Howard D. Palte

Are Ultrasound-Guided Ophthalmic Blocks Injurious to the Eye? A Comparative Rabbit Model Study of Two Ultrasound Devices Evaluating Intraorbital Thermal and Structural Changes

BACKGROUND: Since Atkinsons original description of retrobulbar block in 1936, needle-based anesthetic techniques have become integral to ophthalmic anesthesia. These techniques are unfortunately associated with rare, grave complications such as globe perforation. Ultrasound has gained widespread acceptance for peripheral nerve blockade, but its translation to ocular anesthesia has been hampered because sonic energy, in the guise of thermal or biomechanical insult, is potentially injurious to vulnerable eye tissue. The US Food and Drug Administration (FDA) has defined guidelines for safe use of ultrasound for ophthalmic examination, but most ultrasound devices used by anesthesiologists are not FDA-approved for ocular application because they generate excessive energy. Regulating agencies state that ultrasound examinations can be safely undertaken as long as tissue temperatures do not increase >1.5°C above physiological levels. METHODS: Using a rabbit model, we investigated the thermal and mechanical ocular effects after prolonged ultrasonic exposure to single orbital- and nonorbital-rated devices. In a dual-phase study, aimed at detecting ocular injury, the eyes of 8 rabbits were exposed to continuous 10-minute ultrasound examinations from 2 devices: (1) the Sonosite Micromaxx (nonorbital rated) and (2) the Sonomed VuMax (orbital rated) machines. In phase I, temperatures were continuously monitored via thermocouples implanted within specific eye structures (n = 4). In phase II the eyes were subjected to ultrasonic exposure without surgical intervention (n = 4). All eyes underwent light microscopy examinations, followed at different intervals by histology evaluations conducted by an ophthalmic pathologist. RESULTS: Temperature changes were monitored in the eyes of 4 rabbits. The nonorbital-rated transducer produced increases in ocular tissue temperature that surpassed the safe limit (increases >1.5°C) in the lens of 3 rabbits (at 5.0, 5.5, and 1.5 minutes) and cornea of 2 rabbits (both at 1.5 minutes). A secondary analysis of temporal temperature differences between the orbital-rated and nonorbital transducers revealed statistically significant differences (Bonferroni-adjusted P < 0.05) in the cornea at 3.5 minutes, the lens at 2.5 minutes, and the vitreous at 4.0 minutes. Light microscopy and histology failed to elicit ocular injury in either group. CONCLUSIONS: The nonorbital-rated ultrasound machine (Sonosite Micromaxx) increases the ocular tissue temperature. A larger study is needed to establish safety. Until then, ophthalmic ultrasound-guided blocks should only be performed with ocular-rated devices.

Ultrasound-guided ophthalmic regional anesthesia.

Purpose of reviewNeedle-based and cannula-based eye blocks are ‘blind’ techniques prone to rare but serious complications. Ultrasound, an established adjunct for peripheral nerve block, may be beneficial for ophthalmic anesthesia application. The present review details the evolution of ultrasound-guided eye blocks, outlines safety issues, and reviews recent studies and editorial opinions. Recent findingsUltrasound-assisted ophthalmic regional anesthesia allows imaging of key structures such as the globe, orbit, and optic nerve. Recent findings reveal that needle path is not reliably predictable by clinical evaluation. Needle tips are frequently found to be intraconal, extraconal, or transfixed in the muscle cone independent of the intended type of block. In addition, contemporary human and animal studies confirm that real-time observation of local anesthetic spread inside of the muscle cone correlates directly with block success. SummaryUltrasound-guided ophthalmic regional anesthesia is evolving beyond simple visualization of the anatomy. Recent research emphasizes the imprecision of needle tip location without ultrasound and the key role of imaging local anesthetic dispersion. There is ongoing debate in the literature regarding the utility of routine ultrasound for eye blocks.

Postoperative analgesia and infant vitreoretinal surgery

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Motor-Sparing Surgical Nerve Blocks for Upper Extremity Surgery:Significantly Less Motor Paralysis Using 15 mL versus 30 mL ofMepivacaine 1.5% for Supraclavicular Block - A Prospective RandomizedDouble-Blinded Study

Background and objectives: We performed a prospective randomized double-blinded study evaluating if a reduced volume of local anesthetic would result in operative limb surgical anesthesia while decreasing motor paralysis during an ultrasound-guided supraclavicular nerve block. Current tendencies in clinical practice towards smaller injectate volumes during ultrasound-guided nerve block placement prompted our investigation on its impact regarding block quality. Methods: 43 patients were consented for this prospective, double-blinded randomized clinical trial. Each patient was randomly assigned. Group HIGH received the conventional injection dose of 30 mL of 1.5% Mepivacaine. Group LOW received the reduced volume dose of 15 mL. An ultrasound-guided supraclavicular nerve block was performed on each patient. Motor block and sensory perception to pin-prick were assessed in the nerve distributions for the ulnar, median, radial, and musculocutaneous branches at 5, 10, 15, 20, and 30 minutes post-injection. Results: Complete motor block in the radial, ulnar, musculocutaneous and median nerve distributions at 30 minutes, was present in 55% of patients in Group HIGH versus 10% in Group LOW and was statistically significant between both groups (p<0.01). The anatomic distribution of the observed motor-sparing was statistically significant in the median (p<0.01) and ulnar (p<0.05) nerve branches among those patients who received 15 mL LA boluses. Conclusions: Our study demonstrated that 15 mL vs. 30 mL injections of mepivacaine 1.5% at the supraclavicular approach provide equivalent surgical anesthesia, while reducing the incidence of motor block. These findings may have implications on early postoperative physical therapy for the subset of patients that present with Galeazzi-type fractures, carpal tunnel syndrome, and minimally-displaced distal radius fractures.