The International Association for the Study of Pain – the leading global organisation supporting research into pain – has recently amended their definition of pain to “An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage” (Raja et al., 2020). However, what is arguably of more direct relevance to veterinary professionals working with painful patients are the different types of pain (acute versus chronic) encountered on a day-to-day basis.
Why should I be concerned about the difference between acute and chronic pain?
As veterinary professionals dealing with real-life problems, we are all very aware of the presence of acute pain in our patients. Acute pain (Grichnik and Ferrante, 1991):
- Is provoked by a specific disease or injury
- Serves a useful biological purpose
- Is associated with sympathetic nervous system activation
- Is self-limiting
It is a useful and necessary evolutionary trait that allows us to localise and deal with pain, thus limiting further damage.
In contrast, chronic pain is often pathological in nature and serves no useful biological purpose. We have traditionally based the definition of chronic pain on time (typically pain that lasts between three and six months, based on human studies). Arguably, a better definition would be pain that extends beyond the expected period of healing, as the time course of veterinary conditions is usually very different to humans.
Chronic pain states may be difficult to recognise due to their insidious onset, resulting in gradual changes in behaviour demonstrated by patients
Chronic pain states may be difficult to recognise due to their insidious onset, resulting in gradual changes in behaviour demonstrated by patients. These changes can be easily mistaken for other conditions or primary behavioural alterations.
So, where does osteoarthritis (OA) pain fit into this spectrum of disease? OA pain is chronic in nature but appropriate to the disease, especially during “flare-ups”. In this case, it is appropriate to say that OA pain is nociceptive (caused by inflamed or damaged tissue activating nociceptors) despite the chronic nature of the condition. The pain is “appropriate” to the condition, whereas chronic (pathological) pain is neuropathic (caused by damage to or malfunction of the nervous system).
What can cause the switch from “useful” acute pain to detrimental chronic pain?
It is well documented that untreated or poorly managed acute pain can undergo a biological “switch” into a pathological pain state.
Chronic (pathological) pain may arise from:
- Long-term inflammatory conditions, such as osteoarthritis (Brown et al., 2008)
- Neoplasia resulting in persistent pain (Brown et al., 2009)
- Damage to or pressure on neural tissue (Brisson 2010; Plessas et al., 2012) producing neuropathic pain, typically intervertebral disc disease or nerve root tumours
- Idiopathic pathological pain that persists after the original injury has healed. This is a common presentation in humans (Katz and Seltzer, 2009). While it has not been confirmed in veterinary medicine, it is considered to be a possible scenario
It is well documented that untreated or poorly managed acute pain can undergo a biological ‘switch’ into a pathological pain state
The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor situated in the dorsal horn of the spinal cord has a crucial role in determining the strength of synaptic transmission in many pathways. It is the principal receptor responsible for setting our “baseline” pain transmission (acute pain). If painful inputs persist, the release of substance P, tumour necrosis factor and prostaglandins within the dorsal horn, as well as a failure of inhibitory interneurone activity, reduced descending noxious inhibitory control (DNIC) or glial cell activation, may occur. These lead to the activation of the N-methyl D-aspartate (NMDA) receptor in the dorsal horn of the spinal cord, which is one of the major steps leading to pathological (neuropathic) pain states.
Recognising the causes of this switch emphasises that multimodal treatment of chronic pain should involve not only analgesic therapy but also an assessment of the underlying cause to limit these mechanisms causing chronic pain states to develop.
Assessment of chronic pain is often reported to be more difficult than that of acute pain. In addition, chronic pain can be caused by a wide variety of conditions with different mechanisms of producing pain. For example, consider pain in an old dog suffering from osteoarthritis (inflammatory pain) versus pain in a dog with a splenic tumour that is stretching the splenic capsule (“visceral” pain).
Despite the problems associated with assessing chronic pain, several validated scales do exist for assessment of OA pain (White and Hunt, 2019). Although they are not validated for visceral pain, these scales can be useful to guide treatment (Table 1).
|Pain assessment tool||Comments|
|Liverpool Osteoarthritis in Dogs questionnaire (LOAD)||Owner questionnaire of 23 questions using a 5-point Likert scale. Questions cover lifestyle and mobility|
|Canine Brief Pain Inventory (CBPI) ||Owner questionnaire of 11 questions on an 11 point (0 to 10) numerical scale. Questions cover pain severity and interference|
|Helsinki Chronic Pain Index (HCPI)||Owner questionnaire of 11 questions, covering mood and mobility. Dogs experiencing ongoing painful conditions are differentiated from dogs without pain when they score greater than 12|
|American College of Veterinary Surgeons Canine Orthopaedic Index||Owner questionnaire of 16 questions using a 5-point Likert scale. Questions relate to four domains: stiffness, function, gait and quality of life|
|Client-specific outcome measures (CSOM)||Objective tool designed between owner and veterinary surgeon for each dog to assess measures to which the client can relate|
There are no universally accepted validated scales for OA or chronic pain in cats; however, examples of published ones include the Feline Musculoskeletal Pain Index (FMPI) (Benito et al., 2013) and client-specific outcome measures (CSOM) (Lascelles et al., 2007).
Validated quality of life scales, such as the online Vetmetrica HRQOL system, should also be used when dealing with patients experiencing chronic pain, as these recognise the impact of pain on the individual’s daily life.
While acute pain is a necessary evolutionary trait that allows animals to localise pain and prevent further damage, chronic pain is pathological and has no biological purpose. Although chronic (nociceptive) in nature, osteoarthritis pain is appropriate to the disease rather than neuropathic. It is, however, possible for untreated or poorly managed acute pain to undergo a biological “switch” into pathological pain.
Chronic pain can be difficult to diagnose and assess as the signs are often mistaken for other conditions and behavioural changes
Chronic pain can be difficult to diagnose and assess as the signs are often mistaken for other conditions and behavioural changes. However, several validated scales for canine OA and quality of life assessments can be used to guide the treatment of chronic pain.