Intracranial pressure (ICP) is determined by the production rate and outflow facility of cerebrospinal fluid (CSF) [1–4]; decreased outflow or increased production leads to elevated ICP and increased outflow or decreased production leads to decreased ICP. Since alterations in ICP are implicated in the pathogenesis of a number of life-threatening and vision-threatening conditions, such as traumatic brain injury (TBI) [5–8], certain forms of meningitis [9,10], large hemispheric ischemic strokes [11,12], subdural/epidural/intracerebral hemorrhages [13,14], and idiopathic intracranial hypertension (IIH) [15–19], measurement of ICP is necessary and common . However, all current methods used clinically to measure ICP are invasive and require direct CSF access . Although lumbar puncture is the least invasive procedure to measure ICP, it still carries the risk of significant side effects, such as site pain, low-tension headaches, chronic CSF leaks requiring further intervention, infections, neurologic dysfunction, and brainstem herniation [21–27]. In addition, lumbar puncture is contraindicated in certain conditions [28,29], including the presence of intracranial space-occupying lesions, Arnold-Chiari malformation, coagulopathies, and skin infections at the puncture site, making ICP determination more challenging in these cases.
Accordingly, there is a large body of literature proposing a number of noninvasive modalities to measure ICP [21,30–38] to complement and, in some cases, supplant the use of invasive measurement of ICP. The development of accurate, reproducible, objective, and portable noninvasive modalities to measure and monitor ICP has the potential to dramatically change the management of certain diseases, such as low-tension glaucoma, IIH, and acute TBI.
NONINVASIVE METHODS TO MEASURE INTRACRANIAL PRESSURE
There are two main categories of noninvasive methods to measure ICP, those that rely on changes in ocular structures, such as changes in optic nerve sheath diameter, retinal venous pulsations, and choroidal thickness; and those that utilize changes in nonocular structures, such as otoacoustic emissions, tympanic membrane displacement (TMD), changes in skull compliance, and transcranial Doppler ultrasound.
METHODS BASED ON CHANGES IN NONOCULAR STRUCTURES
Distortion product otoacoustic emissions (DPOAEs), low-intensity sounds generated from the cochlea in response to two tones of specific frequency , were initially developed as a test of auditory function . However, DPOAEs have been proposed as a noninvasive method to assess ICP based upon changes in sound transmission due to ICP-related changes in pressure on the stapes [41,42].
In a recent study, Loiselle et al., reported that changes in DPOAEs were linearly correlated with ICP above 3 mmHg. However, ICP was not directly assessed; the correlation between ICP and changes in DPOAEs was based on adapted data from Lindén et al. and interpolated with the authors’ DPOAE phase shift data. In addition, due to ‘subject specific offset,’ DPOAE data had to be normalized for each patient, suggesting a prominent component of interindividual variability. Indeed, interindividual variability is a common limitation cited in publications regarding DPOAE and is likely responsible for the widely varying reported correlations between ICP and DPOAEs [44–46].
Tympanic membrane displacement
Similar to otoacoustic emissions, TMD attempts to leverage changes in auricular structures as a biomarker for ICP. TMD during reflex or induced contraction of the stapes, as detected by a transducer placed in the outer ear, is an indirect measure of perilymphatic pressure, which is influenced by ICP [35,47,48].
Although several studies have shown a significant relationship between evoked TMD and invasively obtained ICP [35,48–50], several other studies noted high intersubject variability in the TMD measurements, questioning the clinical utility of TMD to measure ICP [35,49,51–53]. However, a 2018 study by Sharif et al. suggested that some of the variability in TMD measurements may be related to an increased vascular pulse amplitude; a large pulse amplitude was associated with increased variability in evoked TMD. Therefore, accounting for the pulse amplitude may improve the reliability of the evoked TMD measurements. Although a ‘corrected’ evoked TMD may be a reliable indicator of ICP status, further studies are necessary to determine if a ‘corrected’ evoked TMD better correlates with invasively obtained ICP measurements than uncorrected evoked TMD. Currently, TMD is not widely used clinically as a noninvasive method to detect or monitor ICP due to large intersubject variability and prior reports of a variable correlation with invasively obtained ICP [48,51,52,54].
Neuroimaging (MRI and computerized tomography)
Radiographic findings, such as a dilated and expanded partially empty/empty sella turcica, a dilated perioptic CSF space, increased vertical tortuosity of the optic nerves, and cerebellar tonsillar descent, are commonly seen in patients with increased ICP [55,56,57▪,58]. However, some of these findings are also relatively common in the general population ; a partially empty sella turcica has been reported in up to 33% of MRI scans performed in the general population [60▪]. In addition to radiographic findings of increased ICP being present in the general population, another potential limitation of using radiographic findings as an indicator of ICP is the persistence of some of these findings long after normalization of ICP , suggesting that MRI findings may not be an ideal method to detect elevated ICP or to assess a response to treatment.
However, a recent publication using displacement encoding with stimulated echoes (DENSE) MRI, an MRI technique highly sensitive to brain motion, has suggested that DENSE MRI analysis of pontine displacement may be better able to qualitatively assess ICP than standard MRI scans/sequences and it may even be able to provide a quantitative assessment of ICP . Patients with elevated ICP were found to have a smaller pontine displacement prior to lumbar puncture than following lumbar puncture . In addition, postlumbar puncture patients had statistically similar pontine displacements to control patients . Therefore, the use of specialized sequencing protocols [61,62], such as the DENSE MRI protocol, may increase the utility of neuroimaging studies in ICP assessment. However, due to cost and access to scanners, it is unlikely that neuroimaging studies will be commonly used clinically to measure ICP, and it would be impractical to use neuroimaging studies for serial ICP assessments.
Near-infrared spectroscopy (NIRS), which uses near-infrared light (700–1000 nm) to penetrate superficial layers of the head, has been developed as a clinical tool to monitor brain oxygenation . Since brain oxygenation demand changes with changes in brain metabolic state, and changes in ICP alter brain metabolism and cerebral perfusion pressure [64,65], it is believed that brain oxygenation changes can be used as a biomarker for ICP. On the contrary, multiple studies have produced either conflicting results or have failed to show a correlation between invasively obtained ICP and changes in NIRS [66,67]. Therefore, it is unlikely that NIRS will be a clinically useful tool to assess or monitor alterations in ICP.
