AUDITORY STEADY STATE RESPONSE (ASSR)

Transcription

1 AUDITORY STEADY STATE RESPONSE (ASSR) Introduction A far-field evoked auditory potential test Principle Similarity to ABR o Sound stimulus converted to electrical impulse pathway EE COLI recording via electrodes on scalp Difference from ABR o Evoked using continuous rather than transient stimulation o Stimulus is amplitude or frequency modulated o Place of maximal stimulation on the cochlea is determined by the choice of carrier frequency o Continuous, sinusoidal nature of the response lends itself to analysis in frequency rather than the time domain o ASSR is converted to frequency domain using FAST FOURIER TRANSFORM (FFT) techniques. o Frequency spectrum is considerably narrower than the frequency spectrum of the tone bursts typically used to elicit the ABR o Additionally continuous rather than a transient stimulus possible to achieve higher stimulation easier to distinguish between severe and profound hearing loss Procedure Headphone,computer for analysis Electrodes o Inverting (-ve) two Ipsilateral mastoid Scalp of neck o Non inverting (+ve) Vertex o Ground Forehead Historical perspective When the ASSR was originally described, modulation frequencies of approximately 40 Hz were generally used to evoke the response, and relatively strong correlations between ASSR thresholds and audiometric thresholds were reported Initial optimism about the correlation between ASSR thresholds and audiometric thresholds waned, however, when it was noted that these responses were adversely affected by sleep and sedation Interest in ASSR was revived when research showed that these problems could be 1

2 avoided if a modulation frequency of approximately 80 Hz rather than 40 Hz was used There have been a number of studies that have reported finding good correlations between ASSR thresholds and audiometric thresholds in both children and adults Uses & advantages Analysis of the ASSR does not require as much training as is currently needed with ABR For estimating frequency-specific thresholds give results similar to PTA To record the ASSR in ears with no measurable ABR at the limits of the equipment Low frequency hearing loss can be recorded For evaluation of children being considered for cochlear implantation VEMP Principle : Sudden Changes in Saccular Activity Evoke Changes in Postural Tone Anatomic and Physiologic Basis of normal saccular function Saccule lies in a parasagittal plane Hair cells of the saccule, are polarized so that they are excited by otoconial mass displacements away from the striola, can sense accelerations up and down Only the sacculus can sense linear accelerations up or down When the head is upright in the gravitational field, the acceleration resulting from gravity (9.8 m/sec2) constantly pulls the saccular otoconial mass toward the earth Afferents in the inferior half of the saccule, whose hair cells are excited by this downward acceleration, have lower firing rates and lower sensitivities to linear accelerations than do those afferents in the upper half Afferents in the upper half are excited by relative upward acceleration of the otoconial mass, such as might occur when the head drops suddenly (e.g., when one is falling) Thus, sudden excitation of hair cells across the saccular macula would likely be interpreted by the brain as a sudden loss of postural tone (i.e., falling) The appropriate compensatory reflex would be one that activates the trunk and limb extensor muscles and relaxes the flexors to restore postural tone Accordingly, the saccular afferents project to the lateral portions of the vestibular nuclei, which give rise to the vestibulospinal tract, in contrast to the utricular afferents, which project more rostrally to areas involved in the VOR 2

