Non-invasive ICP measurement methods
- Two-Depth Transcranial Doppler for measurements of ICP through the Opthalmic Artery. Innovative method using Two-Depth Transcranial Doppler (TDTD) of monitoring intracranial pressure (ICP) relies on the same fundamental principle used to measure blood pressure with a sphygmomanometer. A sphygmomanometer works using a balance principle - an air-filled pressure cuff wrapped around the arm compresses the brachial artery to a point where blood can no longer flow. The examiner slowly releases the air from the cuff and uses a stethoscope to listen for the return of blood flow. At the balance point, where pressure in the cuff equals systolic artery pressure, a ‘whooshing’ noise can be heard as blood flows through the artery again.
- Ultrasound Time of the Flight Techniques; The majority of patented methods for noninvasive monitoring of ICP are based on an assumption that changes in ICP affect the physical dimensions and/or acoustic properties of the cranial vault or intracranial structures (dura, brain tissue, brain ventricles, and/or intracranial vessels). The common drawback of all these methods is that they measure only relative changes of ICP as referenced to a baseline measurement during which absolute ICP is known, i.e. the ultrasound readouts need to be calibrated on each subject against an invasive measurement. Ultrasound ‘time of the flight’ methods for non-invasive ICP monitoring have not been extensively validated and currently the majority of them do not seem to be accurate enough for a routine clinical use. Their original formulations usually do not specify locations for the transducers placement, and do not address how the intentional or accidental use of different locations and/or angles of the transducers will affect the reliability of ICP estimates. It has also remained unexplored how the measurements are affected by the presence of intracranial pathologic masses on the path of the ultrasound wave, or by brain masses shifts.
Transcranial Doppler Ultrasonography;
The TCD measures the velocity of blood flow through the major intracranial vessels by emitting a high frequency (>2MHz) wave from an ultrasound probe and detecting a frequency shift between the incident and reflected wave which directly correlates with the speed of the blood (the so called Doppler effect). The measurement is taken over the regions of the skull with thinner walls (temporal region, back of the head, or through the eye) as the bones strongly attenuate the transmission of the ultrasound at these frequencies. TCD is primarily a technique for diagnosing various intracranial vascular disorders such as emboli, stenosis, or vasospasm, and can be used to identify patients who are at risk of developing cerebral ischemia in early phases of traumatic brain injury or stroke.
Methods from this group attempt to derive ICP from mechanical properties of the skull bones rather than of the intracranial content. The underlying assumption is similar to that of the ultrasound time of the flight techniques: that the skull is not completely rigid, so that changes in ICP result in a small but measurable skull expansion which creates additional stress within the skull bones and modifies their mechanical properties 9.
Tympanic Membrane Displacement;
Tympanic membrane displacement (TMD) technique, proposed nearly twenty years ago by Marchbanks 12 exploits the effect of intracranial pressure on the acoustic reflex, i.e. a reflex contraction of the stapedius and tensor tympani muscles in response to a sound. Normally, vibrations of the tympanic membrane (eardrum) elicited by acoustic stimuli are transmitted through the chain of ossicles (malleus, uncus, and stapes) in the middle ear to the oval window of the cochlea. Vibrations of the footplate of stapes transmit through the oval window to the perilymph, which in turn causes the endolymph, the basilar membrane, and the organ of Corti to vibrate, activating ultimately the acoustic sensor cells, the inner hair cells of the organ of Corti...
- Otoacoustic Emission; A measurable acoustic phenomenon that originates in the inner ear would, at least in theory, allow for more precise assessment of the pressure of the peri- and endo-lymph, and consequently, of ICP. Otoacoustic emission (OAE), which is a sound generated by subtle oscillations of the endo- and perilymph caused by contractions of the outer hairy cells of the inner ear in response to a loud sound, seems to offer such a possibility. The sound is transmitted to the stapes, and further through the ossicles, to the tympanic membrane from which it can be detected with a sensitive microphone inserted into the ear canal.
- Ocular measurements. Eye provides another possible window into the pressure changes in the intracranial compartment thanks to the fact that the space between the optic nerve and its sheath is a continuation of the subarachnoid space, and is consequently filled with cerebrospinal fluid whose pressure is equal to intracranial pressure. Intracranial hypertension will thus manifest in increased diameter of the optic nerve sheath, and will impede the blood flow through the central retinal vein that courses within the sheath, along and in part inside of the optical nerve.