Similar to NIRS, which detects brain oxygenation changes due to altered brain metabolism, electroencephalography may be able to detect changes in neuronal activity related to changes in brain metabolism. Since brain metabolism is altered by changes in ICP , electroencephalography may potentially be useful to measure or monitor ICP [68,69]. Significantly, in a small cohort of patients with TBI or subarachnoid hemorrhage, electroencephalogram (EEG) changes preceded ICP changes, suggesting that EEG changes can be used to predict ICP changes . From a practical perspective, this information could be useful therapeutically, allowing a treatment window for providers to intervene prior to the development of ICP changes. However, it is not clear from the available data if there is a large enough delay between EEG changes and ICP changes to significantly alter patient management . Although EEG changes, such as generalized rhythmic delta activity and attenuation of fast activity, have been reported to correlate with changes in ICP , EEG changes have not been shown to provide a quantitative assessment of ICP, limiting its clinical utility as a noninvasive biomarker to monitor ICP.
Although it is generally assumed that deformations in the skull due to changes in ICP do not occur after fontanelle closure, animal studies have shown that minute deformations in the skull occur due to changes in ICP and that these deformations positively correlate with changes in ICP . Recently, this technology has been applied to humans, where very small variations in skull volume due to ICP pulsations were detected by a noninvasive electric resistance extensometer placed on the scalp at the temporoparietal transition [73–75,76▪▪,77]. The skull deformations are transformed into ICP waveforms in real time, which have been shown to correlate well with invasively obtained ICP waveforms . However, in its present state, this technology has not been quantitatively correlated with ICP status. In addition, there is some inter-individual variability in the magnitude of waveform pattern changes due to changes in ICP. Although a promising noninvasive method to monitor ICP, further validation and calibration of this technique is necessary before it can be considered a viable alternative to invasively obtained ICP.
Transcranial Doppler ultrasound
Ultrasound is readily available and commonly used in emergency departments and in intensive care units as a noninvasive tool to screen patients for internal injuries after trauma (E-FAST; extended focused assessment with sonography in trauma) [79,80] and to monitor complications arising from intracranial injuries, such as cerebral artery vasospasm [81–83]. Several parameters of transcranial Doppler (TCD) ultrasound, such as cerebral blood flow velocity (CBV), pulsatility index, and the ability to measure the diameter of the optic nerve sheath (discussed in a later section), have been studied as noninvasive biomarkers of ICP [84–86]. Although several early studies and a few recent studies have shown a strong correlation between TCD and ICP , a number of studies have shown varying correlations between TCD-determined ICP and invasively obtained ICP or have failed to show a correlation [84,88–91].
In a study of 16 patients with IIH, there was a significant reduction in middle cerebral artery mean flow velocities following lumbar puncture, while there was no significant change in pulsatility index in the middle cerebral artery, the basilar artery, or the vertebral artery following lumbar puncture . In contrast, in a study of 100 TBI patients who had invasive intracranial monitoring, TCD diastolic flow velocities were unable to reliably detect ICP above 20 mmHg; the reported sensitivity and specificity for detecting ICP at least 20 mmHg was 0% and 74.4%, respectively . Similarly, in a study of 262 patients with acute brain injury (TBI, subarachnoid hemorrhage, intracranial hemorrhage, or ischemic stroke), TCD (based on a determination of flow velocities and mean arterial pressure) was able to rule out elevated ICP (ICP > 22; negative predictive value of 95.6%), however, it was unable to reliably detect elevated ICP (ICP > 22; a positive predictive value of 23.0%) .
Although there have been conflicting data regarding the correlation between CBV or pulsatility index and ICP, a recent study using two-depth TCD to compare the flow velocity in the extracranial and intracranial portions of the ophthalmic artery has shown a strong correlation with invasively obtained ICP [93▪▪]. Continuous noninvasively obtained ICP using two-depth TCD of the ophthalmic artery over a 1-h period showed a very strong correlation with invasively obtained ICP (correlation coefficient of 0.94) [93▪▪]. The TCD-predicted ICP changes closely mimicked the real-time changes in ICP over the 1-h period [93▪▪]. Therefore, while TCD-derived pulsatility index or CBV does not appear to consistently correlate with ICP, two-depth TCD has the potential to be a useful noninvasive method to measure and monitor ICP.
METHODS BASED ON CHANGES IN OCULAR STRUCTURES
Optic nerve sheath diameter
Optic nerve sheath diameter (ONSD) is a widely studied noninvasive method for determining ICP and is commonly measured by orbital ultrasound, however, MRI and computerized tomography (CT) have also been used to measure the ONSD. Indeed, a PubMed search for ‘optic nerve sheath diameter intracranial pressure’ yielded over 150 articles in 2021 and 2022.
A number of recent studies have shown a good correlation between ONSD and invasively obtained ICP [84,87,91,94–97]. For example, a recent study showed that measurement of the ONSD 3 mm posterior to the globe in patients with IIH predicted elevated ICP with good sensitivity and specificity (97% sensitivity and 100% specificity) . In addition, a prospective study in patients undergoing lumbar puncture in a general hospital in China reported that ONSD was significantly different between patients with intracranial hypotension (ICP ≤ 60-mm water), control patients, and patients with intracranial hypertension (ICP > 200-mm water) [99▪]. In addition, a strong correlation was reported between ONSDs and lumbar puncture obtained ICP (r = 0.952 for all patients included in the study and r = 0.871 for patients with elevated ICP) [99▪]. Although a few recent studies have failed to show a strong relationship between ONSD and ICP in both pediatric and adult patients [88,100–102], based on the large amount of data suggesting that ONSD can be used to determine ICP, ONSD measurement may be a useful clinical tool to estimate and potentially measure ICP.
However, several significant limitations of ultrasound-determined ONSD exist, which can dramatically alter ONSD measurements, potentially limiting its usefulness as a noninvasive biomarker for ICP. These limitations include ultrasound probe choice, nonstandardized ultrasound gain settings, not having standardized protocols regarding in which planes to obtain ultrasound images, and operator experience [103–109]. Although using MRI or CT scans to measure ONSD eliminates some of these limitations, the expense, time (MRI), availability, and exposure to radiation (CT scan), make MRI-determined or CT-determined ONSD measurements less ideal and less useful methods to measure or serially monitor changes in ICP [110,111].
