Histopathology of the Inner Ear Relevant to Cochlear Implantation

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Joseph B. Nadol Jr.a,b, Donald K. Eddingtona,c,d aDepartment of Otology and Laryngology, Harvard Medical School, bDepartment of Otolaryngology, cCochlear Implant Research Laboratory, Massachusetts Eye and Ear Infirmary, dResearch Laboratory of Electronics, Massachusetts Institute of Technology, Boston, Mass., USA

Abstract

The most common forms of severe hearing loss and deafness are related to morphological changes in the cochlea. Many individuals with such forms of hearing disorders have received cochlear implants. It has been assumed that preservation of spiral ganglion cells is important for success of cochlear implants. Preservation of ganglion cells is negatively correlated with the duration of the hearing loss. It has, however, not been possible to reveal a relationship between the degree of survival of spiral ganglion cells and performance of cochlear implants. It is important to understand the histopathological changes that follow cochlear implantation. Insertion of cochlear implants may cause trauma to the basilar membrane, the spiral lamina, and the spiral ligament. Rupture of the basilar membrane may occur. Over time, new bone forms at the cochleostomy and along the implant track. Further investigation is necessary to evaluate the causes of variability of behavioral measures of performance.

Copyright © 2006 S. Karger AG, Basel

The wide variability of success as measured by open set speech discrimination in individual patients who have undergone cochlear implantation has added renewed relevance to the histopathology of severe to profound deafness. In addition, as temporal bones from individuals who in life had undergone cochlear implantation become available, study of the pathologic changes induced by cochlear implantation and correlation of the success of implantation with histopathology have become possible [Roland and Wright, this vol, pp 11-30]. This paper reviews the pertinent histopathology of severe to profound deafness in the human and what has been learned from study of temporal bones from patients who underwent cochlear implantation during life.

Fig. 1. Cochlear hair cells from a 65-year-old man with cochleosaccular dysplasia and cataracts, a disorder inherited as an autosomal dominant trait, who suffered a progressive bilateral sensorineural hearing loss beginning at age 26. His hearing loss was severe to profound by age 58. The principal correlate of the sensorineural hearing loss was total loss of the organ of Corti (OC) throughout the cochlea. Although the neuronal population was reduced, there were spiral ganglion cells (SPG) throughout the cochlea. a Basal turn. b High power of the region normally occupied by the organ of Corti.

Fig. 1. Cochlear hair cells from a 65-year-old man with cochleosaccular dysplasia and cataracts, a disorder inherited as an autosomal dominant trait, who suffered a progressive bilateral sensorineural hearing loss beginning at age 26. His hearing loss was severe to profound by age 58. The principal correlate of the sensorineural hearing loss was total loss of the organ of Corti (OC) throughout the cochlea. Although the neuronal population was reduced, there were spiral ganglion cells (SPG) throughout the cochlea. a Basal turn. b High power of the region normally occupied by the organ of Corti.

Histopathology of Deafness in Humans

Although some cases of central deafness have been described [1] and evidence that cognitive defects play a role in hearing loss of the aged has accrued [2], the most important correlates of severe to profound deafness in the human are various forms of degeneration of the inner ear. Evidence for this includes histopathology of the ear [3, 4], which has been at least in part corroborated by modern imaging of the auditory cortex [5]. Although originally designed for the study of human presbycusis [6], the categorization of subtypes of hearing disorders by specific cell and tissue targets may be applied more generally to other etiologies of deafness [7, 8]. The principal peripheral cellular targets are auditory hair cells (fig. 1), the stria vascularis (fig. 2) and first order cochlear neurons (fig. 3). More recently, the possible role of dysfunction of the lateral cochlear wall in the causation of sensorineural loss has been identified [8-11].

Degeneration of Spiral Ganglion Cells and Their Processes

Although word recognition scores range widely in all published series of cochlear implantation [12, 13; see also Geers, this vol, pp 50-65], the histopathologic correlates of success are as yet not completely known. However, it is generally assumed that the residual spiral ganglion cell population in the deaf ear is a critical factor in determining the success of implantation [14, 15].

