Local or regional anesthesia is used frequently in ophthalmic surgery, especially in adults and elderly patients for cataract, glaucoma, vitreoretinal and other anterior segment surgeries. The selection of anesthetic method depends on the type and length of surgery, requirement for ocular akinesia, patient compliance, systemic diseases, surgeon preference and institutional practice [1,2]. Regional ophthalmic blocks continue to be popular due to their ability to provide adequate analgesia, decrease the need for general anesthesia, and permit early postoperative recovery [3].

Peribulbar block is a well established regional anaesthetic technique in ophthalmic surgery. It is an injection of local anesthetic into the extraconal orbital space that results in anesthesia and varying degrees of globe akinesia by diffusion of the anesthetic around the orbit [4]. Peribulbar block is generally safer than retrobulbar anesthesia because the needle is not deliberately pushed into the muscle cone; however, it is still a sharp needle block and can have complications of chemosis, subconjunctival hemorrhage, periorbital ecchymosis, raised intraocular pressure, inadequate block, globe perforation, optic nerve injury, retrobulbar hemorrhage, and rarely systemic or brainstem anesthesia [2,5].

Sub-Tenon's block is a regional anesthetic technique that was introduced as an alternative to the above described technique in which local anesthetic is injected into the sub-Tenon's space through blunt cannula after conjunctival and Tenon's capsule access [6]. The technique does not require deep penetration of the sharp needle into the orbit, which makes it a relatively safe procedure, especially for patients in whom needle-related complications are a concern [7]. It offers a reliable analgesia and may be sufficient for many ophthalmic procedures, but minor complications like chemosis and subconjunctival hemorrhage are reported [7,8].

There have been several studies that have compared peribulbar and sub-Tenon's anesthesia in cataract and other ophthalmic surgery. Azmon et al. found that sub-Tenon's anesthesia resulted in surgical conditions similar to peribulbar anesthesia with less intraocular pressure elevation [9]. Iganga et al. reported that peribulbar and posterior sub-Tenon's anesthesia were equally effective in providing analgesia and akinesia for cataract surgery [10]. A more recent comparative study by Antony et al. also indicated that sub-Tenon's block is a good alternative to peribulbar block in small incision cataract surgery, but that pain, akinesia and minor complications would need to be assessed locally [11].

Both techniques are widely used in ophthalmic anesthesia practice but the complication profile may differ depending on the patient, the operator, the amount of anesthetic used, the type of needle or cannula used, the use of anticoagulants, the type of surgery performed, and the standards of monitoring [5,7]. Adverse events reported include chemosis, subconjunctival hemorrhage, which are self-limited, and rare sight-threatening or life-threatening adverse events [5,7]. Cochrane evidence has also highlighted that, although local anesthetic techniques in cataract surgery are generally safe, the available trials may not be adequate to identify rare but clinically significant complications [12].

Systematic documentation of block related complications is important in high volume ophthalmic units, especially in teaching hospitals, to enhance anesthetic safety and institutional practice. In the year 2008-2009, peribulbar and sub-Tenon's blocks were the most frequently used blocks for ophthalmic surgery in S. M. College and Hospital, Uttar Pradesh. But information on the rates and types of complications that occur with these two techniques was sparse locally. The present prospective observational study was thus designed to assess the complications of peribulbar and sub-Tenon's block in patients undergoing ophthalmic surgery in this institution.

OBJECTIVES

1. To determine the frequency and pattern of complications associated with peribulbar block and sub-Tenon’s block in patients undergoing ophthalmic surgery.
2. To compare the complication rates between peribulbar block and sub-Tenon’s block.
3. To assess block-related clinical outcomes, including adequacy of anesthesia, need for supplementary anesthesia, patient discomfort, and intraoperative stability.

