Ultrasound Skills I: Sonographic Intracranial Pressure Monitoring

Identifying which patients have raised intracranial pressure can be difficult. An altered level of consciousness is the crucial indicator, because early symptoms like severe headache or vomiting are non-specific, and most specific signs occur late - signs like an ipsilateral mydriasis (a "blown pupil") from oculomotor nerve compression, which often precedes the classic “down-and-out” gaze palsy, and finally Cushing’s triad (hypertension as a compensatory response to brainstem compression, bradycardia as a vagal response to the hypertension, and irregular breathing due to direct compression of respiratory centres). Even the classic finding of papilloedema takes time to develop, and requires both an ophthalmoscope and a keen eye to appreciate - and is a skill that's rapidly becoming a rarity for clinicians (the author being no exception).

In minor head injury we want to be able to detect if there is a raised ICP. This is because the signs and symptoms of raised ICP are the canary in the coal mine - they portend the intracranial pathology that would require CT and/or neurosurgical input. Assessing whether there is evidence of raised ICP therefore forms a large part of your clinical assessment in the head injured patient, and in resource limited centres, may also help inform triage decisions about which patients we should prioritise scanning.

In severe head injury, we want to be able to monitor the ICP, and because this patient sub-group is often intubated, serial GCS measurement is often no longer possible. In centres like Bara, invasive ICP monitoring with devices like Codman bolts is not the norm. Being able to identify a worsening ICP guides us in whether we need to be escalating our analgesia or sedation, informs decisions about when to start using osmotherapeutic agents like hypertonic saline or mannitol, and may assist us in advocating for further neurosurgical involvement.

In either case - whether it's part of identification in the minor injury, or monitoring in the severe injury - an early, rapid, and non-resource intensive method of ICP estimation would be a very useful tool to have at your disposal.

Ocular ultrasound

Using a high-frequency linear probe (at least 7.5-10 MHz), scan the (closed) eye. You can use a Tegaderm or similar to cover the eye to protect it from ultrasound gel if you so wish (being careful not to trap air pockets beneath it that will interfere with image acquisition). Obviously don't do ocular ultrasound if the patient has a globe rupture, and in all patients try to keep your acoustic output as low as reasonably achievable. Always use the ophthalmic preset, as this will keep the thermal index <1.

(image adapted from Sinai EM)

Optic nerve sheath diameter

The sheath surrounding the optic nerve is continuous with the dura mater and so the pressure within it should closely approximate intracranial pressure. Because the optic nerve is surrounded by CSF - as pressure increases, so too should the volume of the sub-arachnoid space surrounding it. We can measure this volume through a surrogate marker: diameter.

By convention, ONSD is measured 3mm posterior to the retina - this is because the contrast between the echogenicity of structures is often most pronounced here, and because cadaveric studies seem to suggest this area is the most expansile part of the sheath (and therefore, theoretically, the most sensitive place to measure) [1]. Measure from one edge of the hypoechoic sheath to the other, and do this in 2 planes, transverse and sagittal - taking the average of the two values. Repeat this for the other eye.

(Images obtained by the author using a Butterfly iQ+ on the ophthalmic preset (TI 0.01, MI 0.2))

So what's our cut off for a "raised ICP"? A lot of studies have attempted to answer this question - and full disclosure - the answer varies. Like, a lot. ROC curve analysis has shown the optimal threshold for declaring a "raised ICP" could be anywhere between 4.85mm [2] and 5.9mm [3]. This is very confusing, but these studies were all quite small in nature, and used different frequency probes with differing image quality. The best evidence probably comes from the meta-analysis by Ohle et al (n=478) [4]. They found that when compared to CT-proven raised ICP (which is not necessarily the gold standard for ICP measurement), an ONSD >5mm had a sensitivity of 95.6% (95%CI, 87.7%-98.5%) and a specificity of 92.3% (95%CI, 77.9%-98.4%). If expressing this in a more Bayesian way gets you going, then this equates to a +LR of 12.4 and a -LR of 0.05 for this 5mm cut-off (that's pretty good!)

Given the heterogenous nature of the literature, this still wasn't quite good enough for the folks at PulmCrit [5] - who felt while 5mm had sufficient sensitivity, 6mm had greater specificity, and so while <5mm would likely be sufficient in ruling out elevated ICP, only >6mm was sufficient to rule it in.* They suggest using the presence or absence of papilloedema as the deciding factor for those trapped in the clinical nether-regions betwixt the two (discussed further in its own section below).

Some centres have tried to further increase the accuracy of this technique by adjusting for confounders like patient size. This is done by expressing ONSD as a proportion of eyeball transverse diameter (ETD), which can be obtained by simply finding and measuring the point of maximal eyeball width in the transverse plane.

One large-ish study that obtained baseline data in healthy volunteers was Kim et al (n=585) [6], who found a mean ONSD of 4.11mm and the mean ONSD:ETD ratio of 0.18. Zhu et al (n=104) [7] compared comatose patients with supratentorial space-occupying lesions (ONSD:ETD 0.27) and healthy controls (ONSD:ETD 0.22). This shows some promise as a clinical parameter, but at the time of writing no large studies have been conducted to inform what cut-off value is most useful. One small study on paediatric head trauma found that a cut-off of 0.22 had a sensitivity of 100% and specificity of 88% [8] - but I doubt this would be generalisable to adults if the results of Zhu et al are to be believed.

