Author Name: Sarah McAbendroth; BSc (Hons) Veterinary Nursing Science
The aim of this literature review is to investigate the reliability of research into differences between postoperative pain in midline and flank approach feline ovariohysterectomy. Feline ovariohysterectomy is a common surgical procedure and has painful side effects. Pain assessment is vital and using pain-scoring systems is a reliable way of assessing pain. Level of expertise impacts postoperative pain in cats, and longer surgical time correlates with lower postoperative pain. Research has found that age influences pain, and kittens are less affected by pain than adult cats. A limitation of research in this area is that cats are individuals, and pain is subjective. Different analgesic protocols affect pain experienced, but Meloxicam and Carprofen have been found to have similar effects. It has been discovered that analgesic administration route has an effect on the strength of postoperative pain relief. Pain scoring systems influence postoperative pain reported because sometimes cat specific behaviours are not taken into account. A DIVAS which is validated for use in postoperative pain in cats is the most valid and reliable way of assessing pain. Non-blinded assessors have been found to generate biased results meaning blinded assessors provide more reliable and valid results. Overall in the research, postoperative pain scores have been found to be higher in cats having undergone flank approach ovariohysterectomy, but the differences in scores between approaches are not significant.
Elective ovariohysterectomy (OHE) is a common surgical procedure performed in small animal practice and has painful side effects (Barratt, 2013; Romagnoli & Sontas, 2010). OHE involves surgical removal of the ovaries and uterus, and can be performed through a left flank or midline approach (May, 2012; Adams et al., 2010).
Pain is a protective reflex and is part of the ‘fight or flight’ reflex (Stevenson, 2012). Pain sensation arises when sensory information is transmitted to the central nervous system from afferent neurons. The process is called nocioception. The pain sensation is detected by sensory receptors and transmitted through the spinal cord into the grey matter, and results in a reflex action. The pain sensation travels up the spinal cord in the white matter to the brain where it is interpreted as conscious pain (Gwynn, 2012; Stevenson, 2012).
When caring for an animal, pain assessment is vital because treatment of pain is an essential part of patient care. The detrimental effects of pain prolong patients’ recovery and return to normal behaviour and function (Gwynne, 2012; Reina, 2011; Goddard & Philips, 2010). In humans, it is easy to score pain because they can self-assess, however pain assessment in animals is challenging because they are unable to verbalise their suffering (Bortolami & Love, 2015; McKune et al., 2015; Crompton, 2014; Gwynne, 2012).
The most reliable way to assess pain in cats is through behavioural observations and interaction with the animal (Bortolami & Love, 2015). A number of pain scoring systems have been designed for animals, including visual analogue scales (VAS), simple descriptive scales (SDS) and multidimensional composite scales (DIVAS). These provide a more objective scale by giving numerical values to behavioural and clinical signs (Gwynne,2012). VAS use a horizontal 100mm long line beginning with ‘no pain’ and ending with ‘worst pain’. The observer marks where they believe the patients’ pain levels fall (McKune et al., 2015; Crompton, 2014). When using SDS, the observer rates pain using a scale of descriptors varying from no pain to severe pain (DeGouff, 2010). DIVAS use physical interactions with the patient in addition to observations of appearance and behavioural responses (Reid et al., 2013). If a patient is identified as experiencing pain, there is ethical obligation to reduce or eliminate the pain (Gwynne, 2012).
2.0 How reliable is research into differences between postoperative pain in midline and flank approach feline ovariohysterectomy
Grint et al. (2006) conducted a study in Bristol, and found evidence to suggest that postoperative pain (POP) levels are higher in cats having undergone flank approach OHE. Similar findings were reported in studies performed in Brazil by Oliveira et al. (2014) and in the UK by Burrow et al. (2006). However, the exact data of results are not published. Oliveira et al. (2014)’s study was performed in Brazil, so results may not be applicable to the UK due to cultural differences, and are not comparable to studies from the UK (Oliveira et al., 2014; McNeill & Chapman, 2005). Burrow et al. (2006), Grint et al. (2006) and Oliveira et al. (2014) based their findings on sample populations of 14-24 cats, which is low compared with Coe et al. (2006) who performed a study in Bristol with 66 cats. A larger sample size means results are more representative of the population (Petrie & Watson, 2013; Khan, 2008). Coe et al. (2006) found contradicting results and claims that cats experience more POP after midline approach, with an average score of 10.7 compared to 9.9 for the flank approach. Differences in findings may be due to varying levels of expertise, more than one surgeon operating, or individual differences between pain scoring. All studies were field experiments rather than laboratory experiments, meaning there are uncontrolled independent variables which may affect results, such as differing levels of stress of cats in the hospital environment (Oliveira et al., 2014; Burrow et al., 2006; Coe et al., 2006; Grint et al., 2006; McNeil & Chapman, 2005). It can be difficult to distinguish whether behavioural changes are caused by fear, anxiety or pain (Crompton, 2014).