3 Clinical Importance Method Saccular excitation underlies the test of VEMP VEMPs are transient decreases in flexor muscle electromyographic (EMG) activity evoked by loud acoustic clicks or tones applied to the ear Sufficiently loud sounds applied to the ear excite saccular afferents The predicted reflexive response would include relaxation of flexor muscles EMG activity averaged over multiple acoustic stimuli from a tonically contracting flexor muscle will demonstrate a biphasic short-latency relaxation potential EMG activity can be recorded in many different flexor muscles, but SCM responses have been best described Because the saccule is the only end organ that mediates VEMP responses, absence of VEMP responses may indicate saccular dysfunction However, transmission of the VEMP acoustic stimulus is very sensitive to any cause of conductive hearing loss in the middle ear Interestingly, the preservation of VEMP responses in the face of conductive hearing loss implies an abnormally low acoustic impedance of the labyrinth, such as occurs in superior canal dehiscence syndrome or with enlarged vestibular aqueduct syndrome An intense click or tone pip is delivered to an earphone, stimulating sensory tissue (otolith organ) in the saccule This is interesting since the saccule is part of the balance system and not normally thought of as being sensitive to sound.(work on pigeons was done by pierre fluorence) Neural impulses travel from saccule up to inferior division of the vestibular nerve (cranial nerve VIII), to the lateral vestibular nucleus, to the lateral vestibulospinal tract, to the accessory nerve (XI), to the sternocleidomastoid muscle (SCM). To increase sensitivity, the head is turned away from the ear tested (right ear) and elevated to tense the SCM SCM contracts producing a large amplitude potential (compared to ABR) with positive and negative peaks at 13 and 23 ms (P13 and N23) This pathway is called the vestibulo-collic reflex. Presence of the VEMP indicates integrity of the pathway VEMPs may be abnormal (absent, low amplitude, high or enhanced amplitude, or delayed latency) in Meniere's disease, superior canal dehiscence, vestibular neuritis, multiple sclerosis, migraine, spinocerebellar degeneration. See table. If RE and LE represent the VEMP amplitude for the right and left ears, then a 30-47% asymmetry is clinically significant: Asymmetry = 100*(LE - RE)/(LE + RE) 3

4 The following figure illustrates a reduced response obtained from a patient with Meniere's disease (affecting the left ear): Pathology Meniere's disease Superior canal dehiscence syndrome Neurolabyrinthitis Vestibular neuritis Migraine Spinocerebellar degeneration Multiple sclerosis Brainstem stroke Large Vestibular Acqueduct syndrome Conductive deafness VEMP Response Absent, reduced, enhanced Enhanced Absent, reduced Absent, reduced Absent, reduced, delayed Absent, delayed Absent, delayed Absent, delayed Enhanced reduced 4

5 VIDEONYSTAGMOGRAPHY (VNG) TESTING Videonystagmography (VNG) is often used in the evaluation of a patient who presents with vertigo and uses the vestibular-ocular reflex to indirectly measure vestibular function. VNG tests for nystagmus using infrared light and video technology to monitor eye movements during testing. VNG testing is considered the new standard for testing inner ear functions over Electronystagmography (ENG), because VNG measures the movements of the eyes directly through infrared cameras, instead of measuring the mastoid muscles around the eyes with electrodes like the previous ENG version. VNG testing is more accurate, more consistent, and more comfortable for the patient. By having the patient more comfortable and relaxed, consistent and accurate test results are more easily achieved. VNG testing is used to determine if a vestibular (inner ear) disease may be causing a balance or dizziness problem, and is one of the only tests available today that can decipher between a unilateral (one ear) and bilateral (both ears) vestibular loss. VNG testing is a series of tests designed to document a persons ability to follow visual objects with their eyes and how well the eyes respond to information from the vestibular system. This test also addresses the functionality of each ear and if a vestibular deficit may be the cause of a dizziness or balance problem. To monitor the movements of the eyes, infrared goggles are placed around the eyes to record eye movements during testing. VNG testing is non-invasive, and only minor discomfort is felt by the patients during testing as a result of wearing goggles. Appointments usually last about 1.5 hours. 5

6 There are 4 main parts to a VNG test: 1. Occular Mobility You will be asked to have your eyes follow objects that jump from place to place, stand still, or move smoothly. The technician will be looking for any slowness or inaccuracies in your ability to follow visual targets. This may indicate a central or neurological problem, or possibly a problem in the pathway connecting the vestibular system to the brain. 2. Optokinetic Nystagmus 2. You will be asked to view a large, continuously moving visual image to see if your eyes can appropriately track these movements. Like the occular mobility tests, the technician will be looking for any slowness or inaccuracies in your ability to follow visual targets. This may indicate a central or neurological problem, or possibly a problem in the pathway connecting the vestibular system to the brain. 3. Positional Nystagmus The technician will move your head and body into various positions to make sure that there are no inappropriate eye movements (nystagmus), when your head is in different positions. This test is looking at your inner ear system and the condition of the endolymph fluid in your semi-circular canals. The technician is verifying that small calcium carbonate particles called otoconia are not suspended in the fluid and causing a disturbance to the flow of the fluid. 4. Caloric Testing The technician will stimulate both of your inner ears (one at a time) with warm and then cold air. They will be monitoring the movements of your eyes using goggles to make sure that both of your ears can sense this stimulation. This test will confirm that your vestibular system for each ear is working and responding to stimulation. This test in the only test available that can decipher between a unilateral and bilateral loss. Optokinetic Testing using Micromedical Visual Eyes System 6

