RW Cochleostomy: A Cause of Progressive Hearing Loss?
 

Introduction

Up to one-third of recipients who retain residual hearing after CI have progressive low-frequency loss in the weeks or months after surgery (1,2).

One common theory is that following CI electrode insertion, intra-scalar histiocytic and giant-cell infiltration (foreign body reaction), fibrosis and osteoneogenesis lead to the progressive loss (3,4). These inflammatory reactions may destroy residual neural elements or interfere with the fluid-pressure wave as well as basilar membrane vibration. But are intra-cochlear factors the only causes of progressive loss and does the location of the cochleostomy make a difference?

New Information

Stephen O’Leary’s lab at the University of Melbourne recently compared histological and ABR effects of RWM and bony cochleostomy in guinea pigs. The authors also observed the effects of electrode position in Scala tympani on progressive loss (5). Some of their major findings relevant to this post:

1) Bony cochleostomy did not cause progressive low-frequency hearing loss.

2) RWM incision and muscle seal (with or without electrode insertion) resulted in progressive 2 kHz hearing loss (p < 0.01). Mean threshold ABR shifts (from pre-op) at 2kHz were:

  • 1 week 6.3 (+/- 3.7) dB

  • 6 weeks 22.9 (+/- 3.4) dB

3) Intra-scalar electrode position, tissue damage and inflammatory response did not appear to be associated with progressive hearing loss.

Take Home

The cause of progressive, post-CI hearing loss may be fibrosis in the RW niche associated with the muscular tissue seal. This is consistent with a previous theory (6,7) that fibrosis can reduce compliance of the RWM leading to diminished amplitude of the travelling wave, mostly in the apex (low frequency place).

If these results can be replicated and generalized, they call for further study of our techniques of sealing the RWM after electrode insertion. Are mass and/or stiffness of the RWM actually increased by incision and tissue seal; does this result in reduced amplitude of the travelling wave; how can we minimize the mass and stiffness of the RW without compromising the seal (fascia, areolar tissue, growth factors)?

References

 

1. Woodson EA, Reiss LAJ, Turner CW, Gfeller K, Gantz BJ, 2010. The hybrid cochlear implant: a review. Adv. Otorhinolaryngol. 67, 125-134.

2. Gstoettner W, Helbig S, Maier N, Kiefer J, Radeloff A, Adunka O. Ipsilateral electric acoustic stimulation: results of long-term hearing preservation. Audiol. Neurotol. 11(1), 49 – 56.

3. Nadol, J.B., Eddington, D.K., Burgess, B.J., 2008. Foreign body or hypersensitivity granuloma of the inner ear after cochlear implantation: one possible cause of a soft failure? Otol. Neurotol. 29 (8), 1076-1084.,

4. Seyyedi, M., Nadol Jr JB, 2014. Intracochlear inflammatory response to cochlear implant electrodes in humans. Otol. Neurotol. 35 (9), 1545-1551.

5. Rowe D, Chambers S, Hampson A, Eastwood H, Campbell L, O’Leary S. Delayed low frequency hearing loss caused by cochlear implantation interventions via the round window but not cochleostomy. Hear Res 2015 333:49-57.

6. Choi C., Oghalai J. 2005. Predicting the effect of post-implant cochlear fibrosis on residual hearing. Hear. Res.; 205: 193-200.

7. Richards SH, 1981. Congenital absence of the round window treated by cochlear fenestration. Clin. Otolaryngol. 6 (4), 265-269.

 
Spiral Ganglion Cell Survival and CI Performance
 

Background

The key neural elements that are stimulated by cochlear implants (CI) are spiral ganglion cells (SGC). So it would seem logical that the more SGCs that survive, the better CI performance would be. Nonetheless, histopathologic studies have suggested that SGC survival rates do not correlate with CI performance. (1,2)

However, prior temporal bone studies could not control for variables that might affect performance (age, cause of deafness, degree of hearing loss, duration of deafness, cognitive ability, etc.) Failure to control these variables cast some doubt on the validity of the findings above.

In short, the best data available over the past 25 years indicated that SGC survival did not correlate with CI performance, but those studies created an intuitive dissonance. In order to settle the issue, the critical variables would need to be controlled.

New Information

Joe Nadol’s group at Harvard re-investigated this issue by studying temporal bones from bilateral implantees. Seyyedi, Eddington and Nadol (3) first demonstrated that individual temporal bone donors who were deafened bilaterally by the same etiology and had similar hearing loss in each ear, had similar numbers of surviving SGCs.

Based on this information, Seyyedi, Viana and Nadol (4) studied 12 temporal bones from six ‘bilateral’ subjects with both ears deafened by the same etiology. Age and cognitive function were also controlled by their study design that compared right vs. left ears of individual subjects.

The authors found that word recognition scores were directly correlated with SGC counts ((R = 0.934, p = 0.006). There was no significant correlation between CI performance and depth of electrode insertion, duration of CI usage, or age at implantation in this sample.

Take Home

These findings suggest that greater SGC survival may improve CI performance. If that is the case, several corollaries may exist:

  • Hearing-preservation surgical techniques and electrodes may be more important than previously thought, even in profoundly deaf patients.

  • The neurotrophic effects of electrical stimulation that supports SGC survival may be another reason for early implantation.

