Maria Tran, DO
Published April. 26, 2022
Probetrotters | Medical Education
Study Design
The design is a single center prospective observational study.
Objective
Utilizing ocular ultrasound to measure the optic nerve sheath diameter (ONSD) as a potential method to predict mortality and functional outcomes among stroke patients presenting to the emergency department and ICU.
Methods
Bilateral optic nerve sheath diameter (ONSD) was measured on arrival within 6 hours and subsequently on the second day of hospitalization. ONSD was measured bilaterally in transverse and longitudinal orientation for a total of four measurements. Primary outcome was inpatient survival and secondary outcomes were functional status at 3 and 6 months as measured by Cerebral Performance Category (CPD) and modified Rankin scale (mRS). Statistical analysis were student T/Mann-Whitney U tests for continuous variables, while chi-square and/or Fisher exact tests were for categorical variables. Patients were excluded if they had suspected globe trauma, known retinal or optic nerve disease, previous enucleation, were pending emergent surgery or if enrollment into the study would conflict with clinical management or interventions.
What are some possible flaws in the methods used?
Only one center was used, and data cannot be generalized to the rest of the population. Ultrasounds were performed and or supervised by a PI who was ultrasound fellowship trained and cannot be extrapolated to all EM physicians. There were didactic and training sessions for the sonographers. Also, it was a convenience sample when only the PI was available, so may not capture all potential patients.
Meiburger KM, Naldi A, Michielli N, et al. Automatic optic nerve measurement: A new tool to standardize optic nerve assessment in ultrasound B-mode images. Ultrasound in Medicine & Biology. 2020;46(6):1533-1544. doi:10.1016/j.ultrasmedbio.2020.01.034
Results
1, 124 acute ischemic stroke and 278 acute intracranial hemorrhage patients, with the enrollment of 86 subjects. Mean age was 67.2 years and 54.7% male. There was no difference between left and right eye measurements among all stroke subjects, nor between longitudinal and transverse measurements. Subjects with ICH (n= 29) had a significantly larger ONSD (average 0.59 cm left eye, average 0.6 cm right eye, p = 0.001) on day of admission compared with ischemic stroke patients (n= 57). Measurements of ONSD of both right and left optic nerves and on different days not significantly different between groups. The expired patients had higher ONSD, mean ONSD of those that died was 0.6 cm on first day and 0.63 cm on second day (p<0.001). Among ischemic strokes, for every 0.1cm increase in ONSD, the odds of death were 4.457 higher (p = 0.0905). For hemorrhagic stroke patients, risk of mortality was increased 6.22-fold (p = 0.0329). Poorer functional outcomes were also seen with elevated ONSD at 6 months after incidence of stroke. Use of tPa did not affect the ONSD.
Discussion
Normal ONSD in adults is approximately 0.5cm. Patients with stroke had elevated measurements of ONSD range from 0.59 cm to 0.63 cm. The ONSD is a promising marker of elevated ICP since the nerve sheath is continuous with that of the brain’s three layers of meninges- dura mater, arachnoid mater and pia mater. Due to this, CSF can travel freely between the intracranial subarachnoid space and that within the optic nerve sheath, thus a rise or drop in pressure within the cranium would translate to the same pressure changes in the optic nerve sheath. This study helps demonstrate that ocular ultrasound can be performed repeatedly at the bedside, with equipment present in most emergency departments and ICUs for the assessment of brain damage when neurologic exams prove to be difficult. Also, the benefits of ultrasound use can reduce the use of repeated CT imaging or invasive measurement devices.
Limitations
Limitations include that this single site had various increased access to US modalities that could not be present at all facilities. This was a convenience sample based on availability of a PI who is performing and supervising, this limits the patients included in the study. Another limitation is that even though the study demonstrates that 1 mm has a statistical difference, there remains the question of how measurements as small as 1 mm can affect the patient clinically. There is also the limitation of caliper use to measure the ONSD when minor movements and placing the caliper on the correct location can affect the value, especially for a novice sonographer.
Take Home Points
- Normal ONSD in adults are 0.5 cm, ONSD in stroke patients, ischemic or hemorrhagic average an increase in ONSD of 0.59 cm to 0.63 cm (with no difference in either eye).
- Increase in mortality and decrease in function at 6 months post- stroke is seen with increased ONSD
- Among ischemic strokes, for every 0.1cm increase in ONSD, the odds of death were 4.457 higher. For hemorrhagic stroke patients, risk of mortality was increased 6.22 fold.
- Cerebral edema associated with stroke can elevate ICP. Ultrasound use to measure ONSD can be an important noninvasive method to identify elevated ICP which can be associated with worse outcomes.
Does this study influence your practice?
Yes, it would be helpful to incorporate this additional tool in considering patient management and long term care. Using ONSD as an additional piece of information to form decisive management and predict outcomes to manage patient and patient family’s expectations.
[1] Patel R, Chowdhury MAB, Gul S, et al. Ultrasound of Optic Nerve Sheath Diameter and Stroke Outcomes. Crit Care Explor. 2021;3(11):e0565. Published 2021 Nov 11. doi:10.1097/CCE.0000000000000565 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8613366/
[2] Meiburger KM, Naldi A, Michielli N, et al. Automatic optic nerve measurement: A new tool to standardize optic nerve assessment in ultrasound B-mode images. Ultrasound in Medicine & Biology. 2020;46(6):1533-1544. doi:10.1016/j.ultrasmedbio.2020.01.034