The cochlear aqueduct (CA) traverses the otic capsule of the inner ear between the perilymph in the scala tympani of the cochlea and the subarachnoid space of the posterior cranial fossa. Thus, the CA maintains a direct connection between the perilymph and the cerebrospinal fluid (CSF).1,2 The vestibular aqueduct (VA) traverses the otic capsule between the endolymph-filled membranous saccule and the membranous endolymphatic sac which usually sits partly within the VA and partly within the subarachnoid space.1–3 A study on cats with patent versus non-patent CAs revealed the CA and VA to be primary and secondary routes, respectively for pressure transfer from the CSF.3
We delineate the use of the tympanic membrane displacement (TMD) test in non-invasive assessment of intracranial/CSF pressure and compare the TMD test with invasive tests for measuring intracranial pressure (ICP).
In the TMD test, displacement of the tympanic membrane of the middle ear is recorded during elicitation of the acoustic middle-ear reflex (AR). The AR primarily represents contraction of the stapedius muscle to intense acoustic stimulation, with additional involvement of the tensor tympani muscle at the higher stimulus intensities. The stapedius muscle has its origin on the posterior wall of the middle ear cavity and its tendon inserts on the neck of the stapes of the ossicular chain of the middle ear. The tensor tympani muscle has its origin on the anterior wall of the middle ear cavity and its tendon inserts on the neck of the malleus of the ossicular chain; the malleus is attached to the tympanic membrane. The AR is monitored non-invasively with an acoustic-immittance device that records the change in acoustic immittance of the middle ear (increased impedance or decreased admittance) resulting from stiffening of the ossicular chain. Elicitation of the AR affects the ossicular chain and the tympanic membrane, and ultimately causes TMD.
Because the stapes footplate is situated in the oval window, perilymphatic pressure changes can alter the baseline position of the stapes and consequently the movement of the ossicular chain and tympanic membrane during the AR. Increased intracranial/perilymphatic pressure displaces the resting stapes footplate laterally (outward) so that TMD during the AR is medial (inward). Decreased intracranial/perilymphatic pressure displaces the baseline stapes footplate position medially (inward) so that TMD during the AR is lateral (outward). The TMD typically is bidirectional (inwards and then outwards) when intracranial/perilymph pressure is normal.4,5
MMS-10 (MARCHBANKS MEASUREMENT SYSTEMS) TMD ANALYSER
The MMS-10 TMD Analyser (Marchbanks Measurement Systems, Ltd., Lymington, Hampshire, UK) is a computerized electronic device with a transducer probe that is hermetically sealed in the ear canal. The device measures the changes in air pressure associated with TMD resulting from the AR. The device, sensitive to TMDs as small as 1 nanoliter (nL), is based on the methods used by Casselbrandt.6,7 To improve the signal-to-noise ratio of TMD recordings, the responses to 10 stimuli are averaged. The stimulus usually is a 1000-Hz tone (500 ms duration) presented at intensities of 10–25 dB SL re: AR threshold. Before TMD testing, tympanometry (acoustic-immittance pressure function) is performed to establish the tympanometric peak pressure for obtaining AR measurement and to rule out middle-ear problems adversely affecting the AR and TM impedance. Tympanometry is followed by AR threshold measurement; these tests represent conventional measures in audiologic assessment. Because the AR arc involves the middle ear, cochlea (sensory receptor organ of the inner ear), cochleovestibular nerve (C.N. VIII), ventral cochlear nucleus, medial superior olivary nucleus, facial motonucleus, facial nerve (C.N. VII), and the stapedius muscle, pathology (or certain drugs such as barbiturates and alcohol) affecting any aspect of this arc can cause elevation or absence of the AR. At the highest intensities (higher than those that would be involved in TMD testing unless the AR threshold is high and the TMD testing is done at the highest SLs), the AR arc also involves the trigeminal motonucleus and trigeminal nerve (C. N. V), which innervate the tensor tympani. The TMD response parameters are Vi (maximum inward TMD) and Vm (mean TMD from the time Vi occurs to the time of stimulus cessation).
