VEP and BAEP

Amir M. Arain, M.D.

VEP: Modes of stimulation

• Patterned Stimulation( checkerboard): is most commonly used – most sensitive

• Unpatterned Stimulation( strobe): – Less sensitive – responses are variable. – Information qualitative than quantitative.

Unpatterned stimulation

• Sufficient to assess the integrity of the visual pathway: – following trauma – Intraoperative – patient unable to cooperate:

• Coma • general anesthesia • young infants or mentally retarded

Visual Evoked Potentials Full field pattern reversal

• Checkerboard pattern: TV pattern stimulators are more commonly used

• Pattern Presentation: reversal of light and dark elements without change in luminance

Dimensions of pattern

• Visual angle (function of the distance from the subject’s eye to the pattern and its width)

• Visual angle >8 degrees- full field stimulation

• Visual angle > 20 degrees- partial field stim.

Dimensions of pattern

• Check size: smaller checks increase the P100 amplitude

• Smaller check size have greater sensitivity for abnormalities – more likely to be affected by visual acuity

and luminance changes. – at least two check sizes should be used.

Commonly 35' and 70' are used

Characteristics of stimulus

• High contrast ( > 50%) – Contrast <20%: delay in P100

• Check size: 35' and 70' • Field size: 8-20 degrees • Reversal frequency: 4Hz or less • Reversal direction has no significant

effect

Recording Parameters

• Filters: LFF=1 Hz HFF=100 Hz • Sweep duration: 300- 500 msec • Dwell time: 2 msec • Sweep repetition: 100

– at times 200 or 500 sweep repetitions may be necessary if responses are noisy or unclear

Placement and labeling of electrodes

• Recommended electrodes: • Channel 1: Oz to reference • Channel 2: Pz to reference • Channel 3: L5 to reference • Channel 4: R5 to reference • L5 and R5 correspond to 5 cm above

and 5 cm left and right lateral to inion respectively

Partial-field stimulation: Electrodes placement

• Best montage for partial-field stimulation requires six electrodes with Fz as reference because ears may asymmetrically active.

• Channel 1: L10 to reference • Channel 2: L5 to reference • Channel 3: Oz to reference • Channel 4: R5 to reference • Channel 5: R10 to reference • Channel 6: Pz to reference

VEP: Technical factors affecting results

• Luminance: P100 latency is increased with decrease in luminance – Pupillary diameter has an effect on retinal illumination

and so it affects P100 latency • Contrast: Luminance difference b/w light & dark

Luminance sum (b/w light & dark) • Decrease in contrast increases P100 latency

(stable > 40 %) • Field size: reduces the P100 amplitude but

latency is not affected

VEP: Technical factors affecting results

• Check size: In smaller fields, increasing check size increases P100 latency

• Stimulator type: Increasing reversal time increases P100 latency

• Partial Field Stimulation: (field > 20 degrees) – Ipsilateral P100: central areas – Contralateral negativity: Peripheral areas – Transitional zone

VEP: Patient factors affecting results

• Age: Check size affects the P100 latency Amplitude stable during adult life P100 latency changes with age, most evident after 45

yrs. Then it increases by 2-5msec/decade • Gender: Females have a shorter P100

latency • Visual acuity: Changes in P100 latency is

related to check size and retinal illumination • Reliability: with observation and PERG

VEP Analysis of results

• Identification of waveforms, N75, P100, N145 • Measurements: • P100 latency at Oz • Amplitude of P100 at Oz • Interocular latency difference (range 0-6) • Interocular amplitude difference (range 0-5.5) • Interocular amplitude ratio: • Smallest amplitude x 100 Largest amplitude

Abnormal VEPs: types Abnormal distribution of P100

• Scotoma: removal of central-field contribution results in peripheral field activity to predominate

Partial field stimulation can be helpful • Lesion anterior to chiasm- differences b/w

responses from same half field in both eyes • Lesion at chiasm--same abnormality in

different fields of both eyes (crossed asymmetry)

• Lesion posterior to chiasm--same abnormality in homonymous fields of both eyes(uncrossed asym)

