To evaluate morphological changes of cranial nerves such as loss of neural structures or swelling of cranial nerves in Menière's Disease (MD) or in healthy persons in vivo, a protocol of evaluation has been developed using magnetic resonance imaging (MRI). Additional MRI-based confirmation of MD was performed.
Analysis of neural structures in Menière’s Disease (MD) is of importance, since a loss of such structures has previously been proposed for this patient group but has yet to be confirmed. This protocol describes a method of in vivo evaluation of neural changes especially well suitable for cranial nerve analysis using magnetic resonance imaging (MRI). MD-patients and normal hearing persons were examined in a 3-T MR-scanner using a scan protocol including strongly T2-weighted 3D gradient-echo-sequence (3D-CISS). In the patient group, MD was additionally confirmed using MRI-based assessment of endolymphatic hydrops. Morphometric analysis was performed using a freeware DICOM viewer. Evaluation of cranial nerves included measurements of cross-sectional areas (CSAs) of the nerves at different levels as well as orthogonal diametric measurements.
Magnetic resonance imaging (MRI) plays a major role in visualizing and analyzing anatomy as well as physiological and pathological processes in the human body. Since clinical and electrophysiological diagnosis of Menière's Disease (MD) can be challenging, using additional information derived from MRI is more than helpful1,2,3,4. An in vivo method was developed to analyze endolymphatic hydrops in MD and morphometric changes of cranial nerves using MRI. With this combined approach, diagnosis of definite MD was confirmed, and morphometric changes of cranial nerves were studied at different levels throughout the course of the nerves. Etiology of MD is still unclear5,6,7. It was proposed that neural cell loss could be involved in MD, but this has yet to be confirmed.
Suitable cranial nerves for morphometric analysis in MD are the 7th and 8th nerve with its branches, which were analyzed in this study. Only a few studies can be found analyzing morphometric aspects of these nerves using MRI8,9,10. The study by Henneberger et al. analyzed morphometric changes of the 7th and 8th cranial nerve in MD ears compared to normal hearing ears11.
The method presented here enables in vivo visualization and morphometric analysis of the 7th and 8th cranial nerves throughout their course from the brain to the temporal bone. Using this method, we have shown that there are significant differences between the patient group of MD patients and healthy ears. We propose the described method for use in several situations/diseases whenever potential morphometric changes of cranial nerves are of interest. Whether this method will be established in clinical diagnostic workups remains to be evaluated by future studies. Real alternatives to the described method for in vivo evaluation of morphometric changes of cranial nerves are not available, and while computed tomography (CT) has its strengths such as wide availability, speed, and depiction of bony changes, it also exhibits too low tissue contrasts to visualize subtle changes in cranial nerves within the neurocranium and temporal bone. Post mortem analysis of cranial nerve changes in MD patients remains to be studied. With special imaging and evaluation techniques as described here, it is possible to analyze morphometric changes of cranial nerves in MD patients and healthy controls using MRI. Routine MRI workup of the brain often does not include high resolution, strongly T2-weighted imaging techniques, which are mandatory for the evaluation of morphometric changes of cranial nerves 7 and 8.
The developed method may have further diagnostic impact on evaluating different levels of severity in MD, as well as play a role in the evaluation of vertigo, hearing deficits, and tinnitus. Specialized centers for diagnostic and therapeutic workup of vertigo play a major role in today's health care systems and our method could provide specialists with a possible tool for their diagnostic workup12,13,14. Vertigo is a complex symptom occurring in several diseases, requiring a thorough interdisciplinary cooperation between different specialties, as demonstrated in specialized centers for diagnostic and therapeutic workup of vertigo12,13,14.
To our knowledge, there is no method available in the literature for in vivo morphometric analysis of cranial nerves in MD and healthy controls.
All the procedures were approved by the local ethical committee (Institutional review board of the University of Munich/LMU Munich Protocol No. 093-09). All patients gave their informed consent to the performed procedures.
