This article reviews the relevant anatomy, pathophysiology, clinical presentation, radiographic differential, management, outcomes, and rehabilitation of patients with intralabyrinthine schwannomas.
Key points
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Most patients with intralabyrinthine schwannomas (ILS) will present with unilateral, severe sensorineural hearing loss as the primary indication to obtain an MRI.
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The differential diagnosis is predominately radiographic because hearing loss and vestibular symptoms are common to both ILS and other inner ear lesions.
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Interval MRI scanning and observation is currently performed for most patients with ILS, especially those without vestibular complaints.
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Surgical removal of ILS can be safely performed, and surgery may be increasingly indicated if cochlear implantation demonstrates effectiveness for unilateral hearing loss rehabilitation.
Introduction
Vestibular schwannomas are benign, slow-growing tumors that most commonly arise from the vestibular division of the eighth cranial nerve. They account for approximately 80% to 90% of tumors within the cerebellopontine angle (CPA) and approximately 6% of all intracranial tumors. Although there is ongoing debate over the specifics of tumor origin, most would agree that they arise from Schwann cells within the internal auditory canal (IAC). These progenitor Schwann cells are also found beyond the confines of the IAC in the more distal inner ear labyrinth. Tumors arising in this area are far less common than the typical schwannoma involving the IAC. First described by Mayer in 1917, the most recent systematic review of the literature revealed 234 intralabyrinthine schwannomas (ILS), which have now been described.
Before the advent of MRI, these tumors were found exclusively at the time of autopsy or as an unexpected discovery during neurotologic surgery for another reason, such as labyrinthectomy for Ménière disease. Because of their rarity and presentation overlap with Ménière disease, diagnosis can be delayed. Visualizing these tumors is also a challenge because the MRI quality and sequences used are critical in avoiding a missed diagnosis.
Introduction
Vestibular schwannomas are benign, slow-growing tumors that most commonly arise from the vestibular division of the eighth cranial nerve. They account for approximately 80% to 90% of tumors within the cerebellopontine angle (CPA) and approximately 6% of all intracranial tumors. Although there is ongoing debate over the specifics of tumor origin, most would agree that they arise from Schwann cells within the internal auditory canal (IAC). These progenitor Schwann cells are also found beyond the confines of the IAC in the more distal inner ear labyrinth. Tumors arising in this area are far less common than the typical schwannoma involving the IAC. First described by Mayer in 1917, the most recent systematic review of the literature revealed 234 intralabyrinthine schwannomas (ILS), which have now been described.
Before the advent of MRI, these tumors were found exclusively at the time of autopsy or as an unexpected discovery during neurotologic surgery for another reason, such as labyrinthectomy for Ménière disease. Because of their rarity and presentation overlap with Ménière disease, diagnosis can be delayed. Visualizing these tumors is also a challenge because the MRI quality and sequences used are critical in avoiding a missed diagnosis.
Anatomy and pathogenesis
Anatomy
The otic capsule is a dense section of bone within the petrous pyramid that houses the inner ear organs of hearing and balance. Collectively termed the membranous labyrinth, this includes the cochlea, semicircular canals, utricle, saccule, and endolymphatic duct and sac. The radiographic and surgical bony space that contains the utricle and saccule is called the vestibule. The bony labyrinth, containing perilymph, surrounds the more interior membranous portion, which contains endolymph. The neuroepithelium, which is responsible for this sensory system, is contained within the membranous portion.
The cochlea spirals 2 and a half times around a cone-shaped structure of porous bone called the modiolus, with the apex of the cone corresponding to the apex of the cochlea. The basilar membrane essentially separates the cochlea into a double-lumen tube made up of the scala tympani and scala vestibuli ( Fig. 1 ). The scala tympani communicates with the middle ear near its basal turn via the round window. The scala tympani also connects to the posterior fossa via a potential space called the cochlear aqueduct, which, if abnormally enlarged, can be an open connection between the perilymph of the inner ear and cerebrospinal fluid (CSF). The scala media, or the cochlear duct, is an endolymph-containing portion of the membranous labyrinth that connects to the saccule via the ductus reuniens.
Within the modiolus are numerous bony canals called the Rosenthal canals, which are adjacent to the scala tympani and contain the spiral ganglia. The afferent branches of the cochlear nerve originate between the hair cells and the spiral ganglia. These nerve endings pierce the organ of Corti through multiple small openings called the habenula perforata. It is at this most distal location where Schwann cell myelination terminates. The afferent branches then run from the spiral ganglia through multiple small openings at the distal IAC called the cribriform plate. Just proximal to Scarpa’s ganglion and just distal to the cribriform plate is the Schwann-glial cell junction of the cochlear division of the eighth cranial nerve.