CA PATENCY USING THE MMS TMD ANALYSER
Patency of the CA is requisite for TMD to furnish an estimate of ICP. The ICP is physiologically increased in supine as compared with sitting position. Therefore, absence of significant changes in TMD (based on Vm or Vi) with postural changes signifies lack of patency. Indices of TMD used to assess CA patency include: (a) Vm (sitting)/Vm (supine) >0.1 for CA patency5,8; (b) Vi difference between positions/Vi (supine), ΔVi/Vi(supine) >0.1 for CA patency9–11; (c) ΔVi or ΔVm.12
The results of temporal bone studies on age effects on CA patency are controversial with CA non-patency values ranging from 23% to 62%.10,13–15 These findings suggest CA non-patency in many individuals, irrespective of whether an aging effect is present.
COMPARISON OF DIRECT (INVASIVE) AND INDIRECT (TMD) MEASURES OF ICP
The results of studies on TMD have largely shown good test-retest reliability within the same test session with lack of changes in reliability for varying magnitudes of TMD.10,11,16
SHUNTED HYDROCEPHALUS (SH) AND BENIGN INTRACRANIAL HYPERTENSION (BIH)
Patients with SH afford an opportunity for examinations of the relations between TMD and ICP, as such patients may experience increased or decreased ICP with shunt malfunction and resolution of the abnormal ICP with shunt repair. One study on 40 patients with SH with 61 episodes of shunt malfunction employed TMD monitoring and direct ICP monitoring.8 The kappa statistic was significant and indicated very good agreement among categorical assessments (κ=0.88). The ranges showed substantial intersubject variability in Vm and considerable overlap among groups. Similar findings were obtained in another study.5 Although overall sensitivity ranged between 83% and 93%,5,8 the sensitivity for elevated ICP ranged from 55% to 81.5% and was poorer than that (100%) for decreased ICP.
The regression of Vm on invasively measured ICP by other investigators15 revealed that many TMD values failed to fall within expected limits; Vm values substantially overlapped across ICP categories. Also, mean Vm values did not differ significantly among the three patient groups (SH, BIH, and healthy persons). These investigators concluded that the success rate of the TMD test as an indicator of ICP was low.15,17 Serial TMD testing for monitoring changes in ICP may hold greater clinical utility as the patient’s own baseline serves as a reference point against which changes can be compared.5,17 Abnormal acoustic-immittance results precluded TMD testing in 4% and 18% of their control group and group with SH, respectively.15
MÉNIÈRE’S DISEASE (MD)
The relation between the perilymphatic and endolymphatic pressures in endolymphatic hydrops remains to be elucidated.18–20 Mixed findings have been obtained for the ability of TMD to differentiate patients with MD from healthy persons and patients with other inner ear diseases.21–23 Mixed findings also have been obtained on TMD changes following glycerol ingestion (posited to improve perilymphatic pressure).21,24
CONCLUSIONS AND FUTURE DIRECTIONS
Advantages of TMD testing to estimate ICP include its non-invasiveness, good test-retest reliability, and commercial availability of the instrumentation. Disadvantages include adverse effects of abnormal tympanometric findings on TMD results; TMD measurement requires a present AR and patent CA; TMD measurement furnishes relative rather than absolute measure of perilymphatic pressures; TMD intersubject variability is large; and discrepant findings have been reported regarding TMD test performance.
Large-sample research on TMD test performance in healthy persons and patients with various disorders affecting the ICP is needed using direct, invasive measures of the ICP as the gold standard. It has been hypothesized that glaucoma may result from an increased trans-lamina cribosa pressure difference associated with increased intraocular pressure or decreased ICP.25,26 Therefore, research is needed to examine whether non-invasive TMD testing can contribute to elucidating glaucoma pathophysiology.
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