1. Clinical Examination
2. MRI Image Acquisition in Patients Suffering from MD and Healthy Controls
3. MRI Quality Check and Identification of Endolymphatic Hydrops in MRI
4. Image Based Measurements of Cranial Nerves
Statistical analysis was performed using statistical analysis software, and two-sided independent samples t-test was applied. Image evaluation was performed by two readers. A significant difference between the mean values of the patient group (n = 21) and healthy control group (n = 39) can be found for measurements of the CSA of the facial nerve, CN, SVN, and IVN (Table 2). CSA measurements in the patient group showed significantly larger CSA values (Figure 2 and Figure 3). Evaluation of measurements of the LD and SD showed varying results, depending on the site of measurement, and differences in LD and SD between the two groups were found. For example, at the level of the meatus, SD of the SVN was significantly larger in the patient group compared to the healthy control group, whereas LD was found to be not significantly different (Table 3 and Table 4). Mediator-based theories of MD support these findings7,15.
Figure 1: Endolymphatic hydrops in MRI scans. High grade endolymphatic hydrops of the cochlea (straight arrows) and the vestibule (curved arrows) in 3D-FLAIR (A) and 3D-Real-IR (B). Please click here to view a larger version of this figure.
Figure 2: Morphometric evaluation of cochlear nerve. Significant differences of mean values and interquartile ranges of the cross-sectional area (CSA) of the cochlear nerve were found in the patient group compared to healthy controls. The upper and lower green horizontal lines depict minimal and maximal values, connected by the whisker. The purple star shows the arithmetic mean. The green middle line represents the median. The blue error bars depict 1 SD. Please click here to view a larger version of this figure.
Figure 3: Morphometric evaluation of cranial nerve VII. Significant differences of the cross-sectional area (CSA) of the facial nerve at the level of the cerebellopontine angle (CPA) were found in the patient group compared to healthy controls. The upper and lower green horizontal lines depict minimal and maximal values connected by the whisker. The purple star shows the arithmetic mean. The green middle line represents the median. The blue error bars depict 1 SD. Please click here to view a larger version of this figure.
Figure 4: Measurement of the cross-sectional area (CSA) of the cochlear nerve. Measurement performed at the fundus of the internal meatus on a reconstructed slice perpendicular to the nerve's course. Please click here to view a larger version of this figure.
Figure 5: Measurement of the long diameter (LD) and perpendicular short diameter (SD) of the cochlear nerve. Measurement performed at the fundus of the internal meatus on a reconstructed slice perpendicular to the nerve's course. Please click here to view a larger version of this figure.
Mr-sequence parameters | 3D-CISS |
TR | 5.79 ms |
TE | 2.58 ms |
Flip angle | 34° |
Field of view | 160 x 160 mm2 |
Matrix size | 320 x 320 |
Averages | 1 |
Slice thickness | 0.5 mm |
Table 1: MRI sequence parameters. Set MRI sequence parameters as described using Constructive Interference in Steady State (CISS)-sequence technique for achieving strongly T2-weighted image contrast for optimal depiction of nerves surrounded by cerebrospinal fluid.
Table 2: Morphometric analysis results of cross-sectional area measurements (CSA). Comparison of patients vs. healthy controls measuring CSA of the 7th and 8th cranial nerve at different levels through their course. Analysis of unilaterally affected patients, bilaterally affected patients, and healthy controls including mean value, standard deviation, and p-values (independent samples t-test, patient group n = 21, healthy controls n = 39); significant results with p <0.000595 after Bonferroni correction are marked bold.
Table 3: Morphometric analysis results of long diameter (LD). Comparison of patients vs. healthy controls measuring LD of the 7th and 8th cranial nerve at different levels through their course. Analysis of unilaterally affected patients, bilaterally affected patients, and healthy controls including mean value, standard deviation, and p-values (independent samples t-test, patient group n = 21, healthy controls n = 39).
Table 4: Morphometric analysis results of short diameter (SD). Comparison of patients vs. healthy controls measuring SD of the 7th and 8th cranial nerve at different levels through their course. Analysis of unilaterally affected patients, bilaterally affected patients, and healthy controls including mean value, standard deviation, and p-values (independent samples t-test, patient group n = 21, healthy controls n = 39).
