Imaging Endolymphatic Hydrops at 7T MRI: Validation of Internal Inner Ear Structures on T2w Imaging as Markers of Endolymphatic Hydrops and a Comparison of Diagnostic Outcomes With 3T MRI

Meniere's Disease is an inner ear disease characterised by progressive, fluctuating hearing loss and dizziness. Pathologically, it is characterised by endolymphatic hydrops. The diagnosis of Meniere's Disease can be elusive since it relies on the subjective reporting of symptoms and there is no definitive diagnostic test. Non-specific symptoms can occur in the early stages of Meniere's Disease and the disease progression can also fluctuate unpredictably. The cochlear and vestibular compartments may be differentially involved so there maybe clinical variability characterised by solely audiological or vestibular symptoms (1).

The structural correlate of MD is endolymphatic hydrops (EH) , in which the central smaller endolymphatic compartment of the inner ear (including the cochlear duct, saccule and utricle) expands into the surrounding peri-lymphatic chambers. Pathological studies of temporal bones in subjects with MD have demonstrated EH in most cases of Meniere's Disease (2) and its presence is required for, the diagnosis of 'certain Meniere's Disease' according to 1995 American Academy of Otolaryngology-Head and Neck Surgery (AAO-HNS) guidelines (3). An accepted in vivo biological marker for endolymphatic hydrops has yet to be established. Recent developments in MRI technology and techniques have however allowed the demonstration of EH and the clinical application of these imaging studies is now being explored.

High resolution T2-weighted sequences are widely used for the demonstration of labyrinthine anatomy and pathology however they are unable to distinguish the endolymphatic from perilymphatic compartments and hence are generally unable to depict endolymphatic hydrops. The potential role of gadolinium in discriminating the endolymphatic chamber alone first became apparent in animal studies, since gadolinium was seen to accumulate in the perilymph but was excluded from the endolymph by the impermeable tight junctions. Intra-tympanic administration of gadolinium was initially explored, whereby high concentrations could enter the labyrinth through round window diffusion, however the evolution of clinically applicable high resolution sequences at 3T has allowed for gadolinium to be administered by the less invasive intravenous route. Delayed (4 hours post administration) gadolinium enhanced high resolution imaging is now used at a number of centres worldwide for the evaluation of Meniere's Disease patients, although acquisition and analysis methods continue to evolve. This is most frequently performed with 3D Fluid Attenuated Inversion Recovery with variable flip angles turbo spin echo sequence (3D SPACE FLAIR) or 3D real inversion recovery sequence (3D real IR. In order to corroborate the diagnosis of MD, the degree of EH in each cochlea and vestibule is semi-quantitatively graded (4,5) using various scales or may be quantitatively analysed by contouring the size of the two compartments. An ipsilateral increase in the perilymphatic enhancement is an additional MRI feature which is used to help distinguish MD ears.

The identification of cochlear or vestibular hydrops by MRI in vivo may support the diagnosis of Meniere's disease in cases with incomplete phenotypes and where there are other diagnostic considerations such as vestibular migraine or autoimmune ear disease. It may also facilitate early identification, thus guiding future treatment options for patients who have traditionally been required to take a watchful waiting approach to see how their symptoms may evolve or "trial-and-error" approach to management. The identification of bilateral endolymphatic hydrops in a patient with unilateral fluctuating aural symptoms might predict future bilateral disease and potentially influence therapeutic approaches. Establishing imaging as a biomarker in Meniere's disease may allow a contribution to diagnostic criteria and to define homogenous cohorts for longitudinal studies of natural history and treatment response.

However there are deficiencies with this MRI approach which we would like to address with an exploratory study of Meniere's disease patients on 1.5T and 7 Tesla MRI: