title: Sub-Tenon's injection in equine cadaver eyes: MRI visualization of anesthetic fluid distribution and comparison of two different volumes.
abstract: OBJECTIVE
  To compare the localization and distribution of two different anesthetic fluid volumes around equine cadaver eyes to determine an appropriate volume for a single sub-Tenon's injection in horses.
  
  
  METHODS
  A single sub-Tenon's injection of 2% lidocaine was performed in 10 equine cadaver heads (20 eyes) using two different volumes (7 mL on one side and 10 mL on the opposite side). The posterior circular distribution of the anesthetic was quantified in sagittal, dorsal, and transverse MRI (T2W-TSE) sequences and evaluated independently by three board-certified radiologists. The distribution of the two fluid volumes was compared via a paired Student's t-test. The interobserver reliability was evaluated via a Kruskal-Wallis test.
  
  
  RESULTS
  Extension of the injection fluid was observed along the dorsal and temporal quadrants of the globe within the subconjunctival space, the anterior and posterior sub-Tenon's space, and into the muscle sheaths along the extraocular muscles. Accumulation of anesthetic fluid directly surrounding the optic nerve was detected in three of 20 cadaver eyes. Circular distribution of the 7 and 10 mL anesthetic volumes was not significantly different (P = 0.849). More retrograde leakage of the anesthetic was observed using the 10 mL volume. Evaluation of interobserver reliability revealed no significant differences between observers (P = 0.21-0.92).
  
  
  CONCLUSIONS
  Sub-Tenon's anesthesia can have potential as an alternative to retrobulbar anesthesia for ophthalmic surgeries in equines. A 7- to 10-mL injection volume should be appropriate based on the results of this study. The distribution of the anesthetic solution in live tissues, the clinical effects, and the potential for complications will have to be evaluated in vivo.
authors.affiliation: Ophthalmology Unit, Vetsuisse Faculty, Equine Department, University of Zurich, Winterthurerstrasse 260, Zurich, CH-8057, Switzerland.
authors.forename: Silvia
authors.initials: S
authors.lastname: Stadler
authors.affiliation: Diagnostic Imaging Unit, Vetsuisse Faculty, Department for Small Animals, University of Zurich, Zurich, CH-8057, Switzerland.
authors.forename: Matthias
authors.initials: M
authors.lastname: Dennler
authors.affiliation: Institute of Veterinary Pathology, Vetsuisse Faculty, University of Zurich, Zurich, CH-8057, Switzerland.
authors.forename: Udo
authors.initials: U
authors.lastname: Hetzel
authors.affiliation: Diagnostic Imaging Unit, Vetsuisse Faculty, Department for Small Animals, University of Zurich, Zurich, CH-8057, Switzerland.
authors.forename: Francesca
authors.initials: F
authors.lastname: Del Chicca
authors.affiliation: Diagnostic Imaging Unit, Vetsuisse Faculty, Department for Small Animals, University of Zurich, Zurich, CH-8057, Switzerland.
authors.forename: Sèamus
authors.initials: S
authors.lastname: Hoey
authors.affiliation: Ophthalmology Unit, Vetsuisse Faculty, Equine Department, University of Zurich, Winterthurerstrasse 260, Zurich, CH-8057, Switzerland.
authors.forename: Bernhard M
authors.initials: BM
authors.lastname: Spiess
authors.affiliation: Ophthalmology Unit, Vetsuisse Faculty, Equine Department, University of Zurich, Winterthurerstrasse 260, Zurich, CH-8057, Switzerland.
authors.forename: Katrin
authors.initials: K
authors.lastname: Voelter
authors.affiliation: Ophthalmology Unit, Vetsuisse Faculty, Equine Department, University of Zurich, Winterthurerstrasse 260, Zurich, CH-8057, Switzerland.
authors.forename: Simon A
authors.initials: SA
authors.lastname: Pot
mesh_terms: D000779:Anesthetics, Local / Q000009:adverse effects* / Q000493:pharmacokinetics; D000818:Animals; D004305:Dose-Response Relationship, Drug; D006736:Horses; D056965:Injections, Intraocular / Q000662:veterinary*; D008012:Lidocaine / Q000008:administration & dosage* / Q000493:pharmacokinetics; D008279:Magnetic Resonance Imaging / Q000662:veterinary*; D010865:Pilot Projects; D058475:Tenon Capsule
chemical_list: D000779:Anesthetics, Local; D008012:Lidocaine
keywords: equine; injection; local anesthetic; ophthalmic surgery; sub-Tenon's