POCUS and Optic Nerve Sheath Diameter

“Optic nerve sheath diameter (ONSD) measurements with ultrasound have the potential to truly revolutionize the management of stroke in lowresource settings, where no CT scan is available.”

Introduction

Low- and middle-income countries (LMICs) shoulder the vast majority of global stroke burden, including overall stroke prevalence and stroke mortality.¹ Computed tomography (CT) of the brain is a critical diagnostic tool for discerning between the 2 major stroke types, ischemic and hemorrhagic, which differ significantly in their management. Unfortunately, access to CT scan is often very limited— or entirely absent—for many patients living in LMICs. The optic nerve sheath, which lies behind the eye and is continuous with the intracranial cerebrospinal fluid, can be readily measured using point-of-care ultrasound (POCUS) and has been shown to have increased diameter in the setting of increased intracranial pressure.² An ongoing study in Zambia is evaluating whether optic nerve sheath diameter (ONSD) can be used to discern between hemorrhagic and ischemic stroke types— which, if positive, could potentially alter stroke management in LMICs, where no CT scan is available.

Case Example

I was scanning with one of my 3rd-year Family Medicine resident physicians in the admission ward of Zambia’s largest tertiary care center, University Teaching Hospital. We had just started data collection for a study to evaluate the utility of ONSD, as measured by POCUS, in discerning between hemorrhagic and ischemic stroke types. While this hospital had a working CT scanner available for its stroke patients, most government hospitals in Zambia do not. Indeed, throughout my time working as a physician in Zambia, only a minority of my stroke patients ever underwent CT scanning.

Upon placing the Butterfly device over the patient’s closed eyelid, the resident remarked “this looks wide to me.” I took a closer look. “Yes I agree. Let’s try the long axis view.” In fact, the majority of ONSD measurements on this patient were measuring wide.

Something else caught my resident’s eye. “Doc, could this be optic disc elevation?” I took a closer look, feeling pretty skeptical. Optic disc elevation, the ultrasound equivalent of optic disc swelling or papilledema, is an area of ongoing research, but some initial data suggests it may outperform ONSD in detecting increased intracranial pressure.³ While our study was more focused on ONSD, we had agreed that we would also look for optic disc elevation.

I took a close look at the Butterfly images. I couldn’t believe my eyes. “Yes, that is optic disc elevation for sure. Great find!” At this point in our study, we find ourselves in the unique position of finding these positive findings—such as wide ONSD and elevated optic disc—but there’s not enough research yet for us to be able to apply them to patient management. But, as we collect more data on more stroke patients here in Zambia, hopefully that time will come soon.

Sample Imaging Exam

What does this teach us?

POCUS has enormous potential in LMICs, where patients often have limited imaging options. Novel applications —including optic ultrasound as being investigated in this study—have the potential to transform healthcare diagnostics in LMICs globally.

Introduction

1. Lanas F, Seron P. Facing the stroke burden worldwide. Lancet 2021;9:e236. https://doi.org/10.1016/ S2214-109X(20)30520-9
2. Kimberly HH, Shah S, Marill K, Noble V. Correlation of optic nerve sheath diameter with direct measurement of intracranial pressure. Academic Emergency Medicine 2008;15:201-204.
3. Lochner P, et al. Feasibility and usefulness of ultrasonography in idiopathic intracranial hypertension or secondary intracranial hypertension. BMC Neurol. 2016;16:85.

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