require 3 to 5 days of rest. Severe cases may require 7 to 14 days or longer.

When to Resume Hand Walking

Hand walking can begin when the horse is comfortable, eating well, and serum CK is declining. Start with 5 to 10 minutes of slow walking once or twice daily. Increase duration gradually over several days. Stop hand walking if the horse shows signs of stiffness or pain.

The Merck Veterinary Manual advises that exercise should be resumed gradually and only after muscle enzymes have returned to normal [4].

Dietary Management

Low-Starch, High-Fat Diet for PSSM

Horses with PSSM require a diet low in starch and sugar and high in fat. The goal is to reduce glycogen storage and provide an alternative energy source. Key principles include:

• Replace grain with a low-starch feed or a fat supplement such as vegetable oil or rice bran
• Provide grass hay or low-starch hay (less than 12% non-structural carbohydrates)
• Avoid molasses, sweet feed, and high-starch grains
• Add a vitamin E supplement to support antioxidant status
• Provide a balanced mineral supplement

The Equine Veterinary Journal study on PSSM horses showed that a low-starch, high-fat diet improves metabolic response to exercise [10].

Low-Starch Diet for RER

Horses with RER also benefit from a low-starch diet, but the emphasis is on reducing excitability and muscle calcium flux. Key principles include:

• Replace grain with a low-starch feed or beet pulp
• Provide grass hay
• Avoid high-grain meals that cause insulin spikes
• Add a magnesium supplement to support muscle relaxation
• Provide a balanced electrolyte supplement

Electrolyte Supplementation

All horses with ER should have access to free-choice salt. Horses that sweat heavily may require additional electrolyte supplementation. The Merck Veterinary Manual advises that electrolyte imbalances can trigger tying-up episodes [4].

Provide electrolytes according to the manufacturer's directions. Do not overdose, as excessive sodium can cause problems. Consult a veterinarian or equine nutritionist for specific recommendations.

Exercise Reconditioning

Principles of Return to Exercise

Return to exercise must be gradual and structured. The goals are to rebuild muscle fitness, improve metabolic efficiency, and prevent recurrence. Key principles include:

• Wait until serum CK is normal before starting exercise
• Begin with hand walking, then progress to turnout, then ridden work
• Increase duration before intensity
• Provide daily exercise, even on rest days (turnout or hand walking)
• Avoid abrupt changes in exercise intensity or duration

The Merck Veterinary Manual emphasizes that consistent daily exercise is important for horses with recurrent ER [4].

Phase 1: Hand Walking and Turnout

Duration: 1 to 2 weeks

• Hand walk for 10 to 15 minutes twice daily
• Provide daily turnout in a small paddock if the horse is comfortable
• Monitor for stiffness or pain after exercise
• Measure serum CK weekly to confirm normal levels

Phase 2: Walking and Trotting Under Saddle

Duration: 2 to 4 weeks

• Begin with 15 to 20 minutes of walking under saddle
• Add short periods of trotting (2 to 3 minutes) after the first week
• Increase total ride time to 30 minutes by the end of this phase
• Monitor for signs of muscle soreness after rides
• Measure serum CK if episodes occur

Phase 3: Canter and Conditioned Work

Duration: 4 to 8 weeks

• Add canter work in short intervals (2 to 3 minutes)
• Increase trot and canter duration gradually
• Introduce hill work or interval training as fitness improves
• Monitor for recurrence of stiffness or enzyme elevation
• Adjust diet and exercise based on response

Phase 4: Return to Full Work

Duration: 8 to 12 weeks or longer

• Gradually increase intensity and duration to match the horse's discipline
• Maintain consistent daily exercise
• Continue low-starch diet
• Monitor for early signs of recurrence
• Have a plan for rest days (turnout or hand walking)

Common Failure Patterns

• Returning to full work too quickly after an episode
• Inconsistent exercise schedule with days of rest followed by intense work
• Failure to adjust diet to a low-starch, high-fat formulation
• Ignoring early signs of muscle stiffness or enzyme elevation
• Not providing turnout or hand walking on rest days
• Using high-starch treats or supplements that undermine dietary management

Records and Measurements

Daily Monitoring Log

Keep a daily log for horses with recurrent ER. Record:

• Date and time of exercise
• Type and duration of exercise
• Attitude and willingness to work
• Muscle firmness before and after exercise
• Urine color
• Any signs of stiffness or pain
• Diet and supplements given
• Serum CK and AST results

Serum Enzyme Tracking

Measure serum CK and AST at regular intervals:

• After an acute episode: every 24 to 48 hours until normal
• During reconditioning: weekly for the first month, then monthly
• After any change in exercise or diet: within 24 to 48 hours
• If recurrence is suspected: immediately

Track results in a spreadsheet or notebook to identify patterns.