Venous ophthalmodynamometry (ODM) measures central retinal vein (CRV) pressure by determining the force necessary to collapse the CRV; the force necessary to collapse the CRV is equal to the sum of the intraocular pressure (IOP) and a variable force applied externally to the globe [112,113]. Since the CRV is exposed to and influenced by ICP as it leaves the globe and travels within the substance of the optic nerve, CRV pressure is believed to be a useful biomarker for ICP . Although this theory was first proposed in 1925 , it was not formally studied until 2000, when Firsching et al. showed a good correlation between ODM and invasively obtained ICP measurements. Although most early reports suffered from several limitations, such as a variable correlation between CRV pressure and ICP and using estimated IOP rather than true IOP during ODM, Firsching et al. have shown that, due to advances in technique and technology, the sensitivity and specificity of detecting elevated ICP with ODM can be as high as 84.2% and 92.8%, respectively. However, the study did not attempt to quantitatively correlate CRV pressures with ICP. In addition, patients with papilledema were excluded from the study because the effect of papilledema on CRV pressure was unclear . Since ODM may not be able to be reliably performed in patients with papilledema, a common consequence of elevated ICP, ODM may have limited utility as a noninvasive method to measure ICP.
Spontaneous retinal venous pulsations
Similar to CRV pressure, spontaneous retinal venous pulsations (SRVPs), pulsations of the CRV on or near the optic disc, have also been reported to be affected by changes in ICP and may be a useful biomarker to noninvasively determine ICP [117,118]. SRVPs are reported to occur in ∼75–80% of normal patients when detected ophthalmoscopically, however, newer technology, such as imaging the pulsations with video recordings or with optical coherence tomography (OCT), has shown that SRVPs occur in more than 95% of people with normal ICP and IOP .
SRVPs have long been used as a qualitative indicator of ICP, with venous pulsations diminishing or disappearing in patients with elevated ICP. Indeed, acute loss of SRVPs due to an experimentally induced increase in ICP has been confirmed in animal models, showing that changes in SRVPs occur quickly following changes in ICP [120▪]. Similarly, in a recent study of SRVPs in a para-primate animal model, alterations in ICP and IOP were found to acutely alter SRVP presence and SRVP amplitude . A higher IOP was necessary to elicit SRVPs at higher ICPs than at lower ICPs and SRVPs had smaller amplitudes at high ICPs than at lower ICPs . These data demonstrate multiple effects of changes in ICP on SRVPs and suggest that the relationship between SRVPs, IOP, and ICP is not straightforward. Similar findings were reported in a 2019 study by D’Antona et al., where larger SRVP amplitudes were found in patients with lower ICP compared to patients with higher ICP. Although these studies demonstrate a correlation between SRVPs and ICP and suggest that SRVPs may be useful biomarkers for ICP, more work in this area is needed to determine if SRVPs can quantitatively measure ICP and before measurement of SRVPs can potentially be developed as a noninvasive method to determine ICP.
Automated infrared pupillometry provides accurate quantitative measurements of iris function. A neurological pupil index (NPI) is determined by comparing automated pupillometry measurements to a normative model of iris function . Multiple studies have shown that lower NPI values, which indicate abnormal iris function, correlate with a higher ICP [123–126], potentially making the NPI a valuable screening tool to rapidly detect an abnormal ICP. In addition, sustained low NPI values have been shown to be predictive of poor clinical outcomes in patients with severe TBI [127,128]. However, the NPI is not currently able to provide quantitative information about ICP, and several studies have reported that the NPI does not reliably detect changes in ICP, limiting its utility as a clinical tool to measure ICP or to monitor changes in ICP [123,129,130].
Visual evoked potential
Visual evoked potentials (VEPs), electrical potentials extracted from EEG recordings using signal averaging, are used to assess the integrity of the visual pathways from the optic nerves to the occipital cortex . Therefore, any process that affects the integrity of the visual system can affect VEPs, such as myelin plaques from multiple sclerosis, compression of the anterior visual pathways from space-occupying lesions or hydrocephalus, or from cerebral cortical injuries .
In the 1980s, York et al. reported a good relationship between elevated ICP and a shift in the latency of the N2 wave, which occurs at approximately 70 ms and corresponds to cortical potentials. Based on this work, VEPs were further studied as a potential noninvasive method to measure ICP. However, several studies found that VEPs did not change significantly in patients with elevated ICP; that there was a wide range of VEP latencies, amplitudes, and waveforms in normal subjects; and that there was a large variability over time in individual subjects, questioning the utility of VEPs as a noninvasive biomarker for ICP [133,134].
More recent studies have looked at pattern reversal VEPs in patients with craniosynostosis and at sustained giant pattern VEPs (VEP amplitudes that exceeded the upper 97th percentile of VEP amplitudes) as markers of elevated ICP [133,135]. Similar to prior VEP studies, pattern reversal VEPs or sustained giant pattern VEPs were not found to be reliable indicators of elevated ICP . Pattern reversal VEPs were shown to have a sensitivity for elevated ICP that ranged between 58.3 and 70.6%  and sustained giant pattern VEPs incorrectly suggested elevated ICP in at least 20% of pediatric patients . Therefore, it is unlikely that changes in VEP can be used to reliably detect changes in ICP and even less likely that it can provide a quantitative measurement of ICP.
Optical coherence tomography changes
OCT is a noninvasive modality that can image the retina, optic nerve, and choroid with a resolution approaching that of histological sections . Given the high resolution of OCT, certain structural characteristics of the optic nerve head and surrounding retina, such as the peripapillary retinal nerve fiber layer thickness, the size of Bruch's membrane opening (the opening that surrounds the optic nerve), the angle of Bruch's membrane, and the subfoveal choroidal thickness, can be clearly visualized and measured [137–141]. Changes in these structures have been suggested as biomarkers for ICP.