7 Otoacoustic emissions Otoacoustic emissions are low energy sounds produced by the cochlea. They are thought to be acoustic byproducts of the outer hair cells, which are thought to underlie the amplification of the basilar membrane. Clinically, they are most often evoked using transient and distorted product stimulation. The evoking response causes outer hair cell motility which results in a mechanical wave that travels from the cochlea through the middle ear and tympanic membrane to the ear canal where it is recorded. Spontaneous emissions are not present from the cochlea when there is a greater than 25dB hearing loss. Unfortunately, they are not present in all normal ears, which does not make this the test of choice to clinically assess cochlear functioning. Transient stimuli such as clicks evoke emissions from a large portion of the cochlea. The emissions are then sampled and signal-averaged to extract them from background noise. These alternating samples are then stored in one of two memory banks and compared. Reproducibility, expressed as a percentage, is the cross correlation between these two waveforms. A reproducibility score of 50% or greater indicates that a response is present. Waveforms may vary significantly between people, but they are highly reproducible within a given individual. When hearing thresholds are better than 35dB, TEOAEs are generally present. The advantages of TEOAE are that it can separate normal from abnormal ears at 20-30dB and that it is quick. The specificity of clean, dry ears of infants is 95%. The main disadvantage is that it fails to extract responses at higher frequencies. Distorted products are additional tones which are created when two tones, f1/lower frequency & f2/higher frequency, are presented simultaneously to a healthy cochlea. The most robust DPOAE occurs at the frequency determined by the equation 2f1-f2. Due to a nonlinear process within the cochlea, the DPOAE assesses the cochlear integrity of the region near f2. When hearing thresholds are better than 50dB, DPOAEs are generally present. The main advantage is that DPOAEs can recover OAEs above 6000Hz. The transmission properties of the middle ear directly influence the OAE characteristics. The presence of a middle ear effusion, as in otitis media, affects both the forward and backward transmission. Although otitis media often eliminates OAEs, it is possible to record OAEs in some patients with middle ear effusion. OAE characteristics increase significantly over the first few days of life likely as a result of changes in the ear canal and middle ear. Small tympanic perforations will impede the forward transmission. This can usually be overcome with DPOAEs by increasing the amplitude. 7

8 Central Auditory Processing There is no accepted definition of Central Auditory Processing (CAP). In its simplest form, it is what we do with what we hear. The Task Force on CAP Consensus Development defines CAP as the auditory system mechanisms and processes responsible for the following behavioral phenomena: Sound localization Auditory discrimination Auditory pattern recognition Temporal aspects of audition, including temporal resolution and masking Auditory performance decrements with competing and degraded acoustic signals Deficiencies in any of these behaviors are considered central auditory processing disorders (CAPD). Results of CAPD testing have revealed clustering of test results and characteristic behaviors. These four categories are decoding, tolerance-fading memory, integration, and organization. Each of the four categories has been associated with a specific region of the brain. The Buffalo model of CAPD assessment and management takes into account the classification of CAPD as well as speech language evaluation and academic characteristics. It is important to understand that there is no one test that is sensitive enough to detect CAPD, especially in children where the variability of the tests is very wide. Therefore a battery of tests is recommended. In the Buffalo model, the CAP battery always includes the Staggered Spondaic Word (SSW) test, the Phonemic Synthesis (PS) test, a speech-in-noise (SN) test, and the masking level difference (MLD) test. Most patients will have weaknesses in more than one category and the categories are not mutually exclusive. 8