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Pain relief for strabismus surgery in children: a randomised controlled study of the use of preoperative sub-Tenon levobupivacaine
  1. B Morris1,
  2. P Watts1,
  3. T Zatman2,
  4. M Absolom2,
  5. S Haider1,
  6. J Hall2
  1. 1
    Department of Ophthalmology, University Hospital of Wales, Cardiff, UK
  2. 2
    Department of Anaesthesia, University Hospital of Wales, Cardiff, UK
  1. Mr P Watts, Department of Ophthalmology, University Hospital of Wales, Cardiff CF14 4XW, UK; patrick.watts{at}cardiffandvale.wales.nhs.uk

Abstract

Aims: To compare the postoperative pain scores in children undergoing squint surgery administered with preoperative sub-Tenon levobupivacaine for postoperative pain relief versus controls.

Methods: A prospective randomised controlled clinical trial was performed. Children aged 1–16 years undergoing strabismus surgery were recruited. The test group received sub-Tenon levobupivacaine preoperatively and topical anaesthetic eye-drops at the end of the procedure. The control group received topical anaesthetic eye-drops only at the end of surgery. Pain scores were recorded at 30 min, 2, 4, 6 and 24 h postoperatively using the Wong–Baker Pain or FLACC (face, legs, arms, cry, consolability) assessment score. The principal outcome measured was the pain score at each time interval for both groups.

Results: 27 patients received sub-Tenon levobupivacaine, and there were 27 age- and sex-matched controls. The pain score in the test group was not lower than that of the control group (p = 0.22 at 30 min, p = 0.37 at 2 h).

Conclusions: Sub-Tenon levobupivacaine, which is widely used for postoperative pain relief in paediatric strabismus surgery, was not effective when administered preoperatively in this cohort.

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Pain following strabismus surgery is common, with up to 65% of patients experiencing significant pain.1 Up to one-third of surgical patients have moderate-to-severe postoperative pain as a result of inadequate analgesia.2 The undertreatment of postoperative pain is still one of the commonest errors in pain management in children.3 4 More than half of children undergoing day case surgery have clinically significant pain after discharge.1 5 In one study of children undergoing minor surgery, only 17.5% of parents remembered receiving specific instructions for regular medication, and 42% of parents felt that pain medication should be used as a last resort.1 Paediatric strabismus surgery is increasingly carried out as a day case procedure, and parents are expected to manage the child’s pain at home without clinical support. Pain in children can be difficult to assess and pain scales have been developed to assist health professionals in the management of pain. These scales may be based on a simple self-reporting scale (children usually 6 years and older)6 or based on observations of behaviour and features of facial expression (younger children).7 8 The Wong–Baker7 9 and FLACC8 (face, legs, arms, cry, consolability) are two such scales. Previous studies have shown a good correlation between parents and doctors using both objective scoring systems and visual analogue scales.10

Nausea and vomiting are also common after squint surgery, occurring in 18% to 54% of patients.11 12 Postoperative pain appears to be a clear predictor of nausea and vomiting in children.4 Emesis renders oral analgesia inappropriate, and alternatives are not very acceptable in children. Pain, along with postoperative nausea and vomiting, is the main reason for delayed discharge after day surgery as well as contact with the hospital after discharge.2 It is also the major cause of unanticipated hospital admission13 and thus increased costs.

Regional anaesthesia has recently gained favour as an adjunct to systemic analgesia administration for pain relief. Regional anaesthesia used in conjunction with a general anaesthetic gives smoother control of pain and, if given preoperatively, decreases general anaesthetic requirements facilitating faster postoperative recovery, less nausea and vomiting, faster alimentation and discharge.2 5 Postoperative topical tetracaine, sub-Tenon lignocaine and subconjunctival bupivacaine have all been reported to successfully reduce postoperative pain in children </emph>after squint surgery.1417 However, other investigators reported no reduction in pain with the same agents.11 18 We chose to investigate whether sub-Tenon levobupivacaine would provide more effective analgesia than agents previously administered preoperatively.14

PATIENTS AND METHODS

A prospective randomised controlled clinical trial was carried out on children undergoing squint surgery. Approval was granted by the local Ethics Committee and the Research and Development department. Subjects were recruited at the time of their preoperative assessment 1 week prior to the scheduled surgery when the parent or guardian gave informed written and verbal consent. All children aged 1–16 years undergoing squint surgery were invited to take part. Exclusion criteria included allergy to the agents involved, prior squint surgery, learning disorder or development delay. Randomisation was assigned using a locally developed computer-generated program. Patients received either a single injection of levobupivacaine 0.75% (7.5 mg/ml) (Abbott Laboratories, Maidenhead, UK) into the sub-Tenon space preoperatively and topical tetracaine 1% drops into the inferior fornix at the end of the operation (Group 1, subjects) or topical tetracaine 1% only at the end of the operation (Group 2, controls).