Body Condition and Weight

Monitor body condition score and weight monthly. Horses on high-fat diets may gain weight if calories are not adjusted. Adjust feed amounts to maintain ideal body condition.

Welfare and Safety Context

Pain and Distress

Exertional rhabdomyolysis causes significant pain and distress. The World Organisation for Animal Health (WOAH) includes animal health and welfare in its standards, emphasizing the importance of preventing and managing painful conditions [5]. Prompt recognition and treatment of ER are essential for welfare.

Myoglobinuric Nephropathy

Severe rhabdomyolysis can lead to myoglobinuric nephropathy, a potentially fatal kidney injury. Myoglobin from damaged muscle cells can obstruct renal tubules. Signs include dark urine, decreased urine output, and elevated BUN and creatinine. Aggressive fluid therapy is needed to prevent kidney damage.

Risk of Recumbency

Horses with severe ER may become recumbent and unable to rise. This is a medical emergency. Recumbent horses are at risk for pressure sores, nerve damage, and colic. Immediate veterinary intervention is required.

Long-Term Management

Horses with recurrent ER require lifelong management. Owners must commit to dietary changes, consistent exercise, and regular monitoring. Without proper management, episodes will recur and may become more severe.

Professional Escalation Criteria

When to Refer to a Specialist

Refer to a veterinary internal medicine specialist or a veterinary neurologist if:

• Recurrent episodes despite appropriate management
• Persistent elevation of muscle enzymes
• Suspicion of an underlying myopathy that cannot be diagnosed with available tests
• Poor response to dietary and exercise changes
• Need for advanced imaging or specialized biopsy interpretation

The American College of Veterinary Internal Medicine (ACVIM) provides a directory of board-certified specialists [3].

When to Consult an Equine Nutritionist

Consult an equine nutritionist for:

• Formulating a low-starch, high-fat diet for PSSM
• Balancing electrolytes and minerals
• Managing weight on a high-fat diet
• Addressing specific dietary needs for performance

When to Consider Genetic Counseling

Genetic testing for PSSM type 1 is available through several laboratories. Positive horses should not be used for breeding if the goal is to eliminate the mutation. Discuss breeding decisions with a veterinarian or genetic counselor.

Practical Decision Framework for Managing Recurrent Exertional Rhabdomyolysis: The Three-Tier Response System

Managing recurrent exertional rhabdomyolysis requires a structured decision framework that helps owners and trainers respond appropriately to different severity levels while tracking progress over time. This section presents a three-tier response system based on clinical signs, muscle enzyme patterns, and historical episode data. The framework is designed to be used alongside veterinary guidance, not as a substitute for professional diagnosis and treatment.

Tier 1: Subclinical or Mild Episode Recognition

Defining Tier 1 Criteria

A Tier 1 event involves subtle signs that may not be immediately recognized as exertional rhabdomyolysis. The horse shows mild stiffness, a slightly shortened stride, or reluctance to move forward during or after exercise. The horse remains willing to eat and drink. Urine color appears normal. Serum creatine kinase (CK) measured within 4 to 6 hours of exercise shows mild elevation between 300 and 1,000 U/L, though reference ranges vary by laboratory.

The Merck Veterinary Manual notes that mild cases of tying-up may resolve with rest alone, but veterinary evaluation is recommended to confirm the diagnosis and rule out underlying causes [4].

Immediate Response Protocol for Tier 1

When Tier 1 signs are observed, implement the following steps:

1. Stop exercise immediately. Do not continue working the horse.
2. Walk the horse slowly for 5 to 10 minutes if it is willing and comfortable. If the horse resists, allow it to stand quietly.
3. Return the horse to its stall with fresh water and hay.
4. Monitor the horse for 30 to 60 minutes. Look for progression of stiffness, sweating, or changes in urine color.
5. If signs resolve within 60 minutes and the horse appears comfortable, resume normal turnout but do not exercise the horse for 24 to 48 hours.
6. Measure serum CK and AST at 4 to 6 hours after exercise to confirm the episode and establish a baseline for recovery.

