Timothy C. Hain, MD Page last modified: July 16, 2016
Normal membranous labyrinth
Dilated membranous labyrinth in Meniere's disease (Hydrops)
Hydrops means that the pressure in endolymphatic compartment of the inner ear is elevated. Dogma states that all persons with Meniere's disease have hydrops.
Recently MRI resolution has become good enough that it is possible to image the inner ear and diagnose hydrops from normal using imaging. Dye must be used in this situation. Dye can be administered intravenously, or placed within the middle ear so it can diffuse through the round window. The latter method is rarely used, but may increase as recent work suggests that gadolinium is not ototoxic (in mice), (Nonoyama et al, 2016). 3D-Flair imaging is used to minimize undesired ghosts of CSF flow (Yamazaki et al, 2012)
At this writing (2016), it is clear that 3T MRI can be used to identify hydrops, and might be useful in diagnosing Meniere's disease. The current "standard" method appears to include imaging at 4 hours after IV injection in a 3T scanner, using highly T2 weighted Flair. Flair is fluid attenuated inversion recovery. Contrast accumulates in the perilymph which appears bright on Flair. The endolymphatic compartment appears dark.
Several groups suggest using double-dose gadolinium. The method of injection of gadolinium through the TM or through the eustachian tube, though more sensitive, is less frequently used. When it is used, the imaging is done 24 hours later.
This method appears to be very successful in documenting hydrops in Meniere's patients (Ito et al, 2016). This method is "emerging", and not offered by most radiology departments.
Dr. A Sephardi (2015), from UCLA, was kind enough to provide the images below that are examples of the MRI in patients with Meniere's. They use a subtraction technique now that shows the perilymph signal as white, the endolymph as black, and the surrounding bone as a medium gray. It makes it easier to distinguish the membranous labyrinth from the otic capsule in some areas. The text below and images are from Dr. Sephardi:
|Normal saccule and cochlea (image from Dr. A. Sephardi)||Normal Utricle.(image from Dr. A. Sephardi)|
|Dilated cochlear ducts in Meniere's disease and dilated saccule.(image from Dr. A. Sephardi)||Dilated utricle in Meniere's disease. (image from Dr. A. Sephardi)|
In normal patients, the utricle is well seen and occupies about 50% of the vestibule at the level of the lateral semicircular canal, whereas the saccule and cochlear duct are imperceptibly small. These areas appear virtually entirely filled with perilymph (white). With hydrops, the saccule fills the anterior/inferior vestibule, and the cochlear duct dilates to efface the scala vestibuli. The net effect is an appearance of alternating white and black bands in the cochlea. Utricle hydrops is less common, and usually only seen if there is also saccule and cochlear duct hydrops.
|Asymmetry in post-contrast perilymph signal with higher signal on the symptomatic side in unilateral Meniere's disease. (image from Dr. A. Sephardi)|
Dr. Sephardi stated -- "We also routinely assess for asymmetry in post-contrast perilymph signal intensity. We often see higher perilymph signal intensity on the symptomatic side in unilateral MD. We believe this is due to increased blood-labyrinth barrier permeability. This increased post-contrast effect (enhancement of pathologic side) was originally published by Yamazaki et al, 2012.
Rx: 3T MRI of the inner ear, 3D Flair, 4 hours after double dose IV gadolinium.
It is not enough to simply order this test. One must also establish a "protocol" with your radiologists, who also need to do more work to interpret them. We think that ideally one's radiologists should provide the ratio of endolymph volume to perilymph volume, for both the cochlea and vestibule. More about this is found below.
The FLAIR variant is sometimes constant or has a variable flip angle.
Investigators Gad dose Thick (mm) TR (ms) TE (ms) TI (ms) Flip angle Matrix Bandwidth Turbo Misc Barath et al (2014) 0.2 mmol/kg (double) 0.8 6000 177 2000 180 384 213 27 Yamazaki et al (2012) 0.2 mmol (double) 0.8 9000 458 2500 120 256 Ito et al (2016) 0.2 ml/kg (standard) 2250 Subtracted PEI from PPI Sephardi et al(2015) 0.2 mmol/kg (double) 0.8 9000 534 2350 120 320x260 Fov 200x167
There are many options how to go about this as can be seen from the table above.
The imaging plane is axial, not coronal or sagittal.
Most recent authors use "double dose constrast", which is 0.2 mmol/kg. This can be a bit confusing as the ml/kg standard dose is 0.2 ml/kg, which can be mixed up with the 0.2 mmol/kg. A 0.4 mL/Kg body weight dose, is the same as 0.2 mmol/kg body weight. (Yamazaki et al, 2012; Nakashima et al, 2010).
There are innumerable variants of FLAIR with nearly every paper opting for a different combination of parameters. Sephardi et al (2015) noted that the hT2w-3D flair images were preferable to the standard T2 Flair.
Although often not mentioned, one also needs an MRI sequence that shows both perilymph and endolymph to help determine the area of the entire labyrinth.
Ito et al (2016) used "heavily T2 weighted MRI cisternography -- (hT2W MRC) for this purpose.
Barah et al (2014) used a "SPACE" T2 weighted sequence. Sephardi et al (2015) did the same using a SPACE cisternographic 3D turbo spin echo T2 (on a Siemans Skyra scanner).