Study design and setting This prospective observational study was conducted at S. M. College and Hospital, Uttar Pradesh, during 2008–2009. Adult patients undergoing elective ophthalmic surgery under either peribulbar block or sub-Tenon’s block were enrolled and observed for block-related complications and clinical outcomes. Study population and sampling Patients scheduled for cataract surgery, glaucoma surgery, pterygium surgery, or other anterior segment ophthalmic procedures under peribulbar or sub-Tenon’s block were eligible. Consecutive eligible patients were recruited until the required sample size was achieved. Patients were grouped according to the block received. Sample size A total sample size of 160 patients was selected, with approximately 80 patients in each block group. As the expected complication rate was not known, a prevalence of 50% was assumed for maximum sample size estimation. Using n=Z^2 pq/d^2,with Z = 1.96, p = 50%, q = 50%, and allowable error of 8%, the calculated sample size was approximately 150. After allowing for incomplete records or exclusions, the sample size was rounded to 160. Eligibility criteria Patients aged 18 years or older, of either sex, who were scheduled for elective ophthalmic surgery under peribulbar or sub-Tenon's block anesthesia suitable for regional anesthesia and who were willing to provide informed consent were included. Patients were excluded if they had general anesthesia, topical anesthesia alone, emergency surgery, age < 18 years, were unable or unwilling to provide consent, had ocular infection at the block site, allergy to local anesthetic agents, bleeding diathesis or uncontrolled anticoagulation, severe uncontrolled systemic illness, conversion to general anesthesia, or incomplete block-related records. Preoperative assessment Demographic data, systemic comorbidities, medication history, use of anticoagulants or antiplatelets, prior ocular surgery, procedure planned, and eye operated were documented. Pre-block heart rate, blood pressure and oxygen saturation were recorded. Anesthetic technique The choice of block was based on institutional practice, surgeon preference, patient characteristics, and planned surgery. A standard sharp-needle extraconal technique was used for peribulbar block. Sub-Tenon's block was performed under topical anesthesia with conjunctival and Tenon's capsule access and injection of local anesthetic using a blunt cannula. The local anesthetic agent, volume, and need for ocular compression were recorded. All blocks were conducted or monitored by trained staff. Outcome assessment Immediate and intraoperative block-related complications such as chemosis, subconjunctival hemorrhage, periorbital ecchymosis, lid swelling, pain on block administration, inadequate block, need for supplementary anesthesia, retrobulbar hemorrhage, globe perforation, optic nerve injury, vasovagal episode, respiratory compromise, local anesthetic systemic toxicity, brainstem anesthesia, and any other adverse event were observed. Complication was categorized as minor or major. Minor complications included self-limiting events such as chemosis, subconjunctival hemorrhage, mild ecchymosis, lid swelling, and transient discomfort. Major complications were defined as potentially sight threatening or systemic events such as retrobulbar hemorrhage, globe perforation, optic nerve injury, local anesthetic systemic toxicity, brainstem anesthesia, severe respiratory compromise, or abandonment of the planned anesthetic technique. Analgesia, ocular akinesia and patient comfort were used to determine block adequacy prior to surgery. Supplementary anesthesia, patient discomfort and intraoperative stability were documented. Intraoperative stability was considered as a stable heart rate, blood pressure, oxygen saturation, good cooperation and lack of serious block related or systemic adverse events. Statistical analysis Continuous variables were presented as mean ± standard deviation while categorical variables were presented as frequency and percentage. The chi-square test or Fisher exact test was used to compare the outcomes between the peribulbar and sub-Tenon's groups for categorical variables, and the independent t-test was used for continuous variables. A p-value <0.05 was considered statistically significant. Ethical considerations The study was carried out with the approval of the institutional ethics committee. All participants gave written informed consent. Patient confidentiality was respected and the anaesthetic technique and intra-operative care was in line with the standard practice in the institution.

Study population and baseline characteristics

A total of 160 patients undergoing elective ophthalmic surgery under regional ophthalmic anesthesia were included, with 80 (50.0%) patients in the peribulbar block group and 80 (50.0%) in the sub-Tenon’s block group. The overall cohort comprised 82 (51.2%) females and 78 (48.8%) males. Cataract surgery was the most common procedure, accounting for 126 (78.8%) cases. Baseline demographic profile, ASA physical status, planned surgery, operated eye, comorbidities, and pre-block vital parameters were comparable between the two groups.

Table 1. Baseline and preoperative characteristics of the study groups.

Values are presented as mean ± SD or n (%). BP = blood pressure; SpO₂ = peripheral oxygen saturation.

Block technique and immediate block characteristics

The two block techniques differed in several expected procedural characteristics. The mean local anesthetic volume was higher in the peribulbar group (6.68 ± 0.67) than in the sub-Tenon’s group (4.11 ± 0.60; t=25.63, p <0.001). Pain during block administration and time to surgical readiness were also higher in the peribulbar group. Hyaluronidase use and ocular compression were more frequent with peribulbar block.

Table 2. Block technique and immediate block characteristics.

Values are presented as mean ± SD or n (%). VAS = visual analogue scale.

