Excerpt from Labyrinthitis

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Background

Labyrinthitis is an inflammatory disorder of the inner ear or labyrinth. Clinically, this condition produces disturbances of balance and hearing to varying degrees and may affect one or both ears. Bacteria or viruses can cause acute inflammation of the labyrinth in conjunction with either local infection or systemic infection.

Pathophysiology

The anatomic relationships of the labyrinth, middle ear, mastoid, and subarachnoid space are essential to understanding the pathophysiology of labyrinthitis. The labyrinth is composed of an outer osseous framework surrounding a delicate membranous network that contains the peripheral sensory organs for balance and hearing. These sensory organs include the utricle, saccule, semicircular canals, and cochlea. Symptoms of labyrinthitis occur when infectious microorganisms or inflammatory mediators invade the membranous labyrinth and damage the vestibular and auditory end organs.

The labyrinth lies within the petrous portion of the temporal bone adjacent to the mastoid cavity and connects with the middle ear at the oval and round windows. The labyrinth maintains connections with the central nervous system and subarachnoid space by way of the internal auditory canal and cochlear aqueduct. Bacteria may gain access to the membranous labyrinth by these pathways or through congenital or acquired defects of the bony labyrinth. Viruses typically spread to labyrinthine structures hematogenously or by way of the aforementioned preformed pathways.

Viral and bacterial labyrinthitis are sufficiently different to warrant discussing them as separate disease processes. Viral labyrinthitis is characterized by a sudden unilateral loss of vestibular function and hearing. The acute onset of severe, often incapacitating, vertigo, frequently associated with nausea and vomiting, is characteristic of this disorder. The patient is often bedridden while the symptoms gradually subside. Vertigo eventually resolves after several days to weeks; however, unsteadiness and positional vertigo may persist for several months. Hearing loss is also present and may be the primary presenting symptom in many patients. Physical examination findings include spontaneous nystagmus toward the unaffected side with diminished or absent caloric responses in the affected ear. The hearing loss is usually mild to moderate and typically evident in the higher frequencies (>2000 Hz), although any degree or type of hearing loss may be present. 

An upper respiratory tract infection precedes the onset of symptoms in up to 50% of cases. Recurrent attacks are reported but are rare and may be confused with Ménière disease. Resolution of the vertigo and dysequilibrium is common and is due to partial recovery of vestibular function with concurrent central compensation of the remaining unilateral vestibular deficit. Return of hearing usually mirrors the return of vestibular function. 

Viral labyrinthitis is often confused with vestibular neuritis, and the terms are occasionally used interchangeably in the literature. However, most authors agree that vestibular neuritis is a disorder of the vestibular nerve and is not associated with hearing loss. Because the cochlea is affected in labyrinthine inflammation, hearing loss is always present in persons with viral labyrinthitis. 

Vestibular neuritis typically manifests as sudden acute vertigo without hearing loss in an otherwise healthy person. The condition is more common in the fourth and fifth decades of life and affects men and women equally. An upper respiratory tract infection often precedes the condition, and the disorder is more common in the spring and early summer. Histopathologic nerve studies of patients with vestibular neuritis demonstrate axonal loss, endoneurial fibrosis, and atrophy. These findings are consistent with a viral inflammatory etiology. The treatment of vestibular neuritis and viral labyrinthitis is similar. 

A unique form of viral labyrinthitis is herpes zoster oticus, or Ramsay-Hunt syndrome. The cause of this disorder is reactivation of a latent varicella-zoster virus infection occurring years after the primary infection. The initial symptoms are deep, burning, auricular pain followed a few days later by the eruption of a vesicular rash in the external auditory canal and concha. Vertigo, hearing loss, and facial weakness may follow in insolation or collectively. Symptoms typically improve over a few weeks; however, patients often suffer permanent hearing loss and persistent reduction of caloric responses. 

Viral infections can cause both congenital and acquired hearing loss. Rubella and cytomegalovirus are the best-recognized viral causes of prenatal hearing loss. Virally induced hearing loss in the postnatal period is usually due to mumps or measles. Viral infections are also implicated in idiopathic sudden sensorineural hearing loss (SNHL). These topics are covered in other articles (see Pediatrics, Measles; Pediatrics, Mumps; Cytomegalovirus Infection; Pediatrics, Rubella. 

