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(Radiology. 1999;213:819-823.)
© RSNA, 1999

Head and Neck Imaging

MR Imaging of the Cochlear Modiolus: Area Measurement in Healthy Subjects and in Patients with a Large Endolymphatic Duct and Sac1

Shinji Naganawa, MD, Tokiko Ito, MD, Eriko Iwayama, MD, Hiroshi Fukatsu, MD, Takeo Ishigaki, MD, Tsutomu Nakashima, MD and Nobuyasu Ichinose, MS

1 From the Departments of Radiology (S.N., T.Ito, E.I., H.F., T. Ishigaki) and Otolaryngology (T.N.), Nagoya University School of Medicine, 65 Tsurumai-cho, Shouwa-ku, Nagoya 466-8550, Japan, and the Toshiba Nasu Works, Tochigi, Japan (N.I.). From the 1998 RSNA scientific assembly. Received September 22, 1998; revision requested November 4; revision received March 19, 1999; accepted July 1. Address reprint requests to S.N.


   Abstract
TOP
 Abstract
Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
PURPOSE: To evaluate the cochlear modiolus with thin-section magnetic resonance (MR) imaging in healthy subjects and patients with a large endolymphatic duct and sac, and to assess whether the cochlea is normal or abnormal in patients with a large endolymphatic duct and sac.

MATERIALS AND METHODS: MR images were obtained in 10 ears in five volunteers (group 1), 40 ears in 20 patients with bilateral sensory hearing loss (group 2), three ears in two patients with Mondini malformation (group 3), and 12 ears in seven patients with a large endolymphatic duct and sac (group 4).

RESULTS: In groups 1 and 2, all modiolar areas were larger than 4.0 mm2. In group 3, each modiolus was smaller than 2.0 mm2. In group 4, modiolar areas were smaller than 2.0 mm2 in eight ears and were larger than 4.0 mm2 in four ears.

CONCLUSION: Findings in this study confirm that a large endolymphatic duct and sac is frequently associated with modiolar deficiency, but the modiolar area is normal in some cases. This result does not support the recently proposed hypothesis that hearing loss with a large endolymphatic duct and sac is caused by the transmission of subarachnoid pressure forces into the labyrinth through a deficient modiolus.

Index terms: Ear, abnormalities, 213.1499 • Ear, anatomy, 213.92 • Ear, MR, 213.121416 • Magnetic resonance (MR), pulse sequence, 213.121416 • Magnetic resonance (MR), thin section, 213.121416


   Introduction
TOP
 Abstract
 Introduction
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Large vestibular aqueduct syndrome, or large endolymphatic duct and sac syndrome, is one form of the inner ear malformations that manifest as progressive sensorineural hearing loss starting in infancy or childhood (1–3). To our knowledge, the first radiologic description of the large vestibular aqueduct syndrome was reported by Valvassori and Clemis (4) with polytomography in 1978. Since that time, many investigators have described this syndrome with computed tomography (CT) and magnetic resonance (MR) imaging (1–3,5–19). Although several authors have described a large vestibular aqueduct as an isolated radiologic finding associated with sensorineural hearing loss (12,17), recent reports indicate that a large vestibular aqueduct is frequently associated with other inner ear malformations (2,8,16). Lemmerling et al (8) reported that, on the basis of thin-section CT studies, all ears with a large vestibular aqueduct have associated cochlear modiolar deficiencies. They mentioned that although radiologists cannot be blinded to the knowledge of enlargement of the vestibular aqueduct or endolymphatic duct when they assess the cochlear modiolus at either CT or MR imaging, partial modiolar deficits may appear even more conspicuous on MR images acquired with bright-fluid sequences than on CT scans obtained with volume CT technique.

On MR images, it is not the vestibular aqueduct itself that is visualized but its contents, the endolymphatic duct and sac. Therefore, Okamoto et al (7) proposed that the term "large vestibular aqueduct syndrome" should be changed to "large endolymphatic duct and sac syndrome." They also reported that the signal intensity of an enlarged endolymphatic sac was markedly different from that of cerebrospinal fluid, suggesting protein-rich hyperosmolar content of the enlarged sac (7). Microchemistry study of the endolymphatic sac in humans demonstrated a high protein concentration of 1,000–3,000 mg/dL (10–30 g/L), and fluid in the sac is markedly hyperosmolar compared with that in the remainder of the membranous labyrinth (12).