Frequency and pattern of block-related complications

The frequency of any block-related complication was similar between groups, with complications recorded in 38 (47.5%) patients in the peribulbar group and 39 (48.8%) patients in the sub-Tenon’s group (χ²=0.03, p=0.874). Most events were minor and self-limiting. Major complications were rare, with one (1.2%) retrobulbar hemorrhage in the peribulbar group and no major complication in the sub-Tenon’s group. Subconjunctival hemorrhage and chemosis were numerically more frequent after sub-Tenon’s block, whereas periorbital ecchymosis, lid swelling, and moderate or severe pain during block administration were numerically more frequent after peribulbar block.

Table 3. Frequency and pattern of block-related complications.

Values are presented as n (%). Fisher exact test was used for sparse 2×2 comparisons.

Block adequacy, supplementary anesthesia, and intraoperative cooperation

The distribution of analgesia grade did not differ significantly between groups (χ²=0.97, p=0.808). Complete ocular akinesia was more frequent with peribulbar block, being recorded in 61 (76.2%) patients compared with 38 (47.5%) patients in the sub-Tenon’s group (χ²=15.72, p<0.001). Inadequate block was observed in 3 (3.8%) peribulbar cases and 11 (13.8%) sub-Tenon’s cases (χ²=5.01, p=0.025), and supplementary anesthesia was required in 3 (3.8%) and 12 (15.0%) patients, respectively (χ²=5.96, p=0.015). Patient comfort differed between groups, with comfortable intraoperative experience reported by 24 (30.0%) patients in the peribulbar group and 44 (55.0%) patients in the sub-Tenon’s group (χ²=13.36, p=0.001). Intraoperative instability was recorded in 7 (8.8%) and 11 (13.8%) patients, respectively, without a statistically significant difference.

Table 4. Block adequacy and clinical outcomes.

Values are presented as n (%).

Intraoperative physiological parameters and satisfaction

Intraoperative physiological parameters were comparable between groups. Maximum intraoperative heart rate, maximum systolic blood pressure, and minimum oxygen saturation showed no significant between-group differences. Patient satisfaction score was modestly higher in the sub-Tenon’s group (4.26 ± 0.84) than in the peribulbar group (3.99 ± 0.85; U=2616.5, p=0.034), while surgeon satisfaction scores were comparable between groups.

Table 5. Intraoperative physiological parameters and satisfaction scores.

Values are presented as mean ± SD. Satisfaction scores were compared using the Mann–Whitney U test.

In this prospective observational study, the overall complication rate was similar between peribulbar and sub-Tenon's blocks, with a complication rate of 47.5% and 48.8%, respectively. The majority of events were minor and major complications were uncommon (1 retrobulbar hemorrhage in the peribulbar group and none in the sub-Tenon's group). This result corroborates the opinion that both methods are suitable for ophthalmic surgery, however, they have different clinical profiles that are important in terms of patient comfort, block efficiency and surgical needs.

The overall complication rate in the present study is similar to that reported in the systematic review by Wang and Casson, who found that there is no significant difference in overall efficacy between peribulbar and sub-Tenon's anesthesia, but there are potentially serious complications associated with peribulbar sharp-needle techniques [13]. In the current study, this difference was manifested by the presence of retrobulbar hemorrhage in the peribulbar group, but the number of patients was too small to reliably estimate the incidence of rare sight-threatening events.

In the current cohort, pain on block administration was definitely less with sub-Tenon's block. The mean VAS score was 1.50 in the sub-Tenon's group and 2.71 in the peribulbar group. This is close to the findings of Parkar et al., who randomized 168 patients undergoing manual small-incision cataract surgery and reported that 77.5% of sub-Tenon's patients did not experience pain during administration as compared to 35.2% in the peribulbar group [14]. Briggs et al. also reported lower per-operative pain scores with sub-Tenon's anesthesia, with mean per-operative pain scores being 0.5 with sub-Tenon's block and 1.2 with peribulbar block [15]. These comparisons corroborate the notion that the blunt-cannula sub-Tenon's approach is generally better tolerated at administration than a sharp-needle peribulbar injection.

However, the advantage of sub-Tenon’s block in comfort was balanced by less complete motor block. Complete akinesia of the eye was obtained in 76.2% of the patients who received peribulbar block and 47.5% of the patients who received sub-Tenon's block in the present study. There was also a higher rate of inadequate block (13.8%) and need for additional anesthesia (15.0%) with sub-Tenon's block than with peribulbar block (3.8% and 3.8%, respectively). A similar trend was observed in 100 patients undergoing manual small-incision cataract surgery (MSICS) by Singh et al., where complete akinesia was achieved in 86% of patients in the peribulbar group and 68% in the sub-Tenon's group, while supplementary injections were needed in 6% and 24% respectively [16]. These results indicate that peribulbar block may still be the preferred technique in cases where dense akinesia is desired.