  • Pharmaceutical support of SGCs (e.g.: neurotrophins) may play an important role in improving CI performance.

  • Efforts to improve low-trauma electrode arrays should continue.

  • Large cooperative clinical studies comparing performance among categorical etiologies may provide further insight into determinants of outcome.

References

1. Nadol JB Jr, Young YS, Glynn RJ. Survival of spiral ganglion cells in profound sensorineural hearing loss: implications for cochlear implantation. Ann Otol Rhinol Laryngol 1989;98:411-16.

2. Fayad JN, Linthicum FH Jr. Multichannel cochlear implants: relation of histopathology to performance. Laryngoscope 2006;116:1310-20.

3. Seyyedi M, Eddington DK, Nadol JB Jr. Interaural comparison of spiral ganglion cell counts in profound deafness. Hear Res 2011;282:56-62.

4. Seyyedi M, Viana LM, Nadol JB Jr. Within-Subject Comparison of Word Recognition and Spiral Ganglion Cell Count in Bilateral Cochlear Implant Recipients. Otology & Neurotology 2014;35:1446-50.

 
Long Cochlear Implant Electrodes: Incomplete insertion, loss of residual hearing and balance
 

October 16, 2015

Background

Preservation of residual hearing has been a goal of CI surgery since it was first reported in 1989 (1). It is an indication of good surgical technique and may result in enhanced speech perception in conventional (2) and hybrid devices (3). Very-long electrodes (> 28 mm) have been used successfully for over two decades, but continue to be debated due to anecdotal inferences of loss of residual hearing and vestibular function, as well as a high incidence of insertion failure.

New Information

A recent study by Nordfalk et al (4) of Oslo University provides new data to address these issues.

Loss of Residual Hearing

  • In this study, 35 adult subjects received very-long electrodes (31.5 mm and 28 mm.) In addition, four received more typical, 24 mm electrodes of similar design.

  • A statistically significant relationship between loss of residual hearing and very-long electrodes occurred at 250 Hz (p < .05) but not for the low frequency pure tone average (lf-PTA.) A possible ceiling effect is difficult to avoid due to greater pre-op hearing loss at 500 and 1000 Hz.

    Loss of Vestibular Function

  Electrode Length 24 mm 28 mm 31.5 mm  

Vestibular Symptoms   0% 6% 12%

Lost VEMP    0% 36% 64%

The table suggests that very-long electrodes are associated with subjective and objective indications of vestibular system damage proportional to their length. No vestibular symptoms or VEMP abnormalities were seen with the standard-length electrodes

Failure to completely insert long electrodes

Nearly 1/5 (18%) of very-long electrodes could not be fully inserted by this group of experienced surgeons. These findings confirm anecdotal reports and clinical experience of a high rate of full-insertion failure with very-long electrodes. Insertion failure rates for standard-length electrodes is significantly lower, in the range of 3 to 4% (5).

 Take Home

Nordfalk and colleagues at the University of Oslo have shown that very-long CI electrodes (> 28 mm) are associated with a higher incidence of cochlear and vestibular dysfunction when compared to standard-length electrodes (< 24 mm). The authors also demonstrated that failure to completely insert this type of electrode occurred in 18% of recipients, even in highly competent and experienced hands.

 

References

1. Boggess WJ, Baker JE, Balkany TJ. Loss of residual hearing after cochlear implantation. Laryngoscope. 1989; 99:1002-5

2. Sheffield SW, Jahn K, Gifford RH. Preserved acoustic hearing in cochlear implantation improves speech perception. J Am Acad Audiol. 2015 Feb;26 (2):145-54.

3. Roland JT Jr, Gantz BJ, Waltzman SB, et al. United States multicenter clinical trial of the cochlear nucleus hybrid implant system. Laryngoscope. 2015 [Epub ahead of print]

4. Nordfalk K F, Rasmussen KH, Bunne, M et al. Insertion Depth in Cochlear Implantation and Outcome in Residual Hearing and Vestibular Function. Ear and Hear 2015. Epub ahead of Print.

5. Brito R, Alves T…Bento RF. Surgical complications in 550 consecutive cochlear implantations. Braz. J. Otorhinolaryngol. 78; 3: May/June 2012.


 
Neural Plasticity and Age of Implantation
 

Background

In early stages of development, peripheral stimulation helps organize the sensory cortex, a process commonly referred to as neural plasticity. Neural plasticity may be best thought of as continuous reorganization throughout early life in which neural pathways are formed, but abandoned if not frequently used, in favor of other more active pathways.

In the auditory and visual systems, sensory deprivation during CNS development causes deficient organization, or detrimental reorganization of the cortex that may lead to permanent deficits (1). For example, in amblyopia (“lazy eye”), affecting about 2% of children, one eye is less functional than the other. If untreated, the visual cortex will “ignore” the less-coherent signals from the lazy eye and its neural pathways will degenerate, leading to blindness of that eye.

The central auditory system likewise undergoes sensory-deprivation-based detrimental reorganization in the event of peripheral deafness. In the absence of peripheral stimuli, the auditory cortex can be subsumed by the more active processes of the visual system, especially in signers. This cross-modal adaptation has been associated with poor outcomes when cochlear implantation is delayed.2 Although electrical stimulation with cochlear implants can establish central auditory pathways and diminish takeover by the visual system early in development, after 5 years of age, restitution of the auditory cortex may be minimal (3,4).