Documentation Requirements for Tier 1

Record the following information for every Tier 1 event:

• Date and time of exercise
• Type and duration of exercise before signs appeared
• Specific signs observed (stiffness, shortened stride, reluctance to move)
• Ambient temperature and humidity
• Diet and supplements given in the 24 hours before exercise
• Any recent changes in management (new feed, turnout schedule, training intensity)
• Serum CK and AST results with laboratory reference ranges
• Time to resolution of clinical signs

This documentation helps identify patterns that may indicate progression to recurrent disease. The Veterinary Clinics of North America: Equine Practice review on muscle conditions affecting sport horses emphasizes the importance of detailed history in diagnosing recurrent myopathies [9].

When to Escalate from Tier 1 to Tier 2

Escalate to Tier 2 if any of the following occur:

• Two or more Tier 1 events within a 30-day period
• Progression of signs beyond 60 minutes despite rest
• CK elevation above 1,000 U/L
• Any darkening of urine color
• History of recurrent episodes in the same horse
• Known myopathy diagnosis (PSSM or RER)

Tier 2: Moderate Episode Management

Defining Tier 2 Criteria

A Tier 2 event involves clear clinical signs of exertional rhabdomyolysis. The horse shows obvious stiffness, firm and painful muscles over the back and hindquarters, reluctance to move, and may sweat excessively. The horse may refuse to trot or canter. Urine may appear slightly dark or reddish. Serum CK measured at 4 to 6 hours shows elevation between 1,000 and 10,000 U/L.

The Merck Veterinary Manual describes moderate to severe tying-up as requiring veterinary evaluation and treatment [4].

Immediate Response Protocol for Tier 2

When Tier 2 signs are present, implement the following steps:

1. Stop exercise immediately. Do not attempt to walk the horse if it is painful or distressed.
2. Allow the horse to stand quietly in a safe, comfortable location. Provide shade if outdoors.
3. Offer fresh water. Do not force the horse to drink.
4. Call a veterinarian for assessment and treatment. Do not administer any medications without veterinary guidance.
5. While waiting for the veterinarian, monitor the horse's heart rate, respiratory rate, and urine color.
6. If the horse becomes recumbent or shows signs of colic, inform the veterinarian immediately.

Veterinary Medical Therapy for Tier 2

The veterinarian will determine the appropriate medical therapy based on the horse's condition. Common treatments for moderate episodes include:

• Non-steroidal anti-inflammatory drugs (NSAIDs) for pain and inflammation
• Fluid therapy to maintain hydration and support renal function
• Muscle relaxants such as methocarbamol
• Sedatives such as acepromazine or detomidine for severe pain and anxiety
• Electrolyte supplementation if imbalances are present

Drug doses and withdrawal periods must be determined by the attending veterinarian. Do not administer any medications without veterinary guidance.

Stall Rest and Monitoring for Tier 2

After veterinary treatment, confine the horse to a stall with deep bedding. Provide fresh water and hay. Monitor the horse for:

• Willingness to move and eat
• Urine color and output
• Muscle firmness and pain
• Heart rate and respiratory rate
• Signs of colic or dehydration

The duration of stall rest depends on the severity of the episode and the rate of enzyme decline. Moderate cases typically require 5 to 7 days of stall rest before hand walking can begin.

Serum Enzyme Monitoring for Tier 2

Measure serum CK and AST every 24 to 48 hours until values begin to decline. The Merck Veterinary Manual states that CK is the most specific indicator of acute muscle damage and returns to normal within 24 to 48 hours if no further damage occurs [4].

• If CK continues to rise after 24 hours, the episode may be more severe than initially assessed. Escalate to Tier 3.
• If CK begins to decline, continue stall rest and monitor until values approach normal.
• Do not begin hand walking until CK is below 1,000 U/L and the horse is comfortable.

When to Escalate from Tier 2 to Tier 3

Escalate to Tier 3 if any of the following occur:

• CK continues to rise beyond 24 hours after the episode
• CK remains above 10,000 U/L for more than 48 hours
• The horse becomes recumbent or unable to stand
• Dark red or brown urine (myoglobinuria) develops
• Signs of kidney dysfunction (decreased urine output, elevated BUN and creatinine)
• No improvement after 48 hours of veterinary treatment
• History of recurrent episodes despite appropriate management

Tier 3: Severe Episode Emergency Response

Defining Tier 3 Criteria

A Tier 3 event involves severe, life-threatening rhabdomyolysis. The horse shows extreme pain, may be recumbent or unable to stand, has dark red or brown urine, and shows signs of systemic illness including elevated heart rate, respiratory rate, and temperature. Serum CK is typically above 10,000 U/L and may exceed 100,000 U/L. The horse is at risk for myoglobinuric nephropathy, a potentially fatal kidney injury.

