77 Primary Inner Ear (Intralabyrinthine) Schwannomas

10.1055/b-0039-169231

77 Primary Inner Ear (Intralabyrinthine) Schwannomas

Brian A. Neff

77.1 Introduction

Although there is ongoing debate over the specifics of tumor origin, most agree that vestibular schwannomas (VSs) arise from Schwann cells within the internal auditory canal (IAC). As will be discussed, these progenitor Schwann cells are also found beyond the confines of the IAC in the more distal inner ear labyrinth.s. Literatur Tumors arising in this area are termed “primary inner ear schwannomas” or “intralabyrinthine schwannomas (ILSs)” and are far less common than the typical schwannoma involving the IAC. This chapter reviews primary ILS (i.e., schwannomas that develop from within the labyrinth), which are a separate clinical entity from VS that secondarily invade the inner ear via extension from the IAC through the fundus (Fig. 77‑1 and Fig. 77‑2 ). First described by Mayer in 1917, the most recent systematic review of the literature revealed 234 ILSs which have now been described.s. Literatur ^, s. Literatur

**Fig. 77.1** (a) Intravestibular schwannoma and (b) intracochlear schwannoma on axial and coronal postcontrast T1-weighted images and 3D T2-SPACE images demonstrating tumoral enhancement and filling defects (hypointensity) within the bright inner ear fluid signal.

**Fig. 77.2** (a) Transmodiolar and (b) translabyrinthine growth of vestibular schwannomas demonstrated on axial postcontrast T1- and T2-weighted images in a patient with neurofibromatosis type 2.

Prior to the advent of magnetic resonance imaging (MRI), ILS 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’s disease.s. Literatur Due to their rarity and presentation overlap with Ménière’s disease, diagnosis can be delayed. Visualizing these tumors is also a challenge because the MRI quality and sequences utilized are critical in avoiding a missed diagnosis.

77.2 Anatomy

Within the modiolus are numerous bony channels called Rosenthal’s 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 ganglia and just distal to the cribriform plate is the Schwann-glial cell junction of the cochlear division of the eighth cranial nerve.s. Literatur

The bony vestibule, located between the cochlea and three 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 ganglia within the IAC. Unlike the cochlear division, the Schwann-glial cell junction of the vestibular divisions is at the Scarpa ganglia.s. Literatur With this anatomy in mind, it is helpful to refer to the revised Kennedy classification system in order to describe tumor locations (Fig. 77‑3 ).

**Fig. 77.3** Modified Kennedy classification (reproduced with permission from Van Abel et als. Literatur).

77.3 Pathogenesis

Schwann cells occur in higher density within the IAC near the Schwann-glial junction, and controversially, this is felt by some to be the site of origin of the more common VS of the IAC and CPA.s. Literatur ^, s. Literatur 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.s. Literatur

As will be discussed further in section “Clinical Presentation,” these tumors can cause variable symptoms depending on location of origin, as well as pathway of growth. As postulated earlier, it is possible that a majority of these tumors originate in the terminal Schwann cells of the modiolus, and then easily grow directly into the cochlea. This 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.s. Literatur ^, s. Literatur ^, s. Literatur ^, s. Literatur ^, s. Literatur 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. Reports of tumor spreading to the middle ear by passing through the round window or displacing the stapes footplate are relatively rare, termed “tympanolabyrinthine or transotic growth.”s. Literatur ^, s. Literatur However, tumor can easily spread throughout the labyrinth from cochlea to vestibule to semicircular canal and vice versa. Though not occurring frequently, tumor may break through the cribriform plate located at the distal end of the IAC and then grow more proximally toward the CPA.

77.4 Clinical Presentation

The average age for ILS diagnosis is 49 years (range: 14–89), with no male–female predilection. Due to relative rarity, overlap with other diagnoses, and difficult radiologic detection, the current average delay until diagnosis is approximately 7 years.s. Literatur As mentioned previously, the overlap in ILS symptoms with other more common neurotologic diseases such as Ménière’s disease, vestibular migraine, and VS of the IAC and CPA makes diagnosis challenging. As the most common symptoms are hearing loss and vertigo, we will examine these in greatest detail in the following section.

77.4.1 Hearing Loss

Early reports of ILS series demonstrated that nearly all patients initially presented with hearing loss.s. Literatur ^, s. Literatur ^, s. Literatur In 2013, a large literature review examined 234 cases and reported a 99% rate of hearing loss, with the vast majority (87%) presenting with American Academy of Otolaryngology – Head and Neck Surgery (AAO–HNS) class D hearing.s. Literatur ^, s. Literatur The pattern of hearing loss is variable, and may not provide a means of discriminating these tumors from patients with Ménière’s disease. ILSs 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.s. Literatur Another literature review showed that fluctuating hearing loss occurred only 3% of the time for ILS, while many ILS patients (39%) were misdiagnosed with Ménière’s disease.s. Literatur

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 ILSs. The fact that many ILSs do not involve the cochlea (30%), yet 99% of patients report hearing loss, means that other pathophysiologic mechanisms of hearing loss, other than direct cochlear invasion, 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.s. Literatur Others feel that the presence of abnormal metabolites or interruption of potassium cycling within the tumor-containing labyrinth may directly damage or alter function of the organ of Corti.s. Literatur ^, s. Literatur

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.s. Literatur ^, s. Literatur Hamed and Linthicum captured an example of this in a temporal bone histology section of the cochlea and stapes footplate.s. Literatur 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.

77.4.2 Vertigo and Dysequilibrium

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.s. Literatur Vertigo has been reported in 59% of patients, and the vertigo was episodic in all but one 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’s 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.s. Literatur One report found that 26% (7/27) of patients with vertigo had isolated cochlear ILS. One possible explanation for this occurrence was hypothesized from temporal bone examinations. Three of seven cochlear ILS patients had histologic evidence of endolymphatic hydrops, and perhaps, this is the reason for vestibular symptoms in patients having isolated cochlear involvement.s. Literatur

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 VS of the IAC in whom the presentation is often disequilibrium without vertigo.s. Literatur This might be because ILSs 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 the Scarpa ganglion might lead to a more stable, yet reduced, vestibular input to the brainstem.

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