Pre-Operative Audiological Evaluation PDF

PRE-OPERATIVE AUDIOLOGICAL AND NON AUDIOLOGICAL EVALUATION - Mahima Gupta Presentation Number: 19 ----------------------------------------------------------------------------------------------...

PRE-OPERATIVE AUDIOLOGICAL AND NON AUDIOLOGICAL EVALUATION - Mahima Gupta Presentation Number: 19 --------------------------------------------------------------------------------------------------------------------- Audiological Evaluation: Pre-implant audiological tests include unaided and aided detection thresholds for pure tone and warble tone stimuli, respectively. Unaided thresholds are obtained in each ear individually, and aided detection thresholds may be obtained monaurally as well as binaurally. Aided speech perception abilities are often assessed in both monaural and binaural conditions, depending on the use of amplification in each ear. Speech perception measures are conducted in the sound field, typically at a presentation level of 60 dB SPL and include open-set recorded presentation of words and sentences in quiet, and if appropriate, in noise. Other tests include OAEs, and immittance measurements. Electrophysiological measure: It involves interaction with the human physiology. It can be used to evaluate most areas of the auditory pathways. The most commonly used electrically evoked measures are: 1. Electrically evoked stapedial reflex (ESR) 2. Electrically evoked whole nerve/ compound action potential (EAP or ECAP) 3. Electrically evoked auditory brainstem responses (EABR) 4. Electrically evoked middle latency responses (EMLR) 5. Electrically evoked cortical auditory evoked potentials (AEPs) Among the above mentioned measures EABR, EMLR and AEPs can be used for pre-operative measurements. a. EABR: It can be recorded in the pre-operative stage using electrical stimulation presented at the promontory or round window. Along with MRI, EABR can provide valuable information regarding the presence of intact auditory neurons and the suitability of a patient for cochlear implantation. Many characteristics of the EABR are similar to those experienced with the ABR. One of the reasons for this is the comparability of the acoustic click with the electrical biphasic pulse from the cochlear implant. Both these stimuli have a fast onset and short duration & subsequently produce a high level of synchronization of firing of auditory nerve fibres. An early study of evoked potentials to electrical stimulation was reported in Guinea pigs by Meikle et al., Implantable Devices for the Hearing Impaired: Unit II - 6 Page 1 1977. This was followed by a report on electrical brainstem responses in humans by Starr & Brackmann, 1979. Analysis & characteristics of EABR waveform: Although many characteristics of the EABR are similar to the ABR, there are some important differences. First, wave eI is usually obscured by the stimulus artefact & cannot be identified reliably on the EABR waveform. The latencies of the individuals component of the EABR arise 1.0 to 1.5 ms earlier than the ABR (Allum et al., 1990), with wave eV occurring at about 4-5 ms. This is caused primarily by the electrical stimulus bypassing the transmission process of sound through the EAM, ME & along the BM. There is only a small increase in latency with reduction in stimulus intensity. Amplitude of the response may not exhibit saturation even at relatively high stimulus levels, resulting in a steep slope on the I/O function. Close to threshold, a tailing effect (shallow slope on the I/O function) is sometimes observed. Usually wave eV is the component identified for estimation of response threshold, although in some patients wave II and III are equally dominant close to threshold. eII eV EABR eI ABR Preoperative recording of EABR: An electrical stimulus can be applied to the promontory in the middle ear using a transtympanic needle electrode and many patients will experience some degree of auditory sensation. In adults the test procedure is relatively straightforward and can be performed without a general anesthetic. Absence of any acoustic sensation with the preoperative promontory (and round window) stimulus was taken as a contraindication for cochlear implantation (Kilney et al., 1992). Implantable Devices for the Hearing Impaired: Unit II - 6 Page 2 Gantz et al. (1993) reported considerable variability in the ability of preoperative behavioral promontory tests to predict audiological performance with multichannel cochlear implants. The