Another group, Bykowski et al (2015) used Fiesta as a comparison. Fiesta is the GE name for a steady-state gradient echo sequence. The naming of these protocols varies depending on the manufacturer of the MRI device.
Thus what one is looking for is the percent of endolymph of the total fluid space in both the cochlea and vestibule.
In normal subjects, Liu et al (2012) found endolymph (black area) in 20 normal subjects to account for 8-26% of the fluid space within the cochlea, and 20-41% of the vestibule. This study was done using dye inserted through the eustachian tube and 24 delayed imaging -- so somewhat of an apples/oranges compared to the current evolving standard. One would think there would be more dye and better definition than subsequent studies using IV contrast.
The trouble with current methodology is that the imaging is imperfect (fuzzy), and guesswork is needed to estimate these numbers.
|Method of interpreting MRI for hydrops, according to Barath et al (2014).|
The general method of is to make high resolution axial Flair images of the inner ear, digitally magnify them, find known structures (i.e. cochlear, vestibule, semicircular canals), and determine how much of them (i.e. area) is white (perilymph) vs black (endolymph). More black means more hydrops. Bigger "strips" in the cochlea, and a larger black area in the vestibule, and loss of the expected loops of the semicircular canals means more hydrops. There are some practical issues as these protocols may take 15 minutes (Bykowski et al, 2015) -- about 5 minutes for the FLAIR and another 5 minutes for a comparison non-flair sequence. In addition, it takes the radiologists more time to read these as they have to compute areas.
Barath et al (2014) used three sets of axial sections -- "below midmodiolar level", "midmodiolar level", and "above midmodiolar level". In other words, through the center, and (presumably) one section above and below. An example of these is shown below. The utricle is higher and the saccule is lower, and the utricle is more horizontally oriented than the saccule.
As endolymph is black on this type of imaging, one might wonder - - how can one see the endolymph vs. other black structures such as surrounding bone ? Although Barath (2014) is not explicit, from their figure 2, it appears they used a highly T2 weighted sequence (called "SPACE"), at 0.4 mm resolution with the objective of seeing fluid whether or not it has contrast. This should show both endolymph and perilymph at the same time. Thus it appears that they use two different image resolutions, and compared the Flair to T2 when there were difficulties in deciding what was endolymph and perilymph.
Sephardi et al (2015) reported regarding their method that "an axial image through the vestibule at the level of the LSC was identified. A freehand region of interest was drawn around the VES and ROI area was recorded. A second freehand ROI was drawn around the entire vestibule which included both the vestibualr perilymph (bright signal) and the vestibular endolymph (dark signal). The VES/vestibule ratio was then calculated. Thus Sephardi et al quantified vestibular hydrops, but did not measure cochlear hydrops. These authors also noted that endolymph was sometimes difficult to distinguish from bone at some levels, and they recommended computing hydrops at the level of the LSC.
A common feature to rating scales is that they differ between the cochlea and the vestibule. This is because of the difference in volume of the endolymphatic compartment in these two structures in normal subjects.
Nagoya hospital criteria from Nakashima et al, 2009. Scoring of Vestibule from Nakashima et al, 2009. On the right, the area of the darker and enhanced (perilymph) portions of the vestibule are outlined. The paper states that the area ratio here between the inner and outer is 67.5%.
Nakashima et al (2009) used the "2008 Nagoya scale". They compared the ratio of the endolymphatic space to the sum of the endolymphatic and perilymphatic space. Note that the images above were made with contrast in the middle ear rather than with intravenous contrast -- i.e. it is presumably higher concentration.
For the vestibular labyrinth, no hydrops was defined as < 1/3. Mild hydrops between 1/3-1/2, and "significant hydrops", greater than 50%.
For the cochlea, mild hydrops was defined as the area of the endolymphatic space not greater than the area of the scala vestibuli, and in "significant hydrops", the endolymphatic space in the cochlea area exceeded the area of the scala vestibuli. As illustrated above, there is clearly some opportunity for subjective judgement here deciding on the borders of the perilymphatic space, as well as deciding how to separate perilymph as part of the vestibule from perilymph in other structures of the inner ear.
Example of scoring from Barath et al, 2014. On the left is Flair sequence where endolymph is black and perilymph is white, on right is T2 sequence where fluid, endolymph or perilymph, is all white.
Barath et al (2014) also suggested 6 different ratings: Grades 0-2 (normal, mild and severe), and cochear or vestibular.
Barath et al (2014) stated that more than 50% (black) within the total volume of the saccule and utricle was required for their hydrops grade 1 (mild). Grade 2 (severe) was 100% black. Compared to the Nagoya criteria, the Barath "mild" is equivalent to the Nagoya "significant" -- thus the Barath system is more conservative as it requires more hydrops to be scored as abnormal, for the vestibule.
Barath et al did not provide quantitative criteria for rating cochlear hydrops, but presumably it would take more prominent "stripes" of the cochlea to be rated as either mild or severe. As Barath et al did not provide numerical criteria, here one would presumably need to use the Nagoya criteria.
Written By: Timothy C. Hain, MD of Chicago Dizziness and Hearing.