Clinical experience has demonstrated the value of cochlear implantation prior to the age of 18 months and many feel that the ideal age is less than 12 months. Controlled studies confirm the time-sensitive nature of successful implantation. Based on ABR findings, Sharma, Dorman and Spahr demonstrated that progressively worse CI outcomes occur in prelinguistically deaf children who had delayed implantation.

 

  • Ninety-six percent (96%) of early implanted (age less than 3.5 years) children had normal P1 latencies. Only 5% of late implanted children (age greater than 7 years) had normal P1 latencies (5).

What’s (kind of) New

Normal development of the central auditory system may be thought of in three stages: 1. Establishment of connectivity; 2. Pruning of underused synapses; and 3. Consolidation. During the establishment phase, repeated stimulation of adjacent neurons forms a persistent pathway. First described by Hebb in 1949 (6) (clearly not new), Hebb’s postulate explains that when an axon of one cell stimulates another cell persistently, an efficient pathway is formed. Otherwise stated, cells that fire together, wire together. Of course, other forces are also active in establishment, for example growth of axons from the mid-brain (thalamus) to the primary auditory cortex (A1) is targeted to specific cells of the cortex by chemotrophic guidance (chemical attraction).

Lack of stimulation during sensitive periods can result in “negative plasticity”, a term sometimes used to describe lack of Stage 1. stimulus-driven establishment of connectivity, as occurs in sensory deprivation.

Strides have been made in understanding cellular level mechanisms of neural plasticity, but much remains unclear. Short-term potentiation of auditory (and other) synapses is pre-synaptic and results from repeated-use-based calcium ion accumulation that, on a temporary basis, increases release of neurotransmitter. Long-term hebbian potentiation (LTP) is another story, lasting for weeks to months. It involves the post-synaptic glutamine receptor NMDA. Many other receptors are now being recognized for LTP and its opposite, long-term depression of underused synapses. Consolidation, on the other hand, is influenced by an acetylcholine-based neurotransmitter system.

Establishment stage properties of electrical stimulation (electrical neurotrophism) through a cochlear implant have been demonstrated in development of the brainstem, midbrain and cortex. This is especially significant in the cortex where electrical stimulation promotes normal appearing, layer-specific function of A1 in kittens but not in mature cats. This is consistent with findings of CI electrical neurotrophism in primates dating back to the 1983 (7).

 

Take Home

Neural plasticity is a process of peripheral stimulation that guides structural and functional development, and modification, of the central auditory system. Auditory deprivation during early life results in failure to establish neural pathways that are necessary for cochlear implants to provide excellent outcomes (8). In the absence of early implantation, primary and secondary auditory cortices are “taken over” by the visual system. However, early implantation provides electrical neurotrophism sufficient to establish functional central auditory pathways and preclude cross-modal takeover by elements of the visual cortices.

 

References

1. Kaas J., editor. (ed.). (2001). Mutable Brain: Dynamic and Plastic Features of the Developing and Mature Brain. Amsterdam: Harwood Academic Publishers.

2. Sandmann P., Eichele T., … Dillier N., et al. . (2009). Evaluation of evoked potentials to dyadic tones after cochlear implantation. Brain 132, 1967–1979.

3. Kral A., Sharma A. (2012). Developmental neuroplasticity after cochlear implantation. Trends Neurosci. 35, 111–122.

4. Dorman M. F., Sharma A., … Roland P. (2007). Central auditory development: evidence from CAEP measurements in children fit with cochlear implants. J. Commun. Disord. 40, 284–294.

5. Sharma A., Dorman M. F., Spahr A. J. (2002). A sensitive period for the development of the central auditory system in children with cochlear implants: implications for age of implantation. Ear Hear. 23, 532–539.

6. Hebb, D.O. (1949). The Organization of Behavior. New York: Wiley & Sons.

7. Larsen SA, Asher DL, Balkany TJ, Rucker N. Histopathology of the auditory nerve and cochlear nucleus following intracochlear electrical stimulation. Otolaryngol Clin N Am 1983;16:233-248.

8. Gama NM, Lehmann A. Compensatory plasticity: time matters. Front Neurosci 2015, 9:348.

 
Effects of CI Electrode Insertion on Tinnitus
 

Thomas J. Balkany, MD

10.1.15

 

Introduction

Over the past three decades, several papers have demonstrated positive as well as negative effects of cochlear implantation (CI) on tinnitus (1-6).

Theoretical mechanisms of CI effects on tinnitus include electrical stimulation and electrode insertion trauma (EIT). Electrical effects may reduce tinnitus by masking (creating an auditory percept that makes tinnitus inaudible), electrical suppression (directly altering the generation or neural transmission of the tinnitus signal) or by other mechanisms. EIT may cause increasing tinnitus due to neural or metabolic organelle damage that may cause abnormal signal generation. The following discussion addresses a series of patients in whom traumatic, scala transgressing insertion exacerbated tinnitus in comparison to another cohort in whom non-scala transgressing electrodes did not.

