title: A unique method for estimating the reliability learning curve of optic nerve sheath diameter ultrasound measurement.
abstract: BACKGROUND
  Optic nerve sheath diameter (ONSD) measurement using ultrasound has been proposed as a rapid, non-invasive, point of care technique to estimate intra-cranial pressure (ICP). Ultrasonic measurement of the optic nerve sheath can be quite challenging and there is limited literature surrounding learning curves for this technique. We attempted to develop a method to estimate the reliability learning curve for ONSD measurement utilizing a unique definition of reliability: a plateau in within-subject variability with unchanged between-subject variability.
  
  
  METHODS
  As part of a previously published study, a single operator measured the ONSD in 120 healthy volunteers over a 6-month period. Utilizing the assumption that the four measurements made on each subject during this study should be equal, the relationship of within-subject variance was described using a quadratic-plateau model as assessed by segmental polynomial (knot) regression.
  
  
  RESULTS
  Segmental polynomial (knot) regression revealed a plateau in within-subject variance after the 21st subject. However, there was no difference in overall mean values [3.69 vs 3.68 mm (p = 0.884)] or between-subject variance [14.49 vs 11.92 (p = 0.54)] above or below this cutoff.
  
  
  CONCLUSIONS
  This study suggests a significant finite learning curve associated with ONSD measurements. It also offers a unique method of calculating the learning curve associated with ONSD measurement.
authors: Departments of Surgery, University of Manitoba, 409 Tache Avenue, Winnipeg, MB, R2H 2A6, Canada.
authors: Frederick A
authors: FA
authors: Zeiler
authors: Departments of Surgery, University of Manitoba, 409 Tache Avenue, Winnipeg, MB, R2H 2A6, Canada.
authors: Markus T
authors: MT
authors: Ziesmann
authors: Undergraduate Medical Education, University of Manitoba, Winnipeg, MB, Canada.
authors: Patrick
authors: P
authors: Goeres
authors: Medical Education, University of Manitoba, Winnipeg, MB, Canada.
authors: Bertram
authors: B
authors: Unger
authors: Departments of Surgery, University of Manitoba, 409 Tache Avenue, Winnipeg, MB, R2H 2A6, Canada.
authors: Jason
authors: J
authors: Park
authors: Departments of Internal Medicine, University of South Carolina, School of Medicine, Columbia, SC, USA.
authors: Dimitrios
authors: D
authors: Karakitsos
authors: Departments of Internal Medicine and Emergency Medicine, University of South Carolina, School of Medicine, Columbia, SC, USA.
authors: Michael
authors: M
authors: Blaivas
authors: Departments of Surgery, University of Manitoba, 409 Tache Avenue, Winnipeg, MB, R2H 2A6, Canada.
authors: Ashley
authors: A
authors: Vergis
authors: Departments of Surgery, University of Manitoba, 409 Tache Avenue, Winnipeg, MB, R2H 2A6, Canada. gillmanlm@yahoo.ca.
authors: Lawrence M
authors: http://orcid.org/0000-0003-2429-9740
authors: LM
authors: Gillman
keywords: Education; Intracranial pressure; Learning curve; Optic nerve sheath diameter; Point of Care ultrasound; Ultrasound