Aetiology

Viral infections are presumed to be the most common cause of labyrinthitis and vestibular neuritis. Viral illnesses, in particular DNA viruses, have a known association with other mononeuropathies, such as Bell palsy, optic neuritis, and peritoneal neuronitis.

Viral labyrinthitis is typically associated with a preceding upper respiratory tract infection. Aetiological viral agents include varicella zoster virus, cytomegalovirus, mumps virus, measles virus, rubella virus, and HIV.[3][16]

Bacterial labyrinthitis is associated with acute or chronic otitis media, meningitis, and cholesteatoma (a collection or growth of epidermal and connective tissues within the middle ear). Unlike viral labyrinthitis, the bacterial form may affect both ears simultaneously.[6][17] Potential bacterial causes include Treponema pallidum, Haemophilus influenzae, Streptococcus species, Staphylococcus species, and Neisseria meningitidis.[6] Syphilitic labyrinthitis may follow tertiary neurosyphilis that occurs many years after the primary infection, and is not seen with acute primary or secondary syphilis.[3]

Vestibular neuritis is usually associated with a preceding or accompanying viral infection.[18]​ Aetiological viral agents include influenza virus, adenovirus, herpes simplex virus, cytomegalovirus, Epstein-Barr virus, rubella virus, and parainfluenza virus.[18]​ Reactivation of latent herpes simplex virus-1 is one of the most common causes.[1]​ Bacterial infections, protozoal infections, vascular occlusion, and allergic and autoimmune causes have also been implicated in vestibular neuritis.[7][18]

Pathophysiology

Labyrinthitis and vestibular neuritis are subtypes of acute vestibular syndrome, which results from unilateral injury to peripheral or central vestibular organs.[19]​ Both conditions have distinct pathophysiological mechanisms and can be primarily distinguished by the presence or absence of hearing loss.

Labyrinthitis

Infections arising in the middle ear (otitis media) can spread to the inner ear through the oval or round window. Inflammation or infectious agents can spread from the inner ear into the internal auditory canal. Meningitis can spread to the inner ear through the cochlear aqueduct or the cochlear modiolus.[17] Haematogenous spread of infectious agents through the labyrinthine artery to the stria vascularis is theoretically possible but has not been demonstrated.[17]

Infection in the membranous labyrinth may result in a significant inflammatory response with resultant intraluminal fibrosis and possible ossification (i.e., labyrinthitis ossificans).[20][21][22] Bacterial meningitis is associated with a significant risk of hearing loss.[23] Auditory or vestibular symptoms, or both, may be present in as many as 20% of children with meningitis.[24]

Vestibular neuritis

Both superior and inferior nerve divisions are affected in most cases of acute vestibular neuritis, while superior or inferior vestibular nerves may be affected individually in some people.[25]​ Abnormalities are high when the superior vestibular nerve is affected.[25][26]

Inflammation of the vestibular nerve in vestibular neuritis may be due to replication of herpes simplex virus-1, leading to secondary damage of the vestibular ganglion cells and axons in the bony canals.[1]

One systematic review found higher neutrophil to lymphocyte ratio and higher prevalence of vascular risk factors among patients with vestibular neuritis.[27]​ Elevated plasma fibrinogen and C-reactive protein levels and decreased lipoprotein-a levels have been noted in patients presenting with acute vestibular neuritis.[28]​ One study reported significantly elevated C-reactive protein levels and elevated pro-inflammatory and pro-adhesive proteins in peripheral blood mononuclear cells of patients with acute vestibular neuritis.[29]

Classification

Types of labyrinthitis

Serous (viral) labyrinthitis:

  • Caused by inflammation of the labyrinth only

  • Typically presents with less severe hearing loss and vertigo than suppurative labyrinthitis; hearing loss often recovers

HIV-associated labyrinthitis:

A variety of auditory and vestibular complaints, including labyrinthitis, have been reported in patients with AIDS. The relative importance of the HIV infection itself as opposed to its associated opportunistic infections requires further study.

Suppurative (bacterial) labyrinthitis:[2]

  • Follows direct bacterial invasion of the inner ear and therefore is typically unilateral

  • Typically presents with severe to profound hearing loss and vertigo

  • Hearing loss that occurs with suppurative labyrinthitis is typically irreversible

Meningogenic labyrinthitis:

  • Occurs in the setting of meningitis

  • Typically begins at the basal turn of the cochlea adjacent to the opening of the cochlear aqueduct

Syphilitic labyrinthitis:[3]

  • Can follow tertiary neurosyphilis, which occurs many years after primary syphilis infection, and is not seen with acute primary or secondary syphilis

  • Patients can present with progressive hearing loss and pressure- or sound-induced vertigo (Hennebert and Tullio signs)

Labyrinthitis ossificans:

  • Fibrosis of the membranous labyrinth occurs within a few days of acute infection

  • Ossification can occur as early as fibrosis, resulting in complete osseous replacement of the membranous labyrinth

  • Membranous labyrinth can be obstructed in up to 30% of patients with suppurative labyrinthitis

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