·Pre-operative tinnitus in CI candidates has been estimated at 65 – 100%. (1, 2)

·The rate of tinnitus improvement following CI ranges from 50 – 90%. (4,5)

·Tinnitus may be generated or made worse by CI in 0 – 28% of recipients. (5, 6)

New Information

In an important paper by Ingo Todt and colleagues at Unfallkrankenhaus (UFK) in Berlin (7), post-op flat panel CT was used to determine whether or not CI electrodes ruptured inter-scalar partitions and traversed between S. tympani and S. vestibuli. Tinnitus was analyzed by standard metrics: a validated analog loudness scale and a questionnaire. Below is a summary of one of the significant outcomes. In short:

Tinnitus Worse after CI

Electrode traversed scala (n = 19): 16%

Electrode did not traverse (n = 36): 0

 

Take Home

Good surgical technique may reduce post-CI tinnitus. In this study, only when electrodes penetrated from one scala to another (equivalent to Eshraghi-Balkany Grade IV/IV trauma (8)), was tinnitus generated or made worse.

Surgical techniques that prevent or reduce inter-scalar transgression may be expected to reduce post-implant tinnitus. These are the same techniques that are used to preserve residual hearing: adaptive cochleostomy (adapting the cochleostomy to the patient’s cochlear anatomy and the configuration of the electrode to be implanted) and meticulous attention to the electrode insertion trajectory (to avoid deflection of the electrode tip into S. vestibuli.)

References

·When CI electrodes ruptured intrascalar partitions and traversed between the scala, tinnitus had a 16% chance of being generated or becoming worse.

·When electrodes did not traverse scala, tinnitus was not made worse.

(Please see the original paper for a breakdown of all groups and all data. There is interesting information about which electrodes tend to traverse scala and tinnitus suppression differences between peri-modiolar and anti-modiolar electrodes.)

1. Balkany TJ, Bantli H, Vernon J, Douek E. Direct electrical stimulation of the inner ear for the relief of tinnitus. Am J Otol. (now Otol and Neurotol) 1987;207-212.

2. McKerrow WS, Schreiner CE, Snyder RL, Merzenich MM, Toner JG. Tinnitus suppression by cochlear implants. Ann Otol Rhinol Laryngol 1991;100:552–8.

3. Tyler RS, Rubinstein PJ, Pan T, Chang S-A, Gogel SA, Gehringer A, et al. Electrical stimulation of the cochlea to reduce tinnitus. Semin Hear 2008;29:326–32.

4. Mo B, Harris S, Lindbaek M. Tinnitus in cochlear implant patients–a comparison with other hearing-impaired patients. Int J Audiol 2002;41:527–34.

5. Ito J. Tinnitus suppression in cochlear implant patients. Otolaryngol Head Neck Surg 1997;117:701–3.

6. Kompis M, Pelizzone M, Dillier N, Allum J, DeMin N, Senn P. Tinnitus before and 6 months after cochlear implantation. Audiol Neurootol 2012;17:161–8.

7. Ingo Todt, Grit Rademacherb, Sven Mutzeb, Ravi Ramalingam, Selene Wolter, Philipp Mittmann, Jan Wagner & Arne Ernst. Relationship between intracochlear electrode position and tinnitus in cochlear implantees. Acta Oto-Laryngologica 2015;8.

8. Eshraghi AA1, Yang NW, Balkany TJ. Comparative study of cochlear damage with three perimodiolar electrode designs. Laryngoscope. 2003 Mar;113(3):415-9.

 
High Rate of Migration/Partial Extrusion of Straight Cochlear Implant Electrodes
 

Thomas J. Balkany, MD

9.1.15

 

Background

Cochlear implant (CI) electrode migration is generally considered to be very uncommon, the subject of few clinical reports and often not considered when discussing CI complications with patients. However, partial electrode extrusion from the cochlea, enough to reduce function and cause aversive stimuli, may not be so unusual.

Rivas et al.(1) described electrode migration as an important cause of cochlear reimplantation, second only to device failure at Hopkins. And Connell et al. (2) reported a ten year period during which the United States Food and Drug Administration MAUDE database (3) showed 151 reported instances of electrode extrusion (presumably underestimated due to the voluntary nature of reporting and the tendency to report only major extrusions).

Electrode extrusions are thought to have been more common in the generation of stiff, straight electrodes some two decades ago. To reduce the rate of electrode migration at that time, Cohen and Kuzma introduced a titanium clip that attached the electrode cable to the incus bridge (buttress) (4) and Balkany and Telischi described the split bridge technique (a slot in the incus bridge) to fixate the cable in the same location (5). Both were effective in reducing electrode extrusion (6). However, the advent of pre-curved electrodes and more flexible cables reduced the tendency for electrode migration.

New Information


With the current predominance of thin straight electrodes, which have reduced frictional forces to resist extrusion, it is time to reconsider the frequency of CI electrode migration/partial extrusion. Recently, Dietz et al. (7) from the Kuopio University Hospital, the University of Eastern Finland, and Helsinki University Central Hospital reported on electrode migration with straight flexible electrodes. In that study, prospectively collected data on 201 implantations performed between 2002 and 2014 were analyzed. Patients with less than full insertion were excluded. Eighteen devices were noted to have large, progressive increases in impedance values (>75%) or aversive stimuli from the basal electrodes. Cone-beam CT demonstrated extrusion of basal electrodes in 12 of 201 subjects (6%). (This total includes both straight and pre-curved.)

