Two-depth transcranial Doppler: a novel approach for non-invasive absolute intracranial pressure measurement

Authors: Nusbaum DM. (Baylor College of Medicine, Department of Medicine, USA.)

There is a real need in healthcare for a way to rapidly yet safely measure intracranial pressure. The two-depth transcranial Doppler technology shows promise in its ability to measure ICP non-invasively, with accuracy and without the need for individual calibration. With time and improvement, this technology may prove valuable in multiple settings, including aviation, space, and emergency medicine.

Since NASA shifted its focus to long-duration space missions, many astronauts have returned home from missions with visual changes thought to be related to increased intracranial pressure (ICP) associated with spaceflight. There is interest in better understanding the nature of this increased ICP, but there are certain obstacles that stand in the way of conducting the necessary research which must first be overcome. For example, the space environment has certain risks that make performing lumbar punctures infeasible. Similarly, in other clinical settings such as emergency centers, there may be a hesitation to use an invasive lumbar puncture to establish raised ICP in patients who may have only mild symptoms, such as headaches. Even in settings such as the intensive care unit, where invasive monitoring of ICP is used routinely, there is always a risk that hospital-acquired complications will arise from such a procedure. Thus, finding a fast, non-invasive way to measure ICP in lieu of a lumbar puncture would be helpful to better characterize and determine the magnitude of these problems.

There are many new approaches for non-invasive measurement of ICP, including dynamic magnetic resonance imaging, tympanic membrane displacement, ophthalmodynamometry, quantitative pupillometry, otoacoustic emission, pulsatility of the ocular circulation, retinal vein pressure correlation with ICP, and the prediction of ICP based on transcranial Doppler and absolute blood pressure simultaneous measurements. However, there are difficulties with many of these techniques in their present forms that must be overcome before they will be useful in a clinical setting. First, any technology used to measure ICP non-invasively must act on a biophysical parameter that remains stable over time and acts independently from the differences among individuals, whether from normal variation or as a result of pathologic processes (except for those variations that cause a variability in ICP itself). Second, as there is no gold standard for non-invasive ICP monitoring, devices should be able to accurately measure ICP without any need for individual calibration. Two-depth transcranial Doppler technology is a technology that does just this.

Device Characteristics

The two-depth transcranial Doppler device acts in much the same way as traditional, non-invasive absolute blood pressure measurement, where external pressure is applied to an artery until the applied pressure reaches the systolic pressure inside that artery and occlusion occurs. The ophthalmic artery can be used in a similar way. There is an intracranial and extracranial segment of the ophthalmic artery, and both segments are similar in size and dimensions such that they have similar blood flow parameters when exposed to the same forces, including external pressure. The characteristics of these segments can be measured simultaneously by exposing each segment to a different Doppler ultrasound waveform. Thus, exposing the intracranial segment to a certain amount of ICP will result in a particular pulsatile pattern and this pressure can be determined by increasing the pressure on tissues surrounding the eyeball until the extracranial pulsatile index matches that of the intracranial. When the two indices are the same, the external pressure matches that of the ICP (Fig. 1).

Fig.1.
Ragauskas’ (1; with permission) method to determine ICP.

Advantages

As stated before, the two-depth transcranial Doppler technology has the advantage of requiring no individual calibration to accurately measure ICP. The device is safe and requires no more pressure on the ocular tissues than what a person would receive while swimming at a depth of 0.7 m. This pressure is applied to the eyes by a set of goggles. Two preliminary studies have already been published that attempt to validate this technology. One was carried out on 31 healthy young adults (2). The other was carried out on 57 patients with severe, closed traumatic brain injury (1). The studies demonstrated error means in the range of 0.939 mmHg. The device can measure ICP in each eye (and thus, in each hemisphere of the brain) independently. And the technology will lend itself to modification to a smaller, pocket-sized, and battery-powered model. Initial studies demonstrated uncertainty intervals that may have been too large (SD = ±6 mmHg) and would lead to too much uncertainty of the true ICP to be of value in making clinical decisions, especially when dealing with what may be only mild elevations in ICP. However, a newer study coming out involving 65 patients and comparing two-depth ultrasound measurements to invasive measurements of ICP, and incorporating technological improvements in the sensitivity of two-depth Doppler to specifically target blood flow velocity in the ophthalmic artery, has shown improved accuracy (mean systematic error less than 0.2 mmHg) and precision (random error's SD = ±2.28 mm Hg) of two-depth ultrasound ICP measurements to a level of uncertainty similar to that of a lumbar puncture (Ragauskas A. Unpublished data; January 2011).

Current Limitations

There are a few drawbacks. First, the device does not account for the variations that may exist between the intracranial and extracranial segments of the ophthalmic artery, which may result in a degree of uncertainty between intracranial and extracranial pressure readings. Clinical data suggest, however, that such variation is negligible. Second, there have not been external studies of large enough number to properly validate this technology and define the scope of its utility. Third, there have been no studies testing its accuracy in measuring pressures greater than 25 mmHg, so it is unknown how the device will perform at the extremes of ICP. And, finally, the technology is still too big and bulky to be useful in an environment like space where power, mass, and volume are limiting factors. Though it may prove more useful in terrestrial settings such as emergency centers, its utility may still be limited until a less heavy, more mobile version is available.

There is a real need in healthcare for a way to rapidly yet safely measure intracranial pressure. The two-depth transcranial Doppler technology shows promise in its ability to measure ICP non-invasively, with accuracy and without the need for individual calibration. With time and improvement, this technology may prove valuable in multiple settings, including aviation, space, and emergency medicine.

References

  1. Ragauskas A, Daubaris G, Dziugys A, Azelis V, Gedrimas V. Innovative non-invasive method for absolute intracranial pressure measurement without calibration. Acta Neurochir Suppl 2005; 95:357–61.
  2. Ragauskas A, Daubaris G, Zakelis R, Rutkauskas S, Krutulyte G, Kalasauskiene A. Non-invasive absolute intracranial pressure measurement without problem of calibration: Healthy volunteer study. Electronics and Electrical Engineering 2009; 7(95):103–6.

Aviat Space Environ Med. 2011 Nov;82(11):1080-1.

Source: MedConnect