This document discusses somatosensory evoked potentials (SEPs), which are electrical signals generated in the nervous system in response to sensory stimuli. SEPs reflect the activation of neural structures along somatosensory pathways. They are recorded using electrodes on the scalp and spine in response to electrical stimulation of peripheral nerves. SEP waves are labeled according to their polarity and latency. Clinical uses of SEPs include evaluating peripheral nerves and central somatosensory pathways, localizing lesions, and monitoring patients in intensive care and during surgery. Abnormal SEPs can indicate disorders of the peripheral or central nervous system.

1. Somatosensory Evoked
Potentials

2.  Evoked potentials are the electrical signals
generated by the nervous system in response to
sensory stimuli.
 Auditory, visual, and somatosensory stimuli are
used commonly for clinical evoked potential
studies.
 Somatosensory evoked potentials (SEPs) consist
of a series of waves that reflect sequential
activation of neural structures along the
somatosensory pathways.

3.  The somatosensory system sub serves five modalities:
mechanoreception, thermoreception, nociception,
proprioception and visceroception.
 These sub modalities provide conscious perception of sensory
information from the skin, the musculo-skeletal system and
the viscera
 In addition, somatosensory afferents are involved in
numerous motor and autonomous reflex pathways and
feedback loops with reflex centers in the spinal cord,
brainstem and forebrain.
 Somatosensory afferents also provide a powerful excitatory
input to the ascending RAS that regulates sleep and
wakefulness.

5. Methods of recording and
nomenclature
 Stimulation
 Recording
 Wave labeling, generators and normal values

6. Stimulation
 SEPs are usually evoked by bipolar transcutaneous
electrical stimulation applied on the skin over the
selected nerve.
 Monophasic square-wave electrical pulses of 0.1