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A preliminary theoretical model of hydrodynamics in the inner ear.

Abstract

Head movement should create a transient pressure imbalance across the membranous inner ear. We used basic concepts of fluid dynamics to develop a theoretical model of the inner ear. According to this model, two contiguous fluidic systems--the perilymphatic system and the endolymphatic system--are in hydrostatic equilibrium across a compliant membrane. Our model demonstrates that changes in resistance or compliance in one system results in a transient distortion of the membranous inner ear until equilibrium between the two systems is restored. The concept of hydrodynamic pressure changes in the inner ear has received little attention, but it may represent a new approach to understanding the inner ear and treating inner ear diseases.

Introduction

While some cases of vertigo are easily diagnosed and treated, others present a challenge. New approaches to understanding the inner ear should be explored to help us better manage the challenging cases. We have developed a theoretical hydrodynamic model of the inner ear that may be useful in this regard.

Two proposed causes of vertigo are canalithiasis and cupulolithiasis, which are often cited to explain benign paroxysmal positional vertigo (BPPV) in particular. Rajguru et al developed a three-canal mathematical model of canalithiasis to explain why the Epley canalith repositioning maneuver is so effective in treating patients with BPPV. (1) However, the fact that not all patients respond to the Epley maneuver (2) suggests that a different etiology is involved, and this led to the theory of cupulolithiasis, which holds that dislodged otoconia irritate the cupula.

Our hydrodynamic model is based on the relationship between the two fluidic systems in the inner ear: the perilymphatic system and the endolymphatic system. These two systems are separated by a compliant membrane (the membranous labyrinth), and in a steady state, they are in hydrodynamic equilibrium across this membrane. When movement occurs, however, the fluid dynamics of the systems is perturbed and a transient pressure imbalance develops. The time required for equilibrium to be regained depends on two variables: resistance to fluid flow and compliance of the system. With our theoretical model, we analyze the relationships among these variables, and we express these relationships in mathematical formulas. Our model can be applied to the study of challenging cases of vertigo, BPPV, and Meniere disease.

The hydrodynamic model

Applying basic concepts of fluid dynamics, we developed mathematical formulas that describe the relationships among various properties and actions of the perilymphatic and endolymphatic systems. Taking into consideration three variables--volume (V), time to equilibrium (t), and pressure (P)--we can mathematically define flow (F), resistance (R), and compliance (C) thus:

* F = AV/t

* R = [DELTA]P/F

* C = [DELTA]V/AP

If we combine these expressions, the resulting formulas are as follows:

* R = [DELTA]Pt/[DELTA]V = t/C

* t= RC

[FIGURE 1 OMITTED]

We have also expressed our theoretical model in schematic form, as illustrated in figures 1 through 4:

* In figure 1, a rigid tube with elastic caps represents a fluid-filled bony and membranous labyrinth.

* In Figure 2, resistance is applied to the system.

* In figure 3, the two fluidic systems are depicted as a rigid circular tube with two segments.

* In figure 4, resistance is added to one of the segments.

Maintenance of overall system equilibrium across the membranous labyrinth requires that the RC products of both the perilymphatic (p) and endolymphatic (e) systems be equal--that is: [R.sub.p] [C.sub.p] = [R.sub.e] [C.sub.e]. A change in any of these four factors ([R.sub.p], [C.sub.p], [R.sub.e], and [C.sub.e]), such as an anatomic variation or change in viscosity, can result in a transient overall system imbalance and distortion of the membranous labyrinth in the inner ear. Anatomic variations may include the distortion of the membranous labyrinth in Meniere disease, an enlarged vestibular aqueduct, and even dislodged otoliths; in the latter case, the fluid dynamics model complements current theories of BPPV.

[FIGURE 2 OMITTED]

Discussion

The application of basic physical concepts of hydrostatic pressure may play an important role in understanding the inner ear and may augment current theories of inner ear disease. Groen (3) andVan Buskirk et al (4) separately developed torsion-pendulum models of the inner ear in which endolymph provides inertia and viscous drag and the cupula provides the restoring stiffness. In these theoretical models, the membranous semicircular canal acts as a hollow torus in which the angular displacement of the endolymph is related to the angular acceleration of the head. This system functions like a damped torsion pendulum. Hain et al mathematically simulated episodes of canalithiasis when otoconia fell from different positions. (5) In a guinea pig model, pressure changes in the cranium, ear canal, and middle ear have been shown to lead to pressure changes in the inner ear. (6,7) A model involving the use of high-resolution magnetic resonance imaging (MRI) of temporal bones may be useful since studies show that MRI scans can provide a foundation for developing diagnostic criteria for inner ear disease. (8)

Our hydrodynamic model of the inner ear provides a novel approach to understanding inner ear function. As mentioned, a change in resistance or compliance in either the perilymphatic system or the endolymphatic system may occur as a result of anatomic variations, which can affect resistance or compliance, and changes in viscosity, which will affect resistance. These changes can result in a transient distortion of the membranous inner ear.

[FIGURE 3 OMITTED]

The hydrodynamic model may help explain how symptoms of Meniere disease can be caused by obstructing lesions of the endolymphatic sac and how these symptoms are relieved by the Meniett device, an alternating pressure generator. Our model may also explain why some patients are more susceptible to BPPV and more resistant to treatment with canalith repositioning maneuvers.

[FIGURE 4 OMITTED]

In conclusion, the hydrodynamic model of the inner ear demonstrates how variations in the RC product of the two fluidic systems may contribute to inner ear dysfunction. Variations in resistance and compliance can result in a transient pressure imbalance, which might help explain the pathophysiology of conditions such as vertigo, BPPV, and Meniere disease. Future directions include constructing a physical model that can be modified by manipulating the resistance in the endolymphatic or perilymphatic system or by adding canaliths to the system.

References

(1.) Rajguru SM, Ifediba MA, Rabbitt RD. Three-dimensional biomechanical model of benign paroxysmal positional vertigo. Ann Biomed Eng 2004;32(6):831-46.

(2.) Blakley BW. A randomized, controlled assessment of the canalith repositioning maneuver. Otolaryngol Head Neck Surg 1994; 110 (4): 391-6.

(3.) Groen JJ. The semicircular canal system of the organs of equilibrium. II. Phys Med Biol 1957;1(3):225-42.

(4.) Van Buskirk WC, Watts RG, Liu YK. The fluid mechanics of the semicircular canals. J Fluid Mech 1976;78(1):87-98.

(5.) Hain TC, Squires TM, Stone HA. Clinical implications of a mathematical model of benign paroxysmal positional vertigo. Ann N Y Acad Sci 2005;1039:384-94.

(6.) Thalen E, Wit H, Segenhout H, Albers E Inner ear pressure changes following square wave intracranial or ear canal pressure manipulation in the same guinea pig. Eur Arch Otorhinolaryngol 2002;259(4):174-9.

(7.) Wit HP, Feijen RA, Mbers FW. Cochlear aqueduct flow resistance is not constant during evoked inner ear pressure change in the guinea pig. Hear Res 2003;175(1-2):190-9.

(8.) Silver RD, Djalilian HR, Levine SC, Rimell FL. High-resolution magnetic resonance imaging of human cochlea. Laryngoscope 2002;112(10):1737-41.

Vivian M. Yu, MD; Paul J. Strykowski, PhD; Rick M. Odland, MD, PhD

From the Department of Otolaryngology-Head and Neck Surgery, Lahey Clinic, Burlington, Mass. (Dr. Yu); the Department of Mechanical Engineering, University of Minnesota Institute of Technology, Minneapolis (Dr. Strykowski); and the Department of Otolaryngology, University of Minnesota Medical School, Minneapolis (Dr. Odland).

Corresponding author: Vivian M. Yu, MD, St. Elizabeth's Hospital, 736 Cambridge St., SMC8, Boston, MA 02135. E-mail address: v-yu@ northwestern.edu

Previous presentation: The information in this article has been updated from its original presentation at the annual meeting of the American Academy of Otolaryngology-Head and Neck Surgery; Sept. 17-20, 2006; Toronto.
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Title Annotation:ORIGINAL ARTICLE
Author:Yu, Vivian M.; Strykowski, Paul J.; Odland, Rick M.
Publication:Ear, Nose and Throat Journal
Article Type:Report
Geographic Code:1USA
Date:Apr 1, 2010
Words:1344
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