martedì 1 luglio 2008

Labyrinthine ossificans


Increased hazy densities bilaterally in the membranous component of the cochlea. The left tympanic membrane demonstrates focal thickening in the pars flaccidum. There is mild soft tissue density within the left external auditory canal. (Prior L left myringotomy tube).

Differential diagnosis:
- Labyrinthine ossificans
- Cochlear otosclerosis
- Cochlear aplasia
- Labyrinthine schwannoma
- Intravestibular lipoma

Diagnosis: Labyrinthine ossificans


Labyrinthine Ossificans (LO) refers to ossification occurring within the luminal spaces of the labyrinth and cochlea. This typically is secondary to a destructive or inflammatory process and represents a healing response. Specific processes may be infectious, traumatic or surgical in nature. LO is most commonly seen after bacterial meningitis in children. Not surprisingly, LO secondary to bacterial meningitis represents the most common cause of acquired childhood deafness.

The typical clinical presentation is a child between 2-18 months of age with a history of recent meningitis presenting with bilateral sensorineural hearing loss. Although not a common presenting symptom, patients may also present with vertigo. Meningitis in the age group of interest is usually secondary to either streptococcus pneuomoniae or hemophilus influenzae.

Other presentations may include ear infection, bout of viral illness, or severe head trauma.

Ceftazidime is the antibiotic of choice to prevent otogenic and meningogenic labyrinthitis. This is secondary to the high antibiotic concentrations levels that can be reached in the CSF and endolymph. Steroids have shown some promise and are felt to decrease the rate of hearing loss. This is likely secondary to decreased inflammatory response, granulation tissue formation and collagen formation. Cochlear implantation may be an option if the cochlear nerve is still preserved. In cases of severe vertigo, labyrinthectomy may be warranted.

Radiologic overview

High resolution, 1 mm thick coronal and sagittal CT images are recommended. Post contrast images are not needed. The best clue on CT imaging is bone deposition within the membranous labyrinth. Findings vary based on the severity of the disease. Mild cases of LO may simply demonstrate increase haziness of the luminal spaces of the membranous labyrinth. Moderate cases may demonstrate areas of interspersed bone invading the luminal spaces. Severe cases may show complete destruction of the membranous labyrinth with extensive bony replacement of the luminal spaces.

Although CT imaging is used more often, MRI can be also assist in diagnosis. MRI offers the advantage of visualizing fibrous destruction of the membranous labyrinth which may be difficult to appreciate on CT. T2 weighted images are most useful in helping make the diagnosis. In mild LO, there is partial replacement of the hyperintense signal normally seen in the fluid spaces of the membranous labyrinth. In cases of moderate LO, hypointense focal areas are noted corresponding to bony replacement of the fluid spaces. Finally, complete absence of the T2 hyperintensity correlates with complete bony replacement of the fluid spaces seen in severe LO. As with CT, thin cuts on MR imaging are recommended.

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