SOMATOSENSORY EVOKED POTENTIAL

                       

                   Mr. Sunil Mehta -DNEP(INK)
                   Neuroelectrophysiologist.
                   Neotia Getwel Healthcare Centre

SOMATOSENSORY EVOKED POTENTIAL:

vSomato- Body

vSensory- sensation

vEvoked - Image
vPotential- Reading

ØShort latency somatosensory evoked potential are the electrical potential generated mainly by the large diameter peripheral and central sensory pathways in response to sensory stimulus.

large  diameter  peripheral - Large fiber nerve cells have diameters more then about 5 MICROMETERS, AND SMALL FIBER ARE THINNER. THE LARGE FIBER AREBIGGER BECAUSE THE AXON ARE SHEATHED IN FATTY WRAPPINGS CALLED "MYELIN" THIS HELPS THEM TO SEND SIGNALS FASTER THAN SMALL- FIBERS.   

SSEP PATHWAYS:- 

◦Dorsal root ganglia - Gracile and cuneate nuclei in the lower medulla -  fasciculi gracilis and cuneatus in the spinal cord - pons -  medial lemniscus – posterolateral of the thalamus- sensory cortex. 

IMPORTENE PROBLAME IN SSEP RECORDING:-

vExcessive muscle artefact.

vUncomfortable temperature.
vEnvironment temperature should be always 20°C TO 25°C.
vFull relaxation and cooperation.
vThe SSEP study is carried out with the patient supine and proper head support to relax the neck muscles.

vThe SSEP can recorded by stimulating any large nerve although median and posterior tibial nerves are commonly used in clinical practice. 
vSleep may change the apparent N20 latency by increasing the amplitude of the pick component to wake stage.



Median SSEP:-


vElectrode placement :-
  Surface electrode, 1 cm discs filled with conducting jelly or electrode   past, are  recommended. The  impedance should be kept below 5 kΩ.The median nerve was stimulated at each wrist by square-wave electrostimulation at 5 hz and with 0.2 ms duration.

vElectrode placement:-

•Channel 1 -  Active(-) - ipsilateral Erb’s  point(EP1/EPi )
                          Reference(+) -non-cephalic reference.
• Channel 2 – Active(-) - contralateral Erb’s  point.(Ep2 / EPc)
                            Reference(+) - non-cephalic reference.
• Channel 3 – Active(-) - Fifth cervical spinal segment.(C5S / C5SP)
                            Reference(+) - non-cephalic reference.
• Channel 4 –Active(-) Primary sensory cortex, VPL nucleus of                                                                         thalamus(Ci/Cc )
                            Reference(+) -cephalic reference.

 **Non-cephalic reference - common reference. 

Channel (Montage) Setting:-

The montage suggested by IFCN (INTERNATIONAL FEDERATION OF CLINICAL NEUROPHYSIOLOGY) for median SSEP is as follows.

◦Channel 4 :- Cc-Fz
◦Channel 3 :- Cc-Epc
◦Channel 2 :- C5Sp-Epc
◦Channel 1 :- Epi-Epc

*WAVEFORM      GENERATE

◦N9     -         Brachial Plexus.

◦N13   -         Fifth Cervical segment.
◦N18   -         Brainstem nuclei, thalamus.
◦N20   -         Primary sensory cortex.

The recording should be twice averaged to check for reproducibility.  If waveform are not clear, the following methods can be tried. The recording should be twice averaged to check for reproducibility.  If waveform are not clear, the following methods can be tried. 

◦Multiple trials.

◦Increasing the averaging to 2000 trials.
◦Increasing stimulus intensity.
◦Use the sedative in children and in adults if excessive muscle artifacts are present.

◦SSEP are resistant to various  drugs, therefor sedation can be used for improving the recording in uncooperative patient. On administering 10mg diazepam orally, the muscle artifacts of SSEP recording were much reduced. 




Measurement normal values.   

◦Measurement

The following parameters are measure for the analysis of median nerve.

1.Latency
2.Amplitude
3.Interpeak latency.

Normal value for median SSEP by IFCN:-
 N9  - 11.0ms.
 N13- 14.5ms.
 N18- 19.5ms.
 N20-21.5ms.