2.2 Level of expertise
Level of expertise may influence POP experienced in cats. Final year veterinary students with different experiences of the approaches performed surgery in Coe et al. (2006)’s study. Some students had previous experience, while some did not. The median number of cats spayed by students previously in midline approach was 2 and for those who used flank approach it was 3.5. This may have affected POP experienced by cats due to differences in surgical technique. Students read an account of a standardised method in an attempt to standardise surgery, but more than one student was used meaning there were individual differences between surgery (Khan, 2008; Coe et al., 2006). Grint et al. (2006) used a single final year veterinary student with limited experience of either approach. The student may have had rougher tissue handling than an experienced veterinary surgeon, so results cannot be generalised to surgery performed by experienced vets (Babbie,2009). Burrow et al. (2006) and Oliveira et al. (2014) used one surgeon equally familiar with both approaches. This removed the effect of different surgical experience or abilities so results might have greater reliability.
2.3 Surgical time
Burrow et al. (2006) found that midline surgery took an average of 13.6 minutes compared to flank approach, which took a shorter time of 11.1 minutes. It was discovered that longer surgical time correlated with lower POP scores, and this could be because the surgeon took more care and was less rough during longer surgeries. Conversely, the difference in pain scores could be due to midline approach taking longer, but tending to be less painful due to fewer nociceptors in the linea alba (The Cat Group,2011). There is agreement by Burrow et al. (2006), Coe et al. (2006) and Grint et al. (2006) that midline approach takes longer than flank approach. Oliveira et al. (2014) challenged this idea and found that flank approach had longer surgical time than midline. This may be due to cultural differences because surgery was performed in Brazil. In the UK, surgical procedure is frequently performed through a left flank approach, however midline approach is favoured in other parts of the world (May, 2012; Adams et al., 2010). This may mean the surgeon was more familiar with midline approach. Results can only be generalised to the population that the sample was taken from, so results can only be generalised to Brazil (Petrie & Watson, 2013; Wood & Ross-Kerr, 2011).
Coe et al. (2006) and Grint et al. (2006) included cats aged between 6 months and 10 years, and Oliveira (2014) used cats aged up to 3 years. Polson et al. conducted research in 2012 and 2014 and discovered that kittens are less affected by pain than adult cats. This factor was not considered by Coe et al. (2006), Grint et al. (2006) and Oliveira et al. (2014). Polson et al. (2012) included 100 female cats. 25 were kittens younger than 5 months, and this is a small sample size compared to the 2014 study which used 145 female cats. As sample size increases, validity and reliability increases meaning results from a larger sample size are more representative and a better generalisation of the population (Petrie & Watson, 2013; Khan, 2008; McNeill & Chapman, 2005). In both studies, the specific age when pain experienced changes is unknown and non-specific age groups (kittens aged less than 4 months and adults older than 4 months) were compared rather than exact ages. One experienced veterinary surgeon performed all surgeries in both studies, meaning there were no major differences between the degree of trauma caused by surgery due to differences in technique, however they may have still have been learning and improving their technique. A single blinded observer awarded pain scores, reducing differences that may have occurred with more than one observer, or any bias due to influence of knowledge (Hrobjartssonn et al., 2012). These factors meant that external factors affecting POP were reduced, and results are reliable and valid in determining the difference in pain between cats of different ages.
2.5 Health and upbringing
Researchers have tried to use cats of similar health in studies comparing POP. Coe et al. (2006) used healthy animals (American School of Anaesthesiologists Grade 1) and excluded cats who were pregnant, lactating or in oestrus. Burrow et al. (2006) and Grint et al. (2006) followed this protocol, and cats were client owned from the UK. Results are representative of the sample population, so results are representative of the UK. In contrast, Kiani et al. (2014) used feral/ stray cats from Pakistan. This is a limitation because results are biased to strays in Pakistan, and results are not comparable with studies from the UK (Petrie & Watson,2013). A limitation of all research into POP in cats is that all cats have different upbringings and personalities, meaning cats are individuals and none are the same. Pain is subjective, so minor pain to one individual may be intolerable to another, and this has not been taken into consideration by any studies (Stevenson, 2012).
2.6 Pre-medication, analgesic and anaesthetic protocol
Pre-medication, analgesic and anaesthetic protocol may cause differences in POP due to different combinations having varying levels of pain relief (Crompton, 2014). To allow comparison of studies, similar protocols should be used and no studies were identical. Grint et al. (2006) and Coe et al. (2006) used pre-anaesthetic protocols without analgesic properties, whereas Burrow et al. (2006), Kiani et al. (2014) and Oliveira et al. (2014) used protocols with known postoperative analgesic effects. Cats given protocols containing analgesics may have had more analgesic action, which would have affected POP scores (Grint et al., 2006).
