10 Labyrinthitis

The histopathologic findings in both serous and suppurative labyrinthitis in humans have been well described in postmortem temporal bone studies.1,2 Merchant et al reviewed the inner ear findings of 41 patients who had died of meningitis.² Forty-nine percent of involved temporal bones had inflammatory cells present in the inner ear structures consistent with the diagnosis of suppurative labyrinthitis, while 34% had eosinophilic staining of inner ear fluids with an absence of inflammatory cells, findings consistent with serous labyrinthitis.

In cases of suppurative labyrinthitis, the inflammatory changes affected the scala tympani in all specimens, with the scala vestibuli and vestibular labyrinth affected in only 50% of cases. Notably, the cochlear and vestibular endolymphatic spaces were without inflammatory changes in 95% of the bones. In addition, the neurosensory elements of the inner ear were intact in 75% of the specimens with suppurative labyrinthitis. The remaining 25% of specimens in the suppurative labyrinthitis group had destruction of the organ of Corti and/or the spiral ganglion cells, a finding consistent with previous temporal bone studies.¹

The inner ears with serous labyrinthitis had a different pattern of inflammatory changes. Eosinophilic staining affected primarily the vestibular labyrinth in both the perilymphatic and endolymphatic spaces. The cochlea was involved in only 40% of cases, with the scala tympani being the most frequently affected fluid compartment. Similar to the temporal bones with suppurative labyrinthitis, the neurosensory elements of the inner ear were intact in nearly all of the cases of serous labyrinthitis. The endolymphatic duct and sac were normal in cases of both serous and suppurative labyrinthitis of meningogenic origin, although the authors did note that labyrinthitis of tympanogenic origin consistently demonstrates findings of endolymphatic hydrops.

Traditionally, it has been thought that suppurative labyrinthitis causes frank destruction of inner ear neurosensory structures while serous inflammation of the inner ear represents a toxic but reversible insult to the cochlea without loss of epithelial elements. Multiple temporal bone studies have challenged this paradigm, as it has been shown that serous labyrinthitis can, in some cases, result in irreversible hearing impairment due to loss of sensory elements in the organ of Corti and neural elements in the spiral ganglion.^3,4 Conversely, some have speculated that mild cases of suppurative labyrinthitis may resolve with appropriate therapy without a resultant hearing loss.2 Labyrinthitis likely represents a continuum of inflammatory changes of the inner ear producing a range of severity in its manifestations.

Tympanogenic Labyrinthitis

Epidemiology

Tympanogenic labyrinthitis represents the spread of inflammation from the middle ear or perilabyrinthine air cells into the inner ear and can occur in the setting of acute or chronic otitis media. In the pre-antibiotic era, tympanogenic labyrinthitis was reported to have occurred in up to 16% of cases of otitis media and was a more common form of labyrinthitis than the meningogenic form.⁵ Since the advent and widespread use of antibiotics, tympanogenic labyrinthitis occurs less frequently than the meningogenic form. Clinical labyrinthitis has been found in some studies to occur in 5.37% of cases of chronic otitis media⁶ and 5% of cases of acute otitis media.⁷ In retrospective studies as recently as 1999, Osma et al⁶ and Kangsanarak et al⁸ reported that clinical labyrinthitis represented 12% and 34% of all extracranial complications of suppurative otitis media occurring in Turkey and Thailand, respectively. Bluestone et al reported three cases of serous and two cases of suppurative labyrinthitis in a series of 100 pediatric patients with intratemporal complications of otitis media.⁷ Correlative temporal bone studies have found that labyrinthitis, whether serous or suppurative, occurs in 38% of patients with purulent otitis media and is more common in children than in adults.⁹ Paparella et al¹ found evidence of pathologic labyrinthine changes in 82% of temporal bones with acute otitis media and 77% with chronic otitis media, though this is likely an overestimation of the true incidence secondary to the inherent selection bias of the temporal bone study. The average length of hospital stay in children with tympanogenic labyrinthitis is as long as 8 days in some studies,⁷ which underscores the severity of the disease process. Because labyrinthitis is a clinical diagnosis based on a variety of symptoms and signs that occur in the setting of frequently complex otologic disease, a true incidence of the disease may never be clearly delineated.