Sixty-three ears were implanted with the Cochlear Corporation CI422 (Slim Straight electrode) and 64 with the Contour Advance (pre-curved electrode array). Seventy-four ears were implanted with variable lengths of the Med El electrode.

Five of 63 (8%) of Cochlear Corporation CI422 (Slim Straight) electrodes showed partial migration/extrusion compared to 0 of 64 (0%) of Contour Advance (pre-curved) electrodes.

Seven of the 74 (9%) Med El electrodes partially extruded as well.

Take Home

This paper demonstrates that straight flexible electrodes have a high rate of partial extrusion (12 of 137 = 9%). This figure includes all straight electrodes. A contemporaneous cohort of 64 pre-curved electrodes, inserted by the same surgeons, can be seen as a comparison group. In the pre-curved electrode group, there were no extrusions.

It is hoped that this research can be replicated soon to determine generalizability. Nonetheless, based on this paper, surgical precautions to prevent electrode extrusion from the cochlea should be reconsidered.

The authors used tight fascia packing of the cochleostomy sealed with fibrin glue in the middle ear and bone paté and fibrin glue in the facial recess and mastoid cavity. These were not effective.

Methods used prior to the advent of pre-curved electrodes may once again be considered: the split-bridge technique, titanium clip, coiling a loop of electrode cable against the mastoid tegmen, focus on mastoid overhangs. Newer techniques developed for insertion of hybrid electrodes, such as passing the electrode through a loop of suture in the tegmen that is snugged after insertion (suggested by Bruce Gantz) or manufacturing a tab on the cable near the electrode array to lock into the facial nerve-chorda tympani angle (suggested by Thomas Lenarz), might also be considered.

The Dietz study contains clinically important information regarding a high rate of partial electrode extrusion with straight, flexible electrodes. Although not yet replicated, these findings should be disseminated to cochlear implant surgeons.

References

1. Rivas A, Marlowe AL, Chinnici JE, Niparko JK, Francis HW. Revision cochlear implantation surgery in adults: indications and results. Otol Neurotol. 2008 Aug;29(5):639-48.

2. Connell SS1, Balkany TJ, Hodges AV, Telischi FF, Angeli SI, Eshraghi AA. Electrode migration after cochlear implantation. Otol Neurotol. 2008 Feb;29(2):156-9.

3. US Food and Drug Administration. Manufacturer and User Facility Device Experience (MAUDE) Database Search. Available at: http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfMAUDE/search.cfm. Accessed March 1, 2007.

4. Cohen NL, Kuzma J. Titanium clip for cochlear implant electrode fixation. Ann Otol Rhinol Laryngol Suppl 1995;104:402-3.

5. Balkany T, Telischi FF. Fixation of the electrode cable during cochlear implantation: the split bridge technique. Laryngoscope 1995;105:217-8.

6. Roland JT Jr, Fishman AJ, Waltzman SB, et al. Stability of the cochlear implant array in children. Laryngoscope 1998;108:1119-23.

7. Dietz A , Wennström M, Lehtimäki A, Löppönen H, Valtonen H. Electrode migration after cochlear implant surgery: more common than expected? European Archives of Oto-Rhino-Laryngology and Head & Neck 2015:3716. Epub before print: DOI: 10.1007/s00405-015-3716-4

 
Adaptive Cochleostomy
 

Background

Adaptive cochleostomy is the concept that a single type of cochleostomy is not ideal in all cases. Rather, the type of cochleostomy used should be selected based on the anatomy of the patient and the physical characteristics of the electrode to be inserted. In short, the cochleostomy should be adapted to the patient’s anatomy and the electrode used. Cochleostomy is used here to mean an enduring opening into the cochlea through which an electrode is inserted.

Advocates of round window membrane (RWM), extended RWM and bony cochleostomy approaches have claimed that their preferred technique is best. Support for each is provided in the literature (1-4) RWM insertion may minimize drill trauma, prevent bone dust and blood from entering the scala and reduce leakage of perilymph; extended RWM cochleostomy may provide a more favorable insertion trajectory when the RWM is angled too inferiorly; and bony cochleostomy allows better electrode alignment with the axis of the scalar lumen, especially important when inserting larger, stiffer or pre-curved electrodes (1). However, the one-size-fits-all arguments do not take into account common variations in patient anatomy and electrode configuration.

We initially presented the concept of Adaptive Cochleostomy at CI2012 in Baltimore (5), proposing that no single method of cochleostomy was ideal in all cases. Shapira et al. (6) had already demonstrated a normal variation in angulation of the RWM of 27 – 65°. RWM insertion in the case of angulation >45° (13% of patients) can result in modiolar trauma, insertion into the vestibule and electrode transposition into S. vestibuli. In such cases, bony cochleostomy is preferred.

New Information

Recently Sun and colleagues3 from Hualien and Taichung, Taiwan and Hassepass and colleagues (7) from Freiburg compared hearing preservation following implantation via RWM vs. bony cochleostomies.

Sun et al. prospectively studied 20 recipients of a straight, flexible electrode inserted via RWM cochleostomy with 20 age- and sex-matched controls implanted through bony cochleostomy. There was no difference in low frequency hearing preservation between the two groups.

