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Somatosenory Evoked Potentials
Ronald Emerson, MDCornell UniversityHospital for Special SurgeryNew York
Spinal Cord Dorsal Gray
Medial Lemniscus
CPc - Ci
CPi - Epc
SC5 – Epc
Epi - Epc
Median SSEPs
Spinal Cord Dorsal Gray
Medial Lemniscus
CPi – CPc
CPz – Fpz
Fpz– SC5
T12 - IC
Tibial SSEPs
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 MSSEPs – abnormal in 60-70% definite MSBAEPs – abnormal in 20-40% definite MS
Not specific however; multiple EP abnormalities also occur in other disease, including, e.g.
Cerebellar degenerationsHereditary spastic paraparesisSLEVitamin E deficiencyPernicious 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 actionIdentify structuresIdentify 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 NerveCordBrainstremCortex
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 HeadRecorded using Non-Cephalic Reference
Right Parietal
Left Parietal
Is Focal, and Rides AtopWidely Distributed Subcortical
C4 – C3
Brainstem
Cortical + Brainstem
L Parietal
R Parietal
Right – Left
Is recorded alone by subtracting the underlying subcortical signal using bipolar derivation
Cortical SSEP
Mauguiere and Desmedt, 1989
CPc - CPi
CPi - Epc
SC5 - Epc
Epi - Epc
Median SSEPs
“Standard” ACNS Montage Common Alternatives:
Combine Brainstem and Cervical signals: Less data but more noise-immune.
CPc - CpiCpi - SC5Epi - Epc
CPc - CpiSC5 - FpzEpi - Epc
�
CPi – CPc
CPz – Fpz
Fpz– SC5
T12 - IC
Need 2 Channels for Cortical SSEPBecause of variations in normal Topography:
Tibial SSEPs
Normal Subject A
Normal Subject B
�
�
� = often hard to record without sedation
Median SSEP InterpretationPresence of waveforms
Cortical (N20)Subcortical (P14)Cervical cordErbs
Interpeak latenciesInterside – interpeak
Erbs – N20Erbs – P14P14 – N20
�
�
� = may need sedation to record these, “N13” recorded from SC5 –Fpz may substitute.
Tibial SSEPs InterpretationPresence of waveforms
Cortical (P37)Subcortical (P31, N34)Lumbar cord (LP)
Interpeak latenciesInterside – interpeak
LP – P37
�
�
� = may need sedation to record.
Median SSEP Normal Values
Upper Limit Of Normal (ULN)
ULN of R-L diff.
EP Erb’s point 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
CP4 – CP3
CP3 – EPc
SC5 – EPc
Epi-EPc
31 y/o Woman with MS
EP, 9.5 ms
N20, 19 msec
N13, P14 and N18 Absent. EP – N20 IPL Normal.
Left median SSEP
EP, 10 msec
P14, 14.3 msec
47 y/o man with pontine AVM
EP – P14 IPL normal, N18 ~ absent, N20 absent
CP4 – CP3
CP3 – EPc
SC5 – EPc
Epi-EPc
Left median SSEP
66 y/o man with cervical spondylitic myelopathy
EP, 10 msec
P14, 18 msec
N20, 22.5 msec
EP – P14 delayed, P14 – N20 normal
Epi-Epc
SC5-Epc
Cp3-Epc
CP3-CP4
Right median SSEP
1.5 uv / div 5 msec / div
Cc’ – Ci’
Cc’ - Fpz
Ci’ - Epc
SC5 - FPz
Epi-Epc
L Median SSEP R Median SSEP
Cc’ - Epc
SC5 - EPc
30 y/o with movement disorder
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
Post OPPre OP
Cc’-Ci’
Ci’-Epc
Epi-Epc
58 y/o 2 days s/p cardiac arrest
0.7 uv / div 5 msec / div
Cc’-Ci’
Cc’-Epc
Ci’-Epc
SC5-Epc
Epi-Epc
L Median SSEP R Median SSEP
75 y/o 1 day s/p cardiac arrest
1.0 uv / div 5 msec / div
Cc’-Ci’
Cc’-Epc
Ci’-Epc
SC5-Epc
Epi-Epc
L Median SSEP R Median SSEP
14
1924.5
19
25
14
10.5
15
21
10.5
1520
66 y/o s/p cardiac arrest
Epi-Epc
SC5-Epc
CPi-Epc
CPc-CPi
Epi-Epc
SC5-Epc
CPi-Epc
CPc-CPi
L Median SSEP R Median SSEP
33oC
37oC
Day 1
Day 2
2 uV/div
1.0 uv / div 10 msec / div
Ci’ – Cc’
Cz’ - Fpz
Cz’ – SC5
L1 - IC
Pf
L Tibial SSEP R Tibial SSEP
T10 - IC
44 y/o man with back pain and left leg numbness
Ci’ - Fpz
0.7 uv / div 10 msec / div
Ci - Cc
Ci - Fz
Cz – Fpz
L1 - IC
Pf
L Tibial SSEP R Tibial SSEP
T10 - IC
Cz – SC5
55 year old man with back pain and rightleg numbness
1.5 uv / div 5 msec / div
Cc’ – Ci’
Cc’ - Fpz
Ci ‘- Epc
SC5 - Fpz
Epi-Epc
L Median SSEP R Median SSEP
Cc’ - Epc
SC5 - Epc
85 y/o man with compressive spondylitic myelopathy C3-C4Weak and numb left am
1.0 uv / div 5 msec / div
1.5 uv / div 10 msec / div
Ci - Cc
Cz - Fz
Fz – SC5
L1 - IC
Pf
L Tibial SSEP R Tibial SSEP
T10 - IC
69 y/o woman with sensory loss both legs
0.7 uv / div 10 msec / div
Ci - Cc
Ci - Fz
Cz – Fpz
L1 - IC
Pf
L Tibial SSEP R Tibial SSEP
T10 - IC
66 y/o man with spinal stenosis,difficulty walking
Cz – SC5
2 uv / div 10 msec / div
Ci - Cc
Cz - Fz
L1 - IC
Pf
L Tibial SSEP R Tibial SSEP
T10 - IC
38 y/o man with leg numbness