Overview of Syncope (Fainting) in Dogs
The term syncope (or fainting) refers to a brief period of unconsciousness due a lack of blood flow or oxygen to the brain. Syncope causes collapse, which may last from seconds to minutes. Most fainting “spells” are due to low blood pressure or lack of oxygen delivery to the brain (cerebral hypoxia). Syncope is a clinical sign, not a diagnosis or primary form of heart disease.
Blood pressure depends mostly on heart and blood vessel function. Accordingly, disorders of the heart or vessels (cardiovascular system) are the most common causes of syncope. Often, the cause is an electrical disturbance of the heart (slow or irregular heart activity). At other times, there is a structural problem of the heart (such as an abnormality of the heart muscle or a valve) or pericardial disease (the sac around the heart). Recurrent syncopal attacks may cause brain injury.
Other conditions that can lead to syncope include: severe respiratory disease or severe coughing; metabolic (body chemistry) disease; endocrine (hormonal) disorders; dysautonomia (abnormal function of the involuntary nervous system); anemia and drug therapy. The brief event ends with rapid and complete recovery, in most cases.
What To Watch For
Sudden weakness – Syncope often begins with sudden weakness that quickly progresses to ataxia (incoordination); ending with a transient loss of consciousness. Often fainting is precipitated by sudden activity or exercise.
Crying out – Some pets vocalize (cry out) immediately prior to losing consciousness and there may be a “distant” or “glassy-eyed” appearance to the face.
Leg rigidity – The forelimbs may briefly become rigid and the head may be pulled back, causing confusion with a seizure disorder (such as epilepsy).
Incontinence – which is loss of urinary or bowel control.
Diagnosis of Syncope in Dogs
Diagnostic studies should include:
A history (including medication review) and physical examination with an emphasis on stethoscope examination (auscultation) of the heart and lungs.
Exercise test – pre- and post-exercise heart examination
Measurement of blood pressure
Blood tests, including a blood glucose, blood biochemical tests, and complete blood count
Electrocardiogram (EKG) – this can include a routine EKG, an ambulatory (tape-recorded) EKG, or an “event monitor” (an EKG activated by the pet’s owner). The latter two EKG examinations often require referral to a specialist.
A chest X-ray (thoracic radiograph) – especially when indicated from history and physical examination
Pulse oximetry, if there is evidence of lung disease
Heartworm test in appropriate areas
Ultrasound examination of the heart (echocardiogram)
Additional laboratory (blood) tests, such as those evaluating endocrine (hormone) function
Treatment of Syncope in Dogs
The treatment of syncope must be tailored to the underlying cause. In most cases, syncope is an historical complaint, but the cause of the problem must be sought and managed to prevent further occurrences.
Optimal treatment for a pet with syncope requires a combination of home and professional veterinary care. Follow-up can be critical. Administer prescribed medication(s) as directed, and be certain to alert your veterinarian if you are experiencing problems treating your pet. Exact follow-up depends upon the cause.
Prevention of Syncope in Dogs
In general, syncope cannot be prevented unless the precipitating event can be avoided. Try to avoid possible precipitating events, such as excitement or vigorous exercise.
Avoid collars which pull around the neck.
Your veterinarian may also recommend stool softeners or cough suppressants, if needed.
In-depth Information on Syncope in Dogs
Syncope (fainting) is a symptom related to a wide number of disorders.
Causes of Syncope in Dogs
Sometimes the cause is relatively simple, such as moderate to severe anemia, causing inadequate delivery of oxygen to the brain.
Severe respiratory dysfunction or airway obstruction (as with severe tracheal or windpipe collapse) can cause fainting, because the blood oxygen level will fall.
There are some problems with the involuntary (autonomic) nervous system that are difficult to diagnose, which can lead to fainting in pets. In most cases, however, the cause of syncope is traced to cardiovascular disease (abnormal function of the heart or blood vessels).
Heart conditions that may cause syncope include:
Congenital heart defect (birth defects) that obstructs blood flow or prevents the normal movement of blood to the lungs.
Heart failure with inadequate heart action, leading to reduced blood flow (cardiac output).
