pH-adjusted periocular anaesthesia for primary vitreoretinal surgery

T Sharma; L Gopal; S Parikh; MP Shanmugam; SK Saha; KR Sulochana; NS Shetty; BN Mukesh; SS Badrinath

ORIGINAL ARTICLE

Year : 1999 | Volume : 47 | Issue : 4 | Page : 223-227

pH-adjusted periocular anaesthesia for primary vitreoretinal surgery

T Sharma, L Gopal, S Parikh, MP Shanmugam, SK Saha, KR Sulochana, NS Shetty, BN Mukesh, SS Badrinath
Vision Research Foundation, Chennai, India

Correspondence Address:
T Sharma
Vision Research Foundation, Chennai
India

Source of Support: None, Conflict of Interest: None

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PMID: 10892477

Abstract

Purpose: To evaluate the efficacy of pH-adjusted bupivacaine in conjunction with medial orbital periconal block (periocular anaesthesia).
Methods: Sixty consecutive patients undergoing primary vitreoretinal surgery were enrolled prospectively.
Results: Adequate anaesthesia and akinesia with no intraoperative supplementation was achieved in 53 eyes (88.3%). Factors influencing intraoperative supplementation were combined vitrectomy with scleral buckling (p=0.005) and duration of surgery of more than 2 hours (p=0.001). No ocular or systemic complication resulted.
Conclusion: pH-adjusted periocular anaesthesia is safe and effective in patients undergoing primary vitreoretinal surgery.

Keywords: pH-adjusted bupivacaine, periocular anaesthesia, primary vitreoretinal surgery

How to cite this article:
Sharma T, Gopal L, Parikh S, Shanmugam M P, Saha S K, Sulochana K R, Shetty N S, Mukesh B N, Badrinath S S. pH-adjusted periocular anaesthesia for primary vitreoretinal surgery. Indian J Ophthalmol 1999;47:223-7

How to cite this URL:
Sharma T, Gopal L, Parikh S, Shanmugam M P, Saha S K, Sulochana K R, Shetty N S, Mukesh B N, Badrinath S S. pH-adjusted periocular anaesthesia for primary vitreoretinal surgery. Indian J Ophthalmol [serial online] 1999 [cited 2020 Nov 29];47:223-7. Available from: https://www.ijo.in/text.asp?1999/47/4/223/14908

NUMBERS IN PARENTHESES ARE PERCENTAGES; NS IS NOT SIGNIFICANT.

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Two factors that determine the success of regional or local anaesthesia in primary vitreoretinal surgeries are the techniques of administration and the choice of anaesthetic agents. Despite recent advances in the techniques such as peribulbar and parabulbar anaesthesia for primary vitreoretinal surgery, the need for intraoperative supplementation varies from 31-36.8%.[1-3] The commonly used anaesthetic agents are bupivacaine alone1 or a mixture of bupivacaine and lignocaine.[2],[3] Bupivacaine is added to lignocaine to combine the advantages of rapid onset of anaesthesia with lignocaine and the longer duration of anaesthesia with bupivacaine. However, some investigators have questioned the use of mixtures of bupivacaine and lignocaine in ophthalmic anaesthesia.[4] Mixing 2% of lignocaine with 0.5% of bupivacaine in a 1:1 ratio dilutes the anaesthetic mixture to 1% lignocaine and 0.25% bupivacaine resulting in a possible reduction in efficacy of both anaesthetics. Further, the toxicity of the drugs contained in a mixture is "at least additive".[5]

Recent clinical studies have shown that pH-adjusted solutions of local anaesthetics (pH- 6.6-7.0) produce a more rapid onset of blockade compared to unmodified commercial preparations.[6][7] The studies, however, did not mention the type of surgery, the duration of surgery, and dealt with a small sample size (n=20 or less). Since pH-adjusted anaesthetic solutions hasten the onset time, it is open to question whether there is a need to add lignocaine to the bupivacaine solution. Medial orbital periconal block, a technique of anaesthesia, has been found effective in cataract and vitreoretinal surgeries.[2],[8] We therefore undertook this prospective study to evaluate the efficacy of pH-adjusted bupivacaine solution in conjunction with medial periconal block in primary vitreoretinal surgical procedures.

Materials and Methods

With the approval of the institutional review board, the study was conducted in two phases - experimental work in phase 1 and clinical study in phase 2.

Experimental work was performed in phase 1 to determine the exact amount of sodium bicarbonate needed to alkalinize the anaesthetic solution without any micro or macro precipitation. Under sterile conditions, a 5 microlitre pipette was used to add 7.5% w/v sodium bicarbonate solution to 2.5 ml of anaesthetic mixture of 0.5% bupivacaine and hyaluronidase (25 U/ml). The baseline pH of 0.5% bupivacaine increased from 5.8-6.01 to 6.8-7.0 when 0.01 ml of 7.5% w/v sodium bicarbonate was added to 2.5 ml of 0.5% bupivacaine. Thus, 0.08 ml of sodium bicarbonate was mixed to 20 ml vial of 0.5% bupivacaine. The pH of the solution was estimated using a digital pH meter (model L1 120, ELICO, Hyderabad).

