Automated Measurement of Optic Nerve Sheath Diameter

BACKGROUND:

A promising tool under investigation for the non-invasive estimation of ICP is Optic Nerve
Ultrasound (ONUS). Raised ICP results in distension of the optic nerve sheath (ONS), a
continuation of the dura mater. Ocular imaging, performed by clinicians using point-of-care
ultrasound machines, can detect ONS distension behind the eye. A study conducted at the
University of Michigan identified an ONS Diameter (ONSD) cutoff of >0.51cm as having 98%
sensitivity and 91% specificity for the detection of intracranial hypertension, defined as
ICP>25mmHg. The significant variation seen in the optimal ONSD threshold for identification
of high ICP across several studies, however, greatly limits the practical application of this
technique at the bedside. Much of this variation in the optimal ONSD threshold is likely
related to technique, with variation in the margins used to define the ONS on acquired
ultrasound images by different operators. The ONS, visible as a linear hypodense structure
behind the eye can vary in its visualized dimension based on the angle and plane of
insonation. Automated image analysis may permit standardization of ONSD measurement and
thereby minimize interobserver variability.

SPECIFIC AIM:

The investigators goal is to develop a computer image-analysis algorithm to standardize
measurement of the ONSD from ultrasound videos, and to validate against the reference
standard of expert manual measurement of ONSD.

METHODS:

Development of image-analysis algorithm: Videos in DICOM format of prior ONUS studies
performed for clinical purposes in the neurointensive care unit will be deidentified and used
for initial development and modification of the automated image analysis algorithm, prior to
prospective enrollment of subjects for determination of accuracy.

Under IRB approval, the investigators will perform Optic Nerve Ultrasound (ONUS) on eligible
subjects admitted to the ICU following informed consent of the patient or appropriate
surrogate.

Optic Nerve Ultrasound and ONSD measurement: These patients will undergo ONUS in the ICU.
ONUS is performed with the patient's eye closed, and with a linear array transducer placed on
the upper margin of the orbit to obtain a sonographic video clip of the eye, followed by
bedside measurement of ONSD 3mm behind the level of the posterior scleral border. Imaging
will be performed for both eyes for each patient. The expert investigator performing the ONUS
study and manual ONSD measurement will be blinded to the patient's ICP by turning the monitor
away from the sonographer from the time of entry into the patient's room until exit. The
corresponding ICP from the invasive monitor will be separately documented by the bedside
nurse. The ONUS video corresponding to the highest measured ONSD will be submitted in DICOM
format for automated image analysis.

Equipment: Sonosite M-Turbo point-of-care ultrasound machine and an L25 linear array
transducer with an ophthalmic preset.

Statistical Analysis:

- Simple descriptive statistics including means and standard deviations

- Simple Pearson correlation to allow for visual inspection across a range of values

- Tukey mean-difference plot (Bland - Altman plot) will be used to assess agreement
between the two methods, the ultrasound video analysis of optic nerve sheath diameter
and ONSD ultrasound interpretation by a clinician. Limits of agreement will be
calculated with standard errors and 95% confidence intervals.

Inclusion Criteria:

- Admitted to an ICU in the University of Michigan with Acute Brain Injury

- Age> 18 years

- Invasive ICP monitor (external ventricular drain or intraparenchymal catheter) already
in place for a clinical indication.

Exclusion Criteria:

- Pre-existing ocular pathology other than refractive error

- Traumatic globe injury