1Department of Neurology, Jeonbuk National University Hospital, Jeonbuk National University School of Medicine, Jeonju, Korea
2Research Institute of Clinical, Jeonbuk National University Hospital, Jeonbuk National University School of Medicine, Jeonju, Korea
Correspondence to Sun-Young Oh Department of Neurology, Jeonbuk National University Hospital, Jeonbuk National University School of Medicine, 20 Geonji-ro, Deokjin-gu, Jeonju 54907, Korea Tel: +82-63-250-1590 Fax: +82-63-251-9363 E-mail: ohsun@jbnu.ac.kr
• Received: June 18, 2024 • Revised: October 24, 2024 • Accepted: October 28, 2024
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
The vestibular system, essential for balance and spatial orientation, spans from the inner ear to various brain regions. Advances in imaging techniques have significantly enhanced our ability to diagnose and treat vestibular disorders. This review explores the anatomy of the vestibular system and evaluates the roles of high-resolution computed tomography (CT) and magnetic resonance imaging (MRI) in diagnosing structural abnormalities. CT is particularly useful for identifying bony labyrinth anomalies, temporal bone fractures, and superior canal dehiscence, though it has limitations in visualizing membranous labyrinth lesions. MRI, with its superior soft tissue resolution, is preferred for detecting retrocochlear lesions such as vestibular schwannomas, cerebellopontine angle tumors, and demyelinating diseases in the posterior fossa. Functional MRI also offers insights into the vestibular system’s functional aspects. The review emphasizes the increasing importance of imaging diagnostics in the effective management of vestibular system diseases, highlighting both structural and functional imaging modalities to improve patient outcomes.
The vestibular pathway extends from the vestibular apparatus and vestibular nerve (8th cranial nerve) within the temporal bone to the vestibular nuclei in the medulla, and through ascending pathways such as the medial longitudinal fasciculus (MLF), it connects to the oculomotor nucleus and supranuclear integration center in the rostral midbrain. It then reaches various areas of the vestibular cortex through thalamic projections. Imaging tests can identify anatomical structural abnormalities in these vestibular pathways.1 With advancements in imaging techniques, the importance of imaging diagnostics in diagnosing and treating vestibular system diseases has been increasingly emphasized. Specifically, in cases of lesions within the labyrinth itself, such as congenital anomalies of the bony labyrinth, temporal bone fractures, and superior canal dehiscence, computed tomography (CT) of the temporal bone is useful, though CT has limitations in detecting lesions of the membranous labyrinth. On the other hand, for retrocochlear lesions such as cerebrovascular diseases, vestibular schwannomas, cerebellopontine angle tumors, and demyelinating lesions in the posterior fossa, magnetic resonance (MR) imaging is recognized as more sensitive and specific compared to other imaging techniques, including CT.
This review article first describes the imaging anatomy of the vestibular system and then discusses high-resolution CT and various magnetic resonance imaging (MRI) techniques used for structural imaging diagnosis of the vestibular system, as well as functional neuroimaging techniques such as functional MRI (fMRI).2
IMAGING ANATOMY OF THE VESTIBULAR SYSTEM
Semicircular canals and otoliths
The vestibular apparatus of the inner ear, located within the petrous apex, consists of three semicircular canals that detect rotational acceleration and two otolithic organs (saccule and utricle) that detect linear acceleration and head tilt. The vestibular apparatus includes the membranous labyrinth forming the endolymphatic space and the bony labyrinth forming the outer perilymphatic space, with the perilymphatic space situated between the membranous and bony labyrinths. These structures measure about 4 mm in diameter, connecting anteriorly to the cochlea and posteriorly to the semicircular canals. Membranous structures are not distinguishable on CT scans. The saccule and utricle contain maculae that detect linear stimuli, with the saccule being slightly smaller and spherical in shape, located anteroinferior to the elongated utricle. Between these two lies the vestibular crest, which divides into two branches, with the cochlear recess containing the blind end of the cochlear duct. The vestibule connects to the oval window and the vestibular aqueduct.
The semicircular canals are composed of membranous and bony components, with the former being part of the equilibrium sensory apparatus and the latter consisting of a similarly shaped bony canal filled with perilymph. The membranous semicircular canals are closed loops opening into the utricle at both ends and arranged orthogonally to each other, forming approximately 2/3 of a circle. At the junction of each semicircular canal and the utricle is an ampulla containing the crista ampullaris, a sensory receptor detecting rotational stimuli, while maculae in the saccule and utricle detect linear stimuli. The sensory cells in these areas, known as hair cells, are classified into type 1 (calyx type) and type 2 (boutton type) based on the shape of the afferent nerve endings. CT imaging reveals the bony semicircular canals, each about 1 mm in diameter, with the membranous semicircular canals occupying roughly 1/4 of the bony canal’s diameter. The anterior semicircular canal protrudes upwards, with its bony roof forming the arcuate eminence. Its posterior end merges with the posterior semicircular canal to form a common crus that opens into the superior medial vestibule. The posterior semicircular canal runs parallel to the petrous ridge, while the lateral semicircular canal, oriented towards the medial wall of the attic in the middle ear, has anterior and posterior crura opening into the vestibule. The tympanic segment of the facial nerve lies beneath the lateral semicircular canal, which runs parallel to the supraorbital meatal line used as a reference in axial CT scans.