Although most studies using OCT have shown a qualitative correlation with ICP, some recent studies have suggested that OCT can be used to quantitatively determine ICP. In a recent study of 104 patients with IIH, optic nerve head central thickness showed a modest correlation with lumbar puncture-obtained ICP (correlation coefficients for the right and left eye were 0.21 and 0.28 at baseline and were 0.46 and 0.46 at a 24-month follow-up, respectively) . There was no correlation between ICP and retinal nerve fiber layer thickness, ganglion cell layer thickness, or macular volume changes . However, a study of 22 patients with IIH showed that the subfoveal choroidal thickness was significantly higher (258.22 ± 33.9 μm) in IIH patients than in control subjects (228.04 ± 7.94 μm) . In addition, subfoveal choroidal thickness was positively correlated with ICP obtained by lumbar puncture (correlation coefficient of 0.851) . Although this study demonstrated a good correlation between subfoveal choroidal thickness and ICP, spectral domain OCT was used for choroidal thickness measurements. A stronger correlation may be possible with the use of enhanced depth imaging spectral domain OCT, which can more accurately delineate the transition between choroid and sclera.
Although some studies support the use of OCT as a quantitative biomarker for invasively obtained ICP, all OCT studies assess structural changes in the retina and peripapillary regions which may not readily change in response to ICP-lowering treatments . Therefore, OCT may have limited utility as a noninvasive biomarker for invasively obtained ICP.
Multiple biomarkers have been developed as potential noninvasive methods to determine and monitor ICP. Although none of these noninvasive methods are in routine clinical use due to concerns about their accuracy, several of the noninvasive biomarkers, such as skull elasticity, specialized MRI sequences, two-depth TCD, ONSD measurement, and subfoveal choroidal thickness, have shown strong correlations with invasively obtained ICP. Therefore, it is possible that further technology developed around these biomarkers may be able to produce rapid, accurate, reliable, and portable noninvasive devices to measure and monitor ICP. Development of such devices would allow ICP determination to become more commonplace, potentially supplanting the use of invasively obtained ICP measurement in certain clinical situations, and could significantly change how diseases such as IIH, glaucoma, and acute TBI are managed.
Financial support and sponsorship
MD is supported in part by a departmental grant from the Research to Prevent Blindness, by NIH/NEI core grant P30-EY06360 (Department of Ophthalmology, Emory University School of Medicine), by a University Research Committee pilot grant (Emory University School of Medicine), and by a North American Neuro-Ophthalmology Society pilot grant.
Conflicts of interest
There are no conflicts of interest.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
▪ of special interest
▪▪ of outstanding interest
1. Johnston I, Teo C. Disorders of CSF hydrodynamics. Childs Nerv Syst 2000; 16:776–799.
2. Sakka L, Coll G, Chazal J. Anatomy and physiology of cerebrospinal fluid. Eur Ann Otorhinolaryngol Head Neck Dis 2011; 128:309–316.
3. Watters GV, Page L, Lorenzo AV, et al. Relationship between cerebrospinal fluid (CSF) formation, absorption and pressure in human hydrocephalus. Trans Am Neurol Assoc 1969; 94:153–156.
4. Williams MA, Razumovsky AYe. Cerebrospinal fluid circulation, cerebral edema, and intracranial pressure. Curr Opin Neurol 1993; 6:847–853.
5. Kuurne T, Troupp H, Kaste M, Vapalahti M. Clinical and experimental intracranial pressure. Eur Neurol 1972; 8:188–191.
6. Dawes AJ, Sacks GD, Cryer HG, et al. Intracranial pressure monitoring and inpatient mortality in severe traumatic brain injury: a propensity score-matched analysis. J Trauma Acute Care Surg 2015; 78:492–501.
7. Godoy DA, Lubillo S, Rabinstein AA. Pathophysiology and management of intracranial hypertension and tissular brain hypoxia after severe traumatic brain injury: an integrative approach. Neurosurg Clin N Am 2018; 29:195–212.
8. Miller JD, Becker DP, Ward JD, et al. Significance of intracranial hypertension in severe head injury. J Neurosurg 1977; 47:503–516.
9. Niemöller UM, Täuber MG. Brain edema and increased intracranial pressure in the pathophysiology of bacterial meningitis. Eur J Clin Microbiol Infect Dis 1989; 8:109–117.
10. Depreitere B, Bruyninckx D, Güiza F. Monitoring of intracranial pressure in meningitis. Acta Neurochir Suppl 2016; 122:101–104.
11. Beez T, Munoz-Bendix C, Steiger H-J, Beseoglu K. Decompressive craniectomy for acute ischemic stroke. Crit Care 2019; 23:209.
12. Freeman WD. Management of intracranial pressure. Continuum (Minneap Minn) 2015; 21:1299–1323.
13. Hasan D, Lindsay KW, Vermeulen M. Treatment of acute hydrocephalus after subarachnoid hemorrhage with serial lumbar puncture. Stroke 1991; 22:190–194.
14. Zoerle T, Lombardo A, Colombo A, et al. Intracranial pressure after subarachnoid hemorrhage. Crit Care Med 2015; 43:168–176.
15. Burkett JG, Ailani J. An up to date review of pseudotumor cerebri syndrome. Curr Neurol Neurosci Rep 2018; 18:33.
16. Friedman DI. Papilledema and pseudotumor cerebri. Ophthalmol Clin North Am 2001; 14:129–147.
17. Radhakrishnan K, Ahlskog JE, Garrity JA, Kurland LT. Idiopathic intracranial hypertension. Mayo Clin Proc 1994; 69:169–180.
18. Thurtell MJ, Bruce BB, Newman NJ, Biousse V. An update on idiopathic intracranial hypertension. Rev Neurol Dis 2010; 7:e56–e68.
19. Wall M. Idiopathic intracranial hypertension: mechanisms of visual loss and disease management. Semin Neurol 2000; 20:89–95.
20. Vickers A, Donnelly JP, Moore JX, et al. Epidemiology of lumbar punctures in hospitalized patients in the United States. PLoS One 2018; 13:e0208622.
21. Zhang X, Medow JE, Iskandar BJ, et al. Invasive and noninvasive
means of measuring intracranial pressure: a review. Physiol Meas 2017; 38:R143–R182.
22. Adler MD, Comi AE, Walker AR. Acute hemorrhagic complication of diagnostic lumbar puncture. Pediatr Emerg Care 2001; 17:184–188.