In phase 2 clinical study, 60 consecutive patients who underwent primary vitreoretinal surgery between February and April 1997 were recruited. No preoperative sedation was administered. Exclusion criteria included less than 20 years of age, a history of prior retinal surgery, allergy to bupivacaine, mental retardation and patients' preference for general anaesthesia.

A freshly prepared pH-adjusted 0.5% bupivacaine with hyaluronidase (25 U/ml) solution was used in all cases. No separate orbicularis oculi or lid block was given. The patient was positioned on the table and topical 4% Lignocaine was instilled into the conjunctival sac. Periocular anaesthesia was administered at two sites - lower temporal quadrant and nasal to the caruncle. The first injection was given in the lower lid, at the junction of medial 2/3 and temporal 1/3, just above the inferior orbital rim, using a one inch, 23-gauze disposable needle. The needle was advanced perpendicular to the iris plane to its full depth and after aspirating to rule out intravascular placement, 5 ml of anaesthetic mixture was deposited in this location. The second injection was given as described by Hustead et al.[8] The needle was introduced through the small depression immediately nasal to the caruncle and then advanced posteriorly in the transverse plane making an approximately 5° angulation toward the medial orbital wall. This technique avoids damage to the medial rectus muscle and its sheath; 5 ml of anaesthetic mixture was thus injected. A gentle lid massage was given but no super pinky was applied. To maintain uniformity in the technique, all blocks were administered by the same investigator.

The assessment of lid and globe anaesthesia was done at 2-minute intervals for 10 minutes. If adequate anaesthesia and/or akinesia were not achieved at 10 minutes after the injection, an additional 3-5 ml of the anaesthetic mixture was injected. The site of repeat injection was determined by the quadrant in which extraocular movements persisted. Adequacy of anaesthesia was assessed by holding bulbar conjunctiva or muscle insertion with forceps. Adequacy of akinesia was determined by the absence of ocular movements in all directions. Patients were encouraged to communicate with the surgeon regarding pain during surgery. When required, the supplemental anaesthetic mixture was irrigated into the subtenon space. The efficacy of anaesthesia was graded in the way reported by us earlier [Table - 1].[3]

Other variables included preoperative diagnosis, type and duration of surgery, volume of anaesthetic used, onset of anaesthesia and akinesia, onset of pain during surgery, need for intraoperative supplementation and the subjective response of the surgeon. All vital signs were monitored throughout the surgery. Postoperative pain, vomiting, and the need for an analgesic and/or antiemetic were also recorded. The pH of remaining quantity of anaesthetic solution in vial was estimated in each patient.

Continuous variables were analysed by Student's t-test and categoric variables by Chi-square, Fisher's exact and Mantel-Haenszel tests; p-value of less than 0.05 was considered to be statistically significant.

Results

Of the 60 patients, 38 were men and 22 women. The mean age was 51.9 years (range, 22-74). Preoperative diagnosis was nonabsorbing vitreous haemorrhage with or without traction retinal detachment in 56 patients, and dislocated lens, complicated cataract, macular hole and epimacular proliferation respectively in each of the remaining four patients. All patients underwent standard three-port pars plana vitrectomy. Adjunctive procedures included lensectomy, fluid-gas exchange, endolaser and gas or silicone oil tamponade whenever necessary. Pars plana vitrectomy was combined with #240 encircling band in 12 (20%) cases. Mean duration of surgery was 103 minutes (range, 45-240).

The volume of anaesthetic mixture injected was 10 ml in 50 (83.3%) cases, 13 ml in 6 (10%), 15 ml in 3 (5%) and 18 ml in 1 (1.7%). All patients had adequate lid anaesthesia and akinesia after 2 minutes. The onset of ocular akinesia is shown in the Figure. At 6 minutes, 81.7% achieved partial ocular akinesia (at least 2 mm movement in any direction) and 16.7% complete ocular akinesia. However, at 10 minutes, 81.7% attained complete akinesia. In 11 (18.3%) cases, the injection was repeated in the lower temporal quadrant, once in 9 cases and twice in two. After the repeat injection, complete ocular akinesia was obtained at 15 minutes in 6 cases, at 20 minutes in 4 and at 28 minutes in one case.