The modiolus (from the Latin word for "hub of a wheel") is a small, irregular mass of porous bone in the shape of a cone (8,20). In the center of the cochlea, the modiolus occupies the space enclosed by the first two turns of the spiral (20). The osseous spiral lamina of the cochlea is the thin bone shelf that projects from the modiolus and supports the organ of Corti, as well as the termination of the neurovascular supply to this end organ of hearing (Fig 1). The osseous spiral lamina also separates the scala tympani and scala vestibuli (8). For decades, histopathologic reports emphasized the abnormal appearance of the modiolus in association with a malformed cochlea (21).



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  		Figure 1. Photograph of histologic section of the cochlear modiolus. (Reprinted, with permission, from reference 20.) DC = ductus cochlearis, ISS = interscalar septum, M = modiolus, OSL = osseous spiral lamina, RM = Reissner membrane, ST = scala tympani, SV = scala vestibuli.

 
The purpose of this study was to evaluate, with thin-section MR imaging with a three-dimensional fast asymmetric spin-echo sequence and dedicated phased-array coils, the cochlear modiolus in healthy subjects and to assess cochlear normality in patients with a large endolymphatic duct and sac (the contents of the vestibular aqueduct).


   MATERIALS AND METHODS
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
From April 1996 through August 1998, 29 patients (15 men and 14 women; age range, 2–76 years; mean age, 37.7 years) underwent MR imaging at our institution to evaluate hearing loss. In addition, MR imaging was performed in five healthy volunteers (three men and two women; age range, 26–72 years; mean age, 38.6 years ± 17.1 [SD]). This protocol was approved by the institutional medical ethics committee, and all subjects gave their informed consent to participate in the study.

All MR examinations were performed with a 1.5-T MR system (Visart; Toshiba, Tokyo, Japan) with a quadrature surface phased-array coil. A pair of surface coils was used as a phased array, with one coil placed over each ear.

Imaging was performed with a heavily T2-weighted three-dimensional fast asymmetric spin-echo sequence: repetition time msec/echo time msec of 4,000/240, echo train length of 79, field of view of 16 cm, section thickness of 0.8 mm, 512 x 512 x 40-matrix transverse slab, voxel size of 0.3 x 0.3 x 0.8 mm, one signal acquired, and imaging time of 11 minutes 48 seconds. The surface coil and pulse sequence have been described in detail in previous reports (22,23).

Thin-section MR images were obtained in 10 ears in the five volunteers (group 1), in 40 ears in 20 patients with bilateral sensory hearing loss with a normal endolymphatic duct and sac depicted on MR images (group 2: 10 male and 10 female patients; age range, 2–76 years; mean age, 45.5 years ± 16.4), in three ears in two patients with Mondini malformation with sensorineural hearing loss (group 3: two male patients aged 17 and 23 years; one patient had unilateral disease), and in 12 ears in seven patients with a large endolymphatic duct and sac (group 4: three male and four female patients; age range, 4–34 years; mean age, 20.4 years ± 8.6; two patients had unilateral disease). All ears in group 4 showed progressive sensorineural hearing loss. The variability of severity of hearing loss among groups 2–4 was small.

The endolymphatic duct is considered to be dilated when its diameter at the midpoint between the common crus and its external aperture is 1.5 mm or more on thin-section MR images (13). The area of the cochlear modiolus was measured at the transverse section where the cochlear modiolus was visualized at its maximum size on thin-section T2-weighted images.

Two radiologists (S.N., E.I.) performed the measurements at the MR imaging console. For the modiolar area measurement, the distinct low-signal-intensity area with a triangular or trapezoidal shape at the axis of the basal turn or the basal and middle turns of the cochlea was outlined, excluding the free thin part of the osseous spiral lamina and interscalar septum (Fig 2). The measurements were performed where the cochlear modiolus was visualized at its maximum size. At the console, the window width was set at 100% of the range from the lowest to the highest pixel values in the image, and the window center level was set at the midpoint between the lowest and highest pixel values. Each radiologist measured each cochlear modiolus twice, and the average of the two measurements was obtained. The final value was obtained by averaging the values obtained by the two radiologists.



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  		Figure 2a. Heavily T2-weighted transverse MR images (4,000/240, 0.8-mm section thickness) in a 38-year-old male volunteer with a normal ear. (a) Normal modiolus (arrow). (b) Example of area measurement (arrow).