The bony vestibule, located between the cochlea and 3 semicircular canals, contains the utricle and saccule. The ampulated ends of the semicircular canals open into the utricle. The saccule communicates with the scala media of the cochlea via the ductus reuniens. Two small ducts, one each from the saccule and utricle, unite to form the endolymphatic duct. As its name implies, this endolymph-containing duct is also part of the membranous labyrinth and is housed within the bony vestibular aqueduct. The afferent nerve fibers traveling from the vestibular hair cells of the semicircular canals, utricle, and saccule pierce the cribriform plate of the distal IAC en route to the Scarpa’s ganglia within the IAC. Unlike the cochlear division, the Schwann-glial cell junction of the vestibular divisions is at the Scarpa’s ganglia ( Fig. 2 ). With this anatomy in mind, it is helpful to refer to the revised Kennedy classification system in order to describe tumor locations ( Fig. 3 ).
Pathogenesis
Originally, it was thought that ILS occurred mainly in patients with neurofibromatosis type 2 (NF2). However, more recent reviews have shown that the overall number of sporadic cases far outnumber NF2 cases because of the rarity of the latter. The genetics of sporadic ILS are not thought to be different from the more common vestibular schwannoma of the IAC and CPA, although confirmatory studies are still needed. ILS probably occur following 2 separate sporadic mutations within the NF2 tumor suppressor gene; however, the location of the Schwann cells harboring the mutation is located distally within the inner ear. Schwann cells occur in higher density within the IAC near the Schwann-glial junction (Obersteiner-Redlich zone); controversially, this is thought by some to be the site of origin of the more common vestibular schwannoma of the IAC and CPA. Similarly, it has been hypothesized that cochlear ILS might originate in the more distal cochlear nerve Schwann-glial junction in the modiolus, where Schwann cell concentration is also high.
As is discussed further in the clinical presentation section, these tumors can cause variable symptoms depending on location of origin as well as pathway of growth. As postulated earlier, it is possible that most of these tumors originate in the terminal Schwann cells of the modiolus and then easily grow directly into the cochlea. This point could explain why the cochlea, specifically the basal turn, is the most common location for ILS, with many reports of tumors isolated to the modiolus or basal turn of the cochlea. Lastly, the anatomy is relevant in terms of adjacent spaces into which these tumors may spread, thus influencing their presentation. Tumor spread is likely to occur along the path of least resistance and, therefore, is unlikely to spread beyond the highly dense otic capsule. However, the tumor can easily spread throughout the labyrinth: from cochlea to vestibule to semicircular canal and vice versa. Reports of tumors spreading to the middle ear by passing through the round window or displacing the stapes footplate are relatively rare. Although not occurring frequently, the tumor may break through the cribriform plate located at the distal end of the IAC and then grow more proximally toward the CPA.
Clinical presentation
The average age for ILS diagnosis is 49 years (range 14–89 years), with no male-female predilection. Because of the relative rarity, overlap with other diagnoses, and difficult radiologic detection, the current average delay until diagnosis is around 7 years. As alluded to previously, the overlap in ILS symptoms with other more common neurotologic diseases, such as Ménière disease, vestibular migraine, and vestibular schwannoma of the IAC and CPA, makes diagnosis challenging. As the most common symptoms are hearing loss and vertigo, the authors examine these in greatest detail in this section.
Hearing Loss
Early reports of ILS series demonstrated that nearly all patients initially presented with hearing loss. In 2013, a large literature review examined 234 ILS and reported a 99% rate of hearing loss, with most cases (87%) presenting with American Academy of Otolaryngology–Head and Neck Surgery (AAO-HNS) class D hearing. The pattern of hearing loss is variable and may not provide a means of discriminating these tumors from patients with Ménière disease. ILS have presented with progressive, sudden, and fluctuating patterns of hearing loss. One case series found rates of 61% progressive loss, 32% sudden loss, and 7% fluctuating hearing loss. Another systematic review showed that fluctuating hearing loss occurred only 3% of the time for ILS, whereas many patients with ILS (39%) were misdiagnosed with Ménière disease.
Although there have been histologic reports of isolated neural damage at only the tumor site, potentially causing isolated frequencies of hearing loss, this presentation is not seen in many ILS. The fact that many ILS do not involve the cochlea (30%), yet 99% of patients report hearing loss means other pathophysiologic mechanisms of hearing loss must exist. One hypothesis is that isolated tumors of the vestibule and semicircular canals still may cause hearing loss by either direct compression of the ductus reuniens or any other site along the pathway of endolymph flow toward the endolymphatic sac, thus leading to endolymphatic hydrops. Others think that the presence of abnormal metabolites or interruption of potassium cycling within the tumor-containing labyrinth may directly damage or alter the function of the organ of Corti.
