Pain scoring systems

The main objective of any pain scoring system is to assess pain as well as analgesic efficacy in patients (Mathews 2000). Pain rating scales used in animals have simply been adopted from scales developed for measurement of pain in humans. The ability to communicate verbally allows reliable estimation of pain intensity using different scales in adult humans (Flecknell 1999). In the case of children who cannot write or communicate verbally objective demonstration of surgical stress coupled with behavioural reactions like crying, facial

expression, posture etc. served as basic criteria for developing infant pain scoring systems (McGrath 1987). Scales designed for acute procedural pain in prelingual children might therefore serve as a fundamental template for acute pain studies in veterinary patients (Hansen 2003).

In veterinary practice, most of the commonly used scales measure the intensity of pain unlike humans in which other dimensions like affect, quality and location of pain

(multidimensional) are assessed (Holton et al. 2001). Pain intensity is a quantitative estimate of the severity or magnitude of perceived pain (Jensen et al. 1986).

Different scales have been used to measure the pain (acute) intensity in small animals including simple descriptive scale, numerical rating scale and visual analogue scale (VAS) (Lascelles et al. 1994; Holton et al. 1998; Slingsby 1998). Recently, the Glasgow composite measure pain scale-short form (CMPS-SF) has been introduced by Reid et al. (2007) for measuring acute postoperative pain in a routine clinical setting, in dogs.

Simple descriptive scale: This scale usually consists of four or five expressions to describe different levels of pain intensity. These expressions are assigned a number, which represents the pain score for that animal. For example, Holton et al. (1998) looking at dogs after surgery used the following scoring system;

The observer has chosen the most suitable descriptive term, which was then converted into a numerical score like, no pain = 0; mild pain = 1; moderate pain = 2; and severe pain = 3.

Advantages: 1. Simple to use

2. Results will not be affected by visual acuity. Disadvantages:

1. Lacks sensitivity due to limited number of pain descriptors that lead to over/under assessment of pain and analgesic efficacy.

2. Inter-observer variability.

Numerical rating scale: This scale consists of multiple categories with different descriptive definitions of pain behaviours in each category and numbers are assigned to each descriptive level. The number chosen by an observer represents the pain score (Holton et al. 1998).

Advantages:

1. Thorough evaluation of the patient due to several categories 2. Easy method of tabulating the score.

Disadvantages:

1. Lack of accuracy as categories are scored by whole numbers that implies equal differences between categories which may not be true

2. Lack of specificity with descriptive pain behaviours

3. It encourages inappropriate statistical analysis since the results are not normally distributed.

Visual analogue scale: This technique is widely used in human medicine (McQuay & Moore 1999) and has been used in various veterinary analgesic studies. Typically, visual analogue scale (VAS) is a 100 mm straight horizontal or vertical line anchored with descriptors of pain intensity, such that 0 represents no pain and 100 represents the worst possible pain, on either end of the line (figure 1). The observer draws an intersecting line along the scale that best represents his/her estimated level of animal pain.

0 100

No pain Worst possible pain

Figure 1: Visual analogue scale for pain scoring

Measuring the length of line from 0 to the mark made in mm i.e. from extreme left to right gives the pain score for the animal. In addition, the VAS can be used to assess the level of dogs’ sedation in the peri-operative period. In this system, the level of sedation will be assessed by observing dogs’ posture, mental alertness, and its ability to stand and walk. At each assessment, a mark will be made on a 100 mm scale, on which 0 corresponds to ‘no sedation’ and 100 corresponds to ‘fast asleep’ (Lascelles et al. 1994).

Advantages:

1. More sensitive in detecting subtle variations as the scale is not limited to some labelled categories of pain

2. Gives a broad sense of whether pain is improving or getting worse in the course of evaluation, and

3. Avoids the use of indefinite descriptive terms and provides many points to select.

Disadvantages:

1. Inter-observer variability when more than one observer evaluates the same animal, 2. Variation due to visual acuity among observers, and

3. VAS scores are not linear, i.e. the difference in pain between each successive increment is not equal.

The Glasgow composite measure pain scale: This pain scale is relatively new and has been derived from the McGill pain questionnaire (Holton et al. 2001). It has been developed based on words and expressions used for describing dogs’ pain behaviour by a number of practising veterinary surgeons. These words and expressions were consolidated into seven behaviour categories namely posture, comfort, vocalization, attention to wound, demeanour, mobility and response to touch. Each category includes different descriptions of a dog’s pain

behaviour. For a quick and reliable assessment of acute pain in a routine clinical setting this composite scale has been shortened as composite measure pain scale- short form (CMPS-SF - appendix). CMPS-SF includes six behavioural categories with 30 pain descriptors. The

descriptors in each category were ranked numerically based on associated pain intensity. Pain score is the sum of the rank scores which was assigned by the observer after picking the best fit descriptor of the dog’s pain within each category.

Advantages:

1. Measures more than one aspect of pain.

2. Does not require specialised skills and experience to use since the pain

descriptors were taken from practising veterinary surgeons routinely observing acute pain behaviour of dogs.

Disadvantages:

1. Validity of the scale has not been tested in clinical trials after elective surgery.