EABR can be evoked by an extracochlear electrical stimulus presented at the promontory or round window (Kilney et al., 1994) or even by a ball electrode placed inside the cochlea through a cochleostomy (Frohne et al., 1997). A battery-powered, custom-built stimulator is often used to present the biphasic charge-balanced pulse stimulus, since suitable commercial stimulators are not readily available. It must have constant current output so as to overcome changes in contact impedance of the stimulating electrode. Problems of stimulus artefact are exacerbated with extracochlear stimulation compared to intracochlear stimulation by the implant (Kasper et al., 1991). Some of these problems can be addressed using data collection parameters that minimize the effects of the stimulus artefact onthe signal. Evidences from animal studies have suggested that the amplitude I/O function of the EABR can predict neuronal survival (Smith and Simmons, 1983). An analysis of the EABR waveforms should therefore include measurements of these characteristics (Nikolopoulos et al.,1997). For several years the technique was used routinely immediately before implantation, to assist with selection of the ear for implantation (Mason et al., 1997). The ear with ‘best’ prom-EABR was chosen, providing there were no factors influencing the decision, such as ossification of the cochlea or MRI of the nerve bundles in the internal auditory canal. The responses could be absent due to number of cause, such as status of the auditory nerve, lack of sensitivity of the test technique, positioning of the needle electrode, and difficult recording conditions in which interpretation of the waveform was hindered by a large stimulus artefact. The incidence of absent responses is also significantly higher in children deafened after meningitis compared with congenitally deaf children (1999). A study on children with implants (Nikolopoulos et al., 2000) showed that children with no prom-EABR waveform performed as well as children with well-defined responses. Also there was no significant difference between the two groups of children on speech perception and speech intelligibility aspects. This study suggested that the prognostic value of the prom-EABR is limited and absence of a prom-EABR is not, in itself, a contraindication for CI. However, in selected cases (cochlear malformation, suspected cochlear nerve aplasia, narrow internal auditory canals) the presence of a prom-EABR is a positive finding in the assessment of candidates for CI as it confirms the existence of intact auditory neurons. EABR will only assess the integrity of the auditory pathway up to the level of around the lateral lemniscus in the brainstem pathways, whereas other electrically evoked potentials, such as the middle latency response, and late electrically evoked potentials, such as MLR, and late auditory potentials will examine more central pathways. b. EMLR: Implantable Devices for the Hearing Impaired: Unit II - 6 Page 3 The electrically evoked middle latency response (EMLR) has been suggested as a tool to assess the mechanisms that underlie auditory function in cochlear implant users. In a study by Firszt (1998), average latencies across electrode were 15.41 and 26.37 ms for Na and pa, respectively. EMLR is an alternative to shorter latency potentials because of its longer latency, making it less likely to be contaminated by the stimulus artefact that occurs early in the response. EMLR thresholds have been obtained with the use of anaesthetics, without compromising the replicability of waveform (Kileny et al., 1989). There has been good correspondence noted between EMLR thresholds and behavioural thresholds through the implanted electrode array (Gardi, 1985), and through transtympanic electrode (Kileny and Kemink, 1987). A comparison between EABR and EMLR thresholds obtained within the same subject has shown that EMLR thresholds were lower than the EABR thresholds and were closer to behavioural estimates (Gardi, 1985). The latency of component Pa for electrical stimulation is consistently shorter than that for acoustic stimulation (Firszt et al., 2002b). The electrical Pa is larger in amplitude and narrower than its acoustic counterpart. The increased sharpness in the response is most likely due to the greater neural synchrony with electrical stimulation. Studies of the electrical and acoustic MLR within the same subject suggest that both responses are activated by the same neural generators of