title: 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 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.
authors.affiliation: Department of Anesthesiology, Perioperative Medicine and Pain Management, Miller School of Medicine, 900 NW 17th Street, Miami FL 33136, USA. hpalte@miami.edu
authors.forename: Howard D
authors.initials: HD
authors.lastname: Palte
authors.forename: Steven
authors.initials: S
authors.lastname: Gayer
authors.forename: Esdras
authors.initials: E
authors.lastname: Arrieta
authors.forename: Eric
authors.initials: E
authors.lastname: Scot Shaw
authors.forename: Izuru
authors.initials: I
authors.lastname: Nose
authors.forename: Elizabete
authors.initials: E
authors.lastname: Lee
authors.forename: Kristopher L
authors.initials: KL
authors.lastname: Arheart
authors.forename: Sander
authors.initials: S
authors.lastname: Dubovy
authors.forename: David J
authors.initials: DJ
authors.lastname: Birnbach
authors.forename: Jean-Marie
authors.initials: JM
authors.lastname: Parel
mesh_terms: D000818:Animals; D001831:Body Temperature; D004867:Equipment Design; D004869:Equipment Safety; D005123:Eye / Q000000981:diagnostic imaging* / Q000294:innervation / Q000473:pathology; D005131:Eye Injuries / Q000209:etiology* / Q000473:pathology / Q000503:physiopathology; D008422:Materials Testing; D009407:Nerve Block / Q000009:adverse effects / Q000295:instrumentation*; D011817:Rabbits; D018570:Risk Assessment; D012307:Risk Factors; D013314:Stress, Mechanical; D013817:Thermography; D013997:Time Factors; D014159:Transducers / Q000009:adverse effects; D018084:Ultrasonography, Interventional / Q000009:adverse effects / Q000295:instrumentation*