Inter-rater reliability of optic nerve sheath diameter measurement using real-time ultrasonography

The recent spread of ultrasound technology and its adaptation at the bedside by emergency physicians has led to the exploration of a wide number of potential clinical applications [3]. Ocular ultrasonography is a relatively new application in emergency medicine. In 2002, Blaivas et al. published the first series of emergency department patients presenting with ocular symptoms who were evaluated by bedside emergency department ultrasonography [4]. The ability of ultrasound to evaluate the eye and the adjacent structures in a rapid and non-invasive manner is of tremendous value in the setting of a busy emergency department, where subspecialty consultation is not always available and physical exam may be limited by the patient’s condition. For example, a neurologic exam may be unobtainable in the unconscious, intubated and chemically paralyzed patient. The ocular exam may also be limited by severe facial and head trauma. Emergency department ultrasound provides a quick, accurate, well-tolerated and non-invasive tool for evaluating potentially vision-threatening and life-threatening conditions at the bedside [5].

Elevated intracranial pressure is a potentially devastating condition. Rapid detection and subsequent management of elevated intracranial pressure is critical as increased intracranial pressure is associated with increased morbidity, mortality and poor neurologic outcome. The gold standard for measurement of intracranial pressure is invasive and fraught with risk, requiring placement of intraventricular catheters and/or intraparenchymal probes [6].

Ultrasound measurement of the optic nerve sheath diameter (ONSD) is a rapid, non-invasive means to indirectly assess intracranial pressure [7]. The optic nerve is part of and continuous with the central nervous system via the meninges of the brain. Increases in intracranial pressure are transmitted to the retrobulbar region of the optic nerve sheath via cerebrospinal fluid. The ONSD is measured 3 mm posterior to the globe for both eyes. Though various cutoffs have been proposed, an ONSD greater than 5.8 mm is generally considered abnormal and elevated intracranial pressure should be considered [8‐12]. Given the need for accurate and precise measurement to use this as a screening exam, we sought to determine the inter-rater reliability between ONSD measurements obtained in real time by fellowship-trained ultrasound physicians.

The estimated intraclass correlation coefficient was 0.822 (0.625–0.924, 95% CI). Of the 30 measurements that were obtained, the average optic nerve sheath measurement was 4.34 mm, median was 4.30 mm and the standard deviation was 0.47. The measurements ranged from 3.5 mm to 5.2 mm.

In order for ocular ultrasound to serve as a sensitive screening tool in the assessment of elevated ICP, confidence and reliability of the measurement are of paramount importance. While multiple prior studies have detailed the correlation between increased ONSD and elevated ICP, no studies to our knowledge have assessed the ability of emergency physicians to obtain reliable measurements in real time. Oboerfell et al. described the ICC among a large group of emergency physicians at various stages of training measuring ONSD of static images [13]. They found an ICC among ultrasound fellowship-trained emergency physicians of 0.73, while resident emergency physicians had an ICC of 0.50. Because these measurements were based on static, previously obtained images, the ability of emergency physicians to both obtain and interpret ONSD measurements in a reliable manner was not addressed.

In our study, we assessed the ability of three ultrasound fellowship-trained emergency physicians to obtain measurements of ONSD independently and in real time on healthy volunteers. We found a high degree of measurement agreement with an ICC of 0.822. ICC is typically interpreted as optimal (>  0.8), strong (0.7–0.8), moderate (0.5–0.6), fair (0.3–0.4), or poor (0–0.2) [14, 15]. Our results suggest that ultrasound-trained emergency physicians can obtain images and measurements of the ONSD with a high degree of inter-rater reliability.

Our study is limited in its generalizability by the use of ultrasound operators who were fellowship trained in emergency ultrasound. It remains unclear how much specific training in ocular ultrasound is necessary for non-fellowship-trained physicians to accurately measure the ONSD. Additional research is necessary to assess whether non-fellowship-trained physicians can accurately image and measure the ONSD with a high degree of reliability.

Ultrasonographic measurement of ONSD has the potential to serve as a non-invasive screening tool for elevated intracranial pressure. ONSD obtained by ultrasound fellowship-trained emergency medicine physicians is a reliable measurement with a high degree of correlation between scanners.