Cochlea Implant Electrode Histology

DFNA-9 is an autosomal dominantly inherited disorder, which produces adult onset sensorineural loss. The molecular genetic basis of this disorder has been demonstrated [35] as a missense mutation in the cochlear gene COCH. This disorder is of interest not only because its molecular genetics are understood, but

Congenital Deafness

Fig. 8. This 82-year-old woman suffered profound congenital deafness as a consequence of Usher's syndrome type I. The histopathologic correlate of this deafness included atrophy of the stria vascularis (SV), total loss of the organ of Corti (OC), and the severe loss of spiral ganglion cells (SPG). However, there was a residual spiral ganglion cell count of 6,776, approximately 40% of age-matched controls.

Fig. 8. This 82-year-old woman suffered profound congenital deafness as a consequence of Usher's syndrome type I. The histopathologic correlate of this deafness included atrophy of the stria vascularis (SV), total loss of the organ of Corti (OC), and the severe loss of spiral ganglion cells (SPG). However, there was a residual spiral ganglion cell count of 6,776, approximately 40% of age-matched controls.

also because of unique histopathology [36] (fig. 9). The histopathologic correlate of profound sensorineural loss includes marked degeneration of the spiral ligament and severe degeneration of spiral ganglion cells. Specifically, the dendritic processes of spiral ganglion cells, normally seen in the osseous spiral lamina, are missing and replaced by an eosinophilic acellular material, whereas the spiral ganglion cells do remain, albeit in reduced numbers. This is of particular interest because patients with this disorder who have undergone cochlear implantation are in general good implant users [pers. obs.], suggesting that peripheral processes (dendrites) of spiral ganglion cells are not necessary for neural stimulation using a cochlear implant.

Congenital Deafness

Approximately 25% of congenitally deaf individuals have a recognizable malformation of the otic capsule [37]. These abnormalities range from minor defects to total aplasia of the cochlea. These malformations create special challenges to successful implantation including widely patent communications between the spinal fluid space and perilymphatic scalae via a malformed cribrose area, potentially causing a cerebrospinal fluid leak; abnormal juxtaposition of the vestibular apparatus to the cochlea, increasing the possibility of unintended stimulation of vestibular neurons by a cochlear implant; difficult a

Cochlear Implants

Fig. 9. This 59-year-old woman suffered a progressive bilateral sensorineural loss starting at the age of 21 years secondary to an autosomal dominant disorder (DFNA-9). An audiogram at age 50 years demonstrated a severe to profound sensorineural hearing loss in this right ear, with 0% speech discrimination. Pathologic study demonstrated eosinophilic extracellular material infiltrating the spiral ligament (SL) and the osseous spiral lamina (OSL). Although there were residual spiral ganglion cells (SPG) in Rosenthal's canal, there was total atrophy of the peripheral dendrites in the OSL. Hair cells in the organ of Corti (OC) were present in all three turns of the cochlea. a Midmodiolar section. b Higher power of basal turn (line up b with a).

Fig. 9. This 59-year-old woman suffered a progressive bilateral sensorineural loss starting at the age of 21 years secondary to an autosomal dominant disorder (DFNA-9). An audiogram at age 50 years demonstrated a severe to profound sensorineural hearing loss in this right ear, with 0% speech discrimination. Pathologic study demonstrated eosinophilic extracellular material infiltrating the spiral ligament (SL) and the osseous spiral lamina (OSL). Although there were residual spiral ganglion cells (SPG) in Rosenthal's canal, there was total atrophy of the peripheral dendrites in the OSL. Hair cells in the organ of Corti (OC) were present in all three turns of the cochlea. a Midmodiolar section. b Higher power of basal turn (line up b with a).

a access for cochlear implantation due to absence of the round window and/or malpositioned facial nerve, and decreased and anomalously located spiral ganglion cells in a rudimentary modiolus and Rosenthal's canal (fig. 10).

In summary, except in rare cases, the histopathology of severe to profound deafness in the human is located primarily in the inner ear. It is rare for there to be total degeneration of the spiral ganglion cells, although the distribution and total number of these cells vary widely.

Histopathologic Changes in the Cochlea Induced by Cochlear Implantation

The insertion of a cochlear implant electrode array causes a varying amount of immediate trauma and also results in delayed effects.

Immediate Trauma Caused by Cochlear Implants

Depending upon the location and size of the cochleostomy, significant trauma may occur to the basilar membrane and osseous spiral lamina (figs. 11, 12). Displacement of the basilar membrane or fracture-dislocation of the osseous spiral

Fracture Spiral Lamina

Fig. 10. This 85-year-old woman suffered congenital deafness secondary to severe dysplasia of the bony cochlea and vestibular system. There were approximately one to half turns in the rudimentary cochlea (C). There were no hair cells. The modiolus (M) was rudimentary. There were, however, a few spiral ganglion cells remaining. The scala tympani (ST) of the basal turn was markedly reduced in size.