TIBIAL SSEP:-

ØElectrode placement:-

    •Channel 1- Active – Popliteal fossa(PF)

                  Reference –medial surface of the knee(k)

•Channel 2 - Active - Third lumbar vertebra (L3)

                   Reference- First lumbar vertebra (L1)

•Channel 3 - Active -Twelfth thoracic vertebra (T12)

                  Reference -Tenth thoracic vertebra (T10)

•Channel 4 –Active - Cz position (according to 10-20 E.P. system)

                      Reference - Fz position (according to 10-20 E.P. system)

Channel (Montage)Setting:-The montage suggested by IFCN (INTERNATIONAL FEDERATION OF CLINICAL NEUROPHYSIOLOGY) for Tibial SSEP is as follows.
◦Channel 4- Cz-Fz
◦Channel 3- T12-T10
◦Channel 2- L1-L3
◦Channel 1- FP-K

v WAVEFORMS     GENERATORS   
       N8 - Tibial or sciatic nerve
       N22 - Dorsal Gray matter of lumbar spinal cord
       N23 - Dorsal Gray matter of thoracic spinal cord
       N28 - Cervical Spinal Cord
       P37- Primary sensory cortex

NORMAL VALUE (According to IFCN):-
v WAVEFORMS     GENERATORS   
   ◦N8 - 08.05ms-10.05ms
   ◦N22- 22.00ms-24.05ms
   ◦N23- 23.00ms-26.05ms
   ◦P37-  37.05ms-42.00ms

   ***

   ◦N8-P37 ~28.5-32.00ms
   ◦N22-P37~ 15.5-18.5ms

PATIENT RELATED FACTOR:-

◦Age has a significant on SSEP.
◦Children N9 and N13  potential of median SSEP occur quite early while the central conduction is relatively slow.
◦In elderly individuals, the normal limit for most of the latencies are longer by 5-10 % which occur after the age of 55 years.
◦After 50-60 years age, prolongation of latencies was attributed to both peripheral and central changes.
◦Female have a shorter CSCT(CENTRAL SENSORY CONDUCTION TIME) compared to male but the reason for this is not know.

CLINICAL APPLICATION OF SSEP:-

 SSEP have a good correlation impairment of joint position and vibration  sensation. The correlation between SSEP and sensation is batter in spinal cord compare to cerebral lesion for SSEP abnormalities, a significant degree of sensory impairment is necessary. The SSEP are however abnormal in patient with ms (multiple sclerosis). in general latency abnormalities are more pronounced in demyelinating and amplitude in ischemic lesion.

Demyelinating diseases:-

Multiple sclerosis:-
Multiple sclerosis means-scar tissue in multiple areas
MS affects the central nervous system (CNS), but exactly why is happens is
unclear.In the CNS, nerve fibers are surrounded by a myelin sheath, which
protects them. 
Myelin also helps the nerves conduct electrical signals quickly and
efficiently. In MS, the myelin sheath disappears in multiple areas, leaving a scar,
or sclerosis.
When there is no myelin, the fibers cannot conduct the electrical impulses at all.
The messages from the brain to the muscles cannot be transmitted. 

   



*** In MS, the SSEP and invaluable role in detecting clinical silent lesion, objectively documenting the equivocal sensory finding and assessing the the effect of the therapeutic trials. Since the peripheral nervous system is spared in MS, the abnormality manifests with delay in cortical and subcortical potentials resulting in abnormal IPL with normal peripheral potentials. central sensory conduction slowing may also occur because of temporal dispersion as in peripheral demyelinating  
disease. comparing the diagnostic yield of SSEP, VEP, and BAEP in MS, SSEP was most frequently abnormal 60% compare to VEP 32 % and BAER.

Degenerative diseases:-
Degenerative nerve diseases affect many of your body's activities, such as balance, movement, talking, breathing, weakness, Sensory loss, Bladder and bowel function problems 

 such as Cervical spondylosis,  Lumbar spondylosis, Hereditary Ataxia, Myoclonus and Other causes.

1.Cervical spondylosis
2.Lumbar spondylosis
3.Hereditary Ataxia
4. Myoclonus

1.Cervical spondylosis:-