The Veterinary Clinics of North America: Equine Practice review of nonexertional rhabdomyolysis discusses the importance of recognizing severe cases that require intensive care [6].

Immediate Response Protocol for Tier 3

When Tier 3 signs are present, implement the following steps:

1. Call a veterinarian immediately. This is a medical emergency.
2. Do not attempt to move the horse if it is recumbent. Protect the horse from injury by removing obstacles from the stall.
3. Provide deep bedding to prevent pressure sores.
4. Offer fresh water if the horse is able to drink.
5. Monitor heart rate, respiratory rate, and urine color while waiting for the veterinarian.
6. Be prepared to transport the horse to a referral hospital if the veterinarian recommends it.

Veterinary Medical Therapy for Tier 3

Severe rhabdomyolysis requires intensive veterinary care. Treatment may include:

• Aggressive intravenous fluid therapy to maintain hydration and support renal function
• High-dose NSAIDs for pain and inflammation
• Muscle relaxants and sedatives
• Diuretics to promote urine output
• Monitoring of renal function, electrolytes, and acid-base status
• Possible referral to a veterinary hospital for continuous care

The Merck Veterinary Manual advises that severe cases of tying-up require intensive care and may be life-threatening [4].

Complications of Severe Rhabdomyolysis

Severe rhabdomyolysis can lead to several complications:

• Myoglobinuric nephropathy: Myoglobin from damaged muscle cells can obstruct renal tubules, leading to acute kidney injury. Signs include dark urine, decreased urine output, and elevated BUN and creatinine.
• Recumbency: Horses that become recumbent are at risk for pressure sores, nerve damage, and colic.
• Compartment syndrome: Severe muscle swelling can compress blood vessels and nerves, leading to tissue death.
• Disseminated intravascular coagulation (DIC): Severe muscle damage can trigger a systemic clotting disorder.

The World Organisation for Animal Health (WOAH) emphasizes the importance of preventing and managing painful conditions in animals, including severe rhabdomyolysis [5].

Long-Term Recovery from Tier 3

Recovery from a severe episode may take weeks to months. The horse will require:

• Extended stall rest until muscle enzymes return to normal
• Gradual reintroduction of hand walking
• Careful monitoring for recurrence
• Dietary modifications based on the underlying cause
• Long-term exercise reconditioning

Horses that survive a severe episode may have permanent muscle damage and reduced performance capacity. The prognosis depends on the underlying cause and the owner's commitment to long-term management.

Implementing the Three-Tier System in Practice

Record Keeping for the Three-Tier System

Maintain a written or electronic log for each horse with recurrent exertional rhabdomyolysis. Record the following for every episode:

• Date and time of episode
• Tier classification (1, 2, or 3)
• Clinical signs observed
• Serum CK and AST results with laboratory reference ranges
• Veterinary treatments administered
• Duration of stall rest
• Date of return to hand walking
• Date of return to full exercise
• Any changes in diet, exercise, or management

The Journal of Animal Breeding and Genetics published an estimation of heritability for tying-up syndrome in Thoroughbred racehorses, highlighting the importance of tracking episodes for breeding decisions [11].

Using the Three-Tier System for Prevention

The three-tier system helps identify horses at risk for recurrent episodes. Horses that experience two or more Tier 1 events within 30 days should be evaluated for underlying myopathy. Horses that progress from Tier 1 to Tier 2 or Tier 3 require immediate veterinary investigation.

The Equine Veterinary Journal study on skeletal muscle metabolic response to exercise in horses with tying-up due to PSSM showed that dietary and exercise management can reduce episode frequency [10].

Common Failure Patterns in the Three-Tier System

• Ignoring Tier 1 signs: Many owners dismiss mild stiffness as normal muscle soreness. This delays diagnosis and allows the condition to progress.
• Returning to exercise too quickly: Horses that resume full work before muscle enzymes return to normal are at high risk for recurrence.
• Inconsistent monitoring: Owners who do not measure serum CK after episodes miss important diagnostic information.
• Failure to escalate: Horses that experience multiple Tier 1 events without veterinary evaluation may develop more severe episodes.
• Inadequate documentation: Without detailed records, it is difficult to identify triggers and patterns.