the central system (Kileny et al., 1989). The main difference between the two measurements is that the latency of the Pa peak is reduced in the electrical mode relative to the acoustic mode. In a study by Groenen et al. (2000), EMLR amplitude variation (Na to Pa, Nb to Pb) was reported to relate to suprasegmental speech understanding. In addition, EMLR interlatency variations were related to segmental speech processing. Poorer cochlear implant users had Implantable Devices for the Hearing Impaired: Unit II - 6 Page 4 greater variation in amplitude and interlatency measures for the main components of the EMLR compared to better cochlear implant performers. Animal studies suggest that EMLR measures correlates with neural survival (Jyung et al., 1989). It has also been proposed that the EMLR may be useful as both indicator of eighth nerve survival in humans (Gardi, 1985) and to determine efficacy of electrical stimulation in an animal model (Burton et al., 1989). c. Cortical Evoked Potentials: Cortical responses provide a mechanism for understanding how electrical stimuli are registered by the central auditory system of profoundly hearing-impaired individuals. Late potentials used for this purpose includes the N1, P2, P300 and MMN response elicited with tonal stimuli (Oviatt and Kileny, 1991), stimulated electrodes (Ponton and Don, 1995), and speech (Kaga et al., 1991). Several studies suggest that cortical responses can be reliably recorded with electrical stimulation. Mean latency was found to be 86 ms for N1 and 181 ms for P2, when stimulated at upper portions of the electrical dynamic range (Firszt, 1998). The latencies may be slightly earlier than the acoustic counterpart for each component, depending on the stimulus characteristics and subject sample. The late evoked potential may serve as an indication that the subject has detected the signal (N1, P2) and the cognitive evoked potentials (P3, MMN) may be used as an indicator of discrimination or contrast perceptions. Poor speech recognition performance correlated with an absence of the cognitive component of the late auditory evoked potential. One advantage of the cortical auditory evoked potential is the ability to obtain these responses using speech stimuli. The presence of N1-P2 responses to speech stimuli provides information regarding auditory and association cortex activation with long-term preimplant auditory deprivation. These responses (along with cognitive evoked responses) may be a suitable tool for longitudinal monitoring of neurophysiological progress in prelingually deaf, implanted adult. Cortical potentials may reflect both degeneration and remaining plasticity of the auditory system of those with profound hearing loss receiving implant devices. Cortical responses are assumed to provide insight into how the central system detects and discriminates signals from the implants. Radiological assessment: A proper preoperative radiological evaluation identify patients with anatomical contraindications to implantation, and also aid in identifying certain anatomical features that have consequences in selecting the specific device and which ear is more appropriate for surgery. They may also allow the surgical team to anticipate aberrant anatomy and thus reduce the risk of surgical complications. Preoperative imaging will ideally address several basic anatomical considerations: Implantable Devices for the Hearing Impaired: Unit II - 6 Page 5  1. Mastoid and Middle ear: To assess the presence of a low-lying tegmen or otherwise sclerotic and contracted mastoid cavity. Also, the presence of the mastoid inflammation or cholesteatoma could be contraindicative of the surgery. 2. Facial nerve: It is essentially crucial when other labyrinthine anatomical anomalies are known to be present. 3. Vascular anatomy: The orientation and course of all key vascular structures should be accounted for. In particular, a dehiscent jugular bulb may cause difficulty when attempting to access the usual location of electrode insertion at the promontory adjacent to the round window. 4. Otic capsule: The otic capsule should be inspected for evidence of abnormal bone density or other pathological process. When present, either cochlear otosclerosis or Paget’s disease may predict a higher incidence of unwanted implant electrode stimulation of the facial nerve. 5. Internal Auditory Canal (IAC): A narrowed IAC (< 3 mm) is associated with a significant risk of an underlying hypoplastic or absent cochlear nerve. 