The human osseous labyrinth is composed of endosteal, enchondral, and periosteal layers. The endosteal layer consists of bone lined with a single thin layer of cells that have numerous gaps that separate them. The enchondral layer is unique in that it reaches adult size by 23 weeks' gestation and undergoes minimal remodeling after age 2 years. The periosteal layer consists of lamellar bone and is capable of remodeling and repair.
In the absence of a pathologic condition, the lumen within the otic capsule remains stable in size and patency throughout life; however, in various diseases (eg, Paget disease of bone, osteopetrosis, otosclerosis, trauma, inflammatory and infectious conditions), new disorganized bone replaces healthy bone or obliterates spaces within the otic capsule. Labyrinthitis ossificans (LO) is the pathologic formation of new bone within the lumen of the otic capsule and is associated with profound deafness and loss of vestibular function. Cochlear ossification in this disease generally does not cross the endosteal layer or alter the architecture of the enchondral bone.
Labyrinthitis ossificans (LO) most commonly occurs as a sequela of inflammation of the inner ear that results from bacterial meningitis and subsequent purulent labyrinthitis. Other etiopathologic causes of labyrinthitis ossificans (LO) include vascular obstruction of the labyrinthine artery, temporal bone trauma, autoimmune inner ear disease, otosclerosis, leukemia, and tumors of the temporal bone. In addition, suppurative labyrinthitis associated with otitis media can cause labyrinthitis ossificans (LO).
Labyrinthitis ossificans (LO) is the pathologic ossification of spaces within the lumen of the bony labyrinth and cochlea that occurs in response to a destructive or inflammatory process. Regardless of the etiology, the most common region of cochlear ossification is the scala tympani of the basal turn, with the most extensive disease noted in postmeningitic cases. Fibrosis and ossification of the scala tympani are seen in the image below.
Studies of the pathophysiology of deafness after meningitis suggest that an inflammatory labyrinthitis develops from the spread of infection into the inner ear via the cochlear aqueduct or internal auditory canal. In 1991, Bhatt et al proposed an animal model of pneumococcal meningitis that strengthened the hypothesis that the most likely conduit of meningogenic labyrinthitis is extension of the disease through the cochlear aqueduct.  Because the cochlear aqueduct drains into the scala tympani adjacent to the round window, the initial concentration of inflammatory mediators occurs in this region, perhaps explaining the predominant degree of injury in this area. Another possibility for the disproportionate degree of ossification in the scala tympani of the basal turn is the relative decreased blood flow in this area. This decreased perfusion explains the propensity to develop ossification in this area, regardless of the underlying etiology.
Paparella and Sugiura outlined the pathologic stages associated with purulent labyrinthitis and the process leading to ossification of the labyrinth in laboratory animals and human beings.  They divided the evolution of labyrinthitis ossificans (LO) into 3 characteristic stages: acute, fibrous, and ossification. The image below depicts the stages of ossification.
The acute stage is characterized by purulence that fills the perilymphatic spaces but spares the endolymphatic space, followed by serofibrinous exudate. The second stage, or fibrous stage, consists of fibroblastic proliferation within the perilymphatic spaces, which begins approximately 2 weeks after the onset of infection. Angiogenesis is also present. The third, or ossification, stage is characterized by bone formation first observed in the basal turn of the cochlea as early as 2 months after the onset of infection. Ossification of the scala tympani is seen in the image below.
Formation of osteoid with subsequent mineralization and remodeling obliterates the perilymphatic and endolymphatic spaces. Ossification in humans has been noted to occur within a year after meningitis, although the hearing loss may occur as early as 48 hours after infection.
In 1998, Brodie et al developed a gerbil model of labyrinthitis ossificans (LO) subsequent to Streptococcus pneumoniae –induced meningitis.  This model demonstrates 3 main histological features: fibrosis, osteoid deposition, and osteoneogenesis. Osteoid deposition appears as homogenous, eosinophilic, and moderately cellular deposits and occurs more prominently in areas of denser fibrosis. Osteoneogenesis that involves calcification of the bone matrix and subsequent remodeling develops adjacent to the endosteal layer within the cochlea, with preservation of the normal contour of the otic capsule.
Using the same model, Nabili and Brodie documented the occurrence of osteoneogenesis and mineralization as early as 21 days postinfection, and new bone growth was shown to be active for at least 12 months.  This study was extended by Tinling et al in 2004 to show osteoid deposition and mineralization occurring as early as 3 days postinfection and continuing at least through the first 28 days postinfection.  Resorption of new bone and remodeling by 84 days postinfection was not apparent.
In another study, Nadol et al documented that severe inflammation occurs in the scala tympani of the basal turn where the aqueduct enters the cochlea.  They found that reduction in the inflammatory response in the internal auditory canal occurs as it proceeds from medial to lateral. This study also documented the preservation of auditory nerve fibers despite the intense labyrinthitis and ossification with accompanying degenerative changes in the stria vascularis and organ of Corti. The number of remaining spiral ganglion cells was shown to be inversely proportional to the severity of new bone formation.
The phenomenon of labyrinthitis ossificans (LO) was recognized as early as the 19th century; however, the pathogenic mechanisms remain poorly understood. Early theories divided new bone formation into 2 types: metaplastic and osteoplastic. Metaplastic bone originates from scar or granulation tissue that has filled the bony labyrinth. Osteoplastic bone forms as an extension from the endosteum that lines the lumen of the otic capsule.
Bacterial meningitis, which affects an estimated 15,000 infants and children in the United States each year, is the most common cause of both acquired sensorineural hearing loss in childhood and labyrinthitis ossificans (LO). The reported incidence of hearing loss following meningitis ranges from 6-37%, with an estimated 5% suffering from profound deafness. Deafness results from spread of the infection to the labyrinth and consequent end organ damage. Ossification within the labyrinth compounds destruction of neural elements.
Dodge et al reviewed the outcome of 185 infants and children with meningitis and found a 10% overall incidence of hearing loss.  The incidence of hearing loss was greatest with S pneumoniae (31%) infection and lowest with Haemophilus influenzae (6%) infection. The mortality rate of S pneumoniae –induced meningitis (19% in children, 20-30% in adults) also is the highest of the 3 infecting organisms (see the Causes section). As many as 80% of patients with profound postmeningitic deafness have some degree of labyrinthine ossification. Complete ossification is associated with a very poor prognosis for residual hearing.