CIs (Exam 2)

Interpolation of channels

Is used to estimate the stimulation level based on surrounding channels. Can be done separately or in chunks (for kids)

• Requires anchor channels

What happens to intermediate values in interpolation? Why do interpolation?

• An average of the differences between channels measured is taken

• Streamlines the fitting process 

How do charge balanced biphasic current pulses work? Who determines the current output?

• The amount of charge is the product of amplitude and pulse width 

• The clinician determines the current output based on loudness perception described by the pt 

The greater the charge, the louder the 

Percept 

What happens when you begin to reach the limits of the device for volume?

If the max amplitude/height is met, the pulse width increases

The input dynamic range determines what? What happens to sound when they come into the IDR? What is the range?

Determines the intensity range of the acoustic input signal that is mapped into the audible electrical output range

• Any sound below the IDR will NOT be let in

• Any sound above the IDR will be compressed into the pt’s dynamic range

• Range: 40-60dB

Microphone sensitivity

Adjust the range of sound that is picked up by the microphone

Increasing the microphone sensitivity results in what?

• The pt hearing softer sounds 

  • Pts often decrease sensitivity in loud situations

  • Pt preference varies

Automatic gain control components 

Dual action 

• Slow-acting control with low compression ratio for everyday speech

• Fast-acting control with high compression ratio for loud sounds

Linear 

• Peak clipping once input level exceeds criterion 

Changes in volume result in changes to what?

The comfort levels as a percent of the entire dynamic range. At times the software can limit or set the flexibility of the volume control on the speech processor.

• Pts perceive this as a change in loudness

What is frequency allocation? What does it affect? 

• Controls how frequencies are delivered to the active channels. Can be automatic.

• Can affect the upper frequency limit that is allowed to be delivered

Stimulation or repetition rate

The rate at which individual channels are stimulated

• Expressed as pulses per second (pps)

Total stimulation rate

The rate per channel times the number of possible channels

Pulse rates > 2000 pps

Auditory nerve firing more likely to be stochastic

Objective measures include

• Electrode impedance

• Electrically evoked stapedial reflexes

• Electrically evoked compound action potential

• Electrically evoked auditory brainstem response

Telemetry

Process for obtaining data from the implant device and sending it back to external part

• Uses radio frequency link to transmit info

• Tells us about the goodness of the RF link and integrity of the device

• Can transmit electrophysiological data

Electrode impedance measures determine what?

Impedance measures determine whether the intracochlear and extracochlear electrodes are working appropriately

How is electrode impedance measured?

• Small, known amount of current sent to the electrode

• Voltage is also known, so impedance can be measured at each electrode contact

Voltage compliance 

Is the maximum amount of current that can be delivered to an electrode by a power source 

Voltage is determined by?

The battery

Current depends on what?

Impedance

Constant voltage and lower impedance vs constant voltage and higher impedance

• Constant voltage and lower impedance means MORE current flow

• Constant voltage and higher impedance means LESS current flow

CIs have what type of voltage capacity?

Finite, which determines the current it can deliver

How is electrode impedance measured?

Impedance is measured by the current that returns to the reference electrode and distribution of voltage to the selected electrode contact 

Abnormal impedance may mean what?

• Open circuit

• Short circuit

• Partial short circuit

Open circuit

An incomplete path for current to flow/discontinuous circuit. There is infinite resistance or VERY high impedance: clinical software flags values >30 kΩ

Causes:

• Broken electrode contact

• Broken lead wire

• Air bubble

Short circuit 

Low resistance between two points in a circuit that differ in potential. Results in increased current flow for a fixed voltage: characterized by very LOW impedance values <1 kΩ

Causes:

• Electrode contacts coming into contact with each other (array is kinked or curled back on itself)

Partial short circuit and cause

• Characterized by relatively low impedance and increased current flow BUT less than what is seen for a short circuit

• Impedances lower than those for nonaffected electrodes

  • Often now low enough to be flagged by clinical software

  • Should be monitored

• Caused by small tears in the silicone surrounding electrode lead

Identification for partial short circuit 

• Decrease in impedance of the affected electrodes over time 

• Decrease in impedance of affected electrode relative to impedance of unaffected electrodes

• Increase in MAP levels for effected electrodes

• Reduction in speech perception scores

•  Poor sound quality 

• Reduced pitch discrimination 

• Reduced loudness growth 

• Pain or nonauditory stim 

What is the typical time course of impedance changes? At Surgery? Device activated? Few months of use?

• Impedance lowest at surgery

• 2-3 weeks after surgery, impedance increases prior to initial activation

• Once device stimulated, impedance typically decreases

• Impedance stabilizes within a few months of use

Why can impedance increase?

• Periods of non-use

• Re-implantation

• ME involvement and fluid; infection and inflammation

• Operating outside of voltage compliance limits

Clinical uses for impedance measures (6) and possible course of action

• Identify electrode failures 

  • Disable affected electrodes 

  • OR explant if large number of electrodes fail

• Device programing 

  • Malfunctioning electrodes should not be included in pt’s MAP 

• Ensure voltage compliance 

  • If voltage compliance reached, increase pulse width, reduce upper stimulation levels; change electrode coupling 

• Monitor electrode function over time

• Intraoperative to postoperative changes

• Changes across postoperative intervals

Electrically evoked stapedial reflex (ESR)

Electrical counterpart to the AR

• Is muscular contraction within the ME in response to loud sounds- stimulus is electric current 

• Involved afferent and efferent sensory motor neurons 

Measurement of the ESR protocol

• Stimulus is at 500 ms duration pulse train

• Probe is placed in the contralateral ear to the CI

• Select reflex decay setting

• Stimulus presented via the programming software

• ESR appears as a downward deflection, time-locked to each stimulus presentation

Measurement of ESR Threshold protocol

• Use an ascending approach for the stimulus level 

• Begin with a relatively low stimulus level to avoid overstimulation 

• Once a clear response is seen, stimulus level should be decreased in small current step sizes 

• Threshold is the lowest visible repeatable response on the descending run 

• Measured on as many channels as possible 

Clinical uses for ESRTs

• Can confirm device function

• Assist with speech processor programming

• ESRTs tend to appear near the upper comfort levels

ESRTs present in _ % of CI pts

65-85

Setting upper stimulation levels for adults

• Always measure behavioral upper stimulation levels if possible 

• Measure ESRTs if you want an objective measurement of loudness to compare to behavioral thresholds 

Setting upper stimulation levels in children

• ESRTs rec for setting upper levels

• In children, behavioral measures can overestimate comfort levels

Electrically evoked compound action potential (ECAP)

Is a synchronous physiologic response from an aggregate population of nerve fibers in response to electrical stimulation

• Electrical version of Wave I of the ABR

Characterized by

• A negative peak (N1)

• A positive peak or plateau (P2)

ECAP waveform characteristics 

• Amplitude increases w/ stimulus level 

• Amplitudes can be as large as 1-2 mV 

• Faster stimulation rates degrade neural response 

• Very few averages needed 

ECAP stimulus 

Measured using reverse telemetry system (NRT)

• Single biphasic current pulse delivered using monopolar coupling 

• Low stimulation rate

• Parameters set by programming software

• Delivered to the implanted array via the speech processor that is connected to the programming computer 

ECAP recording

• Recorded using cochlear implant electrodes

• Measured ECAP is the voltage change (as a function of time) produced by depolarization of the auditory nerve 

  • Voltage change measured across an intracochlear recording electrode and an extracochlear MP ground electrode

ECAP measurement characteristics

• Pts do not need to sleep, lie still, or be sedated

• Not influenced by maturational effects

• Measured within the cochlea

• Artifact is quite large

Artifact reduction: forward-masking subtraction

Takes advantage of neural refractory properties by separating the ECAP from stimulus artifact

