Lecture 12 - 3rd Asessment - Evoked Potentials

Evoked potentials Auditory, visual, somatosensory Auditory evoked potentials: Assessment of central auditory pathway Early : 0 - 8 ms, brainstem, peaks I – V Middle : 8 - 50 ms, thalamus, thalamocortical pathway, peaks VI, VII Late : 50 - 300 ms, auditory cortex

Brainstem auditory evoked potentials BAEPs, scalp electrodes Far-field potentials : distant origin Evoked potentials : 1 - 2 µV Spontaneous EEG : 20 - 100 µV Computer averaging

Peak I: cochlear nerve Peak II: medulla, cochlear nuclei Peak III: L. pons, sup. olivary complex Peak IV: U. pons, lateral lemniscus Peak V: midbrain, inferior colliculus

A1 : left ear lobe A2 : right ear lobe Cz : vertex Record between : A1 – Cz Record between : A2 – Cz Ground : wrist or contra. ear

Monaural click duration : 0.1 ms Rate : 10/sec Intensity : 70 dB above H. threshold To eliminate response of contra : masking noise 30 dB below click

Normal versus abnormal BAEPs : Interpeak latency is reliable Peak amplitude is small 1 - 2 µV Peak latency is affected by : Stimulation and electrode application

Normal Interpeak Intervals

Left

Right

dBnHl 70

dBnHl 70

Normal Interpeak Intervals: I-III 2.50 ms III-V 2.19 ms I-V 4.44 ms

Peaks I, III, & V are Mainly Analyzed

Calculate the interpeak latencies for : I - III = -------- ms III - V = -------- ms I-V

= -------- ms

Please give your comments.

Interpeak latency (IPL)

Upper limit

I - III

2.6 msec

III - V

2.3 msec

I-V

4.6 msec

Brainstem lesion may be: Stroke Tumor Multiple sclerosis (demyelination)

- M.S., prolonged I - V - Objective hearing assessment disappearance of peak V - Coma, intensive care e.g. if only I & II present, this means damaged pons (III) & midbrain (V) - Tinnitus, to rule out tumor in internal auditory meatus, or a cerebellopontine tumor Tinnitus can be due to arterio-venous malformation

End of Lecture

The BAEP is Warranted for • • • • • • • • •

Tinnitus: to rule out a lesion in auditory N. (medulla) Vertigo: to rule out a lesion in vestibular N. (medulla) Hearing loss: to rule out a lesion in auditory pathway Hearing assessment in children ? Multiple sclerosis (demyelination) ? Acoustic neuroma (tumor in 8thN.or brainstem) ? Cerebellopontine angle tumor ? Stroke in brainstem Facial nerve palsy (pons, facial N. nucleus)

Acoustic tumors : (a) in VIII th cranial nerve (peak I): e.g. absence of peaks I – V (b) in brainstem (begins with peak II): e.g. absence of peaks II – V

Normal Hearing Level H. Threshold (R): 10 dB nHL (2 yrs F)

dBnHL 70 60 50 40 30 20 10

Normal: < 25 dB

Moderate Hearing Loss H. Threshold (L): 50 dB nHL (2 yrs F)

dBnHL 70 60 50

Moderate: 41 - 60 dB

Profound Hearing Loss H. Threshold (R): No Response at 95 dB nHL, Deafness

dBnHL 95 90

Profound: > 91 dB

Medulla lesion : e.g. Vertigo, nystagmus (vestibular, VIII nerve nucleus) Hearing loss (cochlear, VIII nerve nucleus) Pons lesion : e.g. facial palsy (facial, VII nerve nucleus)

Midbrain lesion: e.g. upgase paralysis (oculomotor, III nerve nucleus)

Visual evoked potentials