Overview of Canine Pain
Pain is the unpleasant sensation that develops with the stimulation of specialized sensory nerve endings, called pain receptors. Pain most often develops from damage, irritation or inflammation of tissues or structures of the body. Pain is a protective mechanism. It causes the dog to react and to move away from the source of the stimulus.
Pain receptors are present in many tissues of the body, including the skin, the covering of the bones (periosteum), the walls of arteries, the surfaces of joints, the lining tissues of the chest and abdomen, the cornea and tissues around the eye, and the meninges of the brain and spinal cord.
Types of stimuli that excite the pain receptors include mechanical forces, such as stretching, tearing or fracturing of tissues; thermal stimuli, such as cold or heat; and chemical substances. The chemical substances that stimulate pain receptors are tiny molecules produced in the body when tissues are damaged or inflamed. They include chemicals like serotonin, histamine, prostaglandins, bradykinin, and various enzymes. These chemicals are all mediators of inflammation in the body.
Once a pain receptor is stimulated, information on the pain is transported back to the brain, where the sensation is perceived. Both fast and slow pain fibers exist in the body. The fast transmission of pain signals allows the animal to be rapidly warned about impending damage to some area of the body, and allows them to react quickly. The slow transmission of pain information allows the pain sensation to continue and provides a mechanism for chronic pain to develop.
Pain may be perceived only at the site of the stimulation of the nerve receptors, or may be referred to a nearby area on the body. For example, compression of nerve roots just outside the spinal cord of the neck may result not only in neck pain, but also lameness in the front leg. Pain caused by inflammation within the kidney may be detected as pain along the upper back.
The threshold for perceiving pain is determined by the sensitivity of the pain receptors. Some animals appear to have a higher threshold for pain than others. In general, some breeds of dogs seem to be more stoic with regards to pain than others. In animals, pain thresholds and sensation are difficult to measure because they do not speak. In people various methods have been developed for detecting and measuring pain, but these tests are not often applicable in animals.
Causes of Canine Pain
Any cause of tissue damage or inflammation is potentially capable of causing pain. Examples of such causes include the following:
Trauma – fractures, sprains, dislocations, wounds, muscle tearing, blunt forceful injuries
Exposure to heat – flame, hot water, heating devices
Exposure to extreme cold – frostbite, cold surfaces, cold weather
Tissue inflammation – from infections, from pathologic conditions or diseases, from physical changes
Necrosis of tissue – death of the tissue
Ischemia – loss of blood supply to the tissue
Stretching of tissue – especially of round or hollow organs
Spasm of tissues – especially of muscles or muscular organs
Organs That May Manifest Pain or Become Painful
Joints and ligaments
Skin and soft tissues under the skin
Tissues of the mouth
Portions of the brain and spinal cord
Tissues within and around the eye
Certain components of the ear and ear canals
Certain structures within the chest, particularly the esophagus and tissues lining the chest (pleura)
Many abdominal organs, including those of the gastrointestinal tract, urinary tract, reproductive tract, and the tissues lining the abdomen (peritoneum)
Tissues near the anus and tail
What to Watch For
Manifestations of pain are highly variable in animals. Some signs are obviously related to pain, while others are more subtle. The individual personality of the animal and its tolerance for pain also affect the clinical manifestations of pain. Animals in pain often present with a wide variety of signs. Some signs that are associated with pain include the following:
Altered behavior – quieter than normal, avoidance of other animals or people, hiding, aggressive behavior, fear biting, mental dullness and depression, agitation, restlessness, pacing
Altered movement or gait – lameness, reluctance to move, reluctance to get up, wobbliness, abnormal carriage or use of one or more legs, stiffness
Vocalization – growling, howling, yipping, moaning, groaning
Decreased or lack of appetite (anorexia)
Increased respiratory rate
Increased heart rate
Diagnosis of Pain in Dogs
An important part of evaluating pain is to localize the pain, which involves determining what part of the body is painful. A thorough history and physical examination are performed. These must be done with care in order not to worsen the animal’s pain and to avoid injury to the veterinarian and the veterinary assistants. Depending upon the outcome of the examination, further diagnostic tests may include the following:
Complete eye examination
Complete neurologic examination
Complete orthopedic examination
Thorough oral examination
Thorough examination of the ears
Thorough examination of the external genitalia
Complete blood count (CBC)
Urinalysis and urine culture
X-rays of the chest, abdomen, spine or any area identified as painful
An ultrasound of the abdomen or heart
Cytology and/or biopsy of tissue or fluid
Serologic tests for certain infectious diseases
Bacterial culture of any infected tissues
Cerebral spinal fluid (CSF) tap
Computed tomography (CT scan) or magnetic resonance imaging (MRI)
Treatment of Pain in Dogs
The goals of treatment are to identify the source of pain and remove it, and to use medications to alleviate the pain. Initially the pain must be localized to a specific site, then a diagnosis made as to the cause of the pain. Specific therapy is designed to alleviate the underlying cause.
Alleviation of pain involves the use of analgesics (drugs that numb the pain sensors) and anti-inflammatories. Whenever possible, a diagnosis of the cause of the pain and therapy for that cause should be instituted prior to administration of pain medications. Administration of analgesics and anti-inflammatory drugs as an empirical, symptomatic treatment can be dangerous.
Many different types of analgesic drugs are available for use in dogs. Injectable pain medications that may be used include butorphanol, buprenorphine, hydromorphone, fentanyl, morphine, and oxymorphone. Two oral analgesic drugs that may be used in dogs are butorphanol and Tylenol with codeine.
Anti-inflammatory agents are divided into two categories, steroidal and non-steroidal drugs (NSAIDs). Steroidal agents, such as prednisone and dexamethasone, are reserved for the treatment of certain specific diseases or conditions. Two non-steroidal anti-inflammatory agents that are available in injectable form are flunixin meglumine and ketoprofen. Oral anti-inflammatory agents used in dogs include aspirin, buffered aspirin, carprofen, etodolac, ketoprofen, and deracoxib. The dosages of these medications that are used in dogs are quite different than that used for people, so these drugs should never be given without consulting with your veterinarian. Overdosage with these medications may result in serious illness and side effects.
Supportive care may also be indicated during the period of diagnostic testing and the initiation of therapy. Supportive care may include the use of intravenous fluids, supplemental nutrition, keeping the animal quiet and confined, the use of cold or warm compresses, and altering the temperature of the environment.
Administer any prescribed medication as directed by your veterinarian. Observe your pet’s general activity and appetite, and watch closely for improvement in the signs believed to be associated with the onset of pain. If signs should worsen, contact your veterinarian immediately.