Clinical assessment of noninvasive intracranial pressure absolute value measurement method

We have concerns with the article by Ragauskas et al. who assessed the precision of a method for noninvasive intracranial pressure (ICP) measurement compared with the invasive gold standard CSF pressure measurement. [1]

The authors assume that extracranial ophthalmic arterial blood flow is independent of ICP. We disagree. Both adult and pediatric work in the last decade demonstrated the effect and correlation of ICP on distal blood vessels. [2-5] In 2009, we measured central retinal artery and vein blood flow velocities by spectral Doppler imaging in awake, supine children and determined the velocity of blood flow is reduced by elevated ICP. [5] Our method is noninvasive and can be performed without anesthesia on children.

We also have safety concerns. Transcranial spectral Doppler imaging through bone necessitates the use of higher Doppler power than ocular Doppler imaging. The authors' diagram (Figure 1) places the transcranial Doppler probe precariously close to the lens, but does not publish powers used. Their method would only be applicable in alert and cooperative adults. In addition, applying prolonged pressure to the globe can reduce central retinal artery blood flow and may result in vision loss.

1. Ragauskas A, Matijosaitis V, Zakelis R, et. Al. Clinical assessment of noninvasive intracranial pressure absolute value measurement method. Neurology 2012; 78:1684-1691.

2. Mitra RA, Sergott RC, Flaharty PM, et al. Optic nerve decompression improves hemodynamic parameters in papilledema. Ophthalmology 1993; 100:987-997.

3. Querfurth HW, Lagreze WD, Hedges TR, Heggerick PA. Flow velocity and pulsatility of the ocular circulation in chronic intracranial hypertension. Acta Neurol Scand 2002; 105:431-440.

4. Miller M, Sable C, Chang T. Spectral Doppler imaging of vessels in the optic nerve of children. J Child Neurol 2007; 22:809-811.

5. Miller MM, Chang T, Keating R, Crouch E, Sable C. Blood flow velocities are reduced in the optic nerve of children with elevated intracranial pressure. J Child Neurol 2009; 24:30-35.

For disclosures, contact the editorial office at [email protected].

We thank Miller et al. for their interest in the results of our assessment of non-invasive ICP measurement technology. This method is accurate and sensitivity and specificity are much higher compared with other approaches. [1,2,3]

Blood flow in both segments of the ophthalmic artery (OA) depends on ICP, ambulatory blood pressure, heart rate, intraocular pressure, intraorbital pressure, and other factors. [1]. The benefit of our method is in the equilibration of ICP with Pe. The balance ICP=Pe is not dependent on influential factors. OA blood flow velocities could be very different in different patients [1] but the balance ICP=Pe does not depend on the OA blood flow velocity values. As a result, our method does not need a patient-specific calibration. All "correlation based" non-invasive ICP measurement approaches cannot be used for absolute ICP value measurements because they need patient-specific calibration, which is impossible.

Our two-depth TCD technology complies with all safety standards. The maximum value of Pe is 50 mmHg and so Pe is applied to the orbit for just 90 seconds. We diagnosed vision changes after 115 snapshot ICP measurements. Experts have agreed that our technology has no influence on vision impairment.

1. Ragauskas A, Matijosaitis V, Zakelis R, et al. Clinical assessment of noninvasive intracranial pressure absolute value measurement method. Neurology 2012;78:1684 -1691.

2. Ragauskas A, Bartusis L, Zakelis R, Daubaris G. Non-invasive absolute intracranial pressure value measurements during HUT/HDT tests. Cerebrovascular Diseases 2012; 33(Suppl 1): 78.

3. Ragauskas A, Bartusis L, Zakelis R, et al. Statistical validity of non-invasive absolute intracranial pressure value meter for evidence based medicine. Cerebrovascular Diseases 2012;33(Suppl 1):74.

For disclosures, contact the editorial office at [email protected].

We thank Liebeskind et al. for their interest in the results of our assessment of non-invasive ICP measurement technology, which does not need patient-specific calibration. [1] We respectfully disagree with the assumption that "pressure on the orbit does not balance transmural pressure in the segments of ophthalmic artery (OA)." Statistically significant evidence-based results of clinical [1,3] and healthy volunteer [2] studies show that this assumption has no scientific background. Dr Heyreh demonstrated [2] that intracranial segment (IOA) of the OA is absent in 5% of humans. Our multi-depth TCD technology [1] with increased resolution of OA blood velocity measurements is a tool for accurate experimental identification of the anatomical structure of the IOA and of the OA physiological reactions to ICP. [3] Absolute ICP non-invasive measurements of hundreds of healthy volunteers, neurological and TBI patients showed that our technology can be used on more than 96% of individuals independently of age, gender, or intracraniospinal pathology. Physiological findings [1,3,4] of OA reaction to ICP and pressure on the orbit fully support conclusions of an anatomical study, [2] which showed that the intracranial course of OA may be absent in just 5% of individuals.

