Pediatric Eye Blocks: Threading the Needle
We read with interest the article by Yao et al,1 entitled “Retrobulbar Block in Pediatric Vitreoretinal Surgery Eliminates the Need for Intraoperative Fentanyl and Postoperative Analgesia,” and commend the authors for scientific data that attest to the advantage of regional ophthalmic anesthesia in children.
The benefits of an intraoperative eye block for pediatric patients have been substantiated by others as well. In a study of 50 children, Deb et al2 compared peribulbar block (0.3 mL/kg 2% lidocaine/0.5% bupivacaine) with intravenous meperidine (1 mg/kg) and noted improved postoperative comfort in the block group; indeed, 62% of children reported no pain at 24 hours. Patel et al3 found improved recovery room scores and early return to normal feeding patterns in 6 infants undergoing retinal surgery who received an intraoperative peribulbar block using 0.375% ropivacaine (0.25 mL/kg).
Needle-based ophthalmic anesthesia was for many decades fashioned on Atkinson's4 retrobulbar technique adopting a “deep and steep” injection with a 38-mm needle. This approach occasionally produced catastrophic complications, in particular globe perforation and optic nerve trauma resulting in poor visual outcome. The anatomic foundation of the needle-based block is predicated on the orbital cone, created by the extraocular muscles as they traverse from their origin at the annulus of Zinn to their insertion on the globe. Modern nomenclature provides greater clarity by describing the relationship of the tip of the needle to the globe and its muscle cone. Thus, the terms retrobulbar and peribulbar have been superseded by intraconal and extraconal in order to delineate the site of local anesthetic delivery. Today, the extraconal block is favored because use of a shorter needle (16–32 mm) with minimal angulation may reduce the risk of scleral impingement or penetration into the optic nerve.
We have concerns surrounding technique and needle choice in Yao and colleagues'1 article. The bony orbit is relatively small compared with the adult from birth until early childhood, but the globe achieves full size by 1 year of age. Bentley et al5 studied orbital volumes in 67 healthy children between the ages of 1 month and 15 years by segmentation of magnetic resonance imaging scans. They demonstrated that total orbital volume is approximately 14 cm3 in the first few months of life, achieving adult volumes of approximately 25 cm3 at 15 years. Augusteyn et al6 examined growth of the human globe from early gestation to adult life and found that growth in anteroposterior length and vertical and horizontal diameters stops by age 1 year. The inference drawn from these studies is that, in young children, there is diminished room for safe passage of the block needle because the globe occupies a greater proportion of the orbit. Consequently, we caution against the use of 50-mm needles and the performance of deep retrobulbar blocks in children. Furthermore, we contend that a short-needle extraconal approach may be safer in this population.
Yao et al1 advocate use of a blunt-tip needle; however, the optimal needle bevel morphology is debatable. Sharp-tipped needles are less painful on insertion but pierce sclera more readily. Conversely, blunt-beveled needles require greater force to penetrate the globe, so ocular injury from dull needles tends to be more severe.7 Hay et al8 conducted a retrospective analysis of 23 cases of globe perforation and found that needle entry into the eye occurred with both sharp and blunt needles with no statistical difference in ultimate visual outcome. Therefore, we would not necessarily advocate in favor of blunt needles over sharp needles.
Finally, one may argue that regional anesthesia should be under the purview of anesthesiologists. In meeting this challenge, tomorrow's anesthesiologist must attain competency in regional ophthalmic anesthesia.