There are few things more frightening for an owner than watching their horse collapse without warning or apparent cause. By the same token, unraveling the cause behind an equine collapse often presents veterinarians with a diagnostic challenge, and in many cases the root cause of a horse's collapse can't be determined.

At the 2013 American College of Veterinary Internal Medicine Forum, held June 12-15 in Seattle, Wash., Neil P.H. Hudson, MA, VetMB, PhD, DEIM, DipVetClinStud, MRCVS, a senior lecturer at the University of Edinburgh's Royal (Dick) School of Veterinary Studies, reviewed with veterinarians how to diagnose a collapsed horse and listed some common diagnoses.

Hudson said there are several definitions for equine collapse, but noted for the purpose of his lecture he would define collapse as "an animal becoming acutely recumbent (lying down) with or without the loss of consciousness."

Evaluating a Collapsed Horse

There are two common scenarios a veterinarian might encounter when evaluating a collapsed horse: an emergency setting immediately following collapse or a visit to a horse presenting with a history of episodic collapse.

"In the first instance, the practitioner may be presented with a situation that requires emergency care to stabilize the horse," Hudson said. Above all, he stressed human and equine patient safety as the priority when working with a recumbent or collapsing horse.

When presented with a collapsed horse, the veterinarian will perform first aid and, if needed, work to manage any uncontrolled bleeding, Hudson said. He or she might also need to support the horse's cardiovascular system with fluid therapy, attend to fractures, or administer drugs to control a seizure, he said. Again, he stressed, safety is paramount.

"In an acute setting, obtaining a quick history and performing a clinical examination as best you can are important," Hudson said. "After stabilization and initial examinations, further ancillary tests and treatments can be instituted depending on the detected problems."

Information practitioners should obtain when gathering a detailed history of the horse include:

  • How often and how long episodes of collapse occur and last;
  • What type of environment the horse was in when episodes took place;
  • Any recent management changes;
  • A description of events that took place before, during, and after the episode;
  • Whether the horse was conscious during the collapse;
  • If any other horses experienced similar episodes;
  • If the horse was on any medications at the time of the episode; and
  • Whether the horse acts "normal" between episodes.

"In many cases, the history can be vague or confusing," he cautioned. "It may be helpful to ask the owner to keep a diary and, if possible and safe to do so, to video events."

Hudson said numerous problems affecting many different body systems can lead to collapse. To ensure a veterinarian evaluates all body systems properly, he recommended including the following steps in an examination:

  • Pay special attention to the cardiovascular and nervous systems (two body systems often implicated in causing collapse);
  • Perform a neurologic exam, if possible;
  • Perform ancillary cardiologic exams—such as echocardiographs and electrocardiograms—if the veterinarian suspects a cardiovascular cause;
  • If possible and safe, include exercise testing in the examination;
  • If the veterinarian suspects systemic or metabolic causes, include blood tests and serum biochemical analyses; and
  • Consider radiography, computed tomography, or MRI to obtain additional information.

Once diagnostic tests are complete, the veterinarian can try to determine what caused the horse to collapse.

Common Causes

"Frustratingly, in many cases of collapse a definitive diagnosis is not reached," Hudson said.

He cited one recent study in which researchers evaluated 44 horses presented for cardiologic evaluation after collapse; veterinarians weren't able to determine the cause of collapse in 18% of those horses. In another recent study of 25 horses, veterinarians determined a definitive cause of collapse in 11 cases, a presumptive diagnosis in eight cases, and no cause in the remaining six cases. And finally, he noted, in a postmortem study of 268 racehorses that collapsed and died either during work or within an hour of working, veterinarians weren't able to determine what caused the collapse in 22% of cases.

"Therefore, it can be seen that in a significant proportion of cases of equine collapse, determining the definitive case remains elusive," Hudson said.

Hudson reviewed several common causes of collapse with the veterinary audience.

Syncope—Simply put, syncope is the term used to describe fainting caused by a drop in arterial blood pressure leading to insufficient blood flow to the brain, and the disorder remains poorly understood in horses. Syncope's root is generally in a malfunction in the cardiovascular system, Hudson said, and "syncope in the absence of cardiac failure is rare."

Hudson said during a recent study researchers found that syncope was either the definitive or presumed cause of 12 of 25 equine collapse cases.

He cautioned that differentiating syncope from both seizures and sleep disorders can be difficult.

Seizures—Hudson said equine seizures can manifest as partial (focused in a localized area) or generalized (all-over) events, and can be status epilepticus (a life-threatening condition in which the brain is in a state of persistent seizure). He said repeated seizures are most commonly seen in foals with an underlying disease process (such as sepsis) and in some cases can have a genetic basis. Seizures in adult horses are largely considered acquired, Hudson said, and generally are associated with an underlying disease process.

Hudson cited one recent study in which researchers evaluated 104 horses with histories of seizures and improved the classification system for seizures in equine medicine. Of the 73 horses described as epileptic (having two or more recurrent seizures) in the study, Hudson said, "35.6% were classified as symptomatic (with an underlying identifiable brain pathology), 54.8% were classified as cryptogenic (unknown etiology), and 2.7% as idiopathic (suspected genetic predisposition)."

Sleep Disorders—Equine sleep disorders can also cause a horse to collapse, although they're slightly different than what most people are familiar with in humans.

"In humans, narcolepsy is characterized by chronic sleepiness, a marked disorganization of sleep/wake behavior, and pathological manifestations of rapid eye movement (REM) sleep," Hudson explained, noting that narcolepsy is sometimes, but not always, associated with cataplexy (a sudden but usually brief loss of muscle tone).

Hudson said that while narcolepsy with cataplexy can occur in foals and some pony breeds, sleep attacks (e.g., excessive drowsiness, buckling at the knees, stumbling, and even total collapse) in adult horses are less likely to represent true narcolepsy.

"There is a certain degree of debate regarding nomenclature, but it is possible that some sleep attacks seen in adult horses may be a form of idiopathic hypersomnia (sleep attack) associated with chronic sleep deprivation," Hudson said.

Other Causes—Finally, Hudson touched briefly on three other potential causes of collapse:

  • Coma—Hudson said coma can be caused by central nervous system disorder, including trauma, bacterial infection, parasites, or liver disease.
  • Loss of motor function—He said trauma, botulism, myasthenic syndromes, and hyperkalemic periodic paralysis (commonly known as HYPP) can all cause a loss of motor function.
  • Generalized or metabolic causes—Shock, hypoglycemia, electrolyte abnormalities, endotoxemia, and anaphalaxis can all cause collapse, Hudson said.

Take-Home Message

Equine collapse frequently presents a diagnostic challenge for veterinarians, and many cases go unsolved. However, in some cases practitioners can pinpoint the definitive cause and treat the horse accordingly.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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