The efficacy of periocular anaesthesia was graded as grade 5 in 45 patients (75%), grade 4 in 8 (13.3%), grade 3 in 6 (10%) and grade 1 in one (1.7%). No patient was graded at 0. Ten patients experienced variable amounts of pain during surgery at a mean interval of 132 minutes (range, 50-200) after the administration of anaesthesia. Intraoperative supplementation of anaesthesia was required in 7 patients. Five patients needed local supplementation by sub-tenon irrigation of original anaesthetic mixture. One patient had systemic supplementation (intravenous injection pentazocine and injection phenergan); and another, both local and systemic supplementation.

The surgeons' subjective evaluation of periocular anaesthesia was satisfactory in 45 (75%) cases and partially satisfactory in 15 (25%). Postoperatively in the first 24 hours, ocular pain was experienced in 17 (28.3%) patients and 6 of them received oral analgesics. The pH of the anaesthetic mixture was 6.8 in 52 cases (86.7%), 6.9 in 6 (10%) and 7.0 in 2 (3.3%).

The need for intraoperative supplementation was significantly higher in patients undergoing vitrectomy with buckle (p=0.005) and in patients with longer duration of surgery of more than 2 hours (p<0.001) [Table - 2]. No correlation with respect to age, gender and the onset of ocular anaesthesia/akinesia was noticed.

Discussion

The results of our study demonstrate that the use of pH-adjusted 0.5% bupivacaine-hyaluronidase solution without lignocaine allows the surgeon to perform primary vitreoretinal procedures with no intraoperative supplementation in 88.3% of cases. In contrast, previous studies indicate the need for intraoperative supplementation in 31-36.8% of cases undergoing vitreoretinal surgery under retro, peri and parabulbar anaesthesia.[1][2][3] We attribute the success of our blocks to alkalization of the anaesthetic mixture and the technique of periocular anaesthesia. The physico-chemical basis of altered pH on the efficacy of local anaesthetic solutions has been reviewed by several investigations.[9][10][11][12][13][14] It has been shown that the cation form of local anaesthetic is the active form of the drug.[15] Depending on the pH, local anaesthetic drugs exist in various ratios of cation to noncation concentrations. Alkalinization of a local anaesthetic solution with sodium bicarbonate increases the availability of the noncation form.

It has been reported that the noncation form penetrates soft tissues and the nerve sheath faster.[16] Hence, pH adjustment increases nerve penetration and decreases onset time. In this study, partial loss of ocular movement increased from 11.7% at two minutes to 81.7% at six minutes. In addition, complete loss of ocular movements increased from 3.3% at two minutes to 81.7% at ten minutes. This relatively faster onset of akinesia was also observed by others.6, 7 While alkalinising the anaesthetic solution, it is important to use freshly prepared mixtures without any micro or macroprecipitates, because local anaesthetics tend to become unstable and precipitate when pH increases.17

The technique of periocular anaesthesia, including the medial orbital periconal block, has several advantages and is based on the pioneering work in orbital anatomy of Koorneef.18 The predominant tissue surrounding the globe and extraocular muscles is fat, split into compartments by connective tissue septa of varying densities. These discrete compartments envelope the superior, medial, and inferior muscle groups and extend to the apex of the orbit.[18] A local anaesthetic agent that is injected into the medial orbit immediately nasal to the caruncle gains easy access to these tissue compartments and the orbital apex. This anatomic explanation is further corroborated by the observation of mild proptosis, drooping of the upper lid and gradual fullness of the lower lid as the medial periconal block is administered. No separate orbicukris oculi or lid block was required for our patients due to anatomic advantages of periocular anaesthesia as compared to our earlier experience with peribulbar and parabulbar anaesthesia.1, 3 The relative avascularity of the medial compartment offers additional safety as the superonasal quadrant utilised in the standard peribulbar block is the most vascular area of the anterior orbit and also contains the belly, trochlear pulley and the tendon of the superior oblique muscle.

Intraoperative pain was experienced at a mean interval of 132 minutes by 10 patients. Most of them were noted to exhibit an apprehension toward surgery and probably had a low threshold for pain as the subjective and individual responses to pain are known to vary among individuals. For these patients, preoperative counselling and if required, anxiolytic drugs may be helpful to enhance compliance. On the other hand, it is prudent to supplement the block by subtenon irrigation at 2 hours, particularly in those patients undergoing combined buckling and vitrectomy.

Postoperative ocular pain necessitating analgesics occurred in only 10% of cases in this study as compared to 63.2% of cases after peribulbar[1] and 60% of cases after parabulbar anaesthesia.[3] Presumably, the anaesthetic effect of alkalinized local anaesthetic solution persists in the postoperative period.

In conclusion, the use of a bicarbonate - buffered 0.5% solution of bupivacaine and hyaluronidase without lignocaine not only improves the efficacy of block but also reduces the need for intraoperative supplementation in patients undergoing primary vitreoretinal surgery. The results suggest the need for randomized study to evaluate further the role of alkalinised anaesthetic solutions in posterior segment surgeries.

References

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