Burrow et al. (2006) administered Meloxicam, whereas Coe et al. (2006) and Grint et al. (2006) used Carprofen. Slingsby and Waterman-Pearson (2002) found that Meloxicam and Carprofen have similar effects and are both effective in relieving signs of pain for over 20 hours, with no differences in pain between both groups at any time. Cats had low POP scores at all times, with median values of less than 20 out of a possible 100. This is an older study, performed more than 10 years ago, but the results were significant findings and there is no more recent relevant research in cats. 80 cats were used which is a good sample size and a blinded single assessor was used to reduce variability of scoring that may have occurred with more assessors (Babbie, 2009; McNeil & Chapman, 2005). An anaesthetic protocol with no analgesia meant the only source of analgesia was Meloxicam or Carprofen. This reduced possible variables that may have affected pain scores and increased validity of the study (McNeill & Chapman,2005). This study shows that POP scores of cats receiving Meloxicam and Carprofen would be similar and comparable (Slingsby & Waterman-Pearson,2002).
Burrow et al. (2006) used an intravenous administration route, while Coe et al. (2006) and Grint et al. (2006) injected analgesics subcutaneously. Previous research by Giordano et al. (2010) compared postoperative analgesic effects of intravenous and subcutaneous Buprenorphine administered to cats undergoing ovariohysterectomy. The study has high reliability, as it was a controlled, randomised, blinded clinical trial with a large sample size of 100 cats (Babbie, 2009; Creswell, 2009). A limitation is the large variety in the age of cats used in the study, ranging from 4 to 72 months old which may have affected pain experienced (Polson et al., 2014). Intravenous administration of Buprenorphine provided better postoperative analgesia than subcutaneous administration. This research suggests that cats receiving analgesia administered intravenously would have better POP relief, and studies using different administration routes can not be compared (Giordano et al., 2010). However, no research specifically compares the administration routes of Meloxicam and Carprofen so it is unknown if results can be generalised to other analgesics.
2.7 Pain scoring system and assessor
Different pain-scoring systems have been used to assess POP in cats, and studies researching POP of OHE are not comparable because they use different criteria for assessment. Coe et al. (2006) asked owners to use a VAS to grade the amount of discomfort their cat had experienced. 62.1 per cent of owners responded and more midline approach owners completed the questionnaire, which means results are not reliable due to poor response rate and bias towards completion by owners of midline approach (Babbie, 2009). Owners who completed the questionnaire may not have fully understood the scale, and VAS is subjective so owners may have interpreted pain differently (DeGouff, 2010).
Hróbjartsson et al. (2012) performed a trial to compare results of blinded and non-blinded assessors, and found that non-blinded assessors generated higher amounts of biased results. This means blinded assessors provide more reliable and valid results.
Single non-blinded assessors used VAS in Grint et al. (2006) and Burrow et al. (2006)’s studies. Burrow et al. (2006) also included SDS. The same assessor was used for all cats, which means possibility of individual differences between pain scoring was minimised (Babbie, 2009). A limitation is that the assessor was aware of the surgical approach so personal opinions may have affected pain score given due to ‘experimenter expectancy effect’. This will have affected reliability and external validity (Hrobjartsson et al., 2012; Creswell, 2009; McNeil & Chapman, 2005). VAS and SDS are used in many species, such as dogs, humans and cats. The systems are not created for cats, so specific pain related behaviours might not be taken into account, and this reduces the validity of research (Crompton, 2014). It has been found that using VAS allows greater sensitivity to detect pain compared to SDS, and SDS is most subjective (DeGouff, 2010).
Oliveira et al. (2014) performed a double blind experiment, eliminating the influence of knowledge of the assessor by clipping and surgically preparing cats for both approaches, and creating two sutures on each cat. A DIVAS, which is validated for use in POP in cats, was used meaning results are valid and reliable producing more consistent and accurate results (Brondani et al.,2013). A limitation is that more than one person carried out pain scoring, so there may have been variation between scores but good inter and intra reliability has been found for the system (Brondani et al., 2013).
2.8 Review of authors
Most studies have been published in well known peer-reviewed journals, such as ‘Journal of Feline Medicine and Surgery’, ‘Journal of Small Animal Practice’ and ‘Veterinary Record’, which means that the results and information is trustworthy. The authors are experts in the field of research, and work occurred in places such as the department of clinical veterinary science in Bristol so information should be reliable due to their high level of knowledge. Oliveira et al.’s 2014 study is from an unknown Brazilian journal, which means that trustworthiness is lower than the other studies however the authors are knowledgeable professors.
Most research into differences between POP in cats undergoing midline or flank approach OHE has found that POP scores are higher in cats having undergone flank approach. However, the differences in pain between both approaches are not significant. All studies are not directly comparable, because none of the studies are identical. Factors such as age and analgesic protocol have influenced POP experienced by the cats and these factors have not been standardised or taken into account. Further research could be performed to determine whether there is a difference in POP, and one surgeon equally familiar with both approaches should be used with a standardised pre medication protocol. A single blinded assessor should use a validated DIVAS to gain results. External factors will always influence the research because cats are individuals and this cannot be eliminated. External factors will influence results because studies are field studies, and there are uncontrollable independent variables such as differences in the level of stress experienced by the cats.
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