NON PATHOLOGIC FACTORS IN ECOCHG ANALYSIS

• STIMULUS FACTORS:  

AP latency decreases and amplitude increases as stimulus intensity is increased.

Ø  SP is not observed at lower intensity level  (< 60 dBPESPL)

Ø  Monophasic stimulus polarity produces a CM component and it produces latency difference in the SP and AP components

Ø  Increasing stimulus rate reduces AP amplitude where as the SP characteristics remain unchanged.

Ø  ECochG  morphology changes dramatically for click Vs TB stimuli and also for low Vs high frequency tone burst stimuli.

• ECochG electrode problems:-

            ECochG activity arising from the cochlea and distal eighth nerve is best measured with a near field technique.  Response amplitude and reliability increases substantially as the recording electrode approaches the cochlea, which serves to overcome various clinical measurement prolem.

• Effect of electrode location on ECochG:-

            With an electrode array consisting of a non inverting electrode on the vertex or forehead and an inverting electrode near the ear (mastoid, earlobe or external ear canal, tympanic membrane or promontory) the AP component is positive in voltage.  If the non inverting electrode is relatively close to the cochlea and inverting electrode is relatively far away, a negative going AP will be recorded.  A negative going wave form format is often utilized in recording and reporting ECochG.

• Age:-

            Infancy and childhood.  A clear ECochG N1 component is normally recorded as early as 27 week conceptional age.  In comparison to adult values, latency is prolonged and amplitude reduced (Stockard et al 1983), Application of TT technique in children requires deep sedation or general anesthesia.  A clinical investigation by Schwartz.  Dratt and Schwartz (1989) provided evidence that CM / SP components can be consistently recorded even in pre term new born infants.  Responses were detected with conventional cup electrodes located on the vertex (non inverting) and both the stimulus ipsilateral and contra lateral ear lobes (inverting). A CM and SP appeared in composite wave form for 40 ears of 20 new born infants for single polarity click stimuli.  Detection of CM component in infants is primary objective in the diagnosis of auditory neuropathy.

Advancing age in adults:- 

Chartrian et al (1985) yielded a confusing set of findings that is age related difference were observed but not consistently for each response parameter.

·         SP: - detection level was positively correlated with age.

                      Amplitude decreased as a function of age.

·         AP: Amplitude decreased as a function of age

·         SP/AP ratio increases with age: that is advancing age amp of AP decreases relatively more than amplitude of the SP

• Gender:  

Chartrian et al (1985) stated that No significant difference between male vs female adults for detection of SP, SP onset time ,SP peak latency or for the duration of SP/AP complex.  But SP amplitude was significant larger for females than males only for left ear.

·         Larger AP amplitude was consistently recorded for females Vs males yet the SP/AP ratio was equivalent between gender.

Older subjects would be expected to have high frequency SN hearing deficit. SN hearing deficit and as a consequence show an altered ECOCHG.

• Body temperature:-

            Hypothermia results in reversible reduced amplitude in CM, while CM latency shows little or no change (Coats 1965). variable changes during hypothermia are found for the SP.  Lowered temperature also produces a reversible reduction in VIII th verve compound action potential (ECochG N1) amplitude and a reversible increase N1 latency (Kahana et al 1950).

In addition, lowered temperature produces a reversible reduction in the amplitude of the eighth nerve

compound action potential (ECochG AP component and ABR wave I) and a reversible increase of

AP latency (Gulick and Cutt, 1961; Kahana, Rosenblith and Galambos, 1950). There is a logical

explanation for the effect of decreased body temperature on latency of the AP component. Synaptic

transmission is delayed and axonal conduction velocity is decreased (Benita and Conde, 1972;

deJesus, Hausmanowa-Petrusewicz and Barchi, 1973; Snyder, 1980). Another effect of hypothermia

may be the selective loss of auditory sensitivity for high-frequency signals (Brown, Smith and Nuttall, 1983; Manley and Johnstone, 1974).

• Attention and state of Arousal

Most available clinical evidence indicates no difference in ECOCHG wave forms recorded in the awake Vs natural sleep state for moderate to high stimulus intensity levels (Amadeo and Shagoss 1973) or for low intensity stimuli close to auditory threshold.

Most available clinical evidence indicates no difference in ECochG waveforms recorded in the

awake versus natural sleep state for stimulation at moderate-to-high intensity levels (Amadeo &

Shagass, 1973; Jewett & Williston, 1971; Osterhammel, Shallop & Terkildsen, 1985; Campbell &

Bartoli, 1986; Sohmer et al, 1978; Picton et al, 1974) or for low intensity stimuli close to auditory

threshold (Deacon, Elliott, Bell & Campbell, 1987). Sleep state is best verified and quantified by

EEG recordings interpreted by an experienced electroencephalographer. Even extremely reduced

states of arousal, such as narcolepsy (Hellekson et al, 1979) and metabolic coma (Hall, Huangfu &

Gennarelli, 1982; Hall, Hargadine & Kim, 1985; Hall, Hargadine & Allen, 1984; Hall, 1988; Starr,

1976; Sutton et al, 1982) have no serious effect on ECochG latency or amplitude. The independence

of ECochG and state of arousal constitutes a major advantage for clinical evaluation of peripheral

and central auditory function of a wide variety of patients. The same statement, of course, holds true

for ABR.

• Muscle/movement artifact

This is minimally affected by muscle activity.A quiet patient state contributes to loss background noise and facilitates detection of even small amplitude response.  Random movements related artifact confounds ECOCHG interpretation especially identification of the SP component.In comparison to later latency auditory evoked responses, ECochG components occurring within 2 or 3-ms period after a stimulus are minimally affected by muscle activity. Still, a quiet patient state contributes to less background noise and facilitates detection of even small amplitude responses. Random movement related artifact confounds ECochG interpretation, especially identification of the SP component. The SP often appears as a hump on theupward slope of the AP. False identification of the SP is more likely when excessive movement artifact is present in the waveform, particularly without strict criteria for reliability.