Evoked potentials are electrical signals produced by the nervous system in response to external stimuli. The document discusses several types of evoked potentials including brainstem auditory evoked potentials (BAEP), visual evoked potentials (VEP), and somatosensory evoked potentials (SEP). For BAEP, electrodes are placed on the ear and vertex to assess conduction through the auditory pathway up to the midbrain. VEP involves recording potentials from the scalp in response to visual stimuli to evaluate visual function. SEP evaluates sensory pathways from peripheral nerves to the spinal cord and cortex by stimulating nerves like the median and posterior tibial. The document outlines the electrode placement, stimulation parameters, and normal/abnormal findings for these ev
2. INTRODUCTION
• Evoked potentials (EPs) are the electrical signals produced by the
nervous system in response to an external stimulus.
• The term EPs was originally used to refer to the responses to sensory
stimulation.
• Sensory EPs can be recorded following stimulation in any sensory
modality, but visual EPs (VEPs), auditory EPs (AEPs), and
somatosensory EPs (SEPs) are most often used for clinical diagnosis
and testing, intraoperative monitoring (IOM), and neurophysiological
research.
5. • BAEP are the potentials recorded from the ear and vertex in
response to a brief auditory stimulation to assess the
conduction through auditory pathway up to mid-brain.
• Auditory brainstem response (ABR)
• Brainstem auditory evoked response (BAER)
• Brainstem evoked response audiometry (BERA)
6.
7. METHOD
• Surface electrodes preferred- 1cm disc
electrode
• 2 channels : Ipsilateral (Ai) and
contralateral (Ac) referred to Cz.
• Channel 1: Ai-Cz
• Channel 2: Ac-Cz
• Ground: Fz
• 2000 trials are averaged.
8. Stimulation
• Square wave pulse 0.1
ms duration.
• Click rate 11-31 Hz
• Stimulus intensity : 70dB
• White noise of 30-40 dB
contralat.
Response
• 5-8 vertex positive peaks
• Trough and peaks denoted by roman
numbers.
9.
10. 1) Absolute latency and amplitude
2) Inter peak latencies
3) Amplitude ratio of wave V/I or
IV-V
4) Inter ear peak differences.
What we are
supposed to see
???
11. Clinical uses and positive findings
I-V IPL
- conduction from proximal
VIII nerve through pons to
midbrain.
- normal I-V IPL is 4.5 ms.
- Prolonged in Focal damage,
tumors, hypoxic brain
damage.
I-III IPL
• VIII nerve across subarachnoid space into the
core of lower pons.
• cerebellopontine angle tumors can cause a
delay at this juncture. Infarction can also
result in prolongation
12. • CP angle tumor (71% sen.
74%spe.)
• 1. Unrecordable BAEP
• 2. Only wave-I recordable
• 3. Prolongation of wave III and V
latency
• 4. Prolonged I-III and I-V IPL
• 5. Right to left asymmetry in wave
V latency >0.5 ms
Multiple Sclerosis
1. Amplitude reduction and absence of
wave V
2. Prolongation of III-V IPL
3. Prolongation of I-V IPL
4. Reduction of V/I ratio
5. Absence of wave III
• COMA
• Prognostic predictor
• Better predicting outcome
of severe head injury
compared to GCS , motor
signs and pupillary
reflexes.
14. • Visual evoked potentials
are electrical potential
differences recorded from
scalp in response to visual
stimuli.
• Visual evoked potentials
are very useful in
evaluating visual function.
15. METHOD
• PRETEST EVALUATION
• Explain the test to ensure full cooperation
• Avoid hair spray or oil.
• The usual glasses if any should be put on
• visual acuity, pupillary diameter, and field charts should be reviewed
before starting the test.
• Avoid any miotic or mydriatic drugs 12h before the test.
16. • Standard disc
electrodes are used.
• The recording
electrode at Oz
• The reference is
placed at the FpZ or
12cm above the
nasion.
• The ground electrode
17.
18. • NORMAL VEP
• Represent a mass response
of cortical and possibly the
subcortical areas.
• Consist of a series of
waveform of opposite
polarity.
• The negative waveform is
denoted as N and positive
deflection as P, which is
followed by the approximate
latency in ms.
• Most common N70,
P100,N135
19. VEP ABNORMALITIES
• The unilateral VEP abnormality therefore, obtained by full field monocular
stimulation is likely to be due to prechiasmal lesion.
• The commonest cause of prolonged P100 latency is demyelination in the
optic pathways where the amplitude of P100 remains normal.
• DEMYELINATING
• plaque of 10 mm size would result in VEP delay of 25 ms.
(Multiple sclerosis)
• Optic Neuritis – monocular loss of vision – 20-50 age
• Nutritional and toxic neuropathy
• COMPRESSIVE :
• Extrinsic compression of anterior visual path ways results in loss
of amplitude, distortion of waveform, and prolongation of P100
latency.
22. • SEPs are the electrical potentials generated mainly by the large
diameter sensory fibers in the peripheral and central portion of
the nervous system.
• The short latency potentials, which lie within 50 ms after the
stimulus are of clinical interest.
• The major advantage of SEP lies in evaluating the relatively long
sensory pathway from peripheral nerve to spinal cord and
cerebral cortex.
23.
24. METHOD
• Patient supine
• Mild hypnotics may be used
• Sensory signs/symptoms and peripheral n. injury should
be obtained first.
• Procedure should be explained
• Recorded by stimulating any large nerve : Median and
posterior tibial
25. Median SsEP
• 1 cm disc electrodes
• recording electrodes - the
Erb's point (2-3 cm above
the mid clavicular point),
spinous process of c5 and 2
cm posterior to C3 or C4 .
• Left and right Erb's point
electrodes are des ignated as
EP1 and EP2, respectively,
• The spinal electrode is
designated as C5S or C5Sp.
• The scalp electrodes are
named as C2 C4
• The Fz, electrode is used as
26. • 200 micro.v square pulse
• Current – 5 to 15 mA
• Analysis time – 50-60ms
• Stimulation at wrist – cathode 2cm proximal to wrist crese.
• Rate of stimulation 3-8 Hz.s
• 1000-2000 averaged.
28. TIBIAL SSEP
• Posterior tibial nerve at the ankle.
• Recording electrodes at the popliteal fossa
(PF) 4-6 cm -between semitendinosus and
biceps femoris.
• Reference electrode is placed on the medial
surface of knee (K).
• Spinal recording electrodes - L1 referred to
third lumbar L3 and T 10 referred to T 12
• The scalp-recording electrode is placed 2
cm posterior to Cz referred to Fz
29.
30.
31.
32. • Multiple sclerosis – silent lesisons can be detected.
• Trauma – SCI, root avulsion and plexopathies as SNAP are taken but
after 4-5 days while this can be taken in acute stage too.
• Vascular lesions - Hemorrhage.
• Cervical myelopathy
• Spinal cord tumor.
33.
34.
35.
36. REFERENCES
• Clinical electrophysiology , K. Mishra 2nd edition
• ORIGINAL CONTRIBUTION soleus h-reflex modulation after motor
incomplete spinal cord injury: effects of body position and walking
speed, chetan P. Phadke et.Al , 2009
• ISCEV standard for clinical visual evoked potentials: (2016 update) J.
Vernon odom (2016)
• The electrophysiological assessment of visual function in Multiple
Sclerosis Joshua L. Barton et al. , 2019.
