Pain Assessment in Dogs and Cats

Anusha Balakrishnan, BVSc, and Elana Benasutti, CVT

The greatest evil is physical pain.
– Saint Augustine

Pain has long been recognized in human medicine as having a deleterious effect on several organ systems in the body. It has been shown to:

• Amplify the body’s stress response to traumatic injury
• Cause several metabolic and endocrine derangements
• Impede healing and patient recovery.

Definitions

Pain in veterinary patients has been defined as an aversive sensory and emotional experience associated with actual or potential tissue damage.¹ Potential causes of pain include:

• Trauma
• Major or minor surgery
• Acute or chronic inflammatory conditions.

Pain may also be felt in the absence of any identifiable noxious stimulus. In such cases, the pathology may exist in the central nervous system (neuropathic pain) causing the animal to perceive pain that is inappropriately excessive for the degree of inflammation or tissue trauma.

Pain is typically classified as acute or chronic:

• Acute pain is broadly defined as pain that lasts for a short period of time and usually subsides after the inflammatory and healing processes are completed.
• Chronic pain is a more complex phenomenon, and can last much longer than acute pain (up to lifelong).

Recognition and control of acute pain in the early stages is important, as undermanaged acute pain has a higher likelihood of incomplete resolution, therefore, resulting in chronic pain.²

The Communication Gap

The patient’s inability to speak makes the task of proper pain assessment challenging: the patient cannot communicate about:

• Character of pain
• Location or length of pain
• Whether pain exists.

The veterinary professionals responsible for patient care must, therefore, rely upon the word of the animal owner, their own clinical observations, and the various parameters that have been documented to assess pain in patients.

Pain Scales & Management Guidelines

Several pain scoring scales and sets of pain management guidelines have been developed for use in veterinary patients. They can be used for serial monitoring of pain levels in hospitalized patients, which facilitates optimal pain management for an in-clinic patient, despite different personnel caring for that patient throughout its stay.
Obvious limitations to this approach include the fact that these parameters are affected and altered significantly by trauma, surgery, anesthesia, pharmacologic interventions, and the stress of being handled by veterinary personnel.

Acute & Chronic Pain

Some scoring systems and guidelines have been shown to be useful in assessing and quantifying chronic pain, such as pain associated with osteoarthritis in dogs and cats. These systems/guidelines often rely upon questionnaires provided to animal owners and use easily assessable behavioral characteristics, such as activity levels and ability to exercise, to evaluate pain:

• AAHA/AAFP Pain Management Guidelines for Dogs and Cats¹: aahanet.org/publicdocuments/painmanagementguidelines.pdf
• Questionnaire to Measure the Effects of Chronic Pain on Health-Related Quality of Life in Dogs³

A few systems have been developed to assess acute postsurgical pain, which may be more relevant in a hospital setting:

• Subjective and Objective Measures of Postoperative Pain in Cats⁴
• Canine Acute Pain Scale⁵: ivapm.evetsites.net/refId,20468/refDownload.pml (case-sensitive)
• Feline Acute Pain Scale⁵: ivapm.evetsites.net/refId,20467/refDownload.pml (case-sensitive)
• Acute Postsurgical Pain in Dogs and Cats⁶
• Scale to Evaluate Postoperative Pain in Dogs⁷

Pain Grading

Other systems that have been described include:

• Simple Descriptive Scales: Canine Simple Descriptive Pain Scale, ivapm.evetsites.net/refId,20473/refDownload.pml (case-sensitive)
• Visual Rating Scales

These types of scales allow the user to grade pain as none, mild, moderate, or severe. While relatively easy and straightforward to use, these systems may not be very sensitive in distinguishing subtle changes in pain levels.