Pulmonary hypertension (high resistance to blood flow in the lungs) such as that caused by heartworm disease or pulmonary thromboembolism (abnormal blood clot formation in the blood vessels of the lung).
Occasionally, disease of the pericardium (the space around the heart), the heart muscle (cardiomyopathy), the heart valves, or cardiac tumors lead to syncope.
Electrical disturbances of the heart, including pacemaker malfunction (sinus arrest), abnormal electrical impulse conduction (atrioventricular heart block) or excessively fast, abnormal heart rhythms (supraventricular and ventricular tachycardias).
Neurocardiogenic syncope (slow heart rate and abnormal dilation of blood vessels causing low blood pressure) may be precipitated by sudden standing, activity, excitement, urination, or pressure on the neck (hypersensitive carotid sinus).
Pressure on the neck or collar can cause fainting in some sensitive animals (hypersensitive carotid sinus syndrome).
High blood pressure (hypertension) that can lead to stoke or abnormal heart function.
Some heart conditions are especially common in particular breeds, for example:
The Boxer dog, English bulldog, and Doberman pinscher are often affected with abnormal electrical rhythms, such as ventricular tachycardia (a rapid abnormal rhythm originating from the bottom of the heart) or neurocardiogenic syncope (characterized by a slow heart rate and excessive dilation of blood vessels).
The miniature schnauzer, the West Highland white terrier, and the cocker spaniel are breeds especially prone to disease of the heart “pacemaker” (the “sick sinus” syndrome).
The Labrador retriever is predisposed to supraventricular tachycardia (a rapid abnormal rhythm originating in the top chambers of the heart).
Syncope (fainting) must be distinguished from brain dysfunction or weakness related to neurologic disease (such as epilepsy), stroke, neuromuscular disease, orthopedic (bone and joint) diseases, and metabolic disorders such as liver failure. Syncope can occur in any breed and in pets of any age.
As there are dozens of reasons for syncope, your veterinarian must formulate an often-detailed evaluation to make a correct diagnosis. The conditions most often confused with syncope are seizure disorders (epileptic fits), metabolic (body chemistry and hormone) diseases, and disorders of muscle, bone and joints (musculoskeletal diseases).
Epilepsy and other true seizure disorders are electrical disturbances of the brain.
Narcolepsy/cataplexy – these are rare sleep disorders (inappropriate sleep).
Hepatic encephalopathy – a type of abnormal brain function caused by liver disease or blood vessel malformations involving the liver.
Hypocalcemia – low blood calcium causing muscle tremors (shaking), weakness, collapse or seizures.
Hypoglycemia – low blood sugar from metabolic disease, cancer, infection, insulin overdose in a diabetic pet, or the malicious injection of human insulin. This problem is also seen in hunting dogs (fasted before working) and in dogs with tumors of the pancreas (insulinoma) or liver (hepatoma).
Adverse drug reaction – low blood pressure associated with a medication prescribed for a pet (such as a diuretic drug or vasodilator drug prescribed for heart failure).
Illicit drug intoxication – malicious administration (or exposure to) a drug designed for human use in a pet.
Neuromuscular diseases leading to profound weakness or collapse (examples include myotonia congenita, and myasthenia gravis).
Severe hypokalemia – low blood potassium leading to extreme muscular weakness.
Endocrine diseases such as Addison’s disease (hypoadrenocorticism) in dogs may cause exertional syncope.
You can assist in the diagnosis by observing and writing down answers to the following questions:
Can you describe the fainting (syncopal) event, from beginning to end?
What situation(s) precipitate the fainting?
Is there any relationship to rising, exercise or excitement?
What is the total number of events that have been observed?
Does the event occur immediately after a bout of coughing?
What color is the tongue and mucous membranes – pink (normal), white or blue?
Is there any “paddling” of the legs, facial contractions or excessive salivation?
How does your pet behave after the “spell” – is behavior relatively normal or does your pet seem confused or have other abnormal behavior?
Has the problem been diagnosed or treated before? If so, what was the response to treatment? (take any medication bottles to the veterinarian with you).