23. Alstadhaug KB, Odeh F, Baloch FK, et al. Postlumbar puncture headache. Tidsskr Nor Laegeforen 2012; 132:818–821.
24. del-Rio–Vellosillo M, Garcia-Medina JJ, Pinazo-Duran MD, et al. Ocular motor palsy after spinal puncture. Reg Anesth Pain Med 2017; 42:1–9.
25. Egede LE, Moses H, Wang H. Spinal subdural hematoma: a rare complication of lumbar puncture. Case report and review of the literature. Md Med J 1999; 48:15–17.
26. Oliver WJ, Shope TC, Kuhns LR. Fatal lumbar puncture: fact versus fiction—an approach to a clinical dilemma. Pediatrics 2003; 112:e174–e176.
27. Verslegers L, Schotsmans K, Montagna M, et al. Severe bilateral subdural hematomas as a complication of diagnostic lumbar puncture for possible Alzheimer's disease. Clin Neurol Neurosurg 2017; 152:95–96.
28. Engelborghs S, Niemantsverdriet E, Struyfs H, et al. Consensus guidelines for lumbar puncture in patients with neurological diseases. Alzheimers Dement (Amst) 2017; 8:111–126.
29. Holdgate A, Cuthbert K. Perils and pitfalls of lumbar puncture in the emergency department. Emerg Med 2001; 13:351–358.
30. Bruce BB. Noninvasive
assessment of cerebrospinal fluid pressure. J Neuroophthalmol 2014; 34:288–294.
31. Raboel PH, Bartek J, Andresen M, et al. Intracranial pressure monitoring: invasive versus non-invasive methods—a review. Crit Care Res Pract 2012; 2012:1–14.
32. Kristiansson H, Nissborg E, Bartek J, et al. Measuring elevated intracranial pressure through noninvasive
methods: a review of the literature. J Neurosurg Anesthesiol 2013; 25:372–385.
33. Moreno JA, Mesalles E, Gener J, et al. Evaluating the outcome of severe head injury with transcranial Doppler ultrasonography. Neurosurg Focus 2000; 8:1–7.
34. Schmidt B, Czosnyka M, Raabe A, et al. Adaptive noninvasive
assessment of intracranial pressure and cerebral autoregulation. Stroke 2003; 34:84–89.
35. Shimbles S, Dodd C, Banister K, et al. Clinical comparison of tympanic membrane displacement with invasive intracranial pressure measurements. Physiol Meas 2005; 26:1085–1092.
36. Voss SE, Horton NJ, Tabucchi THP, et al. Posture-induced changes in distortion-product otoacoustic emissions and the potential for noninvasive
monitoring of changes in intracranial pressure. Neurocrit Care 2006; 4:251–257.
37. Voulgaris SG, Partheni M, Kaliora H, et al. Early cerebral monitoring using the transcranial Doppler pulsatility index in patients with severe brain trauma. Med Sci Monit 2005; 11:CR49–CR52.
38. Vijay V, Mollan SP, Mitchell JL, et al. Using optical coherence tomography as a surrogate of measurements of intracranial pressure in idiopathic intracranial hypertension. JAMA Ophthalmol 2020; 138:1264–1271.
39. Harris FP, Lonsbury-Martin BL, Stagner BB, et al. Acoustic distortion products in humans: systematic changes in amplitude as a function of f2/f1 ratio. J Acoust Soc Am 1989; 85:220–229.
40. Lonsbury-Martin BL, Martin GK. The clinical utility of distortion-product otoacoustic emissions. Ear Hear 1990; 11:144–154.
41. Büki B, de Kleine E, Wit HP, Avan P. Detection of intracochlear and intracranial pressure changes with otoacoustic emissions: a gerbil model. Hear Res 2002; 167:180–191.
42. Loiselle AR, De Kleine E, Van Dijk P, Jansonius NM. Noninvasive
intracranial pressure assessment using otoacoustic emissions: an application in glaucoma. PLoS One 2018; 13:1–11.
43. Lindén C, Qvarlander S, Jóhannesson G, et al. Normal-tension glaucoma has normal intracranial pressure. Ophthalmology 2018; 125:361–368.
44. Hvedstrup J, Radojicic A, Moudrous W, et al. Intracranial pressure: a comparison of the noninvasive
HeadSense monitor versus lumbar pressure measurement. World Neurosurg 2018; 112:e576–e580.
45. Garner CA, Neely ST, Gorga MP. Sources of variability in distortion product otoacoustic emissions. J Acoust Soc Am 2008; 124:1054–1067.
46. Sun X-M. Distortion product otoacoustic emission fine structure is responsible for variability of distortion product otoacoustic emission contralateral suppression. J Acoust Soc Am 2008; 123:4310–4320.
47. Mostafa BE, El-Sersy HAA, Hamid TA. Increased intracranial tension and cochleovestibular symptoms: an observational clinical study. Egypt J Otolaryngol 2018; 34:191–193.
48. Reid A, Marchbanks RJ, Burge DM, et al. The relationship between intracranial pressure and tympanic membrane displacement. Br J Audiol 1990; 24:123–129.
49. Gwer S, Sheward V, Birch A, et al. The tympanic membrane displacement analyser for monitoring intracranial pressure in children. Child's Nerv Syst 2013; 29:927–933.
50. Gonzalez Torrecilla S, Avan P. Wideband tympanometry patterns in relation to intracranial pressure. Hear Res 2021; 408:108312.
51. Evensen KB, Paulat K, Prieur F, et al. Utility of the tympanic membrane pressure waveform for noninvasive
estimation of the intracranial pressure waveform. Sci Rep 2018; 8:1–11.
52. Sharif SJ, Campbell-Bell CM, Bulters DO, et al. Does the variability of evoked tympanic membrane displacement data (V m
) increase as the magnitude of the pulse amplitude increases? Acta Neurochir Suppl 2018; 126:103–106.
53. Walsted N, Wagner A. Postural-induced changes in intracranial pressure evaluated noninvasively using the MMS-10 tympanic displacement analyser in healthy volunteers. Acta Otolaryngol 2000; 120:44–47.
54. Samuel M, Burge DM, Marchbanks RJ. Tympanic membrane displacement testing in regular assessment of intracranial pressure in eight children with shunted hydrocephalus. J Neurosurg 1998; 88:983–995.
