Non-invasive intracranial pressure measurement methods

Increased intracranial pressure (ICP) is one of the major causes of secondary brain ischemia that accompanies a variety of pathological conditions, most notably, traumatic brain injury (TBI), stroke, and intracranial hemorrhages. Increased intracranial pressure can cause such complications as VIIP, death, permanent neurological problems, reversible neurological problems, seizures, stroke. However, aside from a few Level I trauma centers, ICP monitoring is rarely a part of the clinical management of patients with these conditions because of the invasiveness of the standard monitoring methods (which require insertion of an ICP sensor into the brain ventricle or parenchymal tissue), additional risks they present for patients, high costs associated with the an ICP sensor's implantation procedure, and the limited access to trained personnel, i.e., a neurosurgeon. Alternative methods have therefore been sought with which ICP can be estimated non-invasively.Absolute majority of the approaches to non-invasive ICP estimation are based on the idea that something in the human head's anatomical structure or in the intracranial and extracranial physiology correlates with ICP. Very important limitation of such ""correlation based"" approaches is caused by the fact that correlation shows some relationship but did not show the slope and bias of such relationship. ""Correlation based"" approaches can reflect ICP changes only with limited accuracy (expressed by systematic error) and precision (expressed by standard deviation of random error) because of that. Such approaches are not able to measure quantitatively an absolute ICP value because of the need for individual patient specific calibration. Calibration is an only procedure for identification of slope and bias of ""correlation based"" association. Calibration of non-invasive ICP meter or monitor is impossible because of impossibility to create a ""gold Standard"" non-invasive ICP meter for calibration purposes.Absolute ICP values in mmHg or other units are needed for patients' treatment decision making. The only accurate, precise and patient specific calibration free non-invasive absolute ICP value measurement method relies not on the correlation but on direct ICP and extracranial pressure comparison principle. Innovative method using Two-Depth Transorbital Doppler (TDTD) of intracranial pressure quantitative absolute (ICP) value measurement relies on the same fundamental principle as used to measure blood pressure with a sphygmomanometer. A sphygmomanometer works using a pressure balance principle - an air-filled pressure cuff wrapped around the arm compresses the brachial artery to a point where blood can no longer flow. Externally applied pressure is equal to systolic blood pressure in this case. The examiner slowly releases the air from the cuff and uses a stethoscope to listen for the return of blood flow. At the pressure balance point, where pressure in the cuff equals systolic artery pressure, a ‘whooshing’ noise can be heard as blood flows through the artery again. Pressure balance based non-invasive blood pressure meter does not need a patient specific calibration.The TDTD method uses Doppler ultrasound to translate pressure balance principle of blood pressure measurement with a sphygmomanometer to the measurement of ICP. Ophthalmic artery (OA) - a unique vessel with intracranial and extracranial segments is used as pressure sensor and as a natural pair of scales for absolute ICP value in mmHg or mmH2O measurement. Blood flow in the intracranial OA segment is affected by intracranial pressure, while flow in the extracranial (intraorbital) OA segment is influenced by the externally applied pressure (Pe) to the eyeball and orbital tissues.As with a sphygmomanometer, a special pressure cuff is used - in this case to compress the tissues surrounding the eyeball and also intraorbital tissues surrounding the extracranial segment of OA. External pressure changes the characteristics of blood flowing from inside the skull cavity into the eye socket. In place of the stethoscope, a Doppler ultrasound beam measures the blood flow pulsations in intracranial and extracranial segments of the Ophthalmic Artery. The non-invasive ICP meter based on this method gradually increases the pressure over the eyeball and intraorbital tissues so that the blood flow pulsation parameters in two sections of OA are equal. At this pressure balance point, the applied external pressure (Pe) equals to the intracranial pressure (ICP). This measurement method eliminates the main limiting problem of all other non-successful approaches to non-invasive ICP measurement - the individual patient calibration problem. Direct comparison of arterial blood pressure (ABP) and externally applied pressure is the basic arterial blood pressure measurement principle which eliminates the need of individual calibration. The same calibration free fundamental principle is used in TDTD non-invasive ICP absolute value measurement method. The mean value of OA blood flow, its systolic and diastolic values, pulsatility and other indexes are almost the same in both OA segments in the point of balance when ICP=Pe. As a result of that all individual influential factors (ABP, cerebrovascular autoregulation impairment, individual pathophysiological state of patient, individual diameter and anatomy of OA, hydrodynamic resistance of eyeball vessels, etc.) do not influence the balance ICP=Pe and, as a consequence, such natural “scales” do not need calibration.Ragauskas A. et al. already published the statistically significant results of prospective clinical study on assessment of the accuracy and precision of proposed non-invasive absolute ICP value measurement method. The study shows that proposed method is the only quantitative noninvasive ICP absolute value (mmHg) measurement method which does not need an individual patient specific calibration. High accuracy, precision, sensitivity and specificity of proposed method are fully acceptable for clinical practice and for very wide applications in neurology, transplantology, intensive care, sport medicine, aerospace medicine and combat casualty care.This method is further developed by Company Vittamed Ltd together with consortium partners in EU FP7 projects BrainSafe Brainsafe, Brainsafe II and TBIcare.