6. Cochlea: The presence of a cochlea must be confirmed, since the Michel anomaly is a contraindication to implantation. A small modiolus (< 4 mm2) is considered a risk factor for the presence of an associated absent or hypoplastic cochlear nerve. A detailed inspection for evidence of cochlear ossification or membranous fibrosis is also critical. 7. Other congenital abnormalities may be present and should be accounted for such as, Mondini deformity, an enlarged vestibular aqueduct etc. High resolution computer tomography (HRCT) scans of the temporal bone help to define the surgical anatomy and provide information about cochlear abnormalities. Temporal bone CT scans reveal anatomical details important in treatment planning: the pattern of mastoid pneumatization, the position of vascular structures, middle ear anatomy, and position of the facial nerve. Scan should also be examined for evidence of cochlear malformation, cochlear ossification, enlarged vestibular aqueduct, and other inner ear and skull base anomalies. HRTC does have limitation, particularly in the assessment of cochlear patency. CT findings of cochlear patency generally correlate with surgical findings, but significant discrepancies have been reported. MRI may be a useful adjunct to HRCT for assessment of implant candidacy. Whereas HRCT reveals detailed bony anatomy, MRI offers imaging of soft tissue structures such as membranous labyrinth and neural substrate, and MRI techniques make it possible to visualize the presence or absence of fluid within the cochlear turns, thus enabling evaluation of cochlear patency with greater resolution. T2 weighted sagittal fast-spin echo (FSE) sequence and/ or CISS (constructive interference of steady state) sequences also afford MRI the advantage of providing very detailed images of the IAC. Some distinct disadvantages of the MRI scanning are that most children and many adult undergoing MRI will need some level of sedation, which is not the case with HRCT. Implantable Devices for the Hearing Impaired: Unit II - 6 Page 6  CT scan MRI Morphology of cochlea & semicircular canal ++ +++ Potency of cochlear duct + ++ Status of cochlear nerve - +++ Anatomy of facial nerve & fallopian canal ++ + Defect of the modiolar + +++ Defect of cribiform area +++ ++ Enlarged vestibular aqueduct ++ +++ Enlarged cochlear aqueduct +++ + Presence of round or oval window +++ - CNS abnormalities +++ Functional MRI attempts to provide an objective assessment of neural activities within the auditory cortex during artificial electrical stimulation of the cochlea. In theory, f-MRI may provide valuable information regarding activity along the central auditory pathway that could potentially provide prognostic information or help in selecting the ideal side for implantation. At present, the role and value of f-MRI in the preoperative assessment of CI candidates is yet undetermined. Mainly, CT displays the anatomy of the mastoid, cochlea, vestibular apparatus, and associated structures such as the vestibular & cochlear aqueducts, the MRI shows soft tissue structure such as nerves and fluid such as CSF and perilymph/ endolymph. Surgical Consideration: 1. Skull thickness: In the thin skull, damage to the dura is possible. 2. Otitis externa: It should be cleared up before implantation because of a risk of post-operative infection. 3. Congenital malformations of the cochlea: Implantable Devices for the Hearing Impaired: Unit II - 6 Page 7  These range from the relatively minor widening of the vestibular aqueduct through mild Mondini malformations, with incomplete partition, to more severe Mondini malformations, and total aplasia of the cochlea (Michel deformity). X- linked deformity is also of relevance in CI surgery, since it commonly gives rise to a surgical ‘gusher’. In cases of Mondini malformation, there may normally be enough spiral ganglion tissue in the incomplete modiolus to justify using a ‘pre-coiled’ electrode with inward-facing terminals. In the common cavity and in severe forms of Mondini malformation, the neural tissue is likely to be lying on the walls of the cavity, making a straight electrode a theoretically more sensible strategy. In children with narrow IAM, the most reliable way of establishing whether or not a cochlear nerve is present is by careful observation of the child’s response to sound with HA. If it is clear that, with the most powerful available HA, the child is failing to obtain anything but vibro-tactile stimuli, then it is unlikely that any cochlear nerve fibres are present. However, if clear aided responses to sound are present, albeit with thresholds that place the child within the criteria for a CI, then cochlear nerve tissue must be present and a cochlear implant is likely to provide a degree of benefit for that child. In the cases with aplasia of the cochlea (Michel deformity), there is clearly no likelihood that a CI would either be possible or helpful. A possible strategy is to use an auditory brainstem implant. 