The important point here is that a single measurement is less important than a dynamic change. Studies have shown fair-to-good interobserver reliability between emergency physicians, ultrasonographers, and ophthalmologists when measuring ONSD [9], but the best test will be a single observer taking serial measurements. A rational approach would be to take a baseline measurement as part of your initial assessment, and subsequent measurements as part of on-going periodic neurological observations. Studies performed on patients immediately before and after lumbar puncture seem to suggest ONSD does react to changes in ICP in real time, so what you're measuring likely does represent the contemporaneous ICP [10]. Serial ONSD measurements have also been used in the identification of high altitude cerebral oedema (HACE) - one case report demonstrated an increase in ONSD from 6mm to 7mm when ascending from 3840m to 4321m [11].

Papilloedema

Unlike ONSD, which is a continuous variable, whether there's papilloedema or not is rather binary. It's either there or it isn't - you kind of just have to (if you'll pardon the pun) eyeball it. Bear in mind that oedema doesn't develop immediately, so an acute rise in ICP may not yet manifest itself in the form of papilloedema, but for established elevations in ICP, ultrasonographic papilloedema does seem to be a fairly sensitive and specific finding [12,13,14].

Take home points:

Ocular ultrasound shows some promise in the detection and monitoring of raised ICP. As is the case for most clinical tools at our disposal, it is but one more addition to our toolkit that requires interpretation alongside the other elements that form our global assessment. It should not be used in place of CT imaging or override the use of validated head injury decision rules, but may be a useful adjunct to assessment, as well as assisting in optimising the care of those patients who have a CT-proven intracranial injury. It's likely that its value is greater in Low and Middle Income Countries (LMIC) where the access to imaging is more limited, diagnostic delays are more common, and where invasive means of ICP monitoring are not available.

Notes

*Note that paediatric cut-offs differ again.

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Dr Nick Chapman is a senior emergency medicine registrar, and has recently passed his written fellowship exams for the Australasian College of Emergency Medicine. He has a strong interest in both trauma and retrieval medicine, and completed his Postgraduate Diploma in Aeromedical Retrieval in 2021. He has previously worked at The Alfred Hospital's Emergency & Trauma Centre, but is hanging up his scrubs in favour of overalls as he heads to the Royal Flying Doctor Service.

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References

[1] Helmke K, Hansen HC. Fundamentals of transorbital sonographic evaluation of optic nerve sheath expansion under intracranial hypertension. I. Experimental study. Pediatr Radiol. 1996;26(10):701-5.

[2] Amini A, Eghtesadi R, Feizi AM, et al. Sonographic optic nerve sheath diameter as a screening tool for detection of elevated intracranial pressure. Emerg (Tehran). 2013;1(1):15-9.

[3] Geeraerts T, Launey Y, Martin L, et al. Ultrasonography of the optic nerve sheath may be useful for detecting raised intracranial pressure after severe brain injury. Intensive Care Med. 2007;33(10):1704-11.

[4] Ohle R, McIsaac SM, Woo MY, Perry JJ. Sonography of the optic nerve sheath diameter for detection of raised intracranial pressure compared to computed tomography: a systematic review and meta-analysis. J Ultrasound Med. 2015;34(7):1285-94.

[5] PulmCrit. Algorithm for diagnosing ICP elevation with ocular sonography. 2017. Available from: https://emcrit.org/pulmcrit/pulmcrit-algorithm-diagnosing-icp-elevation-ocular-sonography/.

[6] Kim DH, Jun JS, Kim R. Ultrasonographic measurement of the optic nerve sheath diameter and its association with eyeball transverse diameter in 585 healthy volunteers. Sci Rep. 2017;7(1):15906.

[7] Zhu S, Cheng C, Zhao D, et al. The clnical and prognostic values of optic nerve sheath diameter and optic nerve sheath diameter/eyeball transverse diameter ratio in comatose patients with supratentorial lesions. BMC neurol. 2021;21(1):259.

[8] Şık N, Ulusoy E, Çitlenbik H, et al. The role of sonographic optic nerve sheath diameter measurements in pediatric head trauma. J Ultrasound. 2022;25(4):957-63.

[9] Le A, Hoehn ME, Smith ME, et al. Bedside sonographic measurement of optic nerve sheath diameter as a predictor of increased intracranial pressure in children. Ann Emerg Med. 2009;53(6):785-91.

[10] Chen L, Wang L, Hu Y, et al. Ultrasonic measurement of optic nerve sheath diameter: a non-invasive surrogate approach for dynamic, real-time evaluation of intracranial pressure. Br J Ophthalmol. 2019;103(4):437-41.

[11] Wipplinger F, Holthof N, Lienert J, et al. Point-of-care ultrasound diagnosis of acute high altitude illness: a case report. Wilderness Environ Med. 2021;32(2):204-9.

[12] Lochner P, Brio F, Zedde ML, et al. Feasibility and usefulness of ultrasonography in idiopathic intracranial hypertension or secondary intracranial hypertension. BMC Neurol. 2016;16:85.

[13] Bäuerle J, Nedelmann M. Sonographic assessment of the optic nerve sheath in idiopathic intracranial hypertension. J Neurol. 2011;258(11):2014-9.

[14] Carter SB, Pistilli M, Livingston KG, et al. The role of orbital ultrasonography in distinguishing papilledema from pseudopapilledema. Eye (Lond). 2014;28(12):1425-30.

[15] Marchese RF, Mistry RD, Scarfone RJ, Chen AE. Identification of optic disc elevation and the crescent sign using point-of-care ocular ultrasound in children. Pediatr Emerg Care. 2015;31(4):304-7.