Meningitis and otitis media may be complicated by bacterial labyrinthitis, which can occur by either direct bacterial invasion (suppurative labyrinthitis) or through the passage of bacterial toxins and other inflammatory mediators into the inner ear (serous labyrinthitis). Meningitis typically affects both ears, whereas otogenic infections typically cause unilateral symptoms. Bacteria spread from the cerebrospinal fluid to the membranous labyrinth by way of the internal auditory canal or cochlear aqueduct. Infections of the middle ear or mastoid most commonly spread to the labyrinth through a dehiscent horizontal semicircular canal. Usually, the dehiscence is the result of erosion by a cholesteatoma.

Suppurative labyrinthitis resulting from otitis is uncommon in the postantibiotic era. When suppurative labyrinthitis occurs, it almost always is associated with cholesteatoma. Profound hearing loss, severe vertigo, ataxia, and nausea and vomiting are common symptoms of bacterial labyrinthitis. 

Treatment of suppurative labyrinthitis is aimed at eradicating the underlying infection, providing supportive care to the patient, draining middle ear effusions or mastoid infections, and preventing the spread of infection. Labyrinthitis ossificans often follows suppurative labyrinthitis. In these cases, therefore, decisions regarding cochlear implantation must be made early. Meningitis also may result in progressive hearing loss secondary to necrosis and fibrosis of the membranous cochlea and labyrinth. 

Serous labyrinthitis occurs when bacterial toxins and host inflammatory mediators, such as cytokines, enzymes, and complement, cross the round window membrane, causing inflammation of the labyrinth in the absence of direct bacterial contamination. This condition is associated with acute or chronic middle ear disease and is believed to be one of the most common complications of otitis media. Toxins, enzymes, and other inflammatory products infiltrate the scala tympani, forming a fine precipitate just medial to the round window membrane. Penetration of the inflammatory agents into the endolymph at the basilar turn of the cochlea results in a mild high-frequency SNHL. Audiography testing reveals mixed hearing loss when a middle ear effusion is present. Vestibular symptoms may occur but are less common. Treatment is aimed at eliminating the underlying infection and clearing the middle ear space of effusion. The hearing loss is usually transient but may persist if the otitis is left untreated.

Frequency

United States

Viral labyrinthitis is the most common form of labyrinthitis observed in clinical practice. One study reported that 37 of 240 patients who presented with positional vertigo had viral labyrinthitis. The prevalence of sudden SNHL is estimated at 1 case in 10,000 persons, with up to 40% of these patients complaining of vertigo or dysequilibrium.¹ Auditory and vestibular symptoms develop in approximately 25% of patients with herpes oticus, in addition to the facial paralysis and vesicular rash that characterize the disease. Bacterial labyrinthitis is rare in the postantibiotic era, although bacterial meningitis remains a significant cause of hearing loss. Auditory symptoms, vestibular symptoms, or both may be present in as many as 20% of children with meningitis.²

Mortality/Morbidity

Deaths associated with labyrinthitis are not reported except in cases of meningitis or overwhelming sepsis. The morbidity of labyrinthitis, especially bacterial labyrinthitis, is significant. In the pediatric population, the risk of hearing loss secondary to meningitis is estimated to be 10-20%.^{2, 3} Streptococcus pneumoniae appears to be the causative agent most likely associated with hearing loss, although some studies place Neisseria meningitidis as the most significant agent.² Bacterial labyrinthitis, regardless of etiology, accounts for one third of all cases of acquired hearing loss. One study reported dizziness in 23% of patients following pneumococcal meningitis. Ménière disease may follow an episode of suppurative or serous labyrinthitis and is most likely due to fibrosis of the endolymphatic sac and altered Na⁺/K⁺ transport.¹

Age

Viral labyrinthitis is usually observed in adults aged 30-60 years and is rarely observed in children. Meningogenic suppurative labyrinthitis is usually observed in children younger than 2 years, which is the population most at risk for meningitis. Otogenic suppurative labyrinthitis can be observed in persons of any age in the presence of cholesteatoma and in untreated acute otitis media.⁴ Serous labyrinthitis is more common in the pediatric age group, in which the vast majority of both acute and chronic otitis media cases are observed.

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