 


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  		Figure 2b. Heavily T2-weighted transverse MR images (4,000/240, 0.8-mm section thickness) in a 38-year-old male volunteer with a normal ear. (a) Normal modiolus (arrow). (b) Example of area measurement (arrow).

 
Intergroup statistical analysis was performed with the Mann-Whitney U test. Interobserver and intraobserver variability were also assessed. The signal intensity of the dilated endolymphatic sac was evaluated in comparison with that of the adjacent cerebrospinal fluid as the standard of reference and was graded qualitatively as higher, comparable, or lower. A P value of .05 was considered statistically significant.

One patient in group 4 underwent 1-mm-thick contiguous-section CT (TCT-900S; Toshiba). On CT images, the modiolus was thought to be normal when the modiolus was seen as a distinct structure with calcific attenuation that was triangular or trapezoidal, lay between the basal and middle turns, and was connected to the bone capsule of the cochlea by the interscalar septum, according to the criteria used by Lemmerling et al (8).


   RESULTS
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
In all ears in healthy volunteers (group 1), the modiolus was visualized as an area larger than 4.0 mm2 (range, 4.1–5.8 mm2; mean, 4.6 mm2 ± 0.5) (Fig 2). In all ears in patients with bilateral sensory hearing loss and a normal endolymphatic duct (group 2), the modiolus was also visualized as an area larger than 4.0 mm2 (range, 4.1–6.2 mm2; mean, 4.9 mm2 ± 0.6). In all ears with Mondini malformation and a normal endolymphatic duct (group 3), the modiolus was smaller than 2.0 mm2 (range, 1.2–1.8 mm2; mean, 1.5 mm2 ± 0.3). In the ears with a large endolymphatic duct and sac (group 4), modiolar areas were smaller than 2.0 mm2 in eight ears (Fig 3) but larger than 4.0 mm2 in four ears (overall mean, 2.4 mm2 ± 1.4). Each modiolar area value is shown in Figure 4.



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  		Figure 3. Heavily T2-weighted transverse MR image (4,000/240, 0.8-mm section thickness) in a 21-year-old female patient with an enlarged endolymphatic duct (arrow). The cochlear modiolus is hypoplastic (arrowheads).

 


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  		Figure 4. Diagram depicts distribution of modiolar area for each study subject. All modiolar areas in groups 1 ({diamondsuit}) and 2 ({diamond}) are larger than 4.0 mm2. In all ears in group 3 ({triangleup}) and in eight ears in group 4 (•), modiolar areas are smaller than 2.0 mm2. However, four ears in group 4 showed modiolar area larger than 4.0 mm2.

 
Mean area differences were statistically significant between groups 1 and 4 (P < .005) and between groups 2 and 4 (P < .001). Intraobserver variability for observer 1 was less than 0.5 mm2 for all ears (range, 0.0–0.4 mm2; mean, 0.2 mm2 ± 0.1) and for observer 2 was less than 0.4 mm2 for all ears (range, 0.0–0.3 mm2; mean, 0.2 mm2 ± 0.1). Interobserver variability was less than 0.5 mm2 for all ears (range, 0.0–0.4 mm2; mean, 0.3 mm2 ± 0.1). Among 12 ears with a large vestibular aqueduct, the signal intensity of the dilated endolymphatic sac was graded as lower than that of cerebrospinal fluid on T2-weighted images for four ears and comparable for eight (Fig 5).



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  		Figure 5. Heavily T2-weighted transverse MR image (4,000/240, 0.8-mm section thickness) in a 14-year-old patient with an enlarged endolymphatic duct and sac. The signal intensity of the enlarged endolymphatic sac (solid arrow) is lower than that of adjacent cerebrospinal fluid (arrowheads). However, the signal intensity of the enlarged endolymphatic duct (open arrow) is comparable to that of cerebrospinal fluid.

 
In the patient in group 4 who underwent CT, a calcified, normal trapezoidal modiolus lying in the center of the middle and basal turns of the cochlea was depicted on CT images in both ears (Fig 6), and the modiolar area was 4.1 mm2 in both ears.



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  		Figure 6a. Images in a 34-year-old female patient with an enlarged endolymphatic duct and sac. (a) Transverse CT scan depicts a large vestibular aqueduct (arrow). (b, c) Heavily T2-weighted transverse images (4,000/240, 0.8-mm section thickness) depict an enlarged endolymphatic duct (arrow in b) and sac (arrowheads in b). The area of the modiolus was 4.1 mm2 (arrow in c). (d) Transverse CT scan depicts a calcified modiolus (arrow).

