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5,000

Cochlear nerve diameter (mm)

Fig. 5. Scatter plot demonstrating the correlation between the spiral ganglion cell count and diameter of the cochlear nerve. Reprinted with permission from Nadol and Xu [27].

implantation and in 5 patients with normal hearing, the maximum diameter of the cochlear, vestibular, and eighth cranial nerves were measured histologically. Although the maximum diameters were significantly smaller in the deaf population as compared to the normal hearing controls, only 25% of the variance of the spiral ganglion cell count was predicted by the maximum diameter of the eighth nerve (fig. 5). Therefore, although imaging studies may be helpful in predicting residual spiral ganglion cell counts, the precision is low [27].

Specific Causes of Deafness of Special Relevance to

Cochlear Implantation

Bacterial Labyrinthitis

Patients deafened by bacterial meningitis are frequent candidates for cochlear implantation. However, new bone formation within the cochlea, or labyrinthitis ossificans, is a common finding in such individuals [28, 29] (fig. 6). The presence of labyrinthitis ossificans is recognizable on imaging of the temporal bone, and creates a mechanical impediment to implantation. In addition, there is a significant negative correlation between percent bony occlusion and the percent of normal spiral ganglion cell counts [30, 31] (fig. 7). In all cases in which the segmental and total bony occlusion was less than 10%, there was at

Cochlear Ossification From Meningitis

Fig. 6. Ossification of the cochlea in a 5-year-old boy who became profoundly and bilaterally deaf as a consequence of streptococcal meningitis at the age of 18 months. Two months following recovery from meningitis, a CT of the temporal bone showed no evidence of labyrinthitis ossificans. However, a repeat CT scan done 2 months thereafter showed new bone growth in both cochleae, worse on the right side. In this right ear, the cochlea is largely replaced by new bone (NB) in all turns. Although there is no recognizable organ of Corti, there were residual spiral ganglion cells (SPG) with a total spiral ganglion cell count of 10,900.

Fig. 6. Ossification of the cochlea in a 5-year-old boy who became profoundly and bilaterally deaf as a consequence of streptococcal meningitis at the age of 18 months. Two months following recovery from meningitis, a CT of the temporal bone showed no evidence of labyrinthitis ossificans. However, a repeat CT scan done 2 months thereafter showed new bone growth in both cochleae, worse on the right side. In this right ear, the cochlea is largely replaced by new bone (NB) in all turns. Although there is no recognizable organ of Corti, there were residual spiral ganglion cells (SPG) with a total spiral ganglion cell count of 10,900.

least 30% of the age-matched normal segmental and total spiral ganglion cell densities. However, even in cases with severe bony occlusion, significant numbers of spiral ganglion cells survive. Total absence of spiral ganglion cells was not found in any specimen with labyrinthitis ossificans secondary to bacterial meningitis.

Genetically Determined Sensorineural Hearing Loss

Postnatal Progressive Deafness

Similar to other causes of deafness, the histopathologic correlates of genetically determined postnatal progressive sensorineural loss are degeneration of various elements of the inner ear. Bony malformation of the cochlea is relatively uncommon. Usher's syndrome and DFNA-9 are presented as examples of syndromic and nonsyndromic causes of postnatal, progressive, sensorineural hearing loss.

Usher's Syndrome

Usher's syndrome is the most common cause of autosomal recessive pattern of syndromic deafness. There are three distinguishable subtypes of Usher's

0 10 20 30 40 50 60 70 80 90 100 Bony occlusion (%)

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