Pathophysiology of Tympanogenic Labyrinthitis

Tympanogenic labyrinthitis occurs as a result of spread of infection or inflammation from the middle ear, mastoid antrum, or petrous apex. The inflammatory changes can affect the inner ear diffusely or can be localized to a limited portion of the labyrinth—so-called circumscribed labyrinthitis.⁵ Suppurative labyrinthitis of tympanogenic origin is generally a diffuse infection of the inner ear structures, with some areas more severely affected than others (see above). Serous labyrinthitis can be either a diffuse process, with generally more acute, severe associated symptoms, or a circumscribed process that is often more chronic in nature, with more mild, insidious symptoms. In some cases, the inflammation spreads through an acquired pathway between the inner and middle ear spaces. Examples of this include cholesteatomatous erosion of the bony labyrinth, temporal bone fracture, or after otologic surgery (stapedotomy, fenestration, cochleostomy, etc.). The most common of these is a fistula in the horizontal semicircular canal secondary to erosion by cholesteatoma, often resulting in soft tissue invasion into the labyrinth with a surrounding zone of inflammation within the inner ear.

In many cases of tympanogenic labyrinthitis, there is no known communication between the middle and inner ear spaces. The mechanism of spread of inflammation from the middle to inner ear in these cases has been the subject of many temporal bone and animal investigations over the last century. Though some have entertained the possibility that labyrinthitis could result from direct or embolic spread of inflammation or infection¹ along microscopic vascular channels, there has been little to no evidence to support such a mechanism. Furthermore, because of the different embryologic origins of the middle ear (endoderm and mesoderm) and inner ear (neurectoderm), no direct vascular channels through the otic capsular bone¹⁰ are known to exist. Similarly, the presence of a congenital perilymphatic fistula has been implicated as a potential route of spread in tympanogenic labyrinthitis, although no histopathologic or radiologic evidence exists in the literature to support this mechanism.

The majority of pathologic and experimental studies in tympanogenic labyrinthitis implicate the round window membrane (RWM) as the primary site of spread of middle ear inflammation into the inner ear. The normal RWM is 40 to 70 µm in thickness in humans^11,12 and sits in the round window niche, which measures 1 mm in depth and 2 mm in diameter. The RWM consists of three layers: 1) an outer epithelial layer of nonciliated mucosal cells contiguous with the middle ear epithelium; 2) a middle fibrous layer of fibrocytes with prominent elastic and collagen bundles; and 3) an inner epithelial layer with thin cytoplasmic extensions.¹ The RWM sits adjacent to the sinus tympani, where purulent material can pool in otitis media, particularly when the patient is in the supine position. Multiple RWM pathologic changes are known to occur in otitis media, including vascular hypertrophy and cystic change in the middle layer, with notable thickening of the membrane.11 Beyond this, many temporal bone studies of patients with tympanogenic labyrinthitis have shown evidence of direct spread of bacteria with associated inflammatory cells through an intact RWM.^{12,13,14,15,16,17} Animal studies have shown that thickening of the RWM results in an increase in permeability of the RWM to macromolecules but a decreased incidence of bacterial invasion into the inner ear.¹⁸ Possibly, the thickening of the RWM that occurs in otitis represents a protective mechanism to prevent the direct spread of bacteria from the middle ear, but it may result in an increased susceptibility to influx of inflammatory mediators into the inner ear.

Meningogenic Labyrinthitis

Epidemiology

Five to thirty-five percent of patients who survive bacterial meningitis will have bilateral sensorineural hearing loss (SNHL), due to spread of infection or inflammatory mediators to the inner ear, with resultant loss of neurosensory elements.^{19,20,21,22,23} In 5% of cases, the damage to the cochlea will result in a profound and permanent loss.^{20,22,24,25,26} In children, bacterial meningitis is the leading cause of SNHL (60–90% of all cases) and has significant associated mortality. Haemophilus influenzae, Streptococcus pneumoniae, and Neisseria meningitidis have historically accounted for 64%, 16%, and 10% of cases of meningitis.²⁷ S. pneumoniae has been found to be a particularly virulent organism, with a mortality rate of 19% in children and 20 to 30% in adults.^28,29 In addition, the rate of profound SNHL after S. pneumoniae meningitis in children is 31 to 57%, the highest incidence of SNHL among the common pathogens seen in meningitis.^21,30 The introduction of H. influenzae and S. pneumoniae vaccines has decreased the incidence of bacterial meningitis from these organisms and overall.^21,31