4. Paget’s disease: There has been a report of successful implantation in Paget’s disease (Bacciu et al., 2004). In the reported case, a full insertion of an electrode array was followed by excellent sentence and word recognition scores, around 100%, without any facial nerve stimulation. 5. Demineralization of the temporal bone: In otospongiosis and generalized osteoporosis, as well as in patients with old fractures of the temporal bone, the lack of electrical insulation normally provided by the solid temporal bone and presence of fluid with saline-like electrical conducting properties makes unwanted stimulation of the facial nerve more likely to occur. 6. Obliteration of the lumen of the cochlea by osteoneogenesis may occur after bacterial meningitis, in some cases of otosclerosis, in cases of autoimmune deafness, such as Cogan’s Syndrome, or after temporal bone fracture in which blood entered the labyrinth. In cases where ossification extends only along the first 3 or 4 mm of the basal turn, either partly or completely obstructing it, the cochlear implant could be given. But, in the ears Implantable Devices for the Hearing Impaired: Unit II - 6 Page 8  with the complete ossification of the basal turn of the cochlea, the electrode cannot be inserted along the cochlea. 7. Neurofibromatosis II, mental retardation, psychosis, organic brain dysfunction also act as contraindications of the cochlear implantation. 8. Also, auditory neuropathy often limits the performance with the implant when compare with other implant patients. Immunization: Preoperative vaccination against streptococcal pneumonia is routinely recommended. It was reported that pediatric and adult patients with cochlear implants are at increased risk of acquiring S. Pneumonia meningitis. In 2002, the CDC issued age-appropriate immunization guidelines for patients who have a cochlear implant or are going to be the recipient. According to CDC persons planning to receive a cochlear implant should be up-to-date on age-appropriate pneumococcal vaccination >2 weeks before surgery. The recommended schedule was as follows; 1. Children aged 2 months after vaccination with PCV7. 3. Persons aged 5--64 years should receive PPV23 according to the schedule used for persons with chronic illnesses; a single dose is indicated References: 1. Isaacson, B. et al. (2010). Cochlear implants, indication. Retrived from http://emedicine.medscape.com/article/857164-overview 2. Cooper, H.R., & Craddock, L.C. (2006). Cochlear implants: a practical guide, 2nd edition. West Sussex, England, Whurr Publishers Limited. 3. Cullington, H.E. (2004). Cochlear implants: objective measures. London & Philadelphia, Whurr publishers. 4. Gulya, A.J., Minor, L.B., & Poe, D.S. (2010). Glasscock- Shambaugh surgery of the ear, 6th edition. CT, USA, People’s Medical Publishing House. 5. Lee, K.J., & Toh, E.H. (2007). Otolaryngology: a surgical notebook. New York, Thieme medical publishers Inc. 6. Salvinelli, F., Trivelli, M., Greco, F., & Linthicum, F.H. (1999). Cochlear implant. Histopathological guide to indications and contraindications: A post mortem study on temporal bones. European Review for Medical and Pharmacologial Science, 4, 217-220. Implantable Devices for the Hearing Impaired: Unit II - 6 Page 9  7. Waltzman, S.B., & Roland, J.T. (2006). Cochlear implants, 2nd edition. New York, Thieme medical publishers, Inc. 8. Zeng, F., Popper, A.N., & Fay, R.R. (2004). Cochlear implants: auditory prostheses & electrical hearing. New York, Spring-Verlag Inc. 9. Snow, J.B., & Wackym, P.A. (2009). Ballenger’s Otorhinolaryngology17 Head and Neck Surgery, central edition. Shelton, CT, People’s medical publishing house. 10. Pneumococcal Vaccination for Cochlear Implant Candidates and Recipients: Updated Recommendations of the Advisory Committee on Immunization Practices. Retrived from http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5231a5.htm Implantable Devices for the Hearing Impaired: Unit II - 6 Page 10