Somatosenory Evoked
Potentials
Ronald Emerson, MD
Cornell University
Hospital for Special Surgery
New York
Median SSEPs
CPc - Ci
CPi - Epc
SC5 – Epc
Medial Lemniscus
Epi - Epc
Spinal Cord
Dorsal Gray
Tibial SSEPs
CPi – CPc
CPz – Fpz
Fpz– SC5
Medial Lemniscus
T12 - IC
Spinal Cord
Dorsal Gray
Clinical Applications of SEPs
Provide information about integrity of the large
fiber sensory system from peripheral nerve
brain.
Extension of exam; may reveal or localize a
lesion
Do not indicate particular disease process.
Normal doesn’t exclude organic cause for
symptoms.
Can be useful in documenting multiple lesions in
MS
VEPs – abnormal in 70-80% definite MS
SEPs – abnormal in 60-70% definite MS
BAEPs – abnormal in 20-40% definite MS
Not specific however; multiple EP abnormalities
also occur in other disease, including, e.g.
Cerebellar degenerations
Hereditary spastic paraparesis
SLE
Vitamin E deficiency
Pernicious Anemia
Coma
Bilateral absence of N20 is highly predictive
of poor neurological outcome or death
following anoxic (Brunko 1987; Bassetti 2002) or
traumatic (Hume 1981) brain injury.
However, presence of cortical responses
does not guarantee a good outcome.
Intraoperative EP Monitoring
Detect adverse event in time for effective corrective
action
Identify structures
Identify ways to improve surgical technique
Stimulation
Nerve: median, tibial
Ground on limb, proximal to stimulation site
Constant current stimulation
Monophasic square wave: 100 - 300 usec
Intensity: Just above motor threshold
Rate: 3 – 5 Hz (not harmonically related to 60 Hz)
Recording
Bipolar and referential derivations designed to
record signals from:
Peripheral Nerve
Cord
Brainstrem
Cortex
Recording
Filters: typically 30 – 3000 Hz (- 6 dB/octave)
Avoid 60 Hz notch filter
Avoid using the “smooth” button”
Analysis time – typically 50 msec for UE,
60 msec for LE
Number of responses averaged: sufficient,
typically 500 – 2000
Replicate
Brainstem SSEPs
Conducted Electrically Over Entire Head
Recorded using Non-Cephalic Reference
*
Cortical SSEP
Is Focal, and Rides Atop
Widely Distributed Subcortical
Right Parietal
Is recorded alone by
subtracting the
underlying subcortical
signal using bipolar
derivation
Brainstem
Left Parietal
L Parietal
Cortical +
Brainstem
Mauguiere and Desmedt, 1989
R Parietal
Right – Left
C4
– C3
Median SSEPs
“Standard” ACNS Montage
CPc - CPi
CPi - Epc
SC5 - Epc
Epi - Epc
Common Alternatives:
Combine Brainstem and
Cervical signals: Less
data but more noiseimmune.
CPc - Cpi
Cpi - SC5
Epi - Epc
CPc - Cpi
SC5 - Fpz
Epi - Epc
Tibial SSEPs
CPi – CPc
CPz – Fpz
Need 2 Channels for Cortical SSEP
Because of variations in normal
Topography:
Normal Subject A
Fpz– SC5
Normal Subject B
T12 - IC
= often hard to record without sedation
Median SSEP Interpretation
Presence of waveforms
Cortical
(N20)
Subcortical (P14)
Cervical cord
Erbs
Interpeak latencies
Interside – interpeak
Erbs – N20
Erbs – P14
P14 – N20
= may need sedation to record
these, “N13” recorded from SC5 –
Fpz may substitute.
Tibial SSEPs Interpretation
Presence of waveforms
Cortical
(P37)
Subcortical (P31, N34)
Lumbar cord (LP)
Interpeak latencies
Interside – interpeak
LP – P37
= may need sedation to record.
Median SSEP Normal Values
Upper Limit
Of Normal
(ULN)
EP Erb’s point
ULN of R-L diff.
12 msec
P14 * Brainstem – Cervical 16.3 msec
N20 Cortical
22.1 msec
EP - P14*
5.2 msec
0.7 msec
P14* – N20
6.8 msec
1.1 msec
EP – N20
10.9 msec
0.8 msec
* Recorded using Scalp – SC5 derivation
Chiappa 1987
Tibial SSEP Normal Values
Upper Limit
Of Normal (ULN)
ULN of R-L diff.
