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Sub-Tenon’s Anesthetic Administration for Cataract Surgery: How Much Stays In?

Patton, Niall, MRCOphth; Malik, Tahira Y., FRCOphth; Aslam, Tariq M., DM

doi: 10.1213/01.ane.0000168448.05964.06
Ambulatory Anesthesia: Research Report
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We performed a prospective, randomized trial assessing the “remaining” volume of anesthetic solution that stays within the sub-Tenon’s space after administration of 2 different volumes: 3 mL and 5 mL. The remaining volume correlated with motor block (r = 0.72; P < 0.001). The volume lost through the incision as a percentage of total volume injected was similar in both groups, suggesting sub-Tenon’s space is not limited to a finite injected volume less than 5 mL and may be capable of receiving larger volumes of anesthetic to improve motor block.

IMPLICATIONS: The volume of anesthetic solution remaining within sub-Tenon’s space correlates with motor block. The amount of volume lost as a percentage of total administered is independent of the volume injected, suggesting sub-Tenon’s space is not limited to a finite injected volume less than 5 mL and may be capable of receiving larger volumes of anesthetic to improve motor block.

Department of Ophthalmology, Princess Margaret Hospital, Dunfermline, Fife, United Kingdom

Accepted for publication April 15, 2005.

Address correspondence and reprint requests to Niall Patton, Department of Ophthalmology, Sir Charles Gairdner Hospital, Nedlands, WA 6009, Australia. Address e-mail to niallpatton@yahoo.co.uk.

Sub-Tenon’s anesthesia is an effective technique in modern intraocular surgery, because it provides good anesthesia without the risk of a sharp needle into the peri-/retro-bulbar space (1–3). Whereas some surgeons do not consider akinesia to be important for cataract surgery and use topical anesthesia routinely for cataract surgery, many others prefer to operate under conditions of at least partially reduced eye movements and hence use local infiltrative techniques such as sub-Tenon. Previous studies have found great variation in intraocular pressure (IOP) after sub-Tenon’s anesthetic (4–6) and have speculated on a “valve” effect through the conjunctival incision, allowing efflux of solution from sub-Tenon’s space as the cause (4). Volumes used in sub-Tenon’s anesthetic vary from 1 mL to 5 mL (4, 7–10). We previously published a study showing significantly increased kinesia using 3 mL rather than 5 mL of solution (5). A valve effect might suggest that there is a finite volume more than which any more anesthetic injected would be effluxed. We performed a prospective, randomized study to assess what the effect of the volume of anesthetic solution that remains (remaining volume) after injection (3 mL and 5 mL) has on the efficacy of the anesthetic. We also compared the amount of volume lost through the incision after administration of 3 mL and 5 mL of solution to examine whether the volume of effluxed solution as a percentage of the total volume injected was larger with the larger-volume group.

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Methods

The study received local IRB approval. Consecutive patients presenting for routine cataract surgery were recruited, and full informed consent was obtained. All patients were randomly assigned by means of a computerized random number generator to receive either 3 mL or 5 mL of sub-Tenon’s anesthesia (50:50 mixture of 0.5% bupivacaine and 2% lidocaine). All patients were masked to group assignment. All anesthetic procedures were performed by one surgeon (TM) using a curved, blunt sub-Tenon’s cannula through a standard minimal-sized inferonasal conjunctival incision (using spring scissors) into sub-Tenon’s space posterior to the equator of the globe. All patients received topical benoxinate 0.5% drops and topical 5% povidone-iodine solution before conjunctival incision. After administration of anesthetic, the eyelid was gently taped down, and no ocular massage was performed.

Before the procedure, the syringe containing the anesthetic solution with the sheathed cannula and the green swab that would be used to absorb any fluid that effluxed from the conjunctival incision during injection were weighed using a micro-precision weighing instrument (Sartorius AG, Goettingen, Germany; accurate to 1μg) under conditions of controlled air microenvironment (Measurement 1). During the administration of the anesthetic solution, the green swab was deposited lateral to the canthus of the operated eye, held in place under the lateral aspect by an ocular speculum. Patients were positioned in a slightly lateral head tilt during administration of anesthetic, and any fluid that effluxed from the eye was collected and absorbed by the swab through the effect of gravity (Fig. 1). At the end of the anesthetic injection, patients were asked to gently close their eyes, and any residual anesthetic collecting on the medial canthus was then effluxed laterally onto the swab. No ocular massage was performed. The used swab and re-sheathed cannula containing the syringe with any residual anesthetic were re-weighed by the same technique (Measurement 2). The remaining volume administered was calculated as the difference between Measurement 1 and Measurement 2.