 


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  		Figure 6b. Images in a 34-year-old female patient with an enlarged endolymphatic duct and sac. (a) Transverse CT scan depicts a large vestibular aqueduct (arrow). (b, c) Heavily T2-weighted transverse images (4,000/240, 0.8-mm section thickness) depict an enlarged endolymphatic duct (arrow in b) and sac (arrowheads in b). The area of the modiolus was 4.1 mm2 (arrow in c). (d) Transverse CT scan depicts a calcified modiolus (arrow).

 


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  		Figure 6c. Images in a 34-year-old female patient with an enlarged endolymphatic duct and sac. (a) Transverse CT scan depicts a large vestibular aqueduct (arrow). (b, c) Heavily T2-weighted transverse images (4,000/240, 0.8-mm section thickness) depict an enlarged endolymphatic duct (arrow in b) and sac (arrowheads in b). The area of the modiolus was 4.1 mm2 (arrow in c). (d) Transverse CT scan depicts a calcified modiolus (arrow).

 


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  		Figure 6d. Images in a 34-year-old female patient with an enlarged endolymphatic duct and sac. (a) Transverse CT scan depicts a large vestibular aqueduct (arrow). (b, c) Heavily T2-weighted transverse images (4,000/240, 0.8-mm section thickness) depict an enlarged endolymphatic duct (arrow in b) and sac (arrowheads in b). The area of the modiolus was 4.1 mm2 (arrow in c). (d) Transverse CT scan depicts a calcified modiolus (arrow).

 

   DISCUSSION
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
The endolymphatic duct and sac is a tiny structure in healthy subjects, and it is frequently invisible on thin-section MR images, not only in patients with Meniere disease but also in healthy subjects (24–29). The cochlear modiolus (Fig 1) is also a tiny structure (20,21,28,30). To our knowledge, the cochlear diameter and height have been measured with an anatomic method (20), but the modiolar area has not been directly measured with either imaging or anatomic methods. Thin-section bone-algorithm CT images do not provide sufficient contrast resolution to allow quantification of the modiolar area (8). In a study with CT, cochlear modiolar deficiency was noted in all patients with a large vestibular aqueduct, on the basis of assessment as visible or not visible (8). In this study, quantification of the cochlear modiolus was carried out with our thin-section MR imaging protocol, and all cochlear modiolar areas in groups 1 and 2 were larger than 4.0 mm2. This 4.0-mm2 area covered 41 pixels, and interobserver and intraobserver variability were both less than 0.5 mm2, which indicates the accuracy of the measurements obtained in this study.

According to results in a histologic study, the modiolus has a diameter of 4 mm at its base, and its height is no more than 3 mm (20). The area of the modiolus is less than 6 mm2, if the modiolus is assumed to be triangular (20). Our measured values in the healthy volunteers were 4.1–5.8 mm2. These values were slightly smaller than this calculated value, but we think they are not far from the real values. We used 0.8-mm-thick sections for area measurement of the modiolus. Even so, the modiolus was typically visualized in only two or three sections.

The maximum cross-sectional area of the modiolus was measured in this study, rather than the volume. This single-section measurement may have excluded some small part of the modiolus. For more detailed evaluation, the volume of the modiolus should be quantified by means of imaging with thinner sections. We traced the region of interest carefully to exclude the free part of the osseous spiral lamina and interscalar septum. Although inter- and intraobserver variability were small, this region-of-interest setting is subjective to some extent and may not be perfect.

Direct comparison between CT and MR images was not carried out in this study, except in one patient with a large endolymphatic duct and sac. A modiolus with an area of more than 4.0 mm2 on MR images in patients with a large endolymphatic duct and sac may not be visible on CT images if the modiolus is not sufficiently calcified. This may be one reason for the discrepancy between findings in the previous CT study (8) and in the present MR study. Our patient who underwent both CT and MR imaging had a trapezoidal calcified modiolus depicted at CT that was graded as normal according to the criteria in the previous CT study (8), and it was depicted as more than 4.0 mm2 in area on MR images.