The Visual Analog Scale (VAS; partnersagainstpain.com/printouts/A7012AS1.pdf) has been widely used in human medicine and has also been evaluated in several veterinary studies.^8-11 The scale consists of a line, most often 100 mm long, with 2 descriptors representing extremes of pain intensity (no pain and extreme pain) at each end. Users make a mark somewhere along the line that represents the pain intensity, and the VAS is scored by measuring the distance from the “no pain” end of the line. While it has been shown to be sensitive and reproducible, a major shortcoming of this scoring system is that it relies heavily on the experience and familiarity of the user with VAS.⁹

Behavior-Based Pain Assessment

A behavior-based acute pain scoring system—the Glasgow Composite Measure Pain Scale (CMPS)—was developed at the University of Glasgow, which takes into consideration several parameters, such as:^12,13

• Demeanor
• Response to people
• Posture, mobility, and activity
• Response to touch
• Attention to painful area
• Vocalization.

This pain scoring system has been well validated for use in veterinary medicine, and has been shown to be reliable and sensitive. Application of scaling models to this score has also enabled the use of this system in clinical research and clinical trials.¹³

A shorter version of the CMPS, the Short Form (CMPS-SF, Figure 1, page 69),¹⁴ was developed in 2007 for the purpose of routine clinical use, with emphasis on speed, ease of use, and guidance for analgesia provision. The CMPS-SF is comprised of 6 behavioral categories with associated descriptive expressions:

1. Vocalization (4 descriptions)
2. Attention to wound (5 descriptions)
3. Mobility (5 descriptions)
4. Response to touch (6 descriptions)
5. Demeanor (5 descriptions)
6. Posture/activity (5 descriptions).

Figure 1. Glasgow Composite Measure Pain Scale–Short Form

Items are placed in increasing order of pain intensity and numbered accordingly. The observer chooses that item within each category that best describes the patient’s behavior and ranked scores are summed; the maximum pain score is 24; 20, if mobility is impossible to assess.

Subjective Pain Assessment

Practical clinical experience is one of the most effective ways to become adept at pain assessment. Signs of pain vary greatly among individual patients, and differences in pain expression may be subtle and difficult to evaluate without sufficient experience or knowledge.¹

Clinical experience is also valuable because some signs of pain must be assessed by touch (tactile versus visual assessment). For example, tenseness of the abdomen is best assessed with gentle palpation.

In the hospital setting, particularly 24-hour facilities where patients may be monitored by several different veterinarians and veterinary technicians, it becomes vital to:

• Discuss pain assessment for each patient among the practice team
• Have the same individual, as often as possible, make serial assessments of a patient’s pain to reduce the possibility of interobserver variability.
• Develop a specific pain assessment protocol that allows all personnel to use the same parameters to assess patient pain.

Visual Pain Assessment

Prior to Examination

• The patient should be evaluated from a distance prior to handling to avoid changes in behavior resulting from human proximity.^15,16
• While observing, the respiratory rate and pattern of breathing should be noted.
• Posture, facial expression, position of ears, and tenseness of body should all be observed as they may change significantly once the caregiver interacts with the patient.

Examination Room

• In an examination room setting, it may be beneficial to observe and examine the patient both with and without the owner present. For example, animals that are frightened or suffer from separation anxiety may be easiest to examine with the owner in the room.
• Pet owners should be asked about their animal’s normal behavior at home and how they have reacted to pain previously. This allows better pain assessment during physical examination and hospital stay.

In the Hospital

• Some hospitalized patients will let down their guard and allow signs of pain to be observed.
• However, there are many that never relax while hospitalized, and subsequently are much more difficult to accurately assess.
• There are also animals that maintain their inherent survival mechanism, which causes them to hide any sign of weakness, including pain. These stoic dogs can appear deceptively relaxed and comfortable, despite being in significant pain.

Physical Examination

A thorough physical examination should be performed after the initial visual examination. However, the importance of physical examinations to assess patient pain must be balanced against the possibility of causing a patient further stress with repeated handling.

• Gentle and compassionate handling of the patient during the examination is vital, particularly if patient pain is anticipated.
• Potentially stressful procedures, such as checking rectal temperature, should be performed at the end of the examination, if possible.
• If the source of pain is already known, the rest of the examination should be completed before assessing it; however, if the source of pain is unknown, potential causes should be determined prior to further examination.

Physiologic Changes
Pain, which causes increased stimulation of the sympathetic nervous system, results in many physiologic changes reflected in physical examination findings. These changes include:²⁰

• Elevated heart rate
• Elevated body temperature
• Elevated respiratory rate +/- increased respiratory effort.

