Tympanic Membrane Displacement
Tympanic membrane displacement (TMD) technique, proposed nearly twenty years ago by Marchbanks 1 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. The transfer function of this complex mechanical system under physiological conditions is modulated by the action of two small muscles of the middle ear, the tensor tympani and stapedius. The tensor tympani arises from the cartilaginous portion of the auditory tube and the osseous canal of the sphenoid and, having sharply bent over the extremity of the septum, attaches to the manubrium of the malleus (hammer); its contraction pulls the malleus medially, away from the tympanic membrane, which tenses the membrane. The stapedius, which emerges from the posterior wall of the tympanic cavity of the middle ear and inserts into the neck of the stapes (stirrup), prevents excess movements of the stapes by pulling it away from the oval window. The action of either muscle therefore dampens vibrations of the ossicles and reduces the amplitude of transmitted sounds for up to 20dB. The muscles normally contract in response to vocalization, jawing and loud external sounds, which is accompanied with a small but measurable displacement of the eardrum from its initial position. Because cerebrospinal fluid and perilymph communicate through the cochlear aqueduct, an increase in intracranial pressure is directly transmitted to the footplate of the stapes, changing its initial position and affecting thereby the direction and magnitude of the displacement of the eardrum in response to a sound. The displacement can be measured with common tympanometers used for impedance audiometry that are portable and relatively inexpensive and easy to use (particularly the modern, computerized tympanometers with fully automated measurement procedure). Inward displacement (negative peak pressure on audiogram) is suggestive of high, and outward of normal or low ICP. The direction and magnitude of TMD, however, depend not only on the initial position of stapes but also on numerous other factors that affect the acoustic impedance (integrity of the eardrum, condition of the ossicles, patency of the Eustachian tube, pressure and eventual presence of fluid or other masses in the middle ear) or the strength of the acoustic reflex (physiological variability of the reflex threshold, functional integrity of the cochlear and facial nerves, degree of eventual sensory hearing loss). In addition, the assumption that the pressure of perilymph is equal to ICP does not hold if the patency of the cochlear aqueduct is compromised, which is often the case in elderly subjects. Accuracy of TMD estimates of ICP was found to be at the order of ±15mmHg 2, which is not sufficient for a reliable quantitative assessment of ICP in clinical practice.
An interesting method that involves direct manipulations on the tympanic membrane rather than relying on the acoustic reflex was proposed as one of the embodiments of a US patent by Ragauskas 3 . First, a measurement of the position of the tympanic membrane needs to be obtained while ICP is zero (denoted as the baseline position). Equalization of ICP to the atmospheric pressure according to the inventor can be achieved non-invasively by tilting the head up, or the measurement can be taken during a neurosurgical operation. Later on, ICP can be measured by exerting an external pressure to the tympanic membrane and applying simultaneously the same pressure onto the oval window and inner ear (e.g. through the Eustachian tube) until the eardrum is moved back to the baseline position, which will happen when the exerted external pressure equals ICP. No data is provided in the patent nor is available from other sources that could support the utility of the concept in clinical practice.
1 Marchbanks, R.J.: US4841986 (1989).
2 Shimbles S, Dodd C, Banister K, et al. Clinical comparison of tympanic membrane displacement with invasive ICP measurements. Physiol Meas 2005; 26:1085-1092.
3 Ragauskas, A.: US20067147605 (2006).