55. Belachew NF, Almiri W, Encinas R, et al. Evolution of MRI findings in patients with idiopathic intracranial hypertension after venous sinus stenting. Am J Neuroradiol 2021; 42:1993–2000.
56. Juhász J, Hensler J, Jansen O. MRI-findings in idiopathic intracranial hypertension (pseudotumor cerebri). Rofo 2021; 193:1269–1276.
57▪. Taşcioğlu T. The diagnostic value of cranial MRI findings in idiopathic intracranial hypertension: evaluating radiological parameters associated with intracranial pressure. Acta Radiol 2022; 63:1390–1397.
58. Barkatullah AF, Leishangthem L, Moss HE. MRI findings as markers of idiopathic intracranial hypertension. Curr Opin Neurol 2021; 34:75–83.
59. Auer MK, Stieg MR, Crispin A, et al. Primary empty sella syndrome and the prevalence of hormonal dysregulation – a systematic review. Dtsch Arztebl Int 2018; 115:99–105.
60▪. Chen BS, Meyer BI, Saindane AM, et al. Prevalence of incidentally detected signs of intracranial hypertension on magnetic resonance imaging and their association with papilledema. JAMA Neurol 2021; 78:718–725.
61. Saindane AM, Qiu D, Oshinski JN, et al. Noninvasive
assessment of intracranial pressure status in idiopathic intracranial hypertension using displacement encoding with stimulated echoes (DENSE) MRI: a prospective patient study with contemporaneous CSF pressure correlation. Am J Neuroradiol 2018; 39:311–316.
62. Burman R, Shah AH, Benveniste R, et al. Comparing invasive with MRI-derived intracranial pressure measurements in healthy elderly and brain trauma cases: a pilot study. J Magn Reson Imaging 2019; 50:975–981.
63. Yu Y, Zhang K, Zhang L, et al. Cerebral near-infrared spectroscopy (NIRS) for perioperative monitoring of brain oxygenation in children and adults. Cochrane Database Syst Rev 2018; 1:CD010947.
64. Ågren-Wilsson A, Eklund A, Koskinen LOD, et al. Brain energy metabolism and intracranial pressure in idiopathic adult hydrocephalus syndrome. J Neurol Neurosurg Psychiatry 2005; 76:1088–1093.
65. Tameemm A, Krovvidi H. Cerebral physiology. Contin Educ Anaesth Crit Care Pain 2013; 13:113–118.
66. de Moraes FM, Silva GS. Noninvasive
intracranial pressure monitoring methods – a critical review. Arq Neuropsiquiatr 2021; 79:437–446.
67. Muellner T, Schramm W, O Kwasny VV. Patients with increased intracranial pressure cannot be monitored using near infrared spectroscopy. Br J Neurosurg 1998; 12:136–139.
68. Sanz-García A, Pérez-Romero M, Pastor J, et al. Identifying causal relationships between EEG activity and intracranial pressure changes in neurocritical care patients. J Neural Eng 2018; 15:066029.
69. Kreitzer N, Huynh M, Foreman B. Blood flow and continuous EEG changes during symptomatic plateau waves. Brain Sci 2018; 8:14.
70. Sanz-Garcia A, Perez-Romero M, Pastor J, et al. Is it possible to extract intracranial pressure information based on the EEG activity? Rev Neurol 2019; 68:375–383.
71. Sheikh ZB, Maciel CB, Dhakar MB, et al. Nonepileptic electroencephalographic correlates of episodic increases in intracranial pressure. J Clin Neurophysiol 2022; 39:149–158.
72. Cabella B, Vilela GHF, Mascarenhas S, et al. Validation of a new noninvasive
intracranial pressure monitoring method by direct comparison with an invasive technique. Acta Neurochir Suppl 2016; 122:93–96.
73. Paraguassu G, Khilnani M, Rabelo NN, et al. Case Report: Untreatable headache in a child with ventriculoperitoneal shunt managed by use of new noninvasive
intracranial pressure waveform. Front Neurosci 2021; 15:1–4.
74. Nagai Ocamoto G, Spavieri Junior DL, Matos Ribeiro JA, et al. Noninvasive
intracranial pressure monitoring in chronic stroke patients with sedentary behavior: a pilot study. Acta Neurochir Suppl 2021; 131:55–58.
75. Rickli C, Cosmoski LD, dos Santos FA, et al. Use of noninvasive
intracranial pressure pulse waveform to monitor patients with end-stage renal disease (ESRD). PLoS One 2021; 16:1–11.
76▪▪. Andrade R, de AP, Oshiro HE, et al. A nanometer resolution wearable wireless medical device for non invasive intracranial pressure monitoring. IEEE Sens J 2021; 21:22270–22284.
77. Dhaese TM, Welling LC, Kosciasnki AM, et al. Noninvasive
intracranial pressure monitoring in idiopathic intracranial hypertension and lumbar puncture in pediatric patient: case report. Surg Neurol Int 2021; 12:493.
78. Frigieri G, Andrade RAP, Wang CC, et al. Analysis of a minimally invasive intracranial pressure signals during infusion at the subarachnoid spinal space of pigs. Acta Neurochir Suppl 2018; 126:75–77.
79. Derinoz Guleryuz O, Akca Caglar A. Focused assessment with sonography for trauma including gastric assessment; E-FAST(ING). Pediatr Emerg Care 2021; 37:e677–e678.
80. Montoya J, Stawicki SP, Evans DC, et al. From FAST to E-FAST: an overview of the evolution of ultrasound-based traumatic injury assessment. Eur J Trauma Emerg Surg 2016; 42:119–126.
81. Sharma S, Lubrica RJ, Song M, et al. The role of transcranial doppler in cerebral vasospasm: a literature review. Acta Neurochir Suppl 2020; 127:201–205.
82. Purkayastha S, Sorond F. Transcranial Doppler ultrasound: technique and application. Semin Neurol 2012; 32:411–420.
83. Bonow RH, Young CC, Bass DI, et al. Transcranial Doppler ultrasonography in neurological surgery and neurocritical care. Neurosurg Focus 2019; 47:E2.
84. Sharawat IK, Kasinathan A, Bansal A, et al. Evaluation of optic nerve sheath diameter and transcranial Doppler as noninvasive
tools to detect raised intracranial pressure in children. Pediatr Crit Care Med 2020; 21:959–965.