1. Ragauskas A, Matijosaitis V, Zakelis R, et al. Clinical assessment of noninvasive intracranial pressure absolute value measurement method. Neurology 2012; 78: 1684-1691.

2. Hayreh S. Orbital vascular anatomy. Eye 2006;20:1130 -1144.

3. Ragauskas A, Bartusis L, Zakelis R, Daubaris G. Non-invasive absolute intracranial pressure value measurements during HUT/HDT tests. Cerebrovascular Diseases 2012;v33(Suppl 1):78.

4. Ragauskas A, Bartusis L, Zakelis R, et al. Statistical validity of non-invasive absolute intracranial pressure value meter for evidence based medicine. Cerebrovascular Diseases 2012;33(Suppl 1): 74.

For disclosures, please contact the editorial office at [email protected].

Ragauskas et al. [1] demonstrated that when external pressure is applied, the orbit tends to equilibrate flow in the ipsilateral ophthalmic artery (OA) when its level approaches intracranial pressure (ICP). The authors detected this relationship by making the OA into a "natural pair of scales, in which the intracranial segment of the OA is compressed by extracranial pressure (Pe) applied to the orbit."

Although the conclusion is interesting, there is an implicit assumption that OA crosses a compartmental barrier, which in this case can only be the dural boundary. Consequently, the proposition that ICP can be measured noninvasively, by merely balancing external pressures between the subdural and extradural parts of OA, assumes that these two segments of the OA exist in the entire-- or at least the majority-- of the population. However, microanatomical studies have shown different configurations and variability [2,3], reporting that incidence of the OA originating from the intradural portion of the internal carotid artery ranges from 4.7%-89.6%.

The described effect is therefore unlikely as it is solely explained by transmural pressure balances across these distinct OA segments, thus leaving the underlying mechanism still unclear.

1. Ragauskas A, Matijosaitis V, Zakelis R, et al. Clinical assessment of noninvasive intracranial pressure absolute value measurement method. Neurology 2012;78:1684-1691.

2. Matsumura Y, Nagashima M. Anatomical variations in the origin of the human ophthalmic artery with special reference to the cavernous sinus and surrounding meninges. Cells Tissues Organs 1999;164:112-121.

3. Huynh-Le P, Natori Y, Sasaki T. Surgical anatomy of the ophthalmic artery: its origin and proximal course. Neurosurgery 2005;57(4 Suppl):236-241.

For disclosures, contact the editorial office at [email protected].

Revascularization including stenting, endarterectomy and bypass surgery has been used to treat cerebrovascular stenosis and occlusion. [1-3] However, cerebral hyper-perfusion syndrome (CHS) can be catastrophic. [3] The proposed noninvasive intracranial pressure (ICP) measurement method is accurate compared with gold standard CSF pressure measured via lumbar puncture. [4] However, It was a pity that individuals with brain lesions such as hemorrhages and infarcts were excluded when Ragauskas et al. tested the noninvasive method. [4] Their phase B clinical study should consider these patients and even patients with the severe stenosis or occlusion of internal carotid artery beyond the ophthalmic artery segment. Regional CBF is determined by local cerebral perfusion pressure (CPP) and local cerebrovascular resistance (CVR): CBF=CPP/CVR. CPP is equal to the difference between the mean arterial pressure (MAP) driving blood into the brain and the venous backpressure. Venous backpressure is negligible unless there is elevated intracranial pressure (ICP) or obstruction of venous outflow. If it were feasible for this noninvasive method to be used in stroke patients, we could monitor and obtain ICP and MAP, which would be helpful to understand the physiological mechanism from the change of ICP and MAP.

1. Chimowitz MI, Lynn MJ, Derdeyn CP, et al. Stenting versus aggressive medical therapy for intracranial arterial stenosis. N Engl J Med 2011;365:993-1003.

2. Powers WJ, Clarke WR, Grubb RJ, Videen TO, Adams HJ, Derdeyn CP. Extracranial-intracranial bypass surgery for stroke prevention in hemodynamic cerebral ischemia: the Carotid Occlusion Surgery Study randomized trial. JAMA 2011;306:1983-1992.

3. De Rango P. Cerebral hyperperfusion syndrome: the dark side of carotid endarterectomy. Eur J Vasc Endovasc Surg 2012;43:377.

4. Ragauskas A, Matijosaitis V, Zakelis R, et al. Clinical assessment of noninvasive intracranial pressure absolute value measurement method. Neurology 2012;78:1684-1691.

For disclosures, contact editorial office at [email protected].