The International Association for the Study of Pain (I.A.S.P.) has defined pain as an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. Pain is unquestionably a sensation in a part or parts of the body. It is always subjective, but it is always unpleasant and therefore, also an emotional experience. Chronic pain is generally recognised as pain that persists for more than three to six months. Myofascial trigger points, a potent source of acute and chronic muscle pain, and chronic pain in general are both poorly diagnosed and treated in veterinary medicine.

Recognition of Chronic Pain in Animals

Veterinarians have long ignored animal pain; although the recognition, alleviation and subsequent prevention of pain in animals have received increased attention in recent years. Veterinarians' ability to manage a patient's pain depends on their ability to adequately evaluate that pain; however, the detection of subtle signs of pain in animals is a skill that veterinarians need to develop. Veterinary undergraduate teaching in pain assessment and management is recognised as inadequate knowledge that practitioners have is typically not acquired in veterinary school. Although many species show distinctive species typical pain behaviours that are generally easy to recognise, acute pain behaviours are better recognised in both animals and humans. Chronic pain behaviours in domestic species are poorly described and more difficult to evaluate, thus, chronic pain in animals is inadequately recognised and treated compared with acute pain. The scant literature describing chronic pain behaviours in companion animals is based in many cases on extensive clinical experience observing animals with pain.

Clinically, lameness in animals is generally accepted as being caused (at least in part) by pain. Lameness in animals can occur in which no abnormality can be identified or final diagnosis reached. Descriptions of animals in chronic pain also include individuals where no obvious cause is present, or in which pain persists after all signs of tissue injury have disappeared. Lameness of unknown origin in animals is frequently categorised as "mystery" lameness). A problem cannot be diagnosed in some animals regardless of the skills of the clinician or technical capabilities of the hospital. Forelimb lameness in adult dogs can be particularly frustrating clinically and it is likely that relatively common causes of lameness remain undescribed.

Chronic lameness of soft tissue origin is very poorly recognised and treated in companion animal species, and, the traditional model of lameness and pain in veterinary medicine is neuro-skeletally biased. Muscles receive little attention as a source of pain or dysfunction in conventional veterinary and medical teaching and texts. Contributing to the confusion, a wide range of known orthopaedic conditions may present without affecting the individual clinically. The severity of change radiographically, and on other forms of imaging exams, is recognised to correlate poorly to pain severity in both animals and humans. Imaging examinations provide an anatomical diagnosis and not a diagnosis of pain.

Behavioural manifestations of pain (pain behaviours) such as lameness, in humans, are thought to be abnormal only when they are not matched by physical disease to a considerable extent. Pain complaints in humans in the absence of clear medical findings, or which persist postoperatively, are believed to be psychogenic or fraudulent, neither of which apply to chronically painful veterinary patients. Behavioural lameness (operantly conditioned) is extremely rare in veterinary medicine and ceases quickly when the animal is separated from the individual that has "trained" the behaviour. Medical findings however, tend to be overweighted in the clinical decision making of human chronic pain patients and pain ratings and disability discounted when medical evidence is lacking. Historically, this is because diagnosis and therapy in medicine have been pathology driven for over 200 years. Wall (2001), though, has suggested "pity those who suffered a disease unaccompanied by an identified pathology" as they are likely to be affected by muscle pain.

Chronic Myofascial Pain

Myofascial pain syndromes (MPS) are the sensory, motor and autonomic symptoms caused by myofascial trigger points (MTrPs). A MTrP is a hyperirritable spot in skeletal muscle that is associated with a hypersensitive palpable nodule in a taut band. The spot is painful on compression and can give rise to characteristic referred pain, referred tenderness, motor dysfunction, and autonomic phenomena. At present, no laboratory test or imaging technique has been established as diagnostic of MTrPs although considerable research provides objective validation of characteristic MTrP phenomena. The characteristic features described for MTrPs in humans are a palpable band, spot tenderness, jump sign, twitch response, muscle weakness, pain recognition, elicited referred pain and tenderness and restricted range of muscle motion. The referred nature (perceived at a distance) of the pain at a MTrP is associated with central sensitisation and a segmental spread within the spinal cord. Inter-rater reliability studies have found good reproducibility of MTrP location, provided the examiners had both training and clinical experience in finding MTrPs.