Integrating the Three-Tier System with Diagnostic Testing

When to Perform Diagnostic Testing Based on Tier

The three-tier system guides the timing and type of diagnostic testing:

• Tier 1 events: Measure serum CK and AST after each episode. If two or more Tier 1 events occur within 30 days, consider genetic testing for PSSM type 1 and muscle biopsy.
• Tier 2 events: Measure serum CK and AST every 24 to 48 hours until normal. Perform genetic testing for PSSM type 1. Consider muscle biopsy if genetic testing is negative or if RER is suspected.
• Tier 3 events: Measure serum CK and AST every 12 to 24 hours until declining. Perform genetic testing and muscle biopsy after the horse is stable. Evaluate renal function with BUN and creatinine.

The Veterinary Clinics of North America: Equine Practice review on muscle conditions affecting sport horses discusses the role of biopsy in diagnosing PSSM and RER [9].

Using the Three-Tier System to Monitor Treatment Response

The three-tier system provides a framework for monitoring response to dietary and exercise management:

• Horses that remain in Tier 1 or have no episodes for 6 months are responding well to management.
• Horses that progress from Tier 1 to Tier 2 despite management changes require adjustment of diet, exercise, or medication.
• Horses that experience Tier 3 episodes despite appropriate management may require referral to a veterinary specialist.

The American College of Veterinary Internal Medicine (ACVIM) provides a directory of board-certified specialists for horses with complex myopathies [3].

Practical Implementation Steps for Owners and Trainers

Step 1: Establish Baseline Values

Measure serum CK and AST at rest for all horses with a history of exertional rhabdomyolysis. Record these values as a baseline. Normal resting CK is typically below 300 U/L, but reference ranges vary by laboratory.

Step 2: Create a Monitoring Schedule

For horses with recurrent episodes, measure serum CK and AST:

• Weekly for the first month after an episode
• Monthly for the next 3 months
• Quarterly for horses that remain episode-free for 6 months
• Immediately after any change in exercise or diet
• If recurrence is suspected

Step 3: Train Staff to Recognize Tier 1 Signs

All staff who handle the horse should be trained to recognize early signs of exertional rhabdomyolysis. These include:

• Mild stiffness after exercise
• Shortened stride
• Reluctance to move forward
• Slight sweating during cool-down
• Firm muscles on palpation

The Merck Veterinary Manual advises that early recognition and treatment improve outcomes [4].

Step 4: Develop a Written Emergency Protocol

Post a written emergency protocol in the barn that includes:

• Contact information for the veterinarian
• Steps for each tier of the response system
• Location of emergency supplies (water, hay, stall with deep bedding)
• Instructions for monitoring and documentation

Step 5: Review and Adjust the Plan Regularly

Review the three-tier response system with the veterinarian every 3 to 6 months. Adjust the plan based on the horse's response to management changes. The Pferdeheilkunde journal discussion of PSSM cases highlights the importance of ongoing management adjustments [12].

Welfare and Safety Considerations in the Three-Tier System

Pain Management

All tiers of exertional rhabdomyolysis involve pain. The World Organisation for Animal Health (WOAH) includes animal health and welfare in its standards, emphasizing the importance of preventing and managing painful conditions [5]. Prompt recognition and treatment of pain are essential for welfare.

• Tier 1: Mild pain may resolve with rest alone. Monitor for progression.
• Tier 2: Moderate pain requires veterinary treatment with NSAIDs and muscle relaxants.
• Tier 3: Severe pain requires intensive veterinary care and may require referral to a hospital.

Risk of Recumbency

Horses in Tier 3 are at high risk for recumbency. Recumbent horses require immediate veterinary intervention. The Merck Veterinary Manual advises that recumbent horses are at risk for pressure sores, nerve damage, and colic [4].

Myoglobinuric Nephropathy

Horses in Tier 2 and Tier 3 are at risk for myoglobinuric nephropathy. Aggressive fluid therapy is needed to prevent kidney damage. Monitor urine color and output closely.

Professional Escalation Criteria for the Three-Tier System

When to Refer to a Veterinary Internal Medicine Specialist

Refer to a veterinary internal medicine specialist if:

• The horse experiences two or more Tier 2 episodes within 6 months despite appropriate management
• The horse experiences any Tier 3 episode
• Diagnostic testing (genetic testing, muscle biopsy) is inconclusive
• The horse does not respond to dietary and exercise changes
• Advanced imaging or specialized biopsy interpretation is needed

The American College of Veterinary Internal Medicine (ACVIM) provides a directory of board-certified specialists [3].