The Freiburg group used a different straight, flexible electrode and selected the cochleostomy site based on this concept of Adaptive Cochleostomy. In their retrospective study of 41 subjects, there were no substantial differences in insertion depth/angle or low frequency hearing preservation between the two groups.

A previous blog addressed the work of Wanna et al. (4) at Vanderbilt who demonstrated equally good results using an extended RWM approach.

Radiographic prediction of RW niche visibility was studied by Kashio and colleagues (8) at the University of Tokyo using standard HRCT with 1 mm slice thickness. The authors were able to categorize RW niche imaging findings into 1. invisible or nearly invisible; 2. partially visible; and 3. fully visible. It is yet to be seen if RWN visibility is related to RWM angulation.

Take Home

Thin, straight, flexible electrodes are ideal for RWM insertion but should be considered for extended RWM or bony cochleostomy insertion when angulation of the RWM is greater than 45° (the RWM diameter appears to the surgeon to be less than that of a 1 mm diamond burr held in the same field of view). Thicker, stiffer or pre-curved electrodes may be best inserted by bony cochleostomy to take advantage of coaxial scalar placement.

References

1. Addams-Williams J, Munaweera L, Coleman B, Shepherd R, Backhouse S. Cochlear implant electrode insertion: in defence of cochleostomy and factors against the round window membrane approach. Cochlear Implants Int. 2011;12 Suppl 2:S36-9.

2. Adunka et al. Cochleostomy Versus Round Window Insertions…. Otology & Neurotology 2014; 35: 613–618

3. Sun CH, Hsu CJ, Chen PR, Wu HP. Residual hearing preservation after cochlear implantation via round window or cochleostomy approach. Laryngoscope 2015; doi: 10.1002/lary.25122. (Epub ahead of print).

4. Wanna GB, Noble JH, Carlson ML, et al. Impact of electrode design and surgical approach on scalar location and cochlear implant outcomes. Laryngoscope 2014. 124: Sup 6; S1-S7.

5. Balkany TJ. Adaptive Cochleostomy. CI2012, Twelfth International Congress on Cochlear Implants and Other Implantable Auditory Technologies. Baltimore, May 2 – 12, 2012.

6. Shapira Y, Eshraghi A, Balkany TJ. The perceived angle of the round window affects electrode insertion trauma in round window insertion – an anatomical study. Acta Oto-Laryngologica 2011;131:284-289.

7. Hassepass F, Aschendorff A, Bulla S, Arndt S, Maier W, Laszig R, Beck R. Otol Neurotol. 2015;36(6):993-1000.

8. Kashio, Akinori; Sakamoto, Takashi; Karino, Shotaro; Kakigi, Akinobu; Iwasaki, Shinichi; Yamasoba, Tatsuya. Predicting Round Window Niche Visibility via the Facial Recess Using High-Resolution Computed Tomography. Otology & Neurotology: 2015 - Volume 36 - Issue 1.

 
Hyaluronic Acid and Hearing Preservation
 

Background

Preserving hearing after CI is a notable accomplishment. Although current studies suggest that CI outcomes may not be improved by hearing preservation (unless there is aidable hearing, especially speech recognition), there is evidence that the electrical dynamic range is larger when some hearing is preserved, which could be advantageous (1). Current methods of preserving residual hearing after CI include surgical technique, electrode design and the use of pharmaceutical agents.

New Information

Ramos and colleagues (2) of the University of Sᾶo Paulo recently described beneficial effects of the topical pharmaceutical hyaluronic acid in preserving low frequency residual hearing. The study was prospective, randomized, and controlled. Eighteen adult subjects (not electroacoustic candidates) were implanted with a Hybrid L24 electrode by a single surgeon. Preoperatively, all subjects received intravenous cefazolin and hydrocortisone (4 mg/kg). Group 1 (n=6) received no other intervention; Group 2 (n=6) also received intra-tympanic dexamethasone; and Group 3 (n=6) also received intra-tympanic dexamethasone plus hyaluronic acid application to the surface of the incised RWM.

Pre-op low frequency PTA (lfPTA; average threshold of 125, 250, 500 Hz) was compared to post-op lfPTA. Both Groups 1 and 2 lost approximately 30 dB. But Group 3, receiving hyaluronic acid, lost only 7 dB (p=0.002). The overall preservation of some residual hearing in the low frequencies was 88%. Also notable, intra-tympanic dexamethasone alone provided no additional benefit over pre-op intravenous steroids.

Take Home

Hyaluronic acid just prior to electrode insertion appears to help preserve residual hearing. Hypothetical mechanisms may include reducing the escape of perilymph, ingress of blood or bone dust and frictional forces of insertion.

1. D’Elia A, Bartoli R, Giagnotti F, Quaranta N. The role of hearing preservation on electrical thresholds and speech performances in cochlear implantation. Otol Neurotol 2012;33:343–7.

2. Ramos BF, Tsuji RK, Bento RF…Brito R.Hearing preservation using topical dexamethasone alone and associated with hyaluronic acid in cochlear implantation. Acta Otolaryngol. 2015 May;135(5):473-7.