We have demonstrated a feasible and accessible method of evaluation of morphometric changes of cranial nerves, as they may occur in several pathophysiological situations, here in MD compared to normal hearing controls.
Modifications and Troubleshooting:
Similar measurements to the ones reported here for the 7th and 8th cranial nerves can be performed using the employed 3D-CISS-sequence scans for all other cranial nerves at different levels, as long as they are still surrounded by cerebral fluid, otherwise contrast issues may arise with the mentioned MR-scanning sequence technique. For morphometric analysis of cranial nerves at levels where they are not surrounded by liquid, changes of the MR scanning protocol become mandatory, e.g., application of intravenous Gadolinium or employment of MRI fat-suppression techniques. Measurements in orthogonal reconstructions throughout the nerves course remain mandatory.
When combining MR-based evaluation of endolymphatic hydrops with morphometric analysis, the MR-scan can not only be used for describing anatomical changes of cranial nerves but can also aid in the diagnosis of MD. In the future, automated morphometric analysis techniques including machine based learning and artificial intelligence may speed up the evaluation and improve consistency of measurements and evaluations.
In a general approach for morphometric analysis of cranial nerves, MRI examinations can be performed "natively" without the use of intravenous or intratympanic administration of MRI contrast agents. In the protocol diluted intratympanic contrast agent has been applied in order to quantify the severity of endolymphatic hydrops occurring in MD, relevant for the diagnosis of the disease. The small amount and small concentration of Gadolinium-based contrast agent via intratympanic application in this study does not show effects on quantitative measurements of signal intensities of cerebrospinal fluid or the nerves when comparing the diseased side and contralateral side in MD patients, a finding corroborated by other studies. Signal intensities, image quality, and contrast do not differ when comparing strongly T2-weighted images of Gadolinium-injected patients with images of the non-injected controls16. Therefore, the effects of the contrast agent on the morphometric measurements do not play a role. Until today no evidence was found that Gadolinium might play a role on brain or cranial nerves with regard to alterations of volume. However, the long-term effects of Gadolinium-based contrast agents on cranial nerves remains to be studied. Intratympanic application of Gadolinium in healthy controls remains unethical and therefore wasn't performed in the normal hearing patients in this study.
Future Applications:
The depicted method allows for comparison of morphometric changes of cranial nerves in a very large variety of diseases and in several neural structures. Future transfer of the method for evaluating morphometric changes of cranial nerves e.g., in chronic pain, Alzheimer's Disease, or multiple sclerosis (MS) and comparing those findings to healthy controls is feasible.
Critical Steps Within the Protocol and Limitations of the Technique:
When evaluating morphometric parameters using the described techniques, set consistent windowing levels throughout the scans and/or let the measurements be performed by two or more readers. To avoid inter-rater variability, let each reader evaluate all scans. Using thin slice thickness and slice orientation perpendicular to the course of the cranial nerves is mandatory, throughout the whole course of the nerves. When comparing different studies performed on different MR-scanners, it must be considered that differences in MR scan parameters can result in differences in partial volume effects, as well as differences with regard to image contrasts and image quality. The levels at which morphometric analysis throughout the course of the cranial nerves was performed in different studies need to be considered when comparing studies.
The authors have nothing to disclose.
Robert Gürkov received fundings from the German Ministry of Research and Education BMBF, Grant No. 01 EO 0901.
MR-scanner, e.g. Siemens Magnetom Verio, or appropriate MR-scans in DICOM format, e.g. 3D-CISS | Siemens Healthcare GmbH, Erlangen, Germany, or MR scans by any other vendor | 1 | Instead of the MR scanner, appropriately acquired MR-scans can be used for morphometric analysis |
Osirix or any other DICOM-Viewer with appropriate evaluation tools | Pixmeo SARL, Geneva, Switzerland | 2 | Software for viewing and evaluating DICOM images |
MedCalc or any other statistical analysis software, e.g. SPSS | MedCalc Software bvba, Ostend, Belgium | 3 | Software for statistical analysis |
Computer running Windows or MacOSX/macOS | e.g. Lenovo, Apple or anything selfmade | 4 | Hardware on which the above software can be employed |