Pathophysiology

Labyrinthitis in the setting of meningitis, whether suppurative or serous, occurs due to the extension of bacteria or inflammatory byproducts from the meninges to the inner ear. Cochlear pathology in meningitis occurs as a progression of changes, with formation of a serofibrinous exudate initially followed by infiltration of inflammatory cells and ultimately granulation formation.³² The route of spread has been investigated in both animal and human temporal bone studies, and the cochlear modiolus, with its multiple perineural and perivascular channels, has been implicated as one of the primary sites involved with transmission of infection in meningogenic labyrinthitis.^{5,33,34,35,36} Merchant and Gopen² found a high degree of correlation between modiolar inflammation and suppurative labyrinthitis in a human temporal bone study on patients who had died with meningogenic labyrinthitis.

The cochlear aqueduct is another site thought to allow the spread of infection from the meninges to the inner ear.^37,38 This theory of pathogenesis is supported by the frequent involvement of the basal scala tympani, where the aqueduct terminates in the inner ear, in cases of meningogenic labyrinthitis. Furthermore, Merchant and Gopen² found inflammatory cells within the lumen of the cochlear aqueduct in 78% of cases of meningogenic labyrinthitis, though the possibility of retrograde involvement of the aqueduct could not be excluded. Interestingly, the presence of labyrinthitis did not correlate with aqueduct patency in this study, raising the possibility that in cases of bony or connective tissue obliteration of the aqueduct, infection may spread through microchannels within the aqueduct not seen on histopathologic examination or through an alternative communication, such as the modiolus.

Another notable finding in the human temporal bone study by Merchant and Gopen² is the lack of inflammatory involvement of the vestibular cribrose area and endolymphatic sac, despite their proximity to the meninges. Differences in the ultrastructural anatomy of these structures, leading to a more efficient sealing of the potential communication, are speculated to be responsible for these observations.

Labyrinthitis ossificans (LO) is a process of new bone deposition within the inner ear and occurs most commonly after bacterial meningitis. LO can also occur after tympanogenic labyrinthitis⁵ and can be thought of as the end stage of inner ear inflammation. Paparella and Sugiura⁵ described the pathology of suppurative labyrinthitis and the subsequent labyrinthine changes that lead to new bone formation within the inner ear. After the acute phase of suppurative labyrinthitis (described previously), wherein a hearing loss can manifest as early as 48 hours after the initial infection, a fibrous stage occurs where granulation tissue, with prominent fibroblasts and vasculature, fills the perilymphatic spaces of the inner ear, with relative sparing of the endolymphatic space.^37,39,40 Disorganized, woven bone is then deposited in the inner ear and with the passage of time, the bone is resorbed and remodeled into dense, lamellar bone that is progressively mineralized.⁴¹ In some cases, complete ossification of the inner ear occurs. The fibrous and osseous stages have been found to occur as early as 2 weeks and 2 months, respectively, after the acute phase of inner ear inflammation.⁵ New bone growth was evident as early as 3 weeks after infection and continued for up to 12 months in one animal study.⁴¹

Labyrinthitis: Diagnosis

Labyrinthitis is characterized by vertigo and hearing loss and is to be differentiated from vestibular neuritis by the presence of cochlear symptoms in addition to vestibular complaints. The vertigo seen in suppurative labyrinthitis is often profound and can be associated with nausea and vomiting, whereas patients with serous labyrinthitis tend to have milder vestibular symptoms in general. The vertigo in labyrinthitis can last days to weeks and often has a waxing and waning course. A postural component to the vertigo may be present, causing patients to seek a motionless environment.10 Patients often have disequilibrium for a period of weeks to months after the acute, fulminant vertigo has diminished.

The hearing loss in suppurative labyrinthitis is profound and generally permanent, whereas serous labyrinthitis often produces a partial loss of hearing, primarily affecting the higher frequencies. Other associated cochlear symptoms, such as tinnitus, aural fullness and otalgia, may be present in patients with labyrinthitis and may be quite severe at times.

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