The recommended minimal criteria for MTrP diagnosis in humans are spot tenderness in a palpable band with subject recognition of the pain. Given the inability of animals to describe their pain, the suggestion that a diagnosis of MTrPs could be made in animals could be easily criticized. Pain has been considered the primary focus in presentation, diagnosis and treatment of human MTrPs hence the name myofascial pain syndrome, however, trigger points have important, and complex, influences on motor function.

Diagnostic Criteria of MTrPs in Animals

Frank (1998) has suggested that animals present a spatial map of their pain in the form of antalgic behaviours which counteract or avoid pain, a posture of gait assumed to lessen pain. She has suggested that skeletal muscle pain and reduced range of motion due to MTrPs leads to both static and dynamic abnormalities. These abnormalities are of posture (standing, sitting and recumbent), movement and function. Observable muscular compensations provide a powerful predictive assessment of MTrP location in the absence of abnormality on standard palpation exam and diagnostic testing. Confirmation of MTrP location, based on a comparison with the human model, has the same criteria excluding pain recognition and elicited referred pain and tenderness. Although unable to confirm referred pain and tenderness in animals, clinical experience suggests active MTrPs are associated with referred pain and tenderness as owners report behaviours (e.g., resent being touched or brushed in an area, foot licking / lick granuloma) which cease during the course of treatment.

The diagnosis of myofascial pain syndromes in animals is based on: detailed history for information suggestive of the presence of MTrPs, pain behaviours and abnormal behaviour for that individual; clinical observation of posture, movement and function; detailed physical examination of the musculoskeletal system; followed by muscle palpation to confirm the diagnostic criteria for the presence of MTrPs.

Identifying Chronic Pain in Animals

Recognition of MTrPs as a source of chronic pain is only one hurdle in the treatment of musculoskeletal pain in animals. The biggest impediment to the optimal treatment of animal pain is suggested to be the difficulty of the clinician in recognising its presence, and requires overcoming personal biases and expectations which may limit its treatment. A major problem occurs in the recognition and subsequent quantification of pain behaviour in animals. Observed behaviour may not reflect accurately the intensity of the pain: a particular behaviour may vary between dogs (e.g., vocalisation may be due to pain, breed variation or the effect of anaesthesia); and different observers may interpret the same behaviour differently. Without training, humans will provide care and grade pain based largely on vocalisations and extreme behavioural manifestations of pain. Owners often comment that their dog "doesn't show pain" or that they "don't think their dog is in pain". On questioning, this is frequently attributed to lack of vocalisation on the dog's part.

Vocalisation/ verbal complaints are considered to be pain behaviours in humans. Vocalisation, although relied on by some veterinarians and owners to signal animal pain, is not a reliable indicator of the presence or absence of pain. However, a very severe transient pain will often elicit a vocalisation especially if the dog is not expecting the stimulus. Some breeds of dog (e.g., Siberian Husky and Alaskan Malamute) may be more likely to vocalise in response to pain. The present author has found that Huskies, Malamutes, Dalmatians and individual dogs of a wide range of breeds which normally vocalise in their daily activities (colloquially referred to as "talkers") will typically vocalise when undergoing palpation/manipulation procedures to localise pain. Caution must be used in order to avoid inadequate treatment of pain in breeds with a reputation of having an exaggerated behavioural response, as well as under treatment of pain in larger working "stoic" breeds that are "able to handle pain".

Owners are best able to perceive the subtle changes of behaviour that may reflect chronic pain. Chronic pain may be harder to recognise than acute pain, due to an insidious onset, varying intensity, and animals' ability to mask pain/discomfort. Pain behaviours may not be recognised by even the most observant owners until an advanced stage. Animals with undiagnosed painful conditions may initially present with non-specific signs (e.g., reduced exercise tolerance, gradually changed behaviour and general lack of enthusiasm for life) that may be attributed to aging unless a complete examination reveals a condition associated with pain. A retrospective assessment of the degree of pain in such chronic circumstances is to observe the change in demeanour after administration of an analgesic agent which will often improve exercise/ activity, performance, mood, and appetite levels back towards normal. Function is suggested by some authors as being a better means of assessing chronic pain than behavioural and physiologic measures. Often the best measure of therapeutic efficacy is the owner's impression of the animal's functionality.