Abnormal VEPs: types Abnormal shape of P100

• W, or bifid pattern produced by: • 1. Upper visual field contributes -vity to the

inion and this may be shifted in latency resulting in two peaks. This can be corrected by stimulating only the lower visual field

• 2. Visual field defects( scotoma) the -vity recorded over contralateral scalp, contributed by peripheral field, may intermix with inion +vity resulting in two peaks. This can be corrected by recording from more lateral electrodes

Abnormal VEPs Clinical correlation

• Optic neuritis PSVEP abnormal in 90% pts. More sensitive than MRI in a study 0f 37

patients (Miller et al 1986) • Prognosis can be studied by doing PERG.The

abnormalities in PERG predict permanent visual defect

• 17 eyes permanent reduction -- poor outcome • 29 eyes PERG normal – good outcome

(Kaufman et al 1988)

Abnormal VEPs Clinical correlation

• Multiple sclerosis: Detect silent lesions: No clinical evidence of

optic neuritis - 50% abnormal PSVEP • In transverse myelitis PSVEP is abnormal in

only 10% compared to progressive myelopathy (35-70%)

• Combined Evoked Potential studies: have a higher yield in diagnosing MS (Noseworthy et al)

• Yield for diagnosis of MS: SSEP>PSVEP>BAEP

Abnormalities in MS

• The most sensitive abnormality on VEP with optic neuritis is interside P-100 amplitude difference

• In clinical practice the most useful abnormality to be detected on VEP with optic neuritis is interside P-100 latency difference

Abnormal VEPs Clinical correlation

• Compression produce PSVEP waveform

distortion and loss of amplitude but less latency delay

Brainstem Auditory Evoked Potentials

Definition- BAEP

• “Electrical events generated by neural elements along the auditory pathway in response to an auditory stimulus.” – A sensitive tool for assessment of brainstem

auditory tracts and nearby structures – Allows localization of conduction defects within

a centimeter – Resistant to systemic effects but not structural

pathology

Clinical applications

• Hearing assessment in infants. • Determining hearing loss in

uncooperative adult patients. • Diagnosing cerebellopontine angle

tumors • Evaluating brainstem function in

suspected MS. • Evaluating neuro-otological disorders. • Intraoperative monitoring.

Short Latency AEP

• Short latency AEP(BAEP): -occurring within the 1st 10

msec. following the stimulus.

-amplitude ~ 0.2 µv. -generated in the brainstem.

ANATOMY- BAEP

NEURAL ELEMENTS I. PERIPHERAL

COCHLEA SPIRAL GANGLION EIGHTH NERVE

General principles of BAEP recording

• Subject: comfortable, recline in chair or lies in bed, relax neck muscles by placing pillows under the head.

• Quiet room. • Earphones applied cautiously in children

to avoid collapse of the outer ear canal. • Subjects encouraged to sleep. • May need using sedatives (infants and

young Children).

Recording parameters- BAEP

• Filter settings: low frequency cutoff 100 Hz, and the high frequency cutoff is 3000 Hz.

• Amplification: potentials are amplified between 0.5 & 1 million times and averaged.

• Sweep length: at least 15 msec. • Averaging: to obtain clean recordings, 1000-

4000 trials should be averaged. At least 2 waveforms should be obtained using the same stimulation conditions to check for reproducibility.

Stimulation Methods- BAEP

• Earphones: most commonly used. • Loudspeakers: used only rarely for acoustic

stimulation. This method does not allow testing of monaural AEPs except in patients with unilateral deafness and in recording the Electrocochleogram (EcochG).

• Bone stimulation: not used routinely but may be helpful when air stimulation cannot be used (ex: intraoperatively, in cases of malformations of the external ear…).

Stimulus Types- BAEP

• Most AEPs are produced by Clicks or by Tone pips. • Clicks: (delivering an electrical square wave of 100-200 msec

duration to an audiologic earphone): -very satisfactory for neurological studies because they produce

sudden excitation resulting in a well defined EP. -Not very suitable for audiological studies, because they contain a

wide range of tone frequencies, mainly high frequency content , and do not test lower frequency range, which is important for speech.