If possible, feel for your pet’s heart rate during the syncopal event, and try to count the number of beats in 15 seconds.
Be aware of your pet’s general activity, exercise capacity, and interest in the family activities.
Keep a record of your pet’s appetite, ability or inability to breathe comfortably, and note the presence of any symptoms, such as coughing or severe tiring.
If fainting occurs with difficult breathing or persistent blueness (cyanosis) of the gums and tongue, it is probably an emergency. See your vet!
A complete medical history should be obtained and your veterinarian should complete a thorough physical examination with an emphasis on stethoscope examination (auscultation) of the heart and lungs. Medical tests are needed to establish the diagnosis, exclude other diseases, and determine the impact of syncope on your pet. The minimum “database” for syncope varies depending on the duration of signs, presence of systemic signs, and physical examination findings. Recommended tests may include:
Exercise test – pre- and post-exercise heart rate and heart rhythm (+/- blood pressure) to determine if routine exercise incites an incident or changes the heart rhythm.
Measurement of blood pressure, in order to exclude excessively high pressure that can lead to stroke. The blood pressure is measured with a special device.
Blood tests, including a blood glucose, blood biochemical tests, and complete blood count.
An electrocardiogram (EKG) is obtained to identify abnormalities in the electrical activity of the heart (arrhythmias). The electrocardiogram is a noninvasive test done by attaching small contact electrodes to the limbs and body. Abnormal rhythms that can cause syncope can be very slow (bradycardias) or fast (tachyarrhythmias).
A chest X-ray is needed to identify heart enlargement and fluid accumulation in the lung, and exclude some of the previously mentioned lung and respiratory conditions.
Pulse oximetry or tests of blood oxygenation (blood gases) are appropriate if there is evidence of lung disease
Heartworm tests should be done in appropriate areas, in pets not taking preventative measures.
Additional laboratory (blood) tests, such as those evaluating endocrine (hormone) function, may be needed.
Your veterinarian may recommend additional diagnostic tests to insure optimal medical care. These are selected on a case-by-case basis (if indicated from the examination, prior test results, or lack of response). When a syncopal patient does not respond to symptomatic therapy or if a definitive diagnosis has not been attained, other diagnostic tests may be recommended. These tests may include:
An ultrasound examination of the heart (echocardiogram) can demonstrate diseases of the pericardium, heart muscle, heart valves, and show heart tumors. This test is often a referral examination. This noninvasive test requires sophisticated equipment that creates high frequency sound waves, much like the sonar of a submarine. An image of the heart is created.
Some arrhythmias occur very intermittently and may require a prolonged ambulatory EKG (called a Holter monitor) to catch the period of abnormal rhythm. There are also special EKGs called “event” monitors, that can be worn by your pet for weeks. When you observe a spell or faint, you press a button that stores the electrical activity surrounding the event. These tests may require referral to a specialist.
Thyroid studies (rare cases of hypothyroidism have severe bradycardia – a slow heart rate – or lead to atherosclerotic disease in the brain).
ACTH stimulation studies (to rule out Addison’s disease).
Blood ammonia or abdominal ultrasound – if laboratory tests suggest hepatic encephalopathy.
Plasma immunoreactive insulin (with paired blood glucose) to rule out insulinoma in hypoglycemic patients.
Electroencephalogram (after referral if neurologic disease is suspected).
Computerized tomography of the brain (after referral, if neurologic disease is suspected).
Methemoglobin determination (to diagnose this rare condition).
Referral to a specialist for cardiology consultation, neurology consultation and/or internal medicine consultation.
The treatment for syncope depends entirely on the underlying cause.
Therapy for hypotension (low blood pressure) – intravenous (IV) fluids may be indicated, unless the patient has congestive heart failure.
Hypoglycemia. Karo syrup can be put on the gums (at home) and intravenous glucose solution can be given in the hospital.
Cardiovascular syncope. Arrhythmias may be treated with drugs or, in some cases, a pacemaker.
Anemia. Your veterinarian will need to determine the cause and possibly administer a blood transfusion.