The petrous bone contains two aqueducts. The cochlear aqueduct, about 6-10 mm long with a middle diameter of 0.1-0.2 mm, is widest at its medial opening. It surrounds the perilymphatic space, emerging anterior to the round window at the start of the scala tympani in the basal turn of the cochlea and running posteriorly and inferomedially to connect with the subarachnoid space at the lateral margin of the jugular fossa. The vestibular aqueduct, carrying the endolymphatic duct and sac, measures 1 mm at its midpoint and 2 mm at the operculum (Fig. 1).3 It begins near the common crus in the vestibule, proceeds posteriorly and inferiorly to the endolymphatic sac, terminating in a blind pouch within the dural space at the posterior margin of the petrous pyramid. The vestibular aqueduct is thought to contribute to normal endolymph absorption, removal of debris, and regulation of cerebrospinal fluid pressure. It runs laterally and posteriorly from the vestibule, connecting with the posterior cranial fossa and observed posterior to the posterior semicircular canal.3
Vestibular nerve
The vestibular nerve is formed by the union of nerves originating from the maculae of the saccule and utricle and the crista ampullaris in the ampullae of the three semicircular canals. The cell bodies of these bipolar neurons are located in the Scarpa’s ganglion. The superior and inferior vestibular nerves, which join to form the vestibular nerve, run posteriorly to the facial and cochlear nerves in the internal auditory canal (IAC), forming the vestibulocochlear nerve. In the IAC, the facial and cochlear nerves are separated superiorly and inferiorly by the crista falciformis, a bony plate visible as a sharp line on CT. The vestibulocochlear nerve runs posterior to the facial nerve in the cerebellopontine cistern, connecting to the four vestibular nuclei located at the pontomedullary junction. The superior vestibular nerve innervates the lateral and anterior semicircular canals and the utricle, while the inferior vestibular nerve innervates the saccule and the ampulla of the posterior semicircular canal. Before entering the saccule, the inferior vestibular nerve branches into the singular nerve, passing through the singular canal to innervate the posterior semicircular canal. High-resolution imaging can sometimes reveal the singular nerve at the base of the IAC, and care should be taken not to mistake the singular canal for a fracture on CT.
Vestibular nuclei and supranuclear pathways
Primary vestibular afferents enter the brainstem through the pontomedullary junction, passing through the inferior cerebellar peduncle to reach the four vestibular nuclei. These nuclei, located bilaterally at the floor of the fourth ventricle at the junction of the medulla and pons, are surrounded by a complex network of afferent and efferent fibers, bounded superiorly by the superior cerebellar peduncle, laterally by the inferior cerebellar peduncle, anteriorly by the trigeminal nucleus and its spinal tract, and medially by the pontine reticular formation (Fig. 2). The four main nuclei are the superior vestibular nucleus (Bechterew’s nucleus), inferior vestibular nucleus (Roller’s nucleus), medial vestibular nucleus (Schwalbe’s nucleus), and lateral vestibular nucleus (Deiter’s nucleus), arranged in two longitudinal columns. The lateral column contains the superior, inferior, and lateral vestibular nuclei, while the medial column contains the medial vestibular nucleus. The superior vestibular nucleus is the highest and lies at the posterior part of the pons, bordering the inferior cerebellar peduncle and the fourth ventricle. The inferior vestibular nucleus extends into the medulla. The medial vestibular nucleus runs vertically along the wall of the fourth ventricle, with the lateral vestibular nucleus situated laterally to the medial nucleus.
The axons of the vestibular nuclei project to the oculomotor nucleus, spinal cord, cerebellum, autonomic medullary centers, and thalamocortical structures via the MLF. The MLF is involved in the vestibulo-ocular reflex by connecting the oculomotor, trochlear, and abducens nerve nuclei, which are responsible for eye movement, with the medial and superior vestibular nuclei. The descending pathways from the vestibular nuclei that project to the white matter of the spinal cord include the lateral vestibulospinal tract, medial vestibulospinal tract, and reticulospinal tract. The cerebellum is closely connected with the vestibular nuclei, exchanging signals and regulating reflex responses. Vestibular information reaches the vestibular cortex from the vestibular nuclei, passing through the upper brainstem and the ventral posterolateral nucleus of the thalamus.
Structures that cause dizziness due to abnormalities in the central vestibular pathways include the vestibular nuclei, the root entry zone where the vestibular nerve enters the brainstem, and the vestibulocerebellum. Lesions in these areas can cause dizziness without other neurological symptoms or signs, so it is important to carefully examine these areas for lesions in patients with dizziness. The vestibulocerebellum comprises the flocculus, paraflocculus, nodulus, and ventral uvula. The flocculus is typically supplied by the anterior inferior cerebellar artery (AICA), and its exact location can be difficult to determine on MRI. The nodulus is supplied by the posterior inferior cerebellar artery (PICA) and is located just below the fourth ventricle at the level where the root of the trigeminal nerve is visible on axial views, and directly below the fourth ventricle on mid-sagittal images. The oculomotor vermis (layers VI and VII) and the fastigial nucleus, which are parts of the dorsal vermis, are involved in the control of eye movements, particularly the regulation of saccadic amplitude, and lesions in these areas typically do not cause severe dizziness.
Blood supply of the vestibular system
The vestibular system’s blood supply is primarily from branches of the basilar and vertebral arteries, with the AICA giving rise to the labyrinthine artery supplying the inner ear. The labyrinthine artery bifurcates into the common cochlear artery and the anterior vestibular artery. The common cochlear artery further divides into the main cochlear artery and the posterior vestibular artery, which supplies the saccule, posterior semicircular canal, and parts of the cochlea. The anterior vestibular artery supplies the utricle, anterior and lateral semicircular canals, and parts of the vestibule. High-resolution imaging techniques such as magnetic resonance angiography (MRA) can delineate these vascular structures (Fig. 3).
CT
CT in vestibular system disorders
CT is the most superior imaging modality for examining the bony structures of the temporal bone and osseous vestibular apparatus related to vestibular abnormalities. CT scans are typically acquired with slice thicknesses and intervals of 1-1.5 mm using a 512 × 512 matrix and high spatial frequency algorithm (bone-detailed algorithm) to achieve high-resolution images with pixel sizes below 0.2 mm. The advent of multidetector row CT has enabled faster scan times and thinner slice acquisition, greatly improving the quality of reconstructed images in any desired plane. Axial and coronal images are typically acquired or reconstructed, with axial images obtained with the patient supine and the head positioned to avoid direct radiation exposure to the lens. Axial images are reconstructed to clearly visualize the lateral semicircular canal in a circular shape, with the supraorbital meatal line as the reference plane.
Clinical application
Congenital malformations
Congenital malformations of the inner ear generally occur during the first trimester of pregnancy due to genetic factors or exposure to teratogens. Over 90% of congenital sensorineural hearing loss (SNHL) is caused by developmental abnormalities of the membranous labyrinth without any bony labyrinth anomalies, making it undetectable via CT scans. In 10-20% of inner ear malformations, underdevelopment of the bony labyrinth accompanies the membranous labyrinth, which can be observed on CT scans.4 About 65% of inner ear malformations are bilateral and symmetrical, affecting parts or the entirety of the cochlea, semicircular canals, and vestibular aqueduct.4
Mondini dysplasia is the most common bony labyrinth malformation, accounting for 55% of cases.5 Classic Mondini dysplasia refers to a cochlear malformation where the cochlea only completes 1.5 turns instead of the normal 2.5, and the middle and apical turns are merged.6 This condition often includes malformations of the vestibular system. CT or MRI scans typically show a diminutive cochlea with loss of the interscalar septum and often reveal enlargement of the vestibular aqueduct (Fig. 4).5 Isolated vestibular anomalies are rare and usually accompany malformations like Mondini dysplasia and cochleovestibular hypoplasia.
