Malignant Hyperthermia in Dogs

Malignant hyperthermia is a grim condition in dogs that can lead to muscular diseases, and is more prominent in Greyhounds.  It is an inherited mutation of the dominant genes that results in serious physical reactions due to certain triggers. 

These can range from a reaction to different medications, gaseous anesthetics, caffeine, hops, and any activity level that is too demanding, or when the dog is stressed out; because of this last reason it is also nicknamed “canine stress syndrome”.  

Depending on the seriousness of malignant hyperthermia affecting your dog, the outlook ranges from fair to guarded, though with proper care and treatment, he can be expected to return to a level of living that is comparable to before malignant hyperthermia affected him.

Malignant Hyperthermia, though uncommon, is a life-threatening condition that is due to a genetic mutation, triggered by different sources. It can have a very sudden onset and can escalate quickly, and proving fatal if not caught and treated promptly.

Symptoms can develop quickly and these include:

• The first sign is usually a sudden and dramatic increase of carbon dioxide
• Fever that elevates quickly and stays elevated up to 113F (45C)
• Rapid heartbeat (tachycardia)
• Rapid breathing (tachypnea)
• Cyanosis (the discoloration of skin due to low oxygen levels)
• Rigid muscles 
• Possible seizures
• Changes in blood pressure
• Fluid in the lungs
• Difficulty to form blood clots
• Kidney failure
• Death

Types

The dogs that are affected by malignant hyperthermia have a predisposition to the condition because of a genetic mutation they inherited. The breeds more prone to malignant hyperthermia are:

• Bichon Frises
• Border Collies
• Doberman Pinschers
• German Shepherd/Doberman Pinscher mixes
• Golden Retrievers
• Greyhounds (this is the most commonly affected breed)
• Labrador Retrievers
• Pointers
• Saint Bernards
• Spaniels
• Springer Spaniels

The biggest cause of malignant hyperthermia is from drugs during anesthesia. The drugs that are unsafe for malignant hyperthermia susceptible dogs are:

• Inhaled general anesthetics
• Desflurane
• Enflurane
• Ether
• Halothane (because of its extreme ability to trigger malignant hyperthermia, this drug is not used in western countries anymore)
• Isoflurane
• Methoxyflurane
• Sevoflurane
• Succinylcholine (because of its extreme ability to trigger malignant hyperthermia, this drug has become restricted over time by international anesthesia societies)

Additional causes for malignant hyperthermia include:

• Stress
• Over-exercising 
• Over-stimulation

Malignant hyperthermia is a difficult mutation to diagnose and tests that are minimally invasive and are helping to highlight the susceptibility a dog may have are being created. 

The best way to diagnose malignant hyperthermia is to conduct a muscle biopsy, through means of a procedure called the In Vitro Contracture Test (IVCT) though this can give both false positive, and false negative results equally. The problem with this method is that the dog must be anesthetized for the biopsy to be collected, and most veterinarians will not put a dog that is suspected for malignant hyperthermia under anesthesia because of the risk associated with the drugs used. 

Another means of diagnosis is through a DNA analysis. This will be done using a blood sample collected from your dog and then sent to a diagnostic laboratory where it will be tested for the RYR1 genetic mutation. 

As malignant hyperthermia can have a sudden and unexpected onset in undiagnosed dogs, the clinical tests used to diagnose the condition will be rendered useless. A dog will only be officially diagnosed as malignant hyperthermia susceptible if one of the 31 known mutations are detected in the RYR1 screen. Other tests used in helping to diagnose are:

• Blood bicarbonate of EDTA blood
• Blood pH on EDTA blood
• Blood pressure test
• Serum chemistry
• Urinalysis
• Urine sedimentation
• Electrocardiography
• Halothane-succinylcholine challenge exposure test (this test can result in death of a malignant hyperthermia positive dog, and because of this, is not a test that veterinarians like to use)
• Caffeine induced contraction test on muscle biopsies
• RBC fragility test

For dogs that are having an attack of malignant hyperthermia, there is unfortunately only one treatment option.  It requires an injection of a muscle relaxer called Dantrolene done intravenously, while being kept on fluids through the IV as well. The dog needs to be instantly taken off of the anesthetics, while having his temperature brought back to a normal level and monitoring his breathing. Malignant hyperthermia, even if treated properly and quickly can still sometimes be deadly.

Recovery from an attack of malignant hyperthermia stands a better chance when it is caught early and the treatment is started immediately. The only way to manage malignant hyperthermia, as there is no cure for it, is to have your dog monitored non-stop before, during, and after any procedure requiring an anesthetic, and employing an anesthetic that will not trigger the disease.