title: Optic nerve sheath fenestration using a Raman-shifted alexandrite laser.
abstract: OBJECTIVE
  Optic nerve sheath fenestration is an established procedure for relief of potentially damaging overpressure on the optic nerve resulting from idiopathic intracranial hypertension. Prior work showed that a mid-IR free-electron laser could be delivered endoscopically and used to produce an effective fenestration. This study evaluates the efficacy of fenestration using a table-top mid-IR source based on a Raman-shifted alexandrite (RSA) laser.
  
  
  METHODS
  Porcine optic nerves were ablated using light from an RSA laser at wavelengths of 6.09, 6.27, and 6.43 μm and pulse energies up to 3 mJ using both free-space and endoscopic beam delivery through 250-μm I.D. hollow-glass waveguides. Waveguide transmission was characterized, ablation thresholds and etch rates were measured, and the efficacy of endoscopic fenestration was evaluated for ex vivo exposures using both optical coherence tomography and histological analysis.
  
  
  RESULTS
  Using endoscopic delivery, the RSA laser can effectively fenestrate porcine optic nerves. Performance was optimized at a wavelength of 6.09 μm and delivered pulse energies of 0.5-0.8 mJ (requiring 1.5-2.5 mJ to be incident on the waveguide). Under these conditions, the ablation threshold fluence was 0.8 ± 0.2 J/cm(2) , the ablation rate was 1-4 μm/pulse, and the margins of ablation craters showed little evidence of thermal or mechanical damage. Nonetheless, nominally identical exposures yielded highly variable ablation rates. This led to fenestrations that ranged from too deep to too shallow-either damaging the underlying optic nerve or requiring additional exposure to cut fully through the sheath. Of 48 excised nerves subjected to fenestration at 6.09 μm, 16 ex vivo fenestrations were judged as good, 23 as too deep, and 9 as too shallow.
  
  
  CONCLUSIONS
  Mid-IR pulses from the RSA laser, propagated through a flexible hollow waveguide, are capable of cutting through porcine optic nerve sheaths in surgically relevant times with reasonable accuracy and low collateral damage. This can be accomplished at wavelengths of 6.09 or 6.27 μm, with 6.09 μm slightly preferred. The depth of ex vivo fenestrations was difficult to control, but excised nerves lack a sufficient layer of cerebrospinal fluid that would provide an additional margin of safety in actual patients.
authors.affiliation: Department of Physics & Astronomy, Vanderbilt University, Nashville, 37235, Tennessee.
authors.forename: John
authors.initials: J
authors.lastname: Kozub
authors.affiliation: Vanderbilt Eye Institute, Vanderbilt University, Nashville, 37232, Tennessee.|Vanderbilt Institute in Surgery and Engineering, Vanderbilt University, Nashville, 37235, Tennessee.
authors.forename: Jin H
authors.initials: JH
authors.lastname: Shen
authors.affiliation: Vanderbilt Eye Institute, Vanderbilt University, Nashville, 37232, Tennessee.|Vanderbilt Institute in Surgery and Engineering, Vanderbilt University, Nashville, 37235, Tennessee.|Vanderbilt Biophotonics Center, Vanderbilt University, Nashville, 37235, Tennessee.
authors.forename: Karen M
authors.initials: KM
authors.lastname: Joos
authors.affiliation: Vanderbilt Eye Institute, Vanderbilt University, Nashville, 37232, Tennessee.
authors.forename: Ratna
authors.initials: R
authors.lastname: Prasad
authors.affiliation: Department of Physics & Astronomy, Vanderbilt University, Nashville, 37235, Tennessee.|Vanderbilt Biophotonics Center, Vanderbilt University, Nashville, 37235, Tennessee.|Vanderbilt Institute for Integrative Biosystem Research & Education, Vanderbilt University, Nashville, 37235, Tennessee.
authors.forename: M Shane
authors.initials: MS
authors.lastname: Hutson
mesh_terms: D000818:Animals; D019299:Decompression, Surgical / Q000379:methods*; D004724:Endoscopy; D053844:Lasers, Solid-State / Q000627:therapeutic use*; D009408:Nerve Compression Syndromes / Q000209:etiology / Q000601:surgery*; D019635:Neurosurgical Procedures / Q000379:methods*; D009900:Optic Nerve / Q000601:surgery*; D011559:Pseudotumor Cerebri / Q000150:complications; D013059:Spectrum Analysis, Raman; D013552:Swine
keywords: ablation; free-electron laser; idiopathic intracranial hypertension; laser surgery; mid-infrared; pseudotumor cerebri