Week 11 objective measurements Flashcards
what are the 3 important uses of objective measurements of CI
- evaluate the CI function
- evaluate the auditory system
- CI programming
electrophysiological objective measures for CI
- electrically evoked stapedial reflexes threshold (eSRT)
- electrically evoked compound action potential (eCAP)
- electrically evoked auditory brainstem response (eABR)
- electrocally evoked auditory cortical potential (eACP)
electrical objective measures for CI
- impedance testing and voltage compliance
- electrical field imaging
- averaged electrode voltage–integrity test
factors affection stapedial reflex threholds (SRT)
- 2% of normal hearing adults have absent reflexes
- the presence of minimal air bone gap could affect he probability of measuring SRT
- –average of 5 ABG results in 50% probability of measuring SRT
- measured reflexes in 85% of children with normal functioning middle ear
- grommits, ME effusion, affected ossicles, no seal, Ad or As tymps, 8th nerve dysfunction
- –history of middle ear issue could cause insignificant increase in ME stiffness, causing reflexes to disappear
eSRT procedure
- stimulus is a 300-500msec pulse train
- relex decay with high frequency probe tone (678 or 1000Hz)
- –could start with 226 but lesser chance of measuring with 226 Hz because you have added mass to the system with the electrode lead
- contra ear gets the stimulus
- –or ear with better ME function for bilateral recipients
- present the pulse train 3-5 times to ensure you are getting a response
- start at a low level and go up
- –the response must be time locked
- make sure response is repeatable on two ascending runs
clinical application of eSRT
- confirm functionality of CI
- strong correlation with C-level or USL
- –correlation coefficient (r) of 0.79 or 0.92
- –varies by electrode but not significant
- –congenitally deaf tend to need more and more power when going from HA to CI so run into problems with compliance and poor resolution because interference (compliance limit, battery life, current spread/low spectral resolution)
- –USL and eSRT increase with time, but not at the same rate
- similar performance with measured USL
issues with eSRT
- different upper stimulation level definitions, therefore
- –Ab: M-level 10% below eSRT
- –Med-el: MCL at or just below (within 10%) of eSRT
- –cochlear: c-level 10-15 CL below eSRT
- not in all patients
- –up to 80% of patients have eSRT
- –measured in the ipsilateral ear, chance of measuring eSRT increases with time
- —–37% of electrodes measured (1 month after activation) to 74% (3 month after activation)
when was eCAP 1st comercially available
*1998 with nucleus CI24M (nucleus 22 is therefore the only one that it wont work for)
what is eCAP and where is it measured from
- N1 (0.2-0.4 ms) -P2 (0.6-0.8 ms)
- –25-30 micro volt at threshold, and as large as 1500 micro volt for a high current
what is eCAP used for
- to check the integrity of the device and of the nerve as well as mapping
- measured in 95-96% of cases
what are the names of eCAP for each manufacturer
*AB: neural response imaging (NRI)
Med-el: auditory nerve response telemetry (ART)
*cochlear: neural response telemetry (NRT)
eCAP electrode coupling
- monopolar
- –bipolar can be used as well, but not as common
eCAP montage
- stimulating electrode, recording is +2 in the apical direction, and ground is generally the extracochlear
- so if the stimulating is electrode 1, the recording would be electrode 4 and the extracochlear would b the ground, and so on depending on what electrode you are stimulating
eCAP stimulus
- biphasic pulse
- rate of 30-80 pps
- pulse width of 25-40 microseconds
eCAP advantages
- anesthesia does not affect
- recording site is very close because you are right on the nerve fibers, this makes the response very large so dont need many sweeps (about 100 sweeps)
- no myogenic noise (because right on the neuron)
- no maturation effect because myelination is not a factor because yet again stimulation does not need to travel because it is right there on the neuron