A-B+C-D= response

Artifact reduction: alternating polarity

• Polarity of stimulus is reversed 

  • Artifact reverses polarity 

  • Physiological response does not change 

  • C and A traces are averaged together

Disadvantages of alternating polarity (3)

• Amplitude and latency of ECAP can differ for anodic vs cathodic leading pulses

• Averaged waveform may have slightly different morphology compared to either polarity alone

• Thresholds obtained may be elevated relative to those obtained with forward masking subtraction method

Threshold and growth of response

• Magnitude of the ECAP increases with greater stimulus level

• Peak-to-peak amplitude plotted as a function of stimulus level

• Plotted as an input-output function

  • Amplitude growth function

Amplitude growth function: slope

Slope

• Rate of ECAP growth

• Calculated by applying linear regression to the data points in AGF

Amplitude growth function: threshold

Minimum amount of current needed to elicit a measurable neural response (ECAP). Two methods

• AGF linear regression line used to extrapolate stimulus current that yields ECAP amplitude of 0

  • Used by all manufacturers

• Visual detection

AGF threshold: linear regression

• Typically results in lower thresholds 

• If noise floor is low, both methods are highly correlated 

• AGF functions can flatter or roll over at top of function or have long shallow tails 

• Need at least 3 suprathreshold measures to reasonably performed regression analysis 

What is a measurable ECAP response?

• Morphology and latency

  • Compare waveform to more robust waveform obtained at higher stimulus level

• Scaling

  • Change amplitude scale and zoom in

• Noise-floor

  • ECAP should be larger than the noise-floor

Clinical uses of ECAP 

• Indicates stimulus level that should be audible 

• Confirms responsiveness of auditory nerve to electrical stimulation 

• Confirms device function

• Objective baseline 

• Weak to moderate correlation with T and C/M levels

• Assist with sound processor programming of T’s and C/M levels 

  • ECAp thresholds are usually above T’s regardless of pulse rate

  • ECAP threshold tends to occur in upper portion of the behavioral DR

Setting MAP levels: Hughes 

• ECAP thresholds measured on all electrodes 

• Behavioral T’s and C/M’s measured on a single electrode in the middle of the array

• ECAP threshold function shifted up (for C/M levels) or down (for T’s) by difference btwn ECAP threshold and measured T’s and C/M’s  

Setting MAP levels: Smoorenburg

• Measure ECAP threshold on every electrode

• T’s and C’s set to same level as ECAP threshold

• T and C- levels decreased until they are inaudible

• Activate live speech

•  T and C globally increased until a behavioral response is noted

  • T level

• C levels increased to a level that should provide adequate stimulation without discomfort

  • Very subjective

Clinical use of ECAP: ability to predict MAP levels etc

• ECAP thresholds alone cannot predict MAP levels with adequate accuracy

• Should never substitute for behavioral measures

• If ESRT cannot be measured (does help with setting upper levels), ECAP can at least provide a level of stimulation that should be audible

• ECAP threshold  closer to C/M levels for adults

Weak relationship b/w ECAP and behavioral measures may be due to

• Orientation of electrode array

• Subject differences in neural refractory properties (ECAP measures using a slow stimulation rate)

• Differences in electrode arrays

• Treating each electrode as an independent data point

AB first generation devices

• Clarion S-series 

• Electrode array with 8 channels 

• CIS and CA processing strategies 

AB second generation

Allow simultaneous stimulation

• Clarion CII

• Electrode array with 16 contacts

• Modified CIS strategy called HiRes

AB current technology

HiRes Ultra 3D Implant System

• Similar to HiRes90K

• 16 electrode contacts

Marvel sound processors

• Naida CI M90

• Naida CI M30

• Sky CI M90

Neptune processors (discontinued)