Pain assessment is facilitated by knowledge of both species specific behaviour and behaviour of the individual before and after the onset of pain. The veterinarian assessing normal vs. abnormal behaviour for the clinical management of pain needs to be familiar not only with normal behaviour for a particular species, breed, and ideally, individual animal, but also typical behaviour in hospital and unfamiliar environments. Environment may also influence pain behaviour in animals. Veterinarians regularly encounter owners who have observed abnormal behaviours at home only to have them disappear on presentation to a veterinary clinic. Unfamiliar environments or people may modify behaviour in animals in a species specific way. Thus, in chronic illness or pain, it is suggested that most information about behavioural changes is best elicited by carefully questioning the owners rather than observing the animal's behaviour in the clinic.

The importance of familiarity with the animal's personality in pain assessment and management is such that the owner may be the best person to asses the level of pain/anxiety the animal is experiencing. Although some owners are uncertain as to the level of their animal's pain, they will be acutely aware of abnormal behaviours that suggest the animal is in pain.

Examples of Chronic Pain Behaviours

 Limping, guarding, carrying a limb

 Altered posture/locomotion

 Stiff/stilted movements

 Licking/rubbing the affected area

 Facial expression changes (fixed stare)

 Unsteady gait, alterations of muscle tone

 Reduced activity and mobility levels

 Reluctance to rise/move/jump/run

 Reduced: appetite (+/- wt loss/gain), drinking

 Reduced grooming, socialising, playing

 Sleep loss, irritability

 Change in behaviour to humans/animals

 Change in temperament

Some Common Features: Dogs with MTrP Pain vs. Human Pain Management Programmes

 Many failed treatments

 Multiple health care providers

 Conflicting advice from health care providers

 Lack of specific diagnoses or physical findings

 Pain behaviours

 Anxiety about pain site

 Inactivity

References

1.  Classification of Chronic Pain, Second Edition, IASP Task Force on Taxonomy, edited by H. Merskey and N. Bogduk, IASP Press, Seattle, 1994, pp 209-214.

2.  Dobromylskyj P, et al. Management of Postoperative and Other Acute Pain. In Pain Management in Animals. P. Flecknell and A. Waterman-Pearson eds. W.B. Saunders, London. 2000: 81-146.

3.  Frank EM. Myofascial Trigger Point Diagnostic Criteria in the Dog. In Muscle Pain, Myofascial Pain, and Fibromyalgia: Recent Advances. L. Vecchiet and M.A. Giamberardino eds. The Haworth Press, N.Y. 1999: 231-237.

4.  Hansen B. Through a Glass Darkly: Using Behaviour to Assess Pain. Seminars in Veterinary Medicine and Surgery (Sm Anim) 12 (2) 1997: 61-74.

5.  Hellabreker LJ. Pain in Animals. In Animal Pain. L.J. Hellebrekers ed. Van Der Wees, Utrecht. 2000: 11-16.

6.  Grimm KA. Recognition of Pain in Small Animals. In Veterinary Anaesthesia and Pain Management Secrets. S.A. Greene ed. Hanley and Belfus, Phil. 2002: 327-30.

7.  Hardie EM. Recognition of Pain Behaviour in Animals. In Animal Pain. L.J. Hellebrekers ed. Van Der Wees, Utrecht. 2000: 51-70.

8.  Lamont LA. Management of Chronic Pain. In Veterinary Anaesthesia and Pain Management Secrets. S.A. Greene ed. Hanley and Belfus, Phil. 2002: 349-52.

9.  Mathews KA. ed Management of Pain. Vet Clin N Amer (Sm Anim Prac). 30 (4): 2000.

10. Mense S, Simons DG. eds. Muscle Pain: Understanding Its Nature, Diagnosis, and Treatment. Lippincott Williams and Wilkins, Phil. 2001.

11. Rollins BE. The Ethics of Pain Control in Companion Animals. In Animal Pain. L.J. Hellebrekers ed. Van Der Wees, Utrecht. 2000: 17-38.

12. Simons DG, Travell JG, et al. (1999). Travell and Simons' Myofascial Pain and Dysfunction: The Trigger Point Manual. 2nd ed. Vol 1. Baltimore, Williams and Wilkins.

13. Tranquilli WJ, et al. Pain Management for the Small Animal Practitioner. Teton New Media, Jackson. 2000.

14. Wall PD. In Muscle Pain: Understanding Its Nature, Diagnosis, and Treatment. Mense. S. and D.G. Simons eds. Lippincott Williams and Wilkins, Phil. 2001: v-vi.