N22 Lumbar
25 msec
P31 Subcortical
34.7 msec
P37 Cortical
43.9 msec
N22 – P31
10.2 msec
1.5 msec
P31 – P38
13.4 msec
1.8 msec
N22 – P38
21.0
2.1 msec
IFCN Guidelines 1999
31 y/o Woman with MS
N20, 19 msec
CP4 – CP3
CP3 – EPc
SC5 – EPc
EP, 9.5 ms
Epi-EPc
Left median SSEP
N13, P14 and N18 Absent. EP – N20 IPL Normal.
47 y/o man with pontine AVM
CP4 – CP3
CP3 – EPc
P14, 14.3 msec
SC5 – EPc
EP, 10 msec
Epi-EPc
Left median SSEP
EP – P14 IPL normal, N18 ~ absent, N20 absent
66 y/o man with cervical spondylitic myelopathy
CP3-CP4
N20, 22.5 msec
Cp3-Epc
P14, 18 msec
SC5-Epc
EP, 10 msec
Epi-Epc
Right median SSEP
EP – P14 delayed, P14 – N20 normal
30 y/o with movement disorder
L Median SSEP
R Median SSEP
Cc’ – Ci’
Cc’ - Fpz
Ci’ - Epc
Cc’ - Epc
SC5 - FPz
SC5 - EPc
Epi-Epc
1.5 uv / div 5 msec / div
Acoustic Neuroma – 55 year old woman
Ai-Cz
Ac-Cz
Ai-Ac
Cc-Ci
Cc-Epc
Ci-Epc
EPI-Epc
9 y/o boy with cervical instability
Cc’-Ci’
Cc’-chin
Ci’-chin
Epi-Epc
3 y/o with Chiari Malformation
Pre OP
Cc’-Ci’
Ci’-Epc
Epi-Epc
Post OP
58 y/o 2 days s/p cardiac arrest
L Median SSEP
R Median SSEP
Cc’-Ci’
Cc’-Epc
Ci’-Epc
SC5-Epc
Epi-Epc
0.7 uv / div 5 msec / div
75 y/o 1 day s/p cardiac arrest
L Median SSEP
R Median SSEP
Cc’-Ci’
Cc’-Epc
Ci’-Epc
SC5-Epc
Epi-Epc
1.0 uv / div 5 msec / div
66 y/o s/p cardiac arrest
14
L Median SSEP
14
R Median SSEP
Epi-Epc
SC5-Epc
Day 1
33oC
CPi-Epc
19
19
Epi-Epc
25
24.5
CPc-CPi
10.5
10.5
Day 2
SC5-Epc
37oC
CPi-Epc
15
15
21
CPc-CPi
20
2 uV/div
44 y/o man with back pain and
left leg numbness
L Tibial SSEP
R Tibial SSEP
Ci’ – Cc’
Ci’ - Fpz
Cz’ - Fpz
Cz’ – SC5
T10 - IC
L1 - IC
Pf
1.0 uv / div 10 msec / div
55 year old man with back pain and right
leg numbness
L Tibial SSEP
R Tibial SSEP
Ci - Cc
Ci - Fz
Cz – Fpz
Cz – SC5
T10 - IC
L1 - IC
Pf
0.7 uv / div 10 msec / div
85 y/o man with compressive spondylitic myelopathy C3-C4
Weak and numb left am
L Median SSEP
R Median SSEP
Cc’ – Ci’
Cc’ - Fpz
Ci ‘- Epc
Cc’ - Epc
SC5 - Fpz
SC5 - Epc
Epi-Epc
1.0 uv / div 5 msec / div
1.5 uv / div 5 msec / div
69 y/o woman with sensory loss both legs
L Tibial SSEP
R Tibial SSEP
Ci - Cc
Cz - Fz
Fz – SC5
T10 - IC
L1 - IC
Pf
1.5 uv / div 10 msec / div
66 y/o man with spinal stenosis,
difficulty walking
L Tibial SSEP
R Tibial SSEP
Ci - Cc
Ci - Fz
Cz – Fpz
Cz – SC5
T10 - IC
L1 - IC
Pf
0.7 uv / div 10 msec / div
38 y/o man with leg numbness
L Tibial SSEP
R Tibial SSEP
Ci - Cc
Cz - Fz
T10 - IC
L1 - IC
Pf
2 uv / div 10 msec / div