The sub-Tenon injection was given after the child was anaesthetised but prior to surgery. The levobupivacaine was warmed to 37.5°C in a theatre warming cabinet and administered in the inferonasal quadrant by the surgeon via a sub-Tenon cannula (fig 1). Where both eyes were being operated on, bilateral blocks were administered. The dose of levobupivacine was titrated according to the child’s body weight to ensure a subtoxic dose of less than 2.5 mg/kg19 (children weighing under 30 kg received 1 ml, children weighing 30–40 kg received 2 ml, and children weighing >40 kg received 3 ml). The efficacy of the block was judged satisfactory if the pupil was widely dilated and fixed, thus confirming ciliary ganglion blockade (fig 1). Both groups received anaesthesia according to anaesthetist preference, with either propofol alone or propofol and remifentanil intravenous induction or sevoflurane inhalational induction. The patients were intubated or had a laryngeal mask airway inserted before being ventilated. Surgery commenced 5 min after the block was given, and all operations were carried out by a single surgeon (PW). A limbal incision was used in all patients and extended to the site of the sub-Tenon block, thus utilising the incision made earlier. All children receiving sub-Tenon levobupivacaine had surgery carried out on the medial rectus. Standard analgesia was administered to all patients peroperatively using rectal diclofenac (0.3 mg/kg) and paracetamol (40 mg/kg as a loading dose). Thereafter, analgesia was prescribed on an “as required” basis and consisted of paracetamol oral suspension (20 mg/kg) or dihydrocodeine phosphate oral suspension (1 mg/kg).

Figure 1 Method of sub-Tenon anaesthesia. Levobupivacaine is injected inferonasally after careful dissection. Pupil dilatation confirms ciliary ganglion blockade and effectiveness of the block.

Pain scores were recorded at 30 min, 2, 4, 6 and 24 h postoperatively with the Wong–Baker (fig 2) and FLACC (table 1) pain assessment scores (fig 2). Wong–Baker is suitable for older children, while FLACC is used for infants and preverbal children. A staff nurse assessed the pain level while the child was in hospital and the parents after discharge. The nurse and parents were unaware of the randomisation status of the child. These scoring systems were chosen, as they are the standard pain scales used in our centre and are simple to use. Parents were fully instructed on their use prior to discharge from the unit. The need for additional analgesia, presence of nausea or vomiting and the time to discharge were recorded. Based on a previous pilot study, a sample size calculation suggests 42 patients with a 10% difference in pain scores would give a power of 80%.

Figure 2 Wong–Baker visual analogue scale. This scoring system was used in older children. The child selects the most appropriate category, and the pain is scored accordingly.
Table 1 FLACC (face, arms, legs, cry, consolability) scale. This is used in preverbal children, and the most appropriate category to describe the child’s behaviour is selected. FLACC (face, legs, activity, cry, consolability) pain-assessment scale

RESULTS

Fifty-four children were enrolled in the study. Levobupivacaine was administered in 27 patients, and the rest served as controls. The groups were matched with regards to age, gender, weight and type of surgery (table 2). There were 15 males in each group. The sub-Tenon levobupivacaine block was effective in all patients in the test group (judged by pupil dilatation). The mean age for the test group was 7 years and 6 years for controls. Fourteen patients in the test group had bilateral surgery and 15 in the control group. There was no significant difference in the pain scores in the two groups in any of the time intervals (p = 0.22 at 30 min, p = 0.37 at 2 h, Student t test (table 2). There was no difference in the pain scores in those that did or did not receive remifentanil.

Table 2 Age, gender, type of surgery and mean postoperative pain score at each time interval for the test and control groups

There was no significant difference in nausea and vomiting or time to discharge between the two groups. The rate of nausea and vomiting was very low (4.55%), and the average time until discharge was 5.4 h (4 h 30 min for the test group, 6 h 20 min for controls). There was no reduction in nausea or vomiting if odansetron was given peroperatively. We also examined pain scores in each group to see if there was any difference between children undergoing unilateral and bilateral surgery. We found no difference in pain score depending on type of surgery (p = 0.89 treated group, p = 0.09 control group, Student t test).