• First fully waterproof device

AB current arrays 

• HiRes Ultra electrode family

  • HiFocus Slim J: lateral wall, preserve cochlear structures

  • HiFocus Precurved Mid Scala array

• Other

  • HiFocus Helix

AB sound processing schemes

• HiRes 120 Fidelity with current steering 

• HiRes Optima 

ClearVoice vs SoftVoice

• ClearVoice: looks at each channels to determine if speech or noise to enhance the channel with speech

• SoftVoice: increase audibility of soft sounds

Legacy internal device: Nucleus C122

• First generation

• Used 22-electrode array, straight, rested against lateral wall after insertion

• Titanium case, receiver magnet holds external transmission unity in place

• Body worn speech processor

• Bipolar or common ground stimulation 

Legacy internal device: Nucleus 24M

• 2nd generation

• 24 electrodes

• Longitudinal array

• Single ball electrode for monopolar stimulation

• Bipolar or monopolar stimuluation

• SPring processor is body worn

• ESPrit processor is ear level

Current internal device: Cochlear Americas 

• Internal device: profile plus series

• Compatible with 

  • Cochlear nucleus Kanso

  • Nucleus Kanso 2

  • Nucleus 7

  • Nucleus 8

• Magnet pocket

Nucleus Legacy sound processors: Spectra processor

• Uses SPEAK or MPEAK

• Directional mic housed in BTE

• 22 electrodes in a longitudinal array

Nucleus legacy sound processors: ESPrit 3 G Processor 

• Designed to use with the Nucleus 22 array and Nucleus 24 array 

• Built in T-coil and wireless Phonak microlink to use with public systems that use FM or infrared tech 

• Can store two programs 

• Info directed to 20 electrodes 

• CIS, SPEAK, or ACE

• Uses high power zine air batteries 

CA current sound processors and strategies

Sound processors

• Nucleus 8

• Nucleus Kanso 2

Strategies

• SPEAK

• ACE

• CIS

NRT telemetry system for Nucleus devices and color codes meaning (green, red, brown)

• Process for obtaining data from the implant device and sending it back to the external part

• Uses RF link to transmit info 

• Tells use about the goodness of the RF link and integrity of the device

• Can transmit electrophysiological data 

  • Green: within operating range

  • Red: short or open circuit

  • Brown: Previously flagged

MED-EL 1st generation internal device, sound processor, and electrode arrays

• Internal device: Combi-40+

  • Ceramic+non-removeable magnet

• Sound processor

  • CIS+

  • OPUS 1 and 2 sound processors are backward compatible

• Electrode arrays

  • Standard

  • Medium

  • Compressed

  • Split

MED-EL current system 

• Internal device: Synchrony 2

  • Titanium

  • MRI safe up to 3 T

• Sound processors 

  • Rondo 3

  • Sonnet 2 

MED-EL Fine structure processing

• Timing of stimulation is used to code the temporal fine structure of the sound signal in the low to mid frequencies

• At the basal end, tonotopic fine structure is achieved by creating virtual channels

• FSP may provide better pitch discrimination and a wider pitch range than CIS 

Reasons for revision implantation

• Infection 

• Rejection of implant by body

• Extruding implant rx

• Electrode array damage

• Implant failure

• Technology upgrade 

What can a pt experience with failure?

• Loud popping sounds

• No sound

• Sound quality changes/speech performance decreases

• Intermittent sound

• Loss of lock to internal device

• Episodes of overly loud stimulation

• Non-auditory symptoms (pain, shocking sensations, vertigo, and/or facial twitching) 

3 categories of CI failures

3-8% of CI are failures

• Hard failure 

• Soft failure 

• Medical failure

Hard failure

Device stops working due to either internal electronic malfunction or a structural problem

• Most common indication for revision 40-80%

• Revision CI can take place expeditiously 

41% of hard failures have a history of what?