- stimulation with Tones (sine wave) are desirable for audiological studies.

Noise: Not used as a stimulus, but is delivered to the nonstimulated ear as a

masking sound to reduce spread of stimulus sound delivered to the opposite ear, so that the stimulation is not bilateral

BAEP montage- ACNS

• Electrode placement – A1 left earlobe / behind the left ear – A2 right earlobe / behind the right ear – Cz vertex (10-20 system)

Montages and polarity convention- BAEP

• Dual-channel recordings are preferred for BAEP: A1-Cz, and A2-Cz.

• A ground electrode placed on the head or other body parts, and connected to the amplifier ground.

• Polarity Convention: most laboratories record the BAEPs so the deflection indicates increased positivity at the vertex electrode. This makes the relevant peaks convex upwards.

• Wave I: -appears more than 1.5 msec

after the stimulus. -in contrast to cochlear

microphonics, does not reverse with reversal of click polarity.

-decreasing click intensity leaves wave I as the last peak in this area.

Cochlear microphonic

• The cochlear microphonic is a receptor potential believed to be generated primarily by outer hair cells of the organ of Corti.

• Its detection in surface recordings has been considered a distinctive sign of outer hair cell integrity in patients with auditory neuropathy.

• Wave V: -most prominent peak after 5.5

msec. -starts above the baseline. -the immediately following trough

is maximum below the baseline. -is the last peak visible in this

area. * Waves IV & V interact to

present a variety of patterns.

• Wave III: -appears between I & V, ~

equidistant unless abnormalities are present.

-the last wave present in this area with decreasing click intensities.

Click intensity- BAEP

• Changes in click intensity produce marked changes in the absolute latency and amplitude of all BAEPs.

• All BAEP peak latencies decrease as the click intensity increases ( 0.03 ms/db).

• However, because all of the BAEPs waves latencies shift by roughly the same amounts, there is very little changes in the interpeak latencies.

• Amplitude: with decreasing the click, all peaks except I, III, and V, tend to disappear.

Intensity-latency curves • Recording at multiple

intensities (stepwise decrease of latency by 20 dB increase in intensity.

• Wave V is the last waveform to disappear at lower intensities

- Identify wave V and confirm abnormalities.

- Evaluate sensorineural hearing loss: abrupt drop in III-V slope at lower intensities.

- Evaluate conduction hearing loss: prolonged or absent I with normal III-V slope.

Click rate- BAEP

• Increasing the click rate increases absolute latency of all BAEP waves and decreases amplitude.

• Interpeak latencies (IPL) increase slightly at higher stimulus rates.

• Clicks are delivered at a rate of 8-10 Hz, allowing reproducible identification of all waves.

Stimulus polarity- BAEP

• Condensation clicks: click stimuli may be produced by electric pulses , which causes an initial deflection of the earphone membrane toward the eardrum (condensing the air or compressing the air in the ear canal), generating condensation or compression click.

Stimulus polarity- BAEP

• Rarefaction clicks: the polarity of electric pulse is reversed, so the pulse produces an initial deflection of the membrane away from the eardrum, rarefying the air in the ear canal.

• BAEPs to R clicks are used more often, because they have shorter latency and clearer definition than BAEPs to C click.

• Ideally, all patients should have BAEPs to alternating C and R clicks.

BAEP stimulus mode: Monaural only

- A normal response generated by a good ear will mask abnormal responses from a bad ear, resulting in significant loss of the test sensitivity.

- Masking noise to contralateral ear

BAEP patient variable: age

• absolute and peak latencies increase with increasing age after childhood.

• Older subjects have greater increase in IPL when stimulus intensity decreased.

• Infants: waveforms are often higher in amplitude due to smaller head size, greater proximity of the recording electrodes to BAEP generators.

• Waveforms have a simpler appearance in prematures and neonates, and an adult configuration reached by 3-6 months of age.

BAEP patient variable: body temperature

• An increase in absolute and IPLs with progressive lowering of body temperature.

• Below 32.5 C, the values are abnormal. • Below 27 C, waveforms become difficult

to identify or disappear. • Changes in BAEPs with hypoglycemia

probably result from changes in body temperature.