The semicircular canals begin to develop between the 6th and 8th weeks of gestation and are completed between the 19th and 22nd weeks. Congenital malformations of the semicircular canals are classified as either aplasia (20%) or varying degrees of hypoplasia (80%). The lateral semicircular canal, which differentiates last embryologically, is the most commonly affected, while the anterior semicircular canal, which differentiates first, is the least commonly affected. Approximately 40% of cochlear malformations are accompanied by hypoplasia of the lateral semicircular canal. Hypoplasia of the semicircular canals is observed on CT or MRI as a short and wide cystic structure merging with the vestibule (Fig. 5). Aplasia of the semicircular canals is much rarer than hypoplasia, can be bilateral, and may accompany an otherwise normal cochlea. If aplasia results from arrest in vestibular differentiation, it must be accompanied by cochlear malformation.
The expansion of the vestibular aqueduct (vestibular aqueduct enlargement) is the most common cause of congenital SNHL identifiable through imaging. Diagnosis can be made by comparing the diameter of the enlarged vestibular aqueduct to the midpoint of the adjacent semicircular canal on CT; an enlargement greater than 1.5 mm in width at the midpoint is diagnostic (Fig. 6). About 60% of cases with enlarged vestibular aqueducts show associated cochlear or semicircular canal malformations on imaging. Large vestibular aqueduct syndrome or enlarged vestibular aqueduct syndrome is typically bilateral and can be diagnosed when imaging shows only enlargement of the vestibular aqueduct without other inner ear malformations, along with clinical features of progressive SNHL from birth.
Temporal bone trauma
Temporal bone fractures are categorized based on their orientation to the petrous ridge into longitudinal fractures (parallel to the long axis, 70-90%) and transverse fractures (perpendicular to the long axis, 10-30%). Mixed fractures, where both longitudinal and transverse components are present, are the most common (Fig. 7). Plain radiographs can detect only 17-55% of temporal bone fractures, while CT is the most accurate diagnostic tool, useful for identifying the fracture and associated complications.7 Longitudinal fractures are best seen on axial CT scans, whereas transverse fractures are best visualized on coronal and axial CT scans (Fig. 7).7 MRI is primarily used to assess associated brain injuries, encephalomeningocele, and vascular injuries in temporal bone trauma but can also be useful in diagnosing cerebrospinal otorrhea or facial nerve injuries. SNHL and vertigo can occur without evidence of fracture or brainstem injury, likely due to labyrinthine concussion or hemorrhage, which may be visible on MRI as high signal intensity on T1-weighted images.8 Longitudinal fractures commonly present with conductive hearing loss (CHL) due to damage to the external auditory canal, tympanic membrane perforation, or ossicular disruption, while inner ear damage is less frequent.8 Conversely, transverse fractures often involve inner ear damage, leading to severe vertigo, spontaneous nystagmus, SNHL, and facial nerve paralysis, with ossicular damage being rare. In cases of inner ear damage or perilymphatic fistula, CT may show air within the labyrinth (pneumolabyrinth).7 To aid in predicting complications and outcomes of temporal bone fractures, a recent classification method based on otic capsule involvement has been proposed. Fractures involving the otic capsule are more likely to involve the cochlea, vestibule, and semicircular canals, and are associated with SNHL, cerebrospinal fluid leaks, facial nerve paralysis, and intracranial injury.8
Otosclerosis
Otosclerosis is a distinctive autosomal dominant dystrophy of the bony labyrinth characterized by the replacement of normal endochondral bone with spongy, vascular bone. Decalcified lesions tend to recalcify, leading to decreased vascularity and formation of sclerotic bone. Otosclerosis typically presents in individuals aged 20-40 years with CHL, SNHL, or mixed hearing loss (MHL), and tinnitus. It is more common in whites than in blacks, Native Americans, and Asians, and more frequently affects women, often bilaterally (85%). Otosclerosis is classified into fenestral and retrofenestral/cochlear types, but isolated cochlear otosclerosis without fenestral involvement is rare, and these types are considered part of a spectrum rather than separate diseases. High-resolution CT is most useful for preoperative diagnosis of otosclerosis, capable of detecting subtle decalcification lesions around the oval window, round window, and stapes footplate when reconstructed with overlapping slices less than 1 mm thick.9,10
Fenestral otosclerosis begins in the fissula ante fenestram, most commonly affecting the anterior part of the oval window and leading to stapes footplate fixation, resulting in CHL. During the active phase, CT shows localized low-density lesions anterior to the oval window, and these lesions exhibit high contrast enhancement on MRI.9 As lesions grow, they can protrude into the middle ear or narrow the anterior oval window, observed on CT as narrowing of the oval window, thickening of the stapes footplate, and small low-density lesions in the lateral wall of the labyrinth. During the inactive phase, CT shows bone density similar to the surrounding bone, with only an enlarged anterior part of the oval window. Preoperative CT for stapes surgery in cases of severe CHL must assess the status of the oval window, facial nerve canal, jugular bulb, middle ear, ossicles, and IAC.10
Cochlear otosclerosis almost always accompanies fenestral otosclerosis and causes sensorineural or MHL. Histologically, decalcified vascular spongy bone starts around the cochlea, gradually progressing to the vestibule, semicircular canals, and IAC, leading to cochlear and spiral ligament damage by proteolytic enzymes, which are associated with SNHL. During the active phase, CT shows a “double ring sign” due to low-density lytic bone lesions around the cochlea, with high contrast enhancement on MRI, attributed to contrast agent retention in the vascular otospongiotic foci, providing information on lesion activity. During the inactive phase, sclerotic lesions appear on CT as thickened parts or the entire circumference of the bony capsule.10 Preoperative MRI is useful for evaluating the membranous labyrinth before cochlear implantation in cases of severe bilateral hearing loss, assessing whether fibrosis or ossification within the labyrinth has caused loss of normal high T2 signal intensity. Differential diagnoses for radiologically active cochlear otosclerosis include incomplete ossification, Paget’s disease, and syphilis. Incomplete ossification occurs earlier than otosclerosis, more often presenting with pure SNHL (10%), and shows more extensive and severe lesions on imaging.