Fig. 10. This 85-year-old woman suffered congenital deafness secondary to severe dysplasia of the bony cochlea and vestibular system. There were approximately one to half turns in the rudimentary cochlea (C). There were no hair cells. The modiolus (M) was rudimentary. There were, however, a few spiral ganglion cells remaining. The scala tympani (ST) of the basal turn was markedly reduced in size.

Bilateral Cochlear Implants

Fig. 11. Example of acute trauma caused by cochlear implantation in a 91-year-old man with bilateral progressive and profound sensorineural hearing loss who underwent cochlear implantation of the left ear 11 years prior to death. The cochlear implant track (CIT) is visible. In this basal turn, the implant array had penetrated the spiral ligament (SL) and was surrounded by a fibrous sheath (FS).

Fig. 11. Example of acute trauma caused by cochlear implantation in a 91-year-old man with bilateral progressive and profound sensorineural hearing loss who underwent cochlear implantation of the left ear 11 years prior to death. The cochlear implant track (CIT) is visible. In this basal turn, the implant array had penetrated the spiral ligament (SL) and was surrounded by a fibrous sheath (FS).

Cochlea Implant Histology Trauma

Fig. 12. Acute trauma from cochlear implantation in a 62-year-old woman who suffered a progressive loss of hearing and profound deafness bilaterally as a consequence of aminoglycoside ototoxicity 5 years prior to implantation. In her implanted right ear, the electrode array (EA) has been maintained in situ. There is fracture-dislocation of the basilar membrane (BM) on the left approximately 11 mm from the round window membrane, and displacement of the basilar membrane (BM) on the right, approximately 18 mm from the round window membrane by the electrode array.

Fig. 12. Acute trauma from cochlear implantation in a 62-year-old woman who suffered a progressive loss of hearing and profound deafness bilaterally as a consequence of aminoglycoside ototoxicity 5 years prior to implantation. In her implanted right ear, the electrode array (EA) has been maintained in situ. There is fracture-dislocation of the basilar membrane (BM) on the left approximately 11 mm from the round window membrane, and displacement of the basilar membrane (BM) on the right, approximately 18 mm from the round window membrane by the electrode array.

lamina is not uncommon [38; see also Roland and Wright, this vol, pp 11-30]. Occasionally, a rupture of the basilar membrane may occur with the passage of a cochlear implant from scala tympani into scala vestibuli.

Damage to the lateral cochlear wall, particularly in the ascending limb of the basal turn, may occur.

Delayed Effects Induced by Cochlear Implantation

New bone formation is a universal finding, particularly near the cochleostomy site following cochlear implantation [38]. A fibrous tissue sheath surrounds the implant electrode within the middle ear and also within the inner ear. New bone formation may extend along the implantation track to a variable length and sometimes may extend apical to the end of the implant array. For examples of delayed effects induced by cochlear implantation, see figures 13, 14.

Effect on Spiral Ganglion Cells

In a study of 11 cochleae from patients who in life had undergone cochlear implantation and in whom the contralateral, nonimplanted ear was available, the mean spiral ganglion cell count for the implanted and nonimplanted ears were not significantly different in the most basal three of four segments of the

Cochlear Spiral

Fig. 13. Delayed effect of cochlear implantation in a 74-year-old man who underwent right cochlear implantation 12 years before death for rehabilitation of a progressive and profound sensorineural hearing loss. A dense fibrous sheath (FS) is present, not only within the cochlea (C), but also extends at least several millimeters into the middle ear (ME).

Fig. 13. Delayed effect of cochlear implantation in a 74-year-old man who underwent right cochlear implantation 12 years before death for rehabilitation of a progressive and profound sensorineural hearing loss. A dense fibrous sheath (FS) is present, not only within the cochlea (C), but also extends at least several millimeters into the middle ear (ME).

New Cochlea Implants

Fig. 14. Delayed effect of cochlear implantation in a 64-year-old woman who suffered a profound bilateral sensorineural hearing loss secondary to cochleosaccular degeneration. A CT scan of the inner ear done 10 years prior to death showed no abnormality of the cochlea. Three months later, she underwent a left cochlear implant with no insertional difficulties. In the right ear, there was evidence of cochlear saccular degeneration, but no evidence of new bone formation. However, in the left ear there was new bone (NB) formation surrounding the electrode array (EA) in the scala vestibuli and extending apical to it. Based on the otologic history, preoperative CT scan, and comparison with the opposite temporal bone, this new bone formation was judged to be a consequence of the cochlear implantation.

Fig. 14. Delayed effect of cochlear implantation in a 64-year-old woman who suffered a profound bilateral sensorineural hearing loss secondary to cochleosaccular degeneration. A CT scan of the inner ear done 10 years prior to death showed no abnormality of the cochlea. Three months later, she underwent a left cochlear implant with no insertional difficulties. In the right ear, there was evidence of cochlear saccular degeneration, but no evidence of new bone formation. However, in the left ear there was new bone (NB) formation surrounding the electrode array (EA) in the scala vestibuli and extending apical to it. Based on the otologic history, preoperative CT scan, and comparison with the opposite temporal bone, this new bone formation was judged to be a consequence of the cochlear implantation.

New Cochlea Implants
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