 
Inferior Cochlear Vein and Cochleostomy
 

Background

Cochleostomy is used here to mean an enduring opening into the cochlea through which a CI electrode is inserted. There are currently three leading categories of cochleostomy:

  • The original round window cochleostomy used with short, single channel implants by Wm. House and currently used with certain electrodes to conserve residual hearing,

  • Traditional bony cochleostomy, suggested by Graeme Clark to avoid misdirection of long multichannel electrodes by the crista fenestra,

  • Extended RW cochleostomy, consisting of extension of the RW by drilling labyrinthine bone antero-inferiorly in cases where the RWM is too small or angled too inferiorly to use effectively.

Helge Rask-Andersen and his colleagues at Uppsala University continue their remarkable work on the anatomy of the hook portion of the cochlea that is relevant to hearing conservation during cochlear implantation. In their detailed description of the hook region in 2014, they conclude that there may be anatomic reasons to prefer RW insertions in hearing conservation cases (1).

New Knowledge

In their most recent work, the Uppsala group focuses on the surgical anatomy of the inferior cochlear vein (ICV) using micro dissection of venous injection molded corrosion casts (2). The ICV is the major venous drainage from the scalae and modiolus of the cochlea and runs in a bony channel, the accessory canal of the cochlear aqueduct (CA). The scalar aperture is located 0.67 mm to 0.81 mm from the RWM in the floor of scala tympani and it terminates in the posterior fossa. The ICV is closer to the RWM than the CA and thus more vulnerable. The authors speculate that little attention is paid to this structure due to the “assumption that venous drainage occurs in analogy with the arterial supply.” However, as the major drainage of the modiolus, surgical damage to the ICV may result in loss of residual ganglion cells and residual hearing.

Gary Wright and Peter Roland previously described venules in the lateral wall and floor of scala tympani that converge on the ICV (3). They also suggested that drilling of a bony cochleostomy may impact the ICV causing loss of residual hearing. The Rask-Andersen lab suggests that risk to the ICV is minimal during RWM cochleostomy, small during standard cochleostomy but may be greater in RWM enlargement techniques that extend the cochleostomy inferiorly.

Some recent studies show no significant functional outcome difference between RWM and traditional bony cochleostomy (4,5). However a study by George Wanna and colleagues at Vanderbilt demonstrated that RWM and extended RW insertions were both associated with better speech reception scores than bony cochleostomy on CNC words (p<0.045) but not on AzBio sentences or HINT. This appeared to be due to a high incidence of bony cochleostomy insertions ending up in scala vestibuli (p<0.001)(6).

Take Home

The ICV is the major vein of the cochlea and is susceptible to injury during drilling of an extended RW cochleostomy. This poses a theoretical risk of damaging residual hearing during cochlear implantation. However, clinical outcome studies to date do not support the notion that extended round window cochleostomy results in poorer CI function or more loss of residual hearing. This may be due to the dozens of clinical variables, each of which may influence conservation of neural elements as well as function. Nonetheless, surgeons should be aware of this relevant anatomy and consider it in cochlear implantation.

References

1. Francesca Atturo, Maurizio Barbara, Helge Rask-Andersen. On the Anatomy of the 'Hook' Region of the Human Cochlea and How It Relates to Cochlear Implantation. Audiology and Neurotology 2014; 19(6): 378-385.

2. Guo R, Zhang H, Chen W, Zhu X, Liu W, Rask-Andersen H. The inferior cochlear vein: surgical aspects in cochlear implantation. Eur Arch Otorhinolaryngol 201; Feb 21.

3. Wright CG, Roland PS. Vascular trauma during cochlear implantation: a contributor to residual hearing loss? Otol Neurotol 2014; 34:402-407.

4. Sun CH, Hsu CJ, Chen PR, Wu HP. Residual hearing preservation after cochlear implantation via round window or cochleostomy approach. Laryngoscope 2015; doi: 10.1002/lary.25122. (Epub ahead of print).

5. Hassepass F, Aschendorff A, Bulla S, et al. Radiologic results and hearing preservation with a straight harrow electrode via round window versus cochleostomy approach at initial activation. Otol Neurotol 2015 (Epub ahead of print).

6. Wanna GB, Noble JH, Carlson ML, et al. Impact of electrode design and surgical approach on scalar location and cochlear implant outcomes. Laryngoscope 2014. 124: Sup 6; S1-S7.

 
The Impact of Maturational Changes in Cochlear Position on Cochlear Implantation
 

Background

Three dimensional CT imaging provides good data regarding the positional changes in normal temporal bone development. Attention has been focused on the position of the basal turn in relation to other structures of the temporal bone. The following studies demonstrate that such changes occur rapidly between birth and 4 years of age and then may occur slowly until adulthood with a possible bump during puberty.

McRackan et al (2012) found age-related variation between adults and children in the orientation of the facial recess to the round window membrane (RWM) (1). Children had greater angulation than adults averaging 6.2°(p=0.01) between adults and children. Thus surgeons can expect a narrower view of the RWM in children and may find slightly more difficulty in electrode insertion. The authors also showed that maturation of the EAC over time simplifies the approach to the facial recess and RWM. The angle between the facial recess and EAC is more acute (tighter) in young children. This supports the findings of Lloyd et al (2010) who showed that the axis of the inferior segment of the basal turn becomes more parallel to the axis of the trans-mastoid approach angle over time (2).

