somatosenory evoked potentials reading session... · epi - epc cpc - cpi sc5 - fpz epi - epc. cpi...

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


Cc’-Ci’

Cc’-Epc

Ci’-Epc

SC5-Epc

Epi-Epc


75 y/o 1 day s/p cardiac arrest


Cc’-Ci’

Cc’-Epc

Ci’-Epc

SC5-Epc

Epi-Epc


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


33oC

37oC

Day 1

Day 2

2 uV/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


Ci - Cc

Ci - Fz

Cz – Fpz

L1 - IC

Pf


T10 - IC

Cz – SC5

55 year old man with back pain and rightleg numbness


Cc’ – Ci’

Cc’ - Fpz

Ci ‘- Epc

SC5 - Fpz

Epi-Epc


Cc’ - Epc

SC5 - Epc

85 y/o man with compressive spondylitic myelopathy C3-C4Weak and numb left am



Ci - Cc

Cz - Fz

Fz – SC5

L1 - IC

Pf


T10 - IC

69 y/o woman with sensory loss both legs


Ci - Cc

Ci - Fz

Cz – Fpz

L1 - IC

Pf


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


T10 - IC

38 y/o man with leg numbness

somatosenory evoked potentials reading session... · epi - epc cpc - cpi sc5 - fpz epi - epc. cpi...

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