BAEP patient variable: peripheral hearing disorder

• Patients with hearing loss needs audiological examination to investigate the peripheral or central causes:

• Peripheral hearing loss: increase BAEP peak latencies rather than IPL, and therefore do not necessarily preclude the identification of central lesions.

Drug effects on BAEP

• IPL of BAEPs are not significantly affected by therapeutic doses of CNS depressants.

• small but statically significant increases in IPLs with high doses of thiopental infusions. (and also decrease in amplitude).

• BAEP abolition with high doses of Lidocaine and thiopental ( reversible).

• Alcohol intoxication may increase BAEP latencies.

Normal BAEP parameters- VUMC

Wave I I-III III-V I-V V/I AMP I-V inter-ear

Male Female 2.10 ms 2.10 ms 2.55 ms 2.40 ms 2.35 ms 2.20 ms 4.60 ms 4.45 ms 0.5 0.5 0.5 ms 0.5 ms

Latency abnormalities (Data analysis)

• Abnormal I-III IPL: conduction defect in the brain stem between the 8th nerve close to the cochlea and the lower pons. The lesion may be either in the nerve or in the brainstem (most common in acoustic neuroma).

• Abnormal III-V IPL: conduction defect between the lower pons and the midbrain.

• Absent wave I, and the III-V separation is normal: usually due to a peripheral hearing disorder.

• Increased I-III & III-V IPL: lesions affects the brainstem at and above the caudal pons with or without involvement of the acoustic nerve.

• Absence of III with normal I & V: normal. • Absence of V with normal I & III: indicates

a lesion above the caudal pons.

Multiple Sclerosis

• Chiappa found 1006 patients with varying classifications of MS and 466 (46%) had abnormal BAEPs.

• Most frequent abnormalities: -decreased amplitude or absence of wave V

(80% of MS patients with abnormal BAEPs). -increased III-V IPL in 1/3 of MS patients with

abnormal BAEPs. -less frequent: absence of wave III. -Chiappa recorded normal BAEPs in ~ 50% of

patients with MS and INO.

Multiple Sclerosis- BAEP

• BAEP are useful in evaluating the patients in which MS is entertained:

-patients without signs or symptoms suggesting brain stem involvement.

-provide objective evidence of brainstem involvement in patients with equivocal findings on clinical examination.

-can be used to monitor effectiveness of an experimental therapy.

Acoustic Neuromas- BAEP

• BAEPs are extremely sensitive in detecting acoustic neuromas:

- Sensitive for lesions of > 2 mm. - Distinguishes from patients with

cochlear loss such as Meniere’s disease and labyrinthine disease who have normal BAEPs.

Acoustic Neuromas: BAEP findings

-ipsilateral I-III and I-V IPL delay (one of the most sensitive

screening tests for acoustic neuromas), seen in 1/3 of patients.

-ipsilateral absence of all peaks following wave I( tumor of sufficient size to produce a conduction block at the compression point), seen in 1/3 of the patients.

-ipsilateral absence of all components including wave I. This most likely because tumor compromised the internal acoustic artery which also runs in the acoustic canal, and supplies the origin of wave I of the BAEPs.

-contralateral BAEPs are usually normal, except when the tumor is large and displaces the brainstem (if abnormality seen, usually III-V prolongation).

Coma and Brain death- BAEP

• Toxic-metabolic coma: normal BAEPs. • Brain death: -Absence of BAEPs waves II through V despite

a normal I wave indicates a significant lack of function of in the brain stem auditory tracts.

-without wave I present, no inferences can be made as to the localization of the interruption of the auditory signal since the integrity of the peripheral apparatus in any individual patient is often not known.

Conclusion- BAEP

• Commonly indicated in evaluation of suspected acoustic neuroma or Multiple sclerosis and intraoperative monitoring.

• Identification of correct waveforms is critical for interpretation

• BAEPs are the most sensitive screening test when small acoustic neuroma is suspected

• The most common BAEP abnormality in patients with CNS disease is a prolonged I-V interpeak latency.