Post Hybrid Cochlear Implant Hearing Loss and Endolymphatic Hydrops

Abstract

Objective: To evaluate for potential causes of delayed loss of residual hearing that variably occurs with hybrid cochlear implants.

Study design: Histopathological evaluation of 29 human temporal bone (HTB) with cochlear implant (CI).

Setting: The Neurotology and House HTB Laboratory of UCLA (House-UCLA).

Subjects and methods: HTB from CI patients from the House-UCLA HTB Laboratory (n = 28) and one courtesy of Massachusetts Eye and Ear Infirmary (MEEI). Histopathological analysis to identify the location of cochleostomy, fibrosis, and bone formation in the scala vestibuli and tympani, and endolymphatic hydrops. Spiral ganglion neuron counts were obtained. Statistical analysis compared presence of cochleostomy and location with the histopathological findings.

Results: Seventeen of 29 bones with fibrosis in the scala vestibule (SV) and tympani had evidence of a cochleostomy involving the SV containing the ductus reunions, all of which had hydrops. Ten of 11 bones had no SV fibrosis, and a cochleostomy limited to the scala tympani, of which all had no hydrops. One HTB had moderate SV fibrosis not involving the ductus reuniens, and was without hydrops. One HTB had a SV cochleostomy but the electrode ruptured Reissner's membrane, and was without hydrops. Cochleostomy was significantly associated with SV fibrosis and hydrops (p < 0.01), those without hydrops had no SV atrophy (p < 0.01). Round window insertion was associated with no fibrosis and no hydrops.

Conclusion: We hypothesize that cochleostomies involving scala vestibuli incite fibrosis, compromising the ductus reuniens, causing hydrops which may cause the delayed loss of residual low frequency hearing in CI.

Figure 1

Cochleostomy into the scala vestibuli. The electrode path is surrounded by fibrosis and new bone. There is hydrops in the upper segments of the cochlea (Hematoxylin and Eosin (H&E) × 100)

Figure 2

Cochleostomy into the scala vestibuli. (Fig. 2). There is fibrosis in the scala vestibuli and shows endolymphatic hydrops in all of the turns of the cochlea. (Hematoxylin and Eosin (H&E) × 100)

Figure 3

Cochleostomy into scala tympani. There is extensive fibrosis and new bone in the scala tympani but none in the scala vestibuli; and no hydrops. (H&E × 100)

Figure 4

Round window insertion after removal of the operculum. There is some debris in the scala tympani but no fibrosis. (H&E × 100)

Figure 5

Bone from MEEI illustrating fibrosis and hydrops in the scala vestibuli. The electrode path is in the scala tympani but ivolved the spiral ligament causing fibrosis in the scala vestibuli. (H&E × 20)