However, these parameters may be altered due to factors, such as anxiety, surgery complications, or clinical conditions (eg, hypotension). If physiologic changes continue once pain is adequately controlled, other causes should be considered.

Tactile Pain Assessment
Several signs of pain can be detected by tactile manipulation. Eliciting pain through palpation of affected body parts may be the only way to recognize that pain exists.

• Most animals will vocalize or show obvious signs of discomfort when the affected area is manipulated.
• Gentle yet thorough abdominal palpation can be extremely useful in detecting focal pain, particularly in conditions, such as pancreatitis, renal pain caused by pyelonephritis or nephroliths, gastrointestinal obstructions, or urethral obstructions.
• Musculoskeletal pain can be detected by a thorough orthopedic examination, including superficial and deep palpation of the muscle and bone, flexion, extension, and range of motion assessment, particularly in the limbs, to identify signs of swelling or joint effusion.

Specific Challenges of Pain Assessment

Due to variability in each animal’s expression of pain, pain assessment in veterinary patients can be very challenging. Signs of pain need to be evaluated in the context of patient history, physical examination, and laboratory findings.

A stoic animal in which pain is suspected but difficult to identify should still be treated appropriately, especially if the disease process or procedure warrants it. Response to therapy can serve as a diagnostic tool for pain assessment, especially if improvement in the patient’s demeanor or posture is noted.

Some signs of pain discussed earlier are subjective and not specific. Several other conditions can cause these signs in patients. Some of the more commonly encountered examples include:

• Elevated heart rate, temperature, blood pressure, or respiratory rate/effort: Caused by a plethora of systemic conditions and the least specific indicators of pain in veterinary patients.^20-24
• Vocalization:
  • Many dogs will whine or whimper quietly in a rhythmic pattern when painful. Loud barking or howling is usually related to anxiety, dysphoria, need to urinate/defecate, or need for attention.
  • However, rhythmic whining may also be noted in, for example, patients that need to have their urinary bladder expressed or those suffering from opioid-induced dysphoria.
  • It can be very difficult to differentiate dysphoria or anxiety from true pain. Often, administering a mild sedative will help resolve dysphoria. Switching analgesics or altering doses may also help distinguish true pain from dysphoria.
  • Dysphoric animals usually do not respond to interaction with caregivers; however, painful animals usually do.
• Trembling: May be caused by pain or anxiety; in the authors’ clinical experience, certain dog breeds, such as Yorkshire terriers or Chihuahuas, have an increased tendency to tremble even when healthy and pain free.
• Pupillary Dilation: Can have multiple etiologies in addition to pain, including various neurologic conditions that involve cranial nerve abnormalities or administration of pharmacologic agents, such as opioids (in cats) or atropine and other mydriatics.

Summary

Accurate pain assessment is becoming a vital prelude to appropriate and adequate pain management. Pain recognition and management, therefore, become essential steps toward our goal of delivering compassionate, humane, and scientifically sound medicine for the animals in our care.

CMPS = Glasgow Composite Measure Pain Scale; CMPS–SF = Glasgow Composite Measure Pain Scale–Short Form; VAS = visual analog scale

Anusha Balakrishnan, BVSc, is a first-year resident in small animal emergency and critical care medicine at University of Pennsylvania School of Veterinary Medicine. Her special interests include sepsis, shock, and coagulopathies. Dr. Balakrishnan received her veterinary degree from Madras Veterinary College (India), followed by a small animal rotating internship at University of Wisconsin–Madison School of Veterinary Medicine.

Elana M. Benasutti, CVT, has been a critical care nurse in the Intensive Care Unit of the University of Pennsylvania Matthew J. Ryan Veterinary Hospital for 10 years. Prior to her current position, Ms. Benasutti was an ultrasound technician in the radiology department. She recently wrote a chapter for Advanced Monitoring and Procedures for Small Animal Emergency and Critical Care, edited by Jamie M. Burkitt and Harold Davis. Ms. Benasutti is a graduate of Harcum Junior College in Bryn Mawr, Pennsylvania. In addition to small animal veterinary medicine, she also pursues an interest in dairy farming.