85. Rasulo FA, Calza S, Robba C, et al. Transcranial Doppler as a screening test to exclude intracranial hypertension in brain-injured patients: the IMPRESSIT-2 prospective multicenter international study. Crit Care 2022; 26:1–12.
86. Pradeep R, Gupta D, Shetty N, et al. Transcranial Doppler for monitoring and evaluation of idiopathic intracranial hypertension. J Neurosci Rural Pract 2020; 11:309–314.
87. Dhanda A, Singh GP, Bindra A. Correlation between invasive and noninvasive
technique of intracranial pressure measurement in children with traumatic brain injury: an observational study. J Neurosurg Anesthesiol 2022; 34:221–226.
88. Chang T, Yan X, Zhao C, et al. Noninvasive
evaluation of intracranial pressure in patients with traumatic brain injury by transcranial Doppler ultrasound. Brain Behav 2021; 11:1–7.
89. Kaloria N, Panda NB, Bhagat H, et al. Pulsatility index reflects intracranial pressure better than resistive index in patients with clinical features of intracranial hypertension. J Neurosci Rural Pract 2020; 11:144–150.
90. Soares MS, de Andrade AF, Brasil S, et al. Evaluation of cerebral hemodynamics by transcranial Doppler ultrasonography and its correlation with intracranial pressure in an animal model of intracranial hypertension. Arq Neuropsiquiatr 2022; 80:344–352.
91. Aslan N, Yildizdas D, Horoz ÖÖ, et al. Evaluation of ultrasonographic optic nerve sheath diameter and central retinal artery Doppler indices by point-of-care ultrasound in pediatric patients with increased intracranial pressure. Turk J Pediatr 2021; 63:300–306.
92. Cardim D, Robba C, Czosnyka M, et al. Noninvasive
intracranial pressure estimation with transcranial Doppler: a prospective observational study. J Neurosurg Anesthesiol 2020; 32:349–353.
93▪▪. Lucinskas P, Deimantavicius M, Bartusis L, et al. Human ophthalmic artery as a sensor for noninvasive
intracranial pressure monitoring: numerical modeling and in vivo pilot study. Sci Rep 2021; 11:1–10.
94. Zhou J, Li J, Ye T, Zeng Y. Ultrasound measurements versus invasive intracranial pressure measurement method in patients with brain injury: a retrospective study. BMC Med Imaging 2019; 19:1–7.
95. Vijay P, Lal BB, Sood V, et al. Dynamic optic nerve sheath diameter (ONSD) guided management of raised intracranial pressure in pediatric acute liver failure. Hepatol Int 2021; 15:502–509.
96. Shrestha B, Shrestha P, Ghale P, Lakshmipathy G. Correlation between invasive intracranial pressure monitoring and optic nerve sheath diameter in patients with traumatic brain injury. Kathmandu Univ Med J 2021; 19:221–224.
97. Robba C, Cardim D, Czosnyka M, et al. Ultrasound noninvasive
intracranial pressure assessment in paediatric neurocritical care: a pilot study. Child's Nerv Syst 2020; 36:117–124.
98. Ertekin T, Boyaci MG, Bilir A, et al. Optic nerve sheath diameter measurement: a means of detecting increased intracranial pressure in pseudotumor cerebri patients. Folia Morphol (Warsz) 2022; 81:567–573.
99▪. Wang L, Zhang Y, Li C, et al. Ultrasonographic optic nerve sheath diameter as a noninvasive
marker for intracranial hypotension. Ther Adv Neurol Disord 2022; 15:175628642110697.
100. Biggs A, Lovett M, Moore-Clingenpeel M, O’Brien N. Optic nerve sheath diameter does not correlate with intracranial pressure in pediatric neurocritical care patients. Child's Nerv Syst 2021; 37:951–957.
101. Zoerle T, Caccioppola A, D’Angelo E, et al. Optic nerve sheath diameter is not related to intracranial pressure in subarachnoid hemorrhage patients. Neurocrit Care 2020; 33:491–498.
102. Bhargava V, Tawfik D, Tan YJ, et al. Ultrasonographic optic nerve sheath diameter measurement to detect intracranial hypertension in children with neurological injury: a systematic review. Pediatr Crit Care Med 2020; 21:E858–E868.
103. De Bernardo M, Livio Vitiello NR. Sonographic evaluation of optic nerve sheath diameter in idiopathic intracranial hypertension. J Clin Neurosci 2020; 73:331–332.
104. Wang N, Xie X, Yang D, et al. Orbital cerebrospinal fluid space in glaucoma: the Beijing intracranial and intraocular pressure (iCOP) study. Ophthalmology 2012; 19:2065–2073.
105. Kishk NA, Ebraheim AM, Ashour AS, et al. Optic nerve sonographic examination to predict raised intracranial pressure in idiopathic intracranial hypertension: the cut-off points. Neuroradiol J 2018; 31:490–495.
106. Raval R, Shen J, Lau D, et al. Comparison of three point-of-care ultrasound views and MRI measurements for optic nerve sheath diameter: a prospective validity study. Neurocrit Care 2020; 33:173–181.
107. Li J, Wan C. Noninvasive
detection of intracranial pressure related to the optic nerve. Quant Imaging Med Surg 2021; 11:2823–2836.
108. Youm JY, Lee JH, Park HS. Comparison of transorbital ultrasound measurements to predict intracranial pressure in brain-injured patients requiring external ventricular drainage. J Neurosurg 2022; 136:257–263.
109. Wang J, Li K, Li H, et al. Ultrasonographic optic nerve sheath diameter correlation with ICP and accuracy as a tool for noninvasive
surrogate ICP measurement in patients with decompressive craniotomy. J Neurosurg 2019; 19:1–7.
110. Lee SH, Kim HS, Yun SJ. Optic nerve sheath diameter measurement for predicting raised intracranial pressure in adult patients with severe traumatic brain injury: a meta-analysis. J Crit Care 2020; 56:182–187.
111. Kim DY, Kim SY, Hong DY, et al. Comparison of ultrasonography and computed tomography for measuring optic nerve sheath diameter for the detection of elevated intracranial pressure. Clin Neurol Neurosurg 2021; 204:106609.