Figure 1.

Figure 1.

Before, immediately after, and 5 min after administration of the anesthetic, the intraocular pressure was recorded by using a Tonopen XL (Mentor, Norwell, MA). In addition, 5 min after anesthetic administration, the degree of motor block, chemosis, and subconjunctival hemorrhage were rated. All ratings were undertaken by one person (NP) who was masked as to which volume of anesthetic the patient received. Motor block was rated on the following scale: complete = no eye movement in all fields of gaze, moderate = small residual movement in one or two fields of gaze, mild = movement in more than two fields of gaze, and none = near normal ductions.

Degree of chemosis and subconjunctival hemorrhage were rated on a subjective scale (none = 0, mild = 1, moderate = 2, and severe = 3). At the end of the procedure, the patient was asked by a trained nurse to mark on a visual analog scale any level of pain they experienced during the operation from 0 (no pain) to 10 (worst pain ever). To detect a difference between the groups of 0.5 (on a motor block ordinal scale of 0–3) required 25 patients per group (α = 0.05, [1 – β] = 0.8). All statistical calculations were performed using SPSS version 11.

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Results

Fifty-two eyes of 52 patients (36 women and 16 men) completed the study. Mean age was 76.2 yr (range, 52–90). Of those that had 3 mL injected into sub-Tenon’s space (n = 25; mean ± sd) remaining volume was calculated as 2.2 ± 0.5 mL (range, 1.2–2.9 mL). For the 5-mL group (n = 27), mean ± sd was 3.7 ± 0.7 mL (range, 1.1–4.9 mL). The mean volume of anesthetic solution that effluxed after administration was 0.8 mL (3-mL group) and 1.2 mL (5-mL group).

The remaining volume of anesthetic solution used correlated with the motor block 5 min after anesthesia administration (r = 0.715; P < 0.001; Spearman correlation coefficient; Fig. 2). There were no other significant associations between the actual volume administered and rating of the anesthetic (Table 1).

Figure 2.

Figure 2.

Table 1

Table 1

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Discussion

The results of this study show, as others have previously speculated 4, that there is a valve effect after sub-Tenon’s administration through the conjunctival excision that allows a variable amount of solution to exit from sub-Tenon’s space. Interestingly, the amount of fluid that exits seems to be broadly a percentage of the amount of fluid that is administered (approximately 25% for both volumes) rather than there existing a finite volume more than which one will see an increase in solution efflux. However, there is great variability with some patients receiving <25% of the injected volume. This may account for the wide variability in IOP changes that occur after sub-Tenon’s anesthesia (4–6,11). This suggests that sub-Tenon’s space is not limited to a finite injected volume less than 5 mL and may be capable of receiving larger volumes of anesthetic to improve motor block

Factors that may play a role in the volume effluxed include the size of the conjunctival incision, the speed with which the solution is administered, and the volume capacity of the surrounding sub-Tenon’s space. Whereas every attempt was made to perform uniform incisions for all patients and perform all injections at uniform speed, it is possible that variations in these two factors may have accounted for some of the differences in amount of anesthetic solution effluxed. In addition, there will be some efflux of injected anesthetic solution through the tear duct rather than laterally onto the swab, as well as some minor tear production by the patients during anesthetic administration.

In conclusion, the remaining volume staying within sub-Tenon’s space after anesthetic administration is closely correlated to motor block. In addition, the volume effluxed through the incision in sub-Tenon’s anesthetic administration as a percentage of the total administered is independent of the total volume injected, thus suggesting that it may be possible to inject larger volumes into sub-Tenon’s space to improve motor block.

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References

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© 2005 International Anesthesia Research Society