However, not all studies have demonstrated better akinesia with peribulbar block. In a prospective comparison of 50 patients, Al-Yousuf found no difference in pain perception between the two techniques, but better control of ocular movement in the sub-Tenon's group, with a significant difference in favor of sub-Tenon's anesthesia [17]. In a randomized study of 50 patients undergoing sequential bilateral cataract surgery, Budd et al. reported no significant difference between lid or globe movement with sub-Tenon's and peribulbar anesthesia, and injection and operative pain scores were low and similar [18]. These differences are likely due to differences in anesthetic volume, posterior spread of sub-Tenon's injection, use of ocular compression, use of hyaluronidase, timing of assessment, and operator technique. The greater peribulbar volume, increased use of hyaluronidase, and increased compression of the eye may have been responsible for the improved akinesia in that group in the present study.

The complication pattern in the present study is also similar to previous reports. Periorbital ecchymosis and lid swelling were more common after peribulbar block, and subconjunctival hemorrhage and chemosis were more common after sub-Tenon's block, numerically. Singh et al. found subconjunctival hemorrhage in 50% of sub-Tenon's patients and 32% of peribulbar patients and chemosis in 32% versus 18%, respectively [16]. These rates are higher than those seen in the current study, in which subconjunctival hemorrhage was seen in 22.5% and 15.0%, and chemosis in 13.8% and 10.0%, respectively. However, the direction of effect is the same and is anatomically plausible as sub-Tenon's block involves dissection of the conjunctiva and passage of cannula through vascular episcleral tissue.

Patient-centered outcomes favoured sub-Tenon’s block. Fifty-five percent of sub-Tenon's patients reported a comfortable intraoperative experience, while only 30.0% of peribulbar patients did, and patient satisfaction was significantly higher with sub-Tenon's anesthesia. This is consistent with Singh et al., who found 60% of sub-Tenon's patients were satisfied or highly satisfied, versus 42% of peribulbar patients [16]. Budd et al. also found a patient preference signal for sub-Tenon's anesthesia, with 10% of patients stating a preference for sub-Tenon's due to a dislike of facial numbness following peribulbar block, although most patients preferred the first technique that they received [18]. So, comfort and satisfaction seem to be in favour of sub-Tenon's anesthesia despite the better akinesia provided by peribulbar block.

In the present study, there were no significant differences between groups in terms of maximum heart rate, maximum systolic blood pressure, or minimum oxygen saturation during surgery. This is clinically relevant as regional ophthalmic anesthesia is frequently performed in elderly patients with systemic comorbidities. Similarly, Rizk et al., in cardiac patients undergoing cataract surgery, advocated the use of both peribulbar and sub-Tenon's blocks with proper monitoring and that differences between the two techniques should be interpreted in the context of patient risk and surgical needs [19]. Both methods were physiologically well tolerated in the present cohort, as there was no major systemic toxicity or persistent respiratory compromise.

Overall, the results indicate a pragmatic compromise and not an absolute advantage for one technique over another in all areas. Peribulbar block resulted in better akinesia, less supplementation, but increased anesthetic volume, increased incidence of ocular compression, increased time to surgical readiness, and increased administration pain. Sub-Tenon's block was more comfortable and more satisfactory, but there were more instances of inadequate block and supplementation. This interpretation is in line with Wang and Casson's review which highlighted broadly similar efficacy and differing risk profiles between techniques [13].

There are limitations to this study. It was performed at a single center and the block type was chosen based on the routine practice and not by randomisation. Technical factors such as anesthetic volume, use of hyaluronidase, ocular compression, and operator experience may have influenced block adequacy and complications. The sample size was also too small to evaluate uncommon serious complications like globe perforation, optic nerve injury, brainstem anesthesia, or systemic local anesthetic toxicity. However, the prospective design, similar group sizes, and baseline characteristics enhance the clinical relevance of the findings.

Peribulbar and sub-Tenon's blocks were appropriate for ophthalmic surgery, and there were few major complications and similar overall complication rates. Peribulbar block was superior in terms of akinesia and less supplementation, while sub-Tenon's block was superior in terms of comfort and patient satisfaction. The choice of block technique should therefore be tailored to the requirement for akinesia, patient comfort, comorbidity profile and local expertise.

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