When to Consult an Equine Nutritionist

Consult an equine nutritionist for:

• Formulating a low-starch, high-fat diet for PSSM
• Balancing electrolytes and minerals for horses with RER
• Managing weight on a high-fat diet
• Addressing specific dietary needs for performance

When to Consider Genetic Counseling

Genetic testing for PSSM type 1 is available through several laboratories. Positive horses should not be used for breeding if the goal is to eliminate the mutation. The Journal of Animal Breeding and Genetics study on heritability of tying-up in Thoroughbreds suggests a genetic component that may influence breeding decisions [11].

Summary of the Three-Tier Response System

This three-tier system provides a practical framework for managing recurrent exertional rhabdomyolysis. It helps owners and trainers respond appropriately to different severity levels, track progress over time, and identify horses that require further diagnostic evaluation. The system should be used alongside veterinary guidance, not as a substitute for professional diagnosis and treatment.

Frequently Asked Questions

What is the difference between tying-up and exertional rhabdomyolysis?

Tying-up is a common term for exertional rhabdomyolysis. Both refer to the same syndrome of muscle pain, stiffness, and damage associated with exercise. The Merck Veterinary Manual uses the term tying-up to describe this condition [4].

How long does it take for muscle enzymes to return to normal after an episode?

Serum CK typically returns to normal within 24 to 48 hours if no further muscle damage occurs. AST takes longer, usually 7 to 14 days. Serial measurements help confirm that muscle damage has stopped.

Can a horse with PSSM ever return to full work?

Yes, many horses with PSSM can return to full work with proper management. The key is a low-starch, high-fat diet and consistent daily exercise. The Equine Veterinary Journal study showed that dietary changes improve metabolic response to exercise in PSSM horses [10].

Is exertional rhabdomyolysis hereditary?

There is evidence for a genetic component in some forms. The GYS1 mutation causes type 1 PSSM and is inherited as an autosomal dominant trait. RER in Thoroughbreds has a heritable component, as shown by a study in the Journal of Animal Breeding and Genetics [11].

What should I feed a horse with recurrent tying-up?

Feed a low-starch, high-fat diet. Replace grain with a low-starch feed or fat supplement. Provide grass hay. Avoid molasses and high-starch grains. Add vitamin E and a balanced mineral supplement. Consult a veterinarian or nutritionist for specific recommendations.

Can a horse die from tying-up?

Severe rhabdomyolysis can be life-threatening. Complications include myoglobinuric nephropathy, recumbency, and severe pain. Prompt veterinary treatment is essential. The Merck Veterinary Manual advises that severe cases require intensive care [4].

How can I prevent tying-up episodes?

Prevention depends on the underlying cause. For sporadic cases, correct management factors such as overexertion, electrolyte imbalances, and poor conditioning. For recurrent cases, maintain a low-starch diet, provide consistent daily exercise, and monitor for early signs.

When should I call a veterinarian for a tying-up episode?

Call a veterinarian immediately if the horse cannot move, has dark urine, shows severe distress, or does not improve with rest. Also call if the horse has a history of recurrent episodes or known myopathy. Mild cases may resolve with rest, but veterinary evaluation is recommended.

Related Veterinary Guides

• California Medical
• Medical Certificate
• Diagnostic Medical Sonography Programs
• Clinical Biochemistry Urinalysis
• Veterinary Clinical Methods Procedures Surgical Interventions

References and Further Reading

• aaep.org
• www.merckvetmanual.com
• www.acvim.org
• Merck Veterinary Manual. Merck Veterinary Manual.
• Animal Health and Welfare. World Organisation for Animal Health.
• Nonexertional Rhabdomyolysis.. The Veterinary clinics of North America. Equine practice, 2025.
• Acute rhabdomyolysis ("tying-up") in standardbred horses. A morphological and biochemical study.. Acta veterinaria Scandinavica, 1974.
• Novel markers for tying-up in horses by proteomics analysis of equine muscle biopsies.. Comparative biochemistry and physiology. Part D, Genomics & proteomics, 2010.
• Muscle Conditions Affecting Sport Horses.. The Veterinary clinics of North America. Equine practice, 2018.
• Skeletal muscle metabolic response to exercise in horses with 'tying-up' due to polysaccharide storage myopathy.. Equine veterinary journal, 1999.
• Estimation of heritability for Tying-up syndrome in the Thoroughbred racehorse by Gibbs sampling.. Journal of animal breeding and genetics = Zeitschrift fur Tierzuchtung und Zuchtungsbiologie, 2005.
• Discussion of polysaccharide storage myopathy (PSSM) on four cases. Pferdeheilkunde, 2001.

This article is educational and is not a substitute for veterinary diagnosis or treatment. Contact a veterinarian for advice about an individual animal.