title: Sonography for determining the optic nerve sheath diameter with increasing intracranial pressure in a porcine model.
abstract: OBJECTIVE
  This study investigated whether it is feasible to use sonography to monitor changes in the optic nerve sheath diameter in a porcine model.
  
  
  METHODS
  A fiber-optic intracranial pressure transducer was surgically placed through the frontal sinus directly into the brain parenchyma of adult Yorkshire pigs (n = 5). A second bolt was placed on the contralateral side for intraparenchymal fluid infusion. Optic nerve sheath diameter measurements were acquired by each of 2 ultrasound operators around the leading edge of the nerve, 3 to 5 mm distal from the origin of the optic nerve. To induce a change in diameter, intracranial pressure was manipulated by injecting normal saline into the intraparenchymal infusion catheter located in the symmetric contralateral position as the pressure-monitoring probe.
  
  
  RESULTS
  Data from 1 pig were unusable because of a cerebrospinal fluid leak into the sinus and orbital fissure. Saline aliquots of 1 to 10 mL were able to generate intracranial pressures typically starting from 10 to 15 mm Hg and increasing to 75 to 90 mm Hg, which eventually evoked a Cushing response. Fluid injection was controlled to increase pressures by 60 mm Hg over a 15- to 20-minute period. Regression analysis of all animals showed that the optic nerve sheath diameter increased by 0.0034 mm/mm Hg of intracranial pressure; however, this slope ranged from 0.0025 to 0.0046, depending on the animal measured. There was no discernible effect of the ultrasound operator on the slope; however, measurements made by 1 operator were consistently higher than the others by about 8% of the overall diameter range.
  
  
  CONCLUSIONS
  These results suggest that the use of the optic nerve sheath diameter to noninvasively confirm acute changes in intracranial pressure over 1 hour is feasible in a porcine model. We recommend that this method be validated in humans using direct intracranial pressure measurement where possible to confirm it as a screening tool for acute and chronically increased diameters secondary to elevated pressure in clinical settings.
authors.affiliation: Wyle Laboratories/ National Aeronautics and Space Administration, Houston, TX 77058, USA. douglas.r.hamilton@nasa.gov
authors.forename: Douglas R
authors.initials: DR
authors.lastname: Hamilton
authors.forename: Ashot E
authors.initials: AE
authors.lastname: Sargsyan
authors.forename: Shannon L
authors.initials: SL
authors.lastname: Melton
authors.forename: Kathleen M
authors.initials: KM
authors.lastname: Garcia
authors.forename: Bill
authors.initials: B
authors.lastname: Oddo
authors.forename: David S
authors.initials: DS
authors.lastname: Kwon
authors.forename: Alan H
authors.initials: AH
authors.lastname: Feiveson
authors.forename: Scott A
authors.initials: SA
authors.lastname: Dulchavsky
mesh_terms: D000818:Animals; D004195:Disease Models, Animal; D005260:Female; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D019586:Intracranial Hypertension / Q000000981:diagnostic imaging* / Q000503:physiopathology; D007427:Intracranial Pressure; D009900:Optic Nerve / Q000000981:diagnostic imaging*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013552:Swine; D014463:Ultrasonography / Q000379:methods