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

Abstract

Background Since Atkinson’s 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 United States Food and Drug Administration have defined guidelines for safe use of ultrasound for ophthalmic examination but most ultrasound devices used by anesthesiologists are not Food and Drug Administration-approved for ocular application because they generate excessive energy. Regulating agencies state that ultrasound examination 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 non-orbital-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 two devices: 1) the Sonosite Micromaxx (non-orbital-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 four rabbits. The non-orbital-rated transducer produced increases in ocular tissue temperature that surpassed the safe limit (increases> 1.50C ) in the lens of three rabbits (at 5.0, 5.5 and 1.5 minutes) and cornea of two rabbits (both at 1.5 minutes). A secondary analysis of temporal temperature differences between the orbital-rated and non-orbital 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 non-orbital-rated ultrasound machine (Sonosite Micromaxx) increases the ocular tissue temperature. A larger study is needed to establish safety. Until then, ophthalmic blocks performed with ultrasound should be performed only with ocular-rated devices.

A randomised controlled trial of periconal eye blockade with or without ultrasound guidance

“…However, atrophy of the optic nerve is a late complication that may also be caused by direct injury to the central retinal artery or injection or bleeding into the nerve sheath . The arguments against ultrasound techniques, such as the time that they take to learn and perform, should be secondary to safety concerns, particularly as we found that ultrasound prolonged blockade by only 15 s. To limit any heat damage and mechanical disruption that ultrasound can cause to the retina, practitioners should abide by the British Medical Ultrasound Society safety limits, which are a thermal index < 1.0 and a mechanical index < 0.23 . Fewer participants were satisfied with the ultrasound‐guided technique, possibly as a result of discomfort caused by pressure exerted on the eye by the probe.…”

“…We placed a 22‐G intravenous catheter and sedated participants with intravenous midazolam 2–3 mg and fentanyl 30–50 μg. We placed a B‐Scan linear array ultrasound transducer at the supraorbital rim (Sonomed Escalon; Master‐Vu USB Ultrasound System, New York City, NY, USA), which emitted waves at 12 MHz with a circular footprint focussed at 6 cm . Longitudinal and axial approaches obtained transocular images of the globe and axial length, respectively.…”

Evaluation of Iridociliary and Lenticular Elasticity Using Shear-Wave Elastography in Rabbit Eyes

“…Another limitation of the present study is that, although the rabbit eye model has been extensively used in experimental studies, it is also known to present distinct anatomical and physiological differences compared with the human eye (20). Finally, the ocular tissues are particularly susceptible to mechanical and thermal injury associated with excessive ultrasound energy, so that the Food and Drug Administration (FDA) and World Federation for Ultrasound in Medicine and Biology have imposed strict thermal index (TI) and mechanical index (MI) limits for diagnostic ocular ultrasound applications (TI < 1.0, MI < 0.23) (21). The fact that the system employed in this study was not ocular-rated implies that the methodology described may not be directly applicable in human eyes in its present form.…”

Ultrasound-Guided Eye Blocks

“…Currently, ultrasound-guided ophthalmic blocks may be impractical because orbitalrated equipment is not readily available in most operating room suites, 6 and technical hurdles may translate into delayed turnover times. We laud the authors for pioneering this technique and stimulating interest for further research.…”