Depiction of the interscalar septum may be another method to assess cochlear normality (8). The interscalar septum becomes thinner toward the apex of the cochlea, however, and depiction of this partition is less consistent than is depiction of the normal modiolus (8). A deficient modiolus is said to allow the transmission of pressure waves from cerebrospinal fluid into the labyrinth, which results in damage to the hair cells in the organ of Corti (8). To evaluate the feasibility of this theory, depiction of the modiolus itself is thought to be more important than is depiction of the interscalar septum. In our MR study, some ears with a large endolymphatic duct and sac had modiolar areas comparable to those in groups 1 and 2. Findings in our study confirm that a large endolymphatic duct and sac is frequently associated with modiolar deficiency, but normal modiolar area is seen in some cases. Results in our study do not necessarily support this hypothesis regarding the cause of progressive hearing loss in the presence of a large vestibular aqueduct.

Another theory suggests that hyperosmolar proteins in the enlarged endolymphatic sac reflux into the ductus cochlearis (scala media) through a widely patent endolymphatic duct, which causes osmotic damage to the neuroepithelium (7). The protein concentration of the endolymphatic sac is very high (1,000–3,000 mg/dL [10–30 g/L]) even in healthy subjects (12,28). The protein concentration of the endolymph from the enlarged endolymphatic sac obtained at surgery in a patient with the large vestibular aqueduct syndrome was 660 mg/dL [6.6 g/L] (31). This high concentration of protein in the sac indicates that the signal intensity of the endolymphatic sac is different from that of cerebrospinal fluid even in healthy subjects. The protein concentration of the endolymph fluid in the ductus cochlearis is far lower than that of the endolymphatic sac and slightly lower than that of the perilymph fluid (12,28). The signal intensity of the endolymph fluid in the ductus cochlearis is considered to be similar to that of cerebrospinal fluid. The signal intensity of the enlarged endolymphatic sac was markedly different from that of cerebrospinal fluid on T2-weighted images in eight of 10 ears with an enlarged endolymphatic sac, which suggests high protein content within the sac (6,7).

In this study, the enlarged endolymphatic sac had lower signal intensity on T2-weighted images in four ears and comparable signal intensity in eight ears. MR imaging is not sensitive enough to depict the subtle differences in protein concentration (7); thus, the eight ears with comparable signal intensity may also have high protein concentration. Although high protein concentration in the sac is not an abnormal finding, our results support the latter theory better, because the widely patent duct may permit reflux of the hyperosmolar fluid in the sac into the cochlea on the occasion of minor head trauma.

Surgery to obliterate the endolymphatic sac stabilizes hearing in patients with a large vestibular aqueduct and progressive hearing loss (5). The results of this surgical study also support the theory that pressure and hyperosmolar fluid reflux into the labyrinth, rather than pressure waves through the deficient modiolus, are responsible for progressive hearing loss in patients with a large vestibular aqueduct. In one recent CT study, the degree of modiolar deficiency was not consistently correlated with hearing loss, although vestibular aqueduct morphology and thickness were very strongly correlated with the severity of hearing loss (18). These observations do not support the hypothesis that hearing loss related to a large vestibular aqueduct is caused by transmission of subarachnoid pressure forces into the inner ear through a deficient cochlear modiolus.

In conclusion, findings in this study confirm that a large endolymphatic duct and sac are frequently associated with modiolar deficiency, but a normal modiolar area is seen in some cases. The results of this study do not necessarily support the recently proposed hypothesis that progressive hearing loss in the presence of a large endolymphatic duct and sac is caused by the transmission of subarachnoid pressure forces into the labyrinth through a deficient cochlear modiolus. Rather, our results support another theory that hyperosmolar proteins in the enlarged endolymphatic sac reflux into the cochlear duct (ductus cochlearis) through a widely patent endolymphatic duct, which causes osmotic damage to the neuroepithelium. Direct comparison between CT and MR images may help clarify the mechanism of progressive hearing loss in patients with a large endolymphatic duct and sac.


   Footnotes
 
Author contributions: Guarantor of integrity of entire study, S.N.; study concepts and design, S.N.; definition of intellectual content, S.N.; literature research, S.N.; clinical studies, S.N., T.Ito, E.I., H.F., T. Ishigaki, T.N., N.I.; data acquisition, S.N., T. Ito, E.I., H.F.; data analysis, S.N., T. Ito, E.I.; statistical analysis, S.N.; manuscript preparation and editing, S.N.; manuscript review, S.N., T. Ishigaki


   References
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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