Fig. 2. Stria vascularis in a 94-year-old woman who suffered a bilateral progressive sensorineural hearing loss beginning at approximately 30 years of age. The family history suggested a genetic etiology. An audiogram at age 89 demonstrated bilateral 80-100 dB sensorineural loss. There was severe atrophy of the stria vascularis (SV) in all turns, partial loss of outer hair cells, and moderate loss of cochlear neurons in the basal turn. The principal histopathologic correlate of the flat sensorineural hearing loss audiometric pattern was severe atrophy of the stria vascularis.

Fig. 2. Stria vascularis in a 94-year-old woman who suffered a bilateral progressive sensorineural hearing loss beginning at approximately 30 years of age. The family history suggested a genetic etiology. An audiogram at age 89 demonstrated bilateral 80-100 dB sensorineural loss. There was severe atrophy of the stria vascularis (SV) in all turns, partial loss of outer hair cells, and moderate loss of cochlear neurons in the basal turn. The principal histopathologic correlate of the flat sensorineural hearing loss audiometric pattern was severe atrophy of the stria vascularis.

Fig. 3. Cochlear degeneration in a 41-year-old woman who suffered a sudden sen-sorineural hearing loss in her right ear at the age of 18 years. Audiometry demonstrated a profound loss in the right ear. Throughout the cochlea there was a total loss of spiral ganglion cells in Rosenthal's canal (RC), whereas the organ of Corti (OC) and stria vascularis (SV) were normal.

Fig. 3. Cochlear degeneration in a 41-year-old woman who suffered a sudden sen-sorineural hearing loss in her right ear at the age of 18 years. Audiometry demonstrated a profound loss in the right ear. Throughout the cochlea there was a total loss of spiral ganglion cells in Rosenthal's canal (RC), whereas the organ of Corti (OC) and stria vascularis (SV) were normal.

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Sudden idiopathic SNHL (n = 6) Aminoglycoside ototoxicity (n =8)

Neoplasm of temporal bone (n = 8) Bacterial labyrinthitis (n = 11) Congenital or genetic (n = 9) Postnatal viral labyrinthitis (n = 8)

Fig. 4. Means and standard deviations of spiral ganglion cell counts in six most common diagnostic categories of etiologies of hearing loss and in 5 individuals with normal hearing. Reprinted with permission from Nadol et al. [4].

Both primary and secondary degeneration of the spiral ganglion cells occur and are more common in the basal as compared to the apical half of the cochlea [16]. Although all the factors that determine degree and speed of secondary degeneration of spiral ganglion cells in the human are unknown, it is most severe when both inner hair cells [17] and outer hair cells are missing [16], and when there has been degeneration of cochlear supporting [18], loss of pillar and Deiter cells [19] or injury of the peripheral terminal processes of cochlear neurons [20]. Primary degeneration of the spiral ganglion cell and its processes may occur with no obvious neuroepithelial changes in such disorders as presby-cusis [6], sudden deafness [21], Friedrick's ataxia [22], Usher's syndrome [23] and some genetically determined disorders [24] such as in the deaf white cat.

In 93 temporal bones from 66 patients who during life had a documented profound sensorineural hearing loss [4], the duration of hearing loss and duration of profound deafness were found to be negatively correlated with residual spiral ganglion cell count. The main determinant of the total spiral ganglion cell count in this study was the cause of deafness (fig. 4). These findings are consistent with previous studies [25, 26], although other authors have found no correlation between residual spiral ganglion cell count and cause of deafness [3]. Although there is significant intersubject variation of spiral ganglion cell counts within diagnostic categories, it is rare for there to be total spiral ganglion counts less than 10,000 [3, 4].

Because of the variability of spiral ganglion cell counts determined by diagnosis, age, and duration of deafness, it is attractive to search for clinical markers such as the diameter of the eighth nerve on MRI imaging that might correlate with residual spiral ganglion cell count. In a temporal bone study of 42 patients who in life were profoundly deaf and would have been candidates for cochlear

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