112. Lo L, Zhao D, Ayton L, et al. Non-invasive measurement of intracranial pressure through application of venous ophthalmodynamometry. Annu Int Conf IEEE Eng Med Biol Soc 2021; 2021:6771–6774.
113. Jonas JB, Pfeil K, Chatzikonstantinou A, Rensch F. Ophthalmodynamometric measurement of central retinal vein pressure as surrogate of intracranial pressure in idiopathic intracranial hypertension. Graefe's Arch Clin Exp Ophthalmol 2008; 246:1059–1060.
114. Golzan SM, Avolio A, Graham SL. Hemodynamic interactions in the eye: a review. Ophthalmologica 2012; 228:214–221.
115. Firsching R, Schütze M, Motschmann M, Behrens-Baumann W. Venous ophthalmodynamometry: a noninvasive
method for assessment of intracranial pressure. J Neurosurg 2000; 93:33–36.
116. Firsching R, Müller C, Pauli SU, et al. Noninvasive
assessment of intracranial pressure with venous ophthalmodynamometry. J Neurosurg 2011; 115:371–374.
117. Moss HE. Retinal vein changes as a biomarker to guide diagnosis and management of elevated intracranial pressure. Front Neurol 2021; 12:1–5.
118. Stockslager MA, Samuels BC, Allingham RR, et al. System for rapid, precise modulation of intraocular pressure, toward minimally-invasive in vivo measurement of intracranial pressure. PLoS One 2016; 11:1–15.
119. McHugh JA, D’Antona L, Toma AK, Bremner FD. Spontaneous venous pulsations detected with infrared videography. J Neuroophthalmol 2020; 40:174–177.
120▪. Ghate D, Kedar S, Havens S, et al. The effects of acute intracranial pressure changes on the episcleral venous pressure, retinal vein diameter and intraocular pressure in a pig model. Curr Eye Res 2021; 46:524–531.
121. Dattilo M, Read AT, Samuels BC, Ethier CR. Detection and characterization of tree shrew retinal venous pulsations: an animal model to study human retinal venous pulsations. Exp Eye Res 2019; 185:107689.
122. D’Antona L, McHugh JA, Ricciardi F, et al. Association of intracranial pressure and spontaneous retinal venous pulsation. JAMA Neurol 2019; 76:1502–1505.
123. Stevens AR, Su Z, Toman E, Belli ADD. Optical pupillometry in traumatic brain injury: neurological pupil index and its relationship with intracranial pressure through significant event analysis. Brain Inj 2019; 33:1032–1038.
124. Soeken TA, Alonso A, Grant A, et al. Quantitative pupillometry for detection of intracranial pressure changes during head-down tilt. Aerosp Med Hum Perform 2018; 89:717–723.
125. Pansell J, Hack R, Rudberg P, et al. Can quantitative pupillometry be used to screen for elevated intracranial pressure? A retrospective cohort study. Neurocrit Care 2022; 37:531–537.
126. Al-Obaidi SZ, Atem FD, Stutzman SE, Olson DM. Impact of increased intracranial pressure on pupillometry: a replication study. Crit Care Explor 2019; 1:e0054.
127. Jahns FP, Miroz JP, Messerer M, et al. Quantitative pupillometry for the monitoring of intracranial hypertension in patients with severe traumatic brain injury. Crit Care 2019; 23:1–9.
128. El Ahmadieh TY, Bedros N, Stutzman SE, et al. Automated pupillometry as a triage and assessment tool in patients with traumatic brain injury. World Neurosurg 2021; 145:e163–e169.
129. McNett M, Moran C, Grimm DGA. Pupillometry trends in the setting of increased intracranial pressure. J Neurosci Nurs 2018; 50:357–361.
130. Giede-Jeppe A, Sprügel MI, Huttner HB, et al. Automated pupillometry identifies absence of intracranial pressure elevation in intracerebral hemorrhage patients. Neurocrit Care 2021; 35:210–220.
131. Creel DJ. Visually evoked potentials. Handb Clin Neurol 2019; 160:501–522.
132. York DH, Pulliam MW, Rosenfeld JG, Watts C. Relationship between visual evoked potentials and intracranial pressure. J Neurosurg 1981; 55:909–916.
133. Rufai SR, Marmoy OR, Thompson DA, et al. Electrophysiological and fundoscopic detection of intracranial hypertension in craniosynostosis. Eye 2022; [Epub ahead of print].
134. Andersson L, Sjölund J, Nilsson J. Flash visual evoked potentials are unreliable as markers of ICP due to high variability in normal subjects. Acta Neurochir (Wien) 2012; 154:121–127.
135. Thompson DA, Marmoy OR, Prise KL, et al. Giant pattern VEPs in children. Eur J Paediatr Neurol 2021; 34:33–42.
136. Malhotra K, Padungkiatsagul T, Moss HE. Optical coherence tomography use in idiopathic intracranial hypertension. Ann Eye Sci 2020; 5:7–17.
137. Kaya FS, Arici C. Assessment of peripapillary choroidal thicknesses and optic disc diameters in idiopathic intracranial hypertension. Can J Ophthalmol 2021; S0008-4182:00399-9.
138. Bingöl Kiziltunç P, Atilla H. A novel biomarker for increased intracranial pressure in idiopathic intracranial hypertension. Jpn J Ophthalmol 2021; 65:416–422.
139. Ozdemir I, Çevik S. Measurement of choroid thickness using optical coherence tomography to monitor intracranial pressure in an idiopathic cranial hypertension model. Neurol India 2020; 68:636.
140. Gampa A, Vangipuram G, Shirazi Z, Moss HE. Quantitative association between peripapillary Bruch's membrane shape and intracranial pressure. Investig Opthalmol Vis Sci 2017; 58:2739.
141. Sibony PA, Kupersmith MJ, Kardon RH. Optical coherence tomography neuro-toolbox for the diagnosis and management of papilledema, optic disc edema, and pseudopapilledema. J Neuroophthalmol 2021; 41:77–92.
142. Vijay V, Mollan SP, Mitchell JL, et al. Using optical coherence tomography as a surrogate of measurements of intracranial pressure in idiopathic intracranial hypertension. JAMA Ophthalmol 2020; 138:1264.