DISCUSSION

This study did not show any significant difference in pain scores between those children receiving preoperative levobupivacaine (0.75%) and those that did not. Possible reasons for this are additional pain incurred from trauma from dissection at site of block or cannulation as well as possible myotoxicity from the local anaesthetic. Interestingly, there was a much lower rate of nausea and vomiting in our study for both groups than similar studies.11 12 We believe this low rate was due in part to the anaesthetic regimen which included propofol.20 In addition, it has been suggested that suppression of the trigeminal reflex by regional anaesthesia in strabismus patients may correlate with a decreased incidence of vomiting.21 There was no difference in discharge time between the group receiving sub-Tenon levobupivacaine and the control group.

There are a number of conflicting reports on the efficacy of ocular anaesthesia in postoperative pain control. Postoperative topical tetracaine compared with topical saline in paediatric strabismus surgery has been shown to provide a short-lived but significantly better pain relief (p<0.001) as judged by both pain score and analgesic requirement.14 A randomised controlled trial investigated the postoperative use of sub-Tenon lignocaine in 111 children having squint surgery. Pain was reduced significantly in the first hour after surgery, but thereafter there was no effect.15 Lignocaine is a shorter-acting anaesthetic with a duration of 1–2 h when given as a sub-Tenon block, while the effect of bupivacaine and levobupivacine lasts for 3–3.5 h.22 Similarly, the administration of postoperative subconjunctival bupivacaine has reduced pain scores after strabismus surgery in children.16 Another study compared subconjunctival bupivacaine to topical tetracaine in children having squint surgery, with both giving effective analgesia.17 However, some investigators found no difference in pain score after paediatric squint surgery using postoperative topical saline, topical tetracaine or subconjunctival bupivacaine.11

Levobupivacaine has a long duration of action and the added advantage of reduced cardiotoxicity compared with bupivacaine.23 Although subconjunctival levobupivacaine has some success in the relief of postoperative pain, the source of pain after strabismus surgery is believed to be receptors in tenon’s fascia and muscle tendons as well as conjunctival receptors.24 25 Thus, we felt that sub-Tenon administration may offer more effective anaesthesia than other local modalities. Regional blocks are associated with fewer episodes of bradycardia and hypertension intraoperatively,26 and so preoperative administration was chosen both for potential reduction in general anaesthetic requirement and less bradycardia intraoperatively. Preoperative sub-Tenon administration also facilitates surgical dissection and delivery of a controlled volume. In the author’s experience, levobupivacaine administered postoperatively is associated with protrusion of Tenon fascia and leakage of anaesthetic through the incision. The administration of a preoperative sub-Tenon block did result in any surgical difficulty from tissue distortion. As levobupivacaine has an onset of action of approximately 20 min,27 administration preoperatively ensures effectiveness as the general anaesthetic wears off. Although sub-Tenon anaesthesia is a safe method of delivering local anaesthetic with a blunt cannula, there remains a small risk of inadvertent damage to the extraocular muscles, haemorrhage or scarring. This was fully explained to all parents at the time of consent. There were no complications resulting from the sub-Tenon injection in this study.

Unlike prior studies, we looked at preoperative administration of a longer-acting anaesthetic in the sub-Tenon space in children. We, however, found no significant reduction in postoperative pain score. Our findings are similar to findings by Carden et al.11 We believe the results of this study are important, as local anaesthetic agents are commonly used in children’s strabismus surgery to alleviate postoperative pain. There has been no prior randomised controlled trial investigating the preoperative use of sub-Tenon levobupivacaine. Levobupivacaine is a newer and safer agent than longer-acting agents previously available. This study did not show any reduction in pain score in patients who received this agent using a preoperative sub-Tenon block. Overall pain scores were reasonably low in this study, and this may well be due to careful perioperative analgesia. However, our study has a number of limitations. A placebo was not used in the control group, and this may have influenced the results. It is possible that that the differing types of surgery performed or the fact that not all patients received exactly the same general anaesthetic agents or additional analgesia may have influenced the results. However, both groups had the same distribution with regards to the type of surgery and anaesthetic agents used. The wide age range of the patients in this group may have influenced the subjective appreciation of pain on the scoring systems used. One of the strengths of this study is that a single surgeon carried out all operations using a standard technique. Further studies investigating the use of larger doses of levobupivacaine, a narrow age range and identical surgery in both groups are warranted. In conclusion, this is the first randomised control study comparing the pain scores in children undergoing strabismus surgery with the preoperative administration of levobupivacaine. In this cohort, the administration of preoperative levobupivacaine was not associated with lower pain scores compared with controls.

REFERENCES

Footnotes

  • Competing interests: None.

  • Patient consent: Ethics approval was provided by the Local Research and Ethics Board, Bro Taf Health Authority, Wales, UK.

  • Patient consent: Obtained from the parents.

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