KeywordsTemporal bone, labyrinthitis, labyrinthitis ossificans, sensorineural hearing loss
Labyrinthitis, also known as otitis interna, is an inflammatory disorder of the inner ear. Inflammation of the perilymphatic spaces results in secondary changes within the membranous labyrinth, the most common symptoms of which are sensorineural hearing loss and vertigo. Causes of labyrinthitis are most commonly classified either by mode of spread (tympanogenic, meningogenic, hematogenic, posttraumatic) or by causative agent (viral, bacterial, autoimmune, syphilitic).
Viruses are the most common cause of labyrinthitis, and viral labyrinthitis typically follows an upper respiratory tract infection. Because the infection is most often self-limited and the associated symptoms are commonly transient, these patients are not routinely imaged. However, recurrent viral labyrinthitis can result in chronic sensorineural hearing loss. Autoimmune labyrinthitis is rare but has been reported in patients with Cogan syndrome, Hashimoto thyroiditis, Sjögren syndrome, Behçet disease, antiphospholipid syndrome, antiocardiolipin syndrome, and ulcerative colitis. Additionally, vasculitis associated with polyarteritis nodosa, lupus, relapsing polychondritis, rheumatoid arthritis, and granulomatosis with polyangiitis can involve the labyrinth and result in labyrinthitis. Syphilitic (luetic) labyrinthitis was historically a more common cause of labyrinthitis; it virtually always occurs in the setting of advanced systemic disease.
Suppurative labyrinthitis is defined by the presence of inflammatory cells (usually leukocytes) within the fluid spaces of the inner ear and is the result of pyogenic bacterial infection. The most common causative bacteria are Streptococcus pneumonia and Haemophilus influenza . The cochlear aperture, the lamina cribrosa of the vestibule, and the cochlear aqueduct are hypothesized portals of entry for meningogenic labyrinthitis, and the round and oval windows are hypothesized portals of entry for tympanogenic labyrinthitis. Persistent sensorineural hearing loss is common following bacterial labyrinthitis.
Although not the focus of this chapter, it is worth noting that the labyrinth can also be involved by congenital infections, such as cytomegalovirus, rubella, and syphilis.
Temporal Evolution: Overview
Four stages of labyrinthitis have been described: (1) the serous stage, (2) the purulent stage, (3) the fibrous stage, and (4) the osseous stage ( Fig. 43.1 ). The serous and purulent stages are together considered acute labyrinthitis, and the fibrous and osseous stages are considered chronic labyrinthitis. Although function may recover and findings on magnetic resonance imaging (MRI) may normalize after acute labyrinthitis, progression to chronic disease is associated with permanent disability and persistent imaging findings ( Fig. 43.2 ).
Temporal Evolution: in Greater Depth
The serous stage is the earliest stage of labyrinthitis, in which only a few pathogens are present at the portal of entry. In response, inflammatory cells are recruited and an immunoglobulin-rich exudate is produced. Depending on the number and virulence of infecting pathogens, the efficacy of the inner ear immune response, and the initiation of appropriate treatment, the labyrinthitis may resolve at this stage and inner ear function may recover. Patients who do not recover from the initial serous stage of labyrinthitis progress to the purulent stage, in which bacteria and leukocytes fill the perilymphatic space, causing reactive endolymphatic changes and often resulting in end-organ damage.
There are no computed tomography (CT) findings of acute labyrinthitis. In the majority of patients, magnetic resonance (MR) examinations of the temporal bones are also normal ; however, enhancement of the fluid-filled spaces of the membranous labyrinth on postcontrast T1-weighted images can be seen ( Fig. 43.3 ). In suppurative labyrinthitis from bacterial meningitis, labyrinthine enhancement may be seen as soon as 1 day after the diagnosis and may persist unchanged for up to 3 weeks.
Treatment of acute labyrinthitis is directed against the inciting pathogen if identifiable (e.g., antibiotics). Steroids are used to treat viral labyrinthitis and may also be given early in the course of bacterial meningitis in an attempt to mitigate hearing loss and reduce the severity of labyrinthine ossification. Despite treatment, a significant number of patients with acute labyrinthitis will progress to fibrous and osseous labyrinthitis. For example, up to 35% of children who develop suppurative labyrinthitis from bacterial meningitis will go on to develop labyrinthitis ossificans.
The fibrous stage is characterized by proliferation of fibroblasts within the perilymphatic spaces and begins approximately 2 weeks following the initial insult. As opposed to the acute stage, in which the normal fluid signal of the membranous labyrinth is preserved on T2-weighted images, in the fibrous stage of labyrinthitis, the normal high fluid signal is replaced by low signal soft tissue ( Fig. 43.4 ). Persistent enhancement may be seen on T1-weighted postcontrast images. There are no CT findings in fibrous labyrinthitis.