New Knowledge

Jackson et al (2014) studied CTs of 713 children to show a significant angular change between the basal turn axis and the trans-mastoid/facial recess approach during the first 4 years of life (3). They found that during normal development, the mastoid—scalar angle becomes more obtuse (nearly straight), reducing insertion complexity. Another factor that can complicate implantation of young children is incomplete mastoid pneumatization. Jackson et al also note that the small mastoid cavities of young children also add complexity.

Hui-Ying et al (2015) reported similar findings and concluded that a more anterior cochleostomy would be useful in many children. More significantly, the authors suggest that pre-op CT should be used to determine the axis of the inferior segment of the basal turn in relationship to the EAC (4).

Take Home

There are many differences in optimum surgical technique for young children and adults, especially in babies between ages 6 and 12 months. Differences may include delicate soft tissues, skull often less than 2 mm thick, active marrow in diploic spaces that bleeds freely and may cause volume problems, constricted mastoid cavity, more acute angle between the EAC and facial recess and, as noted in these papers, a more acute angle of the axis of S. tympani re: the facial nerve.

All of these are manageable, but attention to insertion angle in young children should receive more attention by surgeons. RWM cochleostomies may require a more inferior electrode insertion trajectory in children. Whether bony cochleostomy is preferable in young children due to the greater scalar angulation has not been demonstrated. Similarly, whether routine pre-operative CT should be favored over MR or used in addition to MR has not been addressed adequately.

References

1. McRackan TR, Reda FA, Rivas A, et al. Comparison of cochlear implant relevant anatomy in children versus adults. Otol Neurotol 2012; 33:328–334.

2. Lloyd SK, Kasbekar AV, Kenway B, et al. Developmental changes in cochlear orientation—implications for cochlear implantation. Otol Neurotol 2010; 31:902–907.

3. Jackson NM, Givens VB, Carpenter CC, Allen LM, Morrell BB, et al. Cochlear trajectory in pediatric patients. Laryngoscope 2014. Doi: 10.11102/ lary.24984

4. Hui-Ying L, Ke-Guang C, Lin Y et al. The age-related positional and orientational changes of the human cochlea. Acta Oto-laryngologica 2015. 135:205-210.

 
Deficient Cochlear Nerve
 

4.1.15

The clinical term cochlear nerve deficiency has been established in the otologic literature to indicate a small or absent cochlear nerve on MR imaging. To the best of my knowledge it was first described by the Antwerp cochlear implant group in 1997 (1).

More recently, Buchman, Adunka and colleagues at UNC have shown that cochlear nerve deficiency (CND) is more common than previously recognized in CI candidates. CND may account for up to 20% of those with OAE/SP/ABR indications of auditory neuropathy spectrum disorder (2,3,4).

NEW KNOWLEDGE

A new paper from Chang-Gung College of Medicine in Taiwan by Che-Ming Wu and colleagues makes several important observations based on a literature review and experience with 656 children (5).

    • CND, aplasia is more common than hypoplasia of the cochlear nerve.

    • Small IACs may make it impossible to differentiate the nerves or diagnose CND.

    • CND is often but not always associated with HRCT findings of hypoplastic IAC/cochlear nerve canal.

    • 21.2% of pediatric CI candidates had CND.

    • 20% of CND/aplastic on MR could hear some sounds at < 90 dB.

    • Overall, CND candidates obtain poorer than average CI outcomes.

    • CND/hypoplasia subjects performed better than CND/aplasia subjects.

    • CND/hypoplasia outcomes showed no significant difference (p=.02) from non-CND outcomes.

TAKE HOME

  1. CND is more common than generally recognized

  2. If substantiated by further studies and experience, CND should probably be differentiated into CND/aplasia and CND/hypoplasia due to different prognostic implications.

  3. As an isolated MRI finding, CND/aplastic (unseen cochlear nerve) is not an absolute contraindication to implantation due to the limitations of current imaging.

REFERENCES 

1. Casselman JW, Offeciers FE, Govaerts PJ, et al. Aplasia and hypoplasia of the vestibulocochlear nerve: diagnosis with MR imaging. Radiology 1997;202:773-81.

2. Buchman CA, Copeland BJ, Yu KK, Brown CJ, Carrasco VN, Pillsbury HC III. Cochlear implantation in children with congenital inner ear malformations. Laryngoscope 2004;114:309-16.

3. Oliver F. Adunka, Patricia A. Roush, Holly F. B. Teagle, Carolyn J. Brown, Carlton J. Zdanski, Valerie Jewells, and Craig A. Buchman. Internal Auditory Canal Morphology in Children with Cochlear Nerve Deficiency. Otology & Neurotology (2006) 27:793 Y 801.

4. Buchman CA, Teagle HF, Roush PA, Park LR, Hatch D, Woodard J, Zdanski C,Adunka OF. Cochlear implantation in children with labyrinthine anomalies and cochlear nerve deficiency: implications for auditory brainstem implantation. Laryngoscope. 2011 Sep;121(9):1979-88. doi: 10.1002/lary.22032. Epub 2011 Aug 16. PubMed PMID: 22024855.

5. Che-Ming W, Li-Ang L, Shin-Kuo C, et al. Impact of Cochlear Nerve Deficiency…on Hearing Outcome in Children with Cochlear Implants. O&N (2015) 36:14-21.)