Superior canal dehiscence syndrome (SCDS)
SCDS is characterized by vertigo induced by sound or pressure changes, caused by congenital thinning of the bone overlying the superior semicircular canal, which may erode due to intracranial pressure or minor trauma with age. Reconstructing images along the plane of the superior semicircular canal on coronal high-resolution CT allows accurate assessment of the dehiscence extent (Fig. 8).11 Although high-resolution CT is the most accurate imaging method, it is known for a high false-positive rate, and recent reports suggest that high-resolution MRI provides 100% sensitivity and negative predictive value.11
MRI
MRI in vestibular disorders
MRI does not use ionizing radiation and offers superior soft tissue resolution compared to CT. It is particularly useful for diagnosing lesions of the membranous labyrinth, petrous bone, blood vessels, auditory nerves, brainstem, and supranuclear pathways within the central nervous system. Recent advancements, such as the introduction of 3T machines, fast scanning techniques, three-dimensional imaging methods, and the development of surface coils, have enabled faster acquisition of high-resolution images, allowing for detailed observation of the membranous labyrinth and fine neural tissue lesions.
Patients with ferromagnetic implants or foreign bodies, such as cochlear implants or artificial cardiac pacemakers, which are significantly affected by external magnetic fields, were previously absolute contraindications for MRI. However, with recent technological advancements in these implants, modern otological implants can now undergo MRI at least with 1.5T machines, provided protective bandages are worn or the magnetic components of the implants are removed.
Among the various MRI acquisition techniques, the spin echo method is most commonly used. Depending on the repetition time (TR) and echo time (TE) applied externally, T1-weighted images (with relatively short TR/TE) and T2-weighted images (with long TR/TE) can be obtained. Each anatomical structure in MRI shows different signal intensities based on its composition. Generally, substances with high amounts of free hydrogen atoms, like cerebrospinal fluid or perilymph, appear low in signal intensity on T1-weighted images and high in signal intensity on T2-weighted images. Soft tissues such as neural tissue show intermediate signal intensity on both T1 and T2-weighted images, with the cerebral cortex showing lower signal intensity than the medullary substance on T1-weighted images and higher signal intensity on T2-weighted images. Cortical bone and air, which have almost no free hydrogen atoms, appear as dark, signal-void areas.
Temporal MRI uses a small imaging field of 16-18 cm, with thin slice thicknesses of 3 mm or less, and no gaps between slices, which is advantageous for obtaining transverse and coronal images. High-resolution 3D techniques like fast spin echo with driven equilibrium, constructive interference in the steady state (CISS), balanced fast-field echo, and fast imaging employing steady-state acquisition are useful for evaluating the IAC and cranial nerves. Contrast-enhanced imaging can be obtained to detect small tumors in the inner ear, assess the spread of tumors to the meninges or intracranial structures, and evaluate minute abnormalities in the inner ear and endolymphatic sac. High-sensitivity post-contrast 3D fluid-attenuated inversion recovery (FLAIR) imaging is recently used to evaluate fine lesions in the inner ear and endolymphatic space. Diffusion-weighted imaging (DWI) is useful for differentiating middle ear infections and cholesteatomas or for distinguishing acute central vestibular disorders. DWI measures the irregular movement (diffusion) of water molecules within tissues, making it highly effective in diagnosing acute ischemic strokes where water molecule movement rapidly decreases. DWI, particularly the non-echoplanar imaging (non-EPI) technique, which has fewer MRI artifacts than the standard EPI DWI, is advantageous for differential diagnosis in cholesteatoma. MRA may be used in conjunction to evaluate vascular lesions.
Clinical application
Vascular vertigo
Dizziness and disequilibrium are the most common symptoms of vertebrobasilar ischemia. Typically, dizziness is accompanied by other neurological symptoms and signs in cerebrovascular disorders, but dizziness can also occur as an isolated symptom.12,13 It is crucial to distinguish between central and peripheral vestibular disorders in high-risk patients (Table 1).14
Unlike peripheral vertigo suspected central vertigo necessitates imaging.15 Contrast-enhanced MRI, including DWI, is the most commonly used imaging modality for patients presenting with dizziness. When central vestibular disorders are suspected, at least one imaging sequence that can evaluate the entire brain is needed to thoroughly assess the thalamus and cerebral cortex related to vestibular function.16
Vertebrobasialr insufficiency is a common cause of central vertigo in patients over 50 years with risk factors. Ischemic infarctions of the lateral medulla due to vertebral or PICA insufficiency or thrombosis of the labyrinthine artery (from AICA) can cause severe dizziness.12 These vascular lesions can be precisely evaluated using MRA and computed tomography angiography, which are particularly useful when vertebral artery dissection is suspected. Rotational vertebral artery syndrome, also known as Bow Hunter’s syndrome, is a rare condition in which head rotation leads to compression of the vertebral artery, typically resulting in symptoms of vertebrobasilar insufficiency. This compression reduces blood flow to the posterior circulation of the brain, causing transient ischemic symptoms during certain head or neck movements, where cervical CT and angiography may be helpful.15
DWI is highly effective in diagnosing acute ischemic infarctions presenting with acute dizziness, making it essential to include DWI in the imaging protocol. Recent studies show that the diagnostic rate of CT for acute dizziness is 2.2%, while MRI shows a diagnostic rate of 16%. Despite the high diagnostic accuracy of DWI for acute strokes, 12-18% of patients with central vestibular syndrome caused by stroke may show false-negative results on DWI. This can be attributed to functional impairment without complete stroke, limitations in MRI resolution, increase in infarct size on follow-up, or isolated inner ear infarction extending to the entire AICA territory. Therefore, utilizing high-field MRI, high-resolution, thin slices, and high diffusion b-values can improve diagnostic accuracy, and in suspicious cases, follow-up DWI 2-3 days later can be helpful.
Vestibular schwannoma
Vestibular schwannoma, a benign tumor of the nerve sheath, accounts for 6-8% of all intracranial tumors and 80% of cerebellopontine angle (CPA) tumors. Vestibular schwannomas may remain within the IAC or extend into the CPA. Large tumors can be suspected on CT due to labyrinthine expansion. MRI easily detects small intracanalicular schwannomas, appearing as moderate signal intensity nodules that replace the high signal intensity of the perilymph on T2-weighted images, with significantly higher contrast enhancement on post-contrast MRI compared to labyrinthitis (Fig. 9).17 In patients with neurofibromatosis type II, vestibular schwannomas may occur along with schwannomas of the IAC and CPA. Rarely, primary or metastatic carcinomas, such as squamous cell carcinoma of the external auditory canal, can invade the labyrinth, showing bone destruction on CT and irregular, enhancing masses with loss of perilymph signal intensity on MRI.17
Meniere’s disease
Recent studies actively explore MR imaging of the endolymphatic space to diagnose endolymphatic hydrops, a known cause of Meniere’s disease. There are two methods for MR imaging of endolymphatic hydrops using contrast agents: intratympanic gadolinium injection and delayed MR imaging after intravenous contrast injection. In 2007, Nakashima et al.18 first visualized the endolymphatic space by acquiring FLAIR images with T1 shortening effects after intratympanic gadolinium injection. The membranous labyrinth does not allow the contrast agent to penetrate due to the presence of tight junctions, making the endolymphatic space, which does not enhance, clearly distinguishable from the perilymphatic space, which does enhance, thus diagnosing endolymphatic hydrops.19 When gadolinium is injected intratympanically, diluted at 1/8 or 1/16, optimal contrast is observed 24 hours after injection. However, intratympanic gadolinium injection is considered off-label, invasive, and can only assess one ear at a time.18 In contrast, intravenous gadolinium injection is an approved method, less invasive, and allows simultaneous assessment of both ears. For intravenous imaging, optimal contrast is achieved 4 hours after a single dose of gadolinium contrast agent injection (Fig. 10).19
In most clinical settings, MRI images are taken 4 hours after intravenous gadolinium injection (0.2 mL/kg body weight, or 0.1 mmoL/kg body weight), utilizing 3D-FLAIR and 3D real-IR sequences to diagnose endolymphatic hydrops based on signal differences between the perilymph (enhanced with contrast) and the endolymph (not enhanced). To enhance the resolution of the endolymphatic space, the HYDROPS protocol (subtracting the positive endolymph image from the positive perilymph image) and the HYDROPS-Mi2 protocol (multiplying HYDROPS images by MR cisternography images) are used, providing higher contrast-to-noise ratios than HYDROPS images alone.18 Researchers are developing computer programs to automatically calculate the volume of the endolymphatic space based on HYDROPS-Mi2, applying them in research and imaging analysis (Fig. 11).
The grading of cochlear and vestibular endolymphatic hydrops is classified according to Nakashima et al. [18] In the HYDROPS-Mi2 image, the expanded endolymphatic space appears as a low-intensity (dark) signal space surrounded by high-intensity (bright white) signals of the gadolinium-enhanced perilymphatic space and bony labyrinth.20 The grading of cochlear endolymphatic hydrops is as follows. Grade 0 = no endolymphatic hydrops, no displacement of Reissner’s membrane, only the enhanced perilymphatic space is observed; grade 1 = mild, slight displacement of Reissner’s membrane, the area of the endolymphatic space in the cochlear duct (dark part) is as large as the area of scala vestibuli; grade 2 = severe, significant displacement of Reissner’s membrane, the area of the endolymphatic space in the cochlear duct is significantly expanded beyond the area of scala vestibuli. The grading of vestibular endolymphatic hydrops is assessed at the lowest slice of the vestibule where the inferior part of the lateral semicircular canal is visible. Grade 0 = no vestibular hydrops, the area ratio of the endolymphatic space to the total fluid space is less than 33.3%; grade 1 = mild vestibular hydrops, the area ratio exceeds 33.3% but is less than 50%; grade 2 = severe vestibular hydrops, the area ratio exceeds 50%.20
Vestibular paroxysmia
In 1994, Brandt and Dieterich first described the term vestibular paroxysmia, diagnosing it in patients experiencing frequent, brief vertigo attacks lasting a few seconds to minutes, often triggered by specific head positions, and accompanied by hyperacusis or tinnitus and reduced auditory or vestibular function on examination.21,22
The recent diagnostic criteria proposed by the Bárány Society’s Committee for the Classification of Vestibular Disorders are as follows.21 At least five episodes of vertigo, with criteria A-E met. A) Brief episodes of vertigo lasting from a few seconds to a few minutes. B) Triggered by one or more of the following: 1) occurring at rest; 2) occurring in specific head or body positions (not specific positional changes seen in BPPV); 3) occurring with head or body movements (not specific positional changes seen in BPPV). C) Accompanied by one or more of the following: 1) postural instability; 2) gait disturbances; 3) unilateral tinnitus; 4) unilateral ear fullness/numbness; 5) unilateral hearing loss. D) One or more additional diagnostic criteria: 1) MRI (CISS sequence or time-of-flight [TOF] MR angiography) showing neurovascular cross-compression; 2) hyperventilation-induced nystagmus in ocular motor testing; 3) worsening of vestibular dysfunction observed in follow-up ocular motor recordings; 4) response to anticonvulsant treatment. E) Symptoms cannot be explained by other diseases.
The arterial (rarely venous) neurovascular compression of the eighth cranial nerve can be visualized using MRI 3D CISS or 3D TOF sequences (Fig. 12). Neurovascular compression at the terminal portion of the vestibular nerve was observed in 95% of patients with vestibular paroxysmia, with bilateral compression found in 42% of these cases.22 While such neurovascular compression can also be found in healthy individuals, it can be pathologically diagnosed when accompanied by relevant clinical symptoms. However, there is no large-scale prospective clinical study on the prevalence of this neurovascular contact in healthy adults. In control groups of patients with trigeminal neuralgia, asymptomatic neurovascular contact of the eighth nerve was found in about 35% of cases. The vestibular nerve may be most susceptible to vascular compression in the intraventricular region covered by central nervous system myelin of oligodendrocytes, located 10-15 mm from the brainstem.
Inflammatory lesions of the vestibular nerve
The vestibular nerve can be invaded by inflammatory lesions such as viral or bacterial infections and meningitis in the IAC or cerebellopontine angle. Enhanced MRI may show localized enhancement of the vestibular nerve in inflammatory lesions, making it challenging to differentiate from vestibular schwannomas.23 Inflammatory lesions generally show less enhancement compared to vestibular schwannomas. Ramsay Hunt syndrome, caused by herpes zoster virus infection, presents with sudden onset facial nerve paralysis and painful vesicles in the external auditory canal, often involving the cochlear nerve. MRI may show abnormal enhancement of the facial nerve, similar to Bell’s palsy, and often also shows enhancement of the vestibular nerve or membranous labyrinth.23
FUNCTIONAL NEUROIMAGING
Functional neuroimaging in vestibular system disorders
fMRI detects differences in magnetic resonance signal intensity due to the blood oxygenation level-dependent effect, which occurs when activated brain regions require more oxygen, leading to a higher concentration of oxyhemoglobin compared to deoxyhemoglobin in the blood.
Electrophysiological animal studies on vestibular cortical areas indicate that, unlike the primary visual or auditory cortex, it is uncertain whether humans possess a primary vestibular cortex. Instead, vestibular function relies not only on vestibular input but also on visual and somatosensory inputs.24 Therefore, the vestibular cortex is characterized by its multimodality, integrating vestibular, visual, and somatosensory information to provide comprehensive information about the body’s position and movement in external space.2,24
In normal subjects, fMRI following vestibular stimulation shows activation in the posterior insula, retroinsular region, superior temporal gyrus, part of the inferior parietal lobule, intraparietal sulcus, postcentral gyrus, precentral gyrus, anterior insula and adjacent inferior frontal gyrus, anterior cingulate gyrus, precuneus, and hippocampus. Specifically, parts of the posterior insula and retroinsular region, where the temporal, parietal, and occipital lobes converge, are considered the core vestibular cortical areas in humans, known as the parieto-insular vestibular cortex.25,26
Activation is not symmetrically observed in both cerebral hemispheres; stronger activation is seen in the non-dominant hemisphere, ipsilateral to the stimulated inner ear, and ipsilateral to the slow phase of vestibular caloric nystagmus. Concurrently, areas of visual and somatosensory systems in both hemispheres show deactivation during vestibular stimulation, suggesting reciprocal inhibitory cortical interaction between the visual/somatosensory and vestibular systems. For example, the deactivation of the visual cortex during vestibular stimulation suppresses visual motion input (e.g., the disturbing oscillopsia caused by retinal slip during vestibular nystagmus). In various vestibular disorders, the activation/deactivation patterns of these visual-vestibular interactions vary depending on the lesion’s location.26
CONCLUSION
Recent advancements in imaging techniques have significantly improved the diagnosis and treatment of vestibular system disorders. High-resolution CT and MRI, along with functional neuroimaging techniques, have enhanced the ability to detect anatomical and functional abnormalities, leading to better patient outcomes.
Notes
Conflicts of Interest
I declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Funding
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (Ministry of Science and ICT) (No. 2022R1A2B5B01001933) and by the Fund of Biomedical Research Institute, Jeonbuk National University Hospital.
Fig. 1.
(A) Axial CT scan of the temporal bone showing the vestibule (a), lateral semicircular canal (b), posterior semicircular canal (c), and vestibular aqueduct (d) with the endolymphatic sac (e), sigmoid sinus (f), and internal auditory canal (g), epitympanum (h), apex of temporal bone (i). (B) Axial CT scan showing the facial nerve originating from the internal auditory canal (a) and progressing to the labyrinthine segment (b), geniculate ganglion (c), and tympanic segment (d), with views of the malleus (e) and incus (f). CT, computed tomography.
(A) Magnetic resonance angiography of the posterior circulation and vascular territories of the cerebellum (B) and inner ear (C). PCA, posterior cerebral artery; SCA, superior cerebellar artery; BA, basilar artery; AICA, anterior inferior cerebellar artery; PICA, posterior inferior cerebellar artery; VA, vertebral artery; MCP, middle cerebellar peduncle; IAA, internal auditory artery; CCA, common cochlear artery; AVA, anterior vestibular artery; ASC, anterior semicircular canal; HSC, horizontal semicircular canal; PSC, posterior semicircular canal.
Fig. 4.
A 1-year-old female with Mondini malformation. Axial temporal bone CT demonstrates hypoplasia of the left cochlea, which exhibits only 1.5 turns (arrow), consistent with Mondini dysplasia. In contrast, the right cochlea has a normal 2.5-turn configuration. An associated finding includes a dilated left vestibule (arrowhead), while the left semicircular canals appear normal (not shown). CT, computed tomography.
Fig. 5.
Lateral semicircular canal hypoplasia. (A, B) Axial CT scans show the lateral semicircular canal as a short, widened cystic structure (solid arrow) merging with the vestibule. 27 CT, computed tomography.
Fig. 6.
Enlargement of the vestibular aqueduct in a 23-year-old woman. Axial temporal bone CT reveals an enlarged right vestibular aqueduct. The left vestibular aqueduct demonstrates normal caliber. CT, computed tomography.
Fig. 7.
Traditional classification of temporal bone fractures into longitudinal and transverse fractures. (A) Axial CT images from two different patients show (B) a longitudinal fracture parallel to the long axis of the petrous ridge (arrow) and (C) a transverse fracture perpendicular to the long axis of the petrous ridge (arrowhead), crossing the basal turn of the cochlea (arrow).7 CT, computed tomography.
Fig. 8.
Superior semicircular canal dehiscence (SSCD). (A, B) CT images obtained in planes parallel and perpendicular to the superior semicircular canal (SSC) demonstrate dehiscence of the left SSC, with absence of the overlying bony roof (arrow and arrowhead, respectively). (C) Coronal 3D-CISS MR image reveals left-sided SSCD (arrowhead), indicated by the subtle loss of the normal low-signal intensity at the canal roof. The right SSC shows a normal intact osseous roof with preserved low-signal margin (arrow). CT, computed tomography; 3D-CISS, 3-dimensional-constructive interference in steady state; MR, magnetic resonance.
Fig. 9.
Vestibular schwannoma. MRI scan shows a round, solitary tumor with a size of 5×5 mm inside the right internal acoustic meatus (arrow). MRI, magnetic resonance imaging.
Fig. 10.
(A) Axial 3D-FLAIR image of a patient diagnosed with Meniere’s disease taken 4 hours after intravenous gadolinium injection shows cochlear hydrops (arrowheads, grade 1) on the right (small arrows) and prominent bilateral perilymphatic enhancement in the cochlea (small arrows) and vestibule (large arrows) on the right. (B) 3D real-IR sequence 4 hours after intravenous gadolinium injection shows significant expansion of the saccule and utricle occupying most of the vestibule (long arrow) and significant dilation of the cochlear duct (short arrows). (C) Concept of HYDROPS-Mi2 image extraction, with the positive endolymph image subtracted from the positive perilymph image to create the HYDROPS image, which is then multiplied by T2 MR cisternography to generate the HYDROPS-Mi2 image. The HYDROPS-Mi2 image shows the endolymph space as dark areas (arrows) and the perilymph space as white areas, with very strong contrast between the two.
Fig. 11.
Grading of cochlear and vestibular endolymphatic hydrops based on HYDROPSMi2. The degree of endolymphatic hydrops in the cochlea (pink semicircle, A-C) and vestibule (orange circle, D-F) is classified according to Nakashima et al.18 The HYDROPS-Mi2 image shows the expanded endolymphatic space as low-intensity (dark) signal areas surrounded by high-intensity (bright white) signals from the gadolinium- enhanced perilymphatic space and bony labyrinth. The grading of cochlear endolymphatic hydrops (A-C) and vestibular endolymphatic hydrops (D-F) is shown.20
Fig. 12.
Vestibular paroxysmia. High-resolution axial T2-weighted MRI demonstrates the internal auditory meatus (IAM) with a prominent vessel (arrow A) extending into the left IAM, consistent with neurovascular contact. This vessel abuts the cisternal segment of the left eighth cranial nerve (arrow B), suggesting a neurovascular conflict as the likely etiology of vestibular paroxysmia. MRI, magnetic resonance imaging.
Table 1.
Differential diagnosis of central and peripheral acute vestibular syndrome14,15
Lesion
PICA-inferior cerebellar and nodulus, lateral medulla
AICA-ponse, root entry zone of vestibular nerve, inner ear
Vestibular nuclei
Vestibular neuritis
Isolated vertigo
Possible, common
Possible, rare
Possible, rare
Almost always
Horizontal semicircular canal paresis
None
Common
Abnormal if related to medial subnuclei
Abnormal if related to superior vestibular nerve or whole vestibular nerve
Head impulse test
Normal
Abnormal
Abnormal if related to medial subnuclei
Abnormal if related to superior vestibular nerve or whole vestibular nerve
Hearing loss
None
Common
None
Rare
Spontaneous nystagmus
Ipsilesional (cerebellum), ipsilesional or contralesional (lateral medulla)
4. Lemmerling MM, Mancuso AA, Antonelli PJ, Kubilis PS. Normal modiolus: CT appearance in patients with a large vestibular aqueduct. Radiology 1997;204:213-219.
5. Gharib B, Esmaeili S, Shariati G, Mazloomi Nobandegani N, Mehdizadeh M. Recurrent bacterial meningitis in a child with hearing impairment, mondini dysplasia: a case report. Acta Med Iran 2012;50:843-845.
6. Graham JM, Ashcroft P. Direct measurement of cerebrospinal fluid pressure through the cochlea in a congenitally deaf child with Mondini dysplasia undergoing cochlear implantation. Am J Otol 1999;20:205-208.
7. Hiroual M, Zougarhi A, El Ganouni NC, Essadki O, Ousehal A, Tijani Adil O, et al. High-resolution CT of temporal bone trauma: review of 38 cases. J Radiol 2010;91:53-58.
8. Kurihara YY, Fujikawa A, Tachizawa N, Takaya M, Ikeda H, Starkey J. Temporal bone trauma: typical CT and MRI appearances and important points for evaluation. Radiographics 2020;40:1148-1162.
12. Lee JO, Park SH, Kim HJ, Kim MS, Park BR, Kim JS. Vulnerability of the vestibular organs to transient ischemia: implications for isolated vascular vertigo. Neurosci Lett 2014;558:180-185.
18. Nakashima T, Naganawa S, Sugiura M, Teranishi M, Sone M, Hayashi H, et al. Visualization of endolymphatic hydrops in patients with Meniere's disease. Laryngoscope 2007;117:415-420.
19. Iwasa YI, Tsukada K, Kobayashi M, Kitano T, Mori K, Yoshimura H, et al. Bilateral delayed endolymphatic hydrops evaluated by bilateral intratympanic injection of gadodiamide with 3T-MRI. PLoS One 2018;13:e0206891.
20. Oh SY, Dieterich M, Lee BN, Boegle R, Kang JJ, Lee NR, et al. Endolymphatic hydrops in patients with vestibular migraine and concurrent meniere’s disease. Front Neurol 2021;12:594481.
22. Hüfner K, Barresi D, Glaser M, Linn J, Adrion C, Mansmann U, et al. Vestibular paroxysmia: diagnostic features and medical treatment. Neurology 2008;71:1006-1014.
23. Iwasaki H, Toda N, Takahashi M, Azuma T, Nakamura K, Takao S, et al. Vestibular and cochlear neuritis in patients with Ramsay Hunt syndrome: a Gd-enhanced MRI study. Acta Otolaryngol 2013;133:373-377.
26. Bucher SF, Dieterich M, Wiesmann M, Weiss A, Zink R, Yousry TA, et al. Cerebral functional magnetic resonance imaging of vestibular, auditory, and nociceptive areas during galvanic stimulation. Ann Neurol 1998;44:120-125.
Fig. 1. (A) Axial CT scan of the temporal bone showing the vestibule (a), lateral semicircular canal (b), posterior semicircular canal (c), and vestibular aqueduct (d) with the endolymphatic sac (e), sigmoid sinus (f), and internal auditory canal (g), epitympanum (h), apex of temporal bone (i). (B) Axial CT scan showing the facial nerve originating from the internal auditory canal (a) and progressing to the labyrinthine segment (b), geniculate ganglion (c), and tympanic segment (d), with views of the malleus (e) and incus (f). CT, computed tomography.
Fig. 2. Vestibular nuclei and supranuclear pathways.27
Fig. 3. (A) Magnetic resonance angiography of the posterior circulation and vascular territories of the cerebellum (B) and inner ear (C). PCA, posterior cerebral artery; SCA, superior cerebellar artery; BA, basilar artery; AICA, anterior inferior cerebellar artery; PICA, posterior inferior cerebellar artery; VA, vertebral artery; MCP, middle cerebellar peduncle; IAA, internal auditory artery; CCA, common cochlear artery; AVA, anterior vestibular artery; ASC, anterior semicircular canal; HSC, horizontal semicircular canal; PSC, posterior semicircular canal.
Fig. 4. A 1-year-old female with Mondini malformation. Axial temporal bone CT demonstrates hypoplasia of the left cochlea, which exhibits only 1.5 turns (arrow), consistent with Mondini dysplasia. In contrast, the right cochlea has a normal 2.5-turn configuration. An associated finding includes a dilated left vestibule (arrowhead), while the left semicircular canals appear normal (not shown). CT, computed tomography.
Fig. 5. Lateral semicircular canal hypoplasia. (A, B) Axial CT scans show the lateral semicircular canal as a short, widened cystic structure (solid arrow) merging with the vestibule. 27 CT, computed tomography.
Fig. 6. Enlargement of the vestibular aqueduct in a 23-year-old woman. Axial temporal bone CT reveals an enlarged right vestibular aqueduct. The left vestibular aqueduct demonstrates normal caliber. CT, computed tomography.
Fig. 7. Traditional classification of temporal bone fractures into longitudinal and transverse fractures. (A) Axial CT images from two different patients show (B) a longitudinal fracture parallel to the long axis of the petrous ridge (arrow) and (C) a transverse fracture perpendicular to the long axis of the petrous ridge (arrowhead), crossing the basal turn of the cochlea (arrow).7 CT, computed tomography.
Fig. 8. Superior semicircular canal dehiscence (SSCD). (A, B) CT images obtained in planes parallel and perpendicular to the superior semicircular canal (SSC) demonstrate dehiscence of the left SSC, with absence of the overlying bony roof (arrow and arrowhead, respectively). (C) Coronal 3D-CISS MR image reveals left-sided SSCD (arrowhead), indicated by the subtle loss of the normal low-signal intensity at the canal roof. The right SSC shows a normal intact osseous roof with preserved low-signal margin (arrow). CT, computed tomography; 3D-CISS, 3-dimensional-constructive interference in steady state; MR, magnetic resonance.
Fig. 9. Vestibular schwannoma. MRI scan shows a round, solitary tumor with a size of 5×5 mm inside the right internal acoustic meatus (arrow). MRI, magnetic resonance imaging.
Fig. 10. (A) Axial 3D-FLAIR image of a patient diagnosed with Meniere’s disease taken 4 hours after intravenous gadolinium injection shows cochlear hydrops (arrowheads, grade 1) on the right (small arrows) and prominent bilateral perilymphatic enhancement in the cochlea (small arrows) and vestibule (large arrows) on the right. (B) 3D real-IR sequence 4 hours after intravenous gadolinium injection shows significant expansion of the saccule and utricle occupying most of the vestibule (long arrow) and significant dilation of the cochlear duct (short arrows). (C) Concept of HYDROPS-Mi2 image extraction, with the positive endolymph image subtracted from the positive perilymph image to create the HYDROPS image, which is then multiplied by T2 MR cisternography to generate the HYDROPS-Mi2 image. The HYDROPS-Mi2 image shows the endolymph space as dark areas (arrows) and the perilymph space as white areas, with very strong contrast between the two.
Fig. 11. Grading of cochlear and vestibular endolymphatic hydrops based on HYDROPSMi2. The degree of endolymphatic hydrops in the cochlea (pink semicircle, A-C) and vestibule (orange circle, D-F) is classified according to Nakashima et al.18 The HYDROPS-Mi2 image shows the expanded endolymphatic space as low-intensity (dark) signal areas surrounded by high-intensity (bright white) signals from the gadolinium- enhanced perilymphatic space and bony labyrinth. The grading of cochlear endolymphatic hydrops (A-C) and vestibular endolymphatic hydrops (D-F) is shown.20
Fig. 12. Vestibular paroxysmia. High-resolution axial T2-weighted MRI demonstrates the internal auditory meatus (IAM) with a prominent vessel (arrow A) extending into the left IAM, consistent with neurovascular contact. This vessel abuts the cisternal segment of the left eighth cranial nerve (arrow B), suggesting a neurovascular conflict as the likely etiology of vestibular paroxysmia. MRI, magnetic resonance imaging.
Fig. 1.
Fig. 2.
Fig. 3.
Fig. 4.
Fig. 5.
Fig. 6.
Fig. 7.
Fig. 8.
Fig. 9.
Fig. 10.
Fig. 11.
Fig. 12.
Imaging of vestibular system
Lesion
PICA-inferior cerebellar and nodulus, lateral medulla
AICA-ponse, root entry zone of vestibular nerve, inner ear
Vestibular nuclei
Vestibular neuritis
Isolated vertigo
Possible, common
Possible, rare
Possible, rare
Almost always
Horizontal semicircular canal paresis
None
Common
Abnormal if related to medial subnuclei
Abnormal if related to superior vestibular nerve or whole vestibular nerve
Head impulse test
Normal
Abnormal
Abnormal if related to medial subnuclei
Abnormal if related to superior vestibular nerve or whole vestibular nerve
Hearing loss
None
Common
None
Rare
Spontaneous nystagmus
Ipsilesional (cerebellum), ipsilesional or contralesional (lateral medulla)
Contralesional
Direction-changing (bruns)
Ipsilesional or contralesional
Gaze-evoked nystagmus
Variable, generally direction-changing
Fixed direction
Variable
Fixed direction
Skew deviation
Variable, generally lateral medullary lesion
Variable
Variable
Occasional
Trunk tilt
Ipsilesional > contralesional
Ipsilesional
Ipsilesional
Ipsilesional
Postural imbalance
Variable, may fall without support
Variable
Variable
Mild to moderate
Common cause
Ischemic
Ischemic, demyelinating
Ischemic
Viral, idiopathic
Table 1. Differential diagnosis of central and peripheral acute vestibular syndrome14,15