Lastly, the clinician should be aware of the possibility of a mixed hearing loss occurring in a small subset of patients with ILS. Stapes footplate fixation or displacement into the middle ear from extension of intracochlear tumors can occur. Hamed and Linthicum captured an example of this in a temporal bone histology section of the cochlea and stapes footplate. In this patient, tumor was discovered filling the vestibule, primarily, but had extended into the proximal scala vestibuli. The tumor extended along the superior vestibular nerve as far as its entry point into the IAC but abruptly stopped at that point. Instead of growing into the IAC, stapes footplate dislocation into the middle ear had occurred. These examples of inner ear conductive hearing loss would appear as a mixed pattern on audiologic testing.
Vertigo and Disequilibrium
Patients with ILS may describe variable vestibular symptoms, including vertigo, motion-induced dizziness, and disequilibrium. Abnormal balance and unsteadiness were found to be more common than vertigo. Vertigo has been reported in 59% of patients; the vertigo was episodic in all but 1 patient, who described constant vertigo. The relatively high rate of episodic vertigo described by these patients, in addition to the near-uniform presence of sensorineural hearing loss (SNHL) and tinnitus, makes differentiating these lesions from Ménière disease challenging.
The pathophysiologic mechanism to explain episodic vertigo is not definitively established. A greater rate of vestibular subsite involvement has been reported in patients presenting with vestibular symptoms, but vertigo can also occur with isolated cochlear lesions. One report found that 26% (7 of 27) of patients with vertigo had isolated cochlear ILS. One possible explanation for why this occurs was hypothesized from temporal bone examinations. Three of 7 patients with cochlear ILS had histologic evidence of endolymphatic hydrops, and perhaps this is the reason for vestibular symptoms in patients having isolated cochlear involvement.
Lastly, it should be noted that the percentage of patients with ILS having vertiginous symptoms seems to be higher than that of patients with the more common vestibular schwannoma of the IAC in whom the presentation is often disequilibrium without vertigo. This higher rate might be because ILS arise from a more distal site within the vestibular system. Distal vestibular damage might create abnormal or fluctuating neural input to the more proximal vestibular system, whereas IAC schwannoma originating near Scarpa’s ganglion might lead to a more stable, yet reduced, vestibular input to the brain stem.
Physical Findings
In general, physical examination findings in patients with intralabyrinthine tumors are rare, and most are nonspecific. For example, patients with unsteadiness or imbalance may have a positive Romberg test or may have gait instability on ambulatory testing. A patient experiencing an episodic flare of vertigo may have nystagmus. There are isolated reports of tumors presenting as a middle-ear mass and one reaching as far as the external auditory canal ( Fig. 4 ). In most patients, physical examination findings are lacking.
Imaging
The diagnosis of an ILS is ultimately made with MRI in the context of the clinical presentation. Unfortunately, there are no pathognomonic clinical symptoms, physical examination findings, or audiologic criteria that will definitively suggest the presence of an ILS. Before the routine use of MRI for evaluation of inner ear and retrocochlear pathology, computed tomography (CT) was occasionally used but was often not helpful because of poor soft tissue resolution or lack of specificity in evaluating the labyrinth and IAC. There have been isolated reports of successful CT use in preoperatively diagnosing ILS but several others reporting their failure. CT findings have been described as soft tissue masses within the cochlea with absence of the normal radiolucency within the basal turn. Without bone erosion, or extension into the IAC or middle ear, the use of CT to diagnose ILS is limited.
Doyle and Brackmann published their results of a series of 8 patients in whom MRI was used for preoperative diagnosis in all patients. They suggested MRI for preoperative localization and surgical planning or as a method of following the growth of stable tumors. Subsequently, there have been several published reports on the MRI findings of ILS. Many describe a lack of normal fluid density on T2-weighted imaging, with corresponding enhancement seen on gadolinium-enhanced T1-weighted scans. However, other specific MRI sequences are also important for accurate diagnosis. The standard 2-dimensional (2D) fast-spin T2 sequence, typically acquired at a slice thickness of 3 mm, may miss smaller tumors. Three-dimensional (3D) imaging techniques to include constructive interference in a steady state (3D CISS) and variable flip-angle 3D fast spin echo T2-weighted sequences (T2 SPACE), acquired at a 1-mm slice thickness or less, have become critical for the evaluation of pathology involving the membranous labyrinth and inner ear. At the authors’ institution, they also use variable flip-angle 3D fast spin echo (FSE) T1-weighted sequences (T1 SPACE), which can be acquired at 0.6-mm sections through the labyrinth. Using both the 3D T2-weighted sequences and fat-saturated postgadolinium 3D T1-weighted images allows for precise localization of tumor margins. Physicians evaluating for ILS must make sure these sequences are performed because many routine MRI protocols may not include these sequences unless they are specifically requested.
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