Head trauma

Soft failure

Occurs when a device malfunction is suspected but cannot be proven by the manufacturer’s test 

• Gradual decreases in performance scores (speech perception)

Non-auditory complaints for soft failure

• Facial nerve stimulation 

• Vertigo 

• Tinnitus

• Intermittent functioning 

Soft failure consensus of 2005 defined as:

Soft failure

• Uncommon occurence

• Device malfunction is suspected but cannot be proven using currently available methods 

• Reimplantation with subsequent alleviation of symptoms strongly supports the dx but cannot conclusively confirm a device malfunction

Medical/audiological assessment of soft failure in pts

• Symptoms 

  • Auditory vs non auditory 

• Medical eval 

  • Electrode position 

  • Infection 

  • Neural 

  • Intracochlear 

• Audio eval 

  • Drop in best performance 

  • Failure to achieve expected benefit 

  • Change in sensitivity 

• Device assessment 

  • Internal 

  • Faulty external hardware 

  • Device integrity testing 

Suggested checklist of soft failures for adults

• Auditory

• Nonauditory 

• Performance

• Mapping 

  • Loss of channels

  • Changes in impedance 

  • Short/open circuits 

• Hardware

  • Replacement of all externals

• Objective assessment 

  • Neural response measures

  • Stimulus artificat 

  • Evoked potentials 

Suggested checklist of soft failures for young children

• Behavioral 

• Teacher/therapist concerns 

• Other factors 

  • Educational placement 

  • Type and amount of therapy

  • Family involvement

  • Puberty

Present auditory symptoms, no non-auditory symptoms, and present decrease in performance 

Consider revision if symptoms are severe and persistent 

Present auditory symptoms, non non-auditory symptoms, no decline in performance

Monitor

No auditory symptoms, present non-auditory symptoms, no drop in performance

Consider revision if symptoms are serve and persistent

No auditory symptoms, no non-auditory symptoms, decline in performance 

Remap, replace external hardware, re-eval in 1-3 months 

Criteria for determining soft failure in children are determined by what outcomes?

• Inner ear development

• Central auditory pathways

• Cognitive abilities 

• During post implant follow up it is difficult to provide a concise definition of clinical improvement 

Soft failures commonly due to which medical outcomes?

50%

• Meningitis 

• Congenital inner ear malformation 

• Asthma 

Facial nerve stimulation soft failure protocol 

• Deactivation of 5 or more electrodes may suggest impeding device failure 

• When reprogramming or deactivation of electrodes does not solve FNS consider revision 

  • Allows broader programming options

  • Replace an outer wall straight electrode with newer perimodiolar array

Most common causes for medical failures 

• Device extrusion

• Trauma to site 

• Wound infection 

Consider what for device salvage after infection?

• Infectious agent 

• Severity of the infection 

• Response of the antibiotic tx 

Extrusion

Revision of scalp flap and initiate antibiotics

• Receiver stimulator may need to be moved to a different location on the skull 

• Delayed reimplantation of a new device 

Electrode extrusion causes and theories

• Cause is unknown but common in cases of ossification

• Theories

  • Occurs with skull growth

  • Force placed on the outer wall by the electrode force it out of the cochlea

Silicone allergy

• Suspect in cases of delayed device extrusion with negative cultures or non responsive to antibiotics

• Custom made implants can be designed for reimplantation 

Technology upgrade

• May not mean improved performance

  • May be due to reduced plasticity of nervous system or inability to adjust completely to the new device 

CI revision outcomes

• Several hundred revision CIs have been performed 

• Revision implantation is generally well tolerated and shows stable or improved outcomes 

• Mean threshold and speech performance scores are either the same or improved

Children < 18 years of age that underwent revision cochlear implant surgery evaluated for open speech rec, subjective reports, and symptom resolution 

• Previous peak performance was more likely to be achieved or exceeded in younger than in older children

• A decline in speech perception was a stronger predictor of successful outcome than chronic underperformance- children often did well after re-implantation

Suspicion of soft failure?

• Contact the Clinical Application specialist from the manufacturer to complete integrity testing

• Consult with CI team and Patient

• Surgeon

  • CT scan, primary care clearance, cardiology clearance

• Audiologist

  • Complete speech performance measures

  • Realistic expectations