The human bony labyrinth is composed of endosteal, enchondral and perisoteal layers. The endosteal layer consists of bone lined with a single layer of cells that have numerous gaps separating them. This layer is significantly thin. The enchondral layer is rather unique in that it reaches the adult size by 23 weeks of gestatation and undergoes minimal remodelling after the age of 2. The periosteal layer is composed of lamellar bone and is capable of bone remodelling and repair.
Labyrinthitis ossificans is the pathologic new bone formation within the lumen of otic capsule. This condition is always associated with profound deafness and loss of vestibular function. Cochlear ossification in this condition doesnot cross the endosteal layer or alter the architecture of the enchondral bone.
Causes of labyrinthitis ossificans:
- Bacterial meningitis
- Vascular obstruction to labyrinthine artery
- Temporal bone trauma
- Autoimmune inner ear disease
- Tumors of temporal bone
- Suppurative labyrinthitis following CSOM
Commonly involved portion of cochlea in this condition is scala tympani portion of the basal turn of the cochlea. This is more severe in patients with bacterial meningitis. Infections from inflammed meninges reach the inner ear via the cochlear aqueduct. Usually the cochlear aqueduct drains into the scala tympani close to the round window region. This is the reason why predominant ossification of the inner ear occurs in this area following bacterial meningitis. This creates lot of problem during cochlear implant electrode introduction via the round window. This ossified area should be drilled out with a microdrill inorder to insert the electrode in these patients.
According to Paperella labyrinthitis ossificans can be divided into three stages:
Acute stage – This stage is characterised by pus which fills up the perilymphatic spaces, sparing the endolymphatic space. This is followed by the formation of serofibrinous exudate.
Stage of fibrosis – This stage is characterised by fibroblastic proliferation within the perilymphatic spaces which usually begins two weeks after the onset of infection. This stage is also associated with new bone formation (angiogenesis).
Stage of ossification – This stage is characterised by bone formation and is first observed at the basal turn of the cochlea.
Theories proposed to explain the pathogenesis of labyrinthitis ossificans:
- Paperella's theory: This theory was proposed by Paperella and Sigiura in 1967. They hypothesized that bone lining cells of the cochlea are plueripotent stem cells that remain uncomitted till they are stimulated to differentiate into osteoblasts.
- Linthicum theory: Linthicum in 1985 said that bone originates from osteoblasts within the otic capsule. He also suggested that ectopic bone forms on the endosteal layer after an inflammatory insult.
- Pericyte theory: This theory suggests that pericytes associated with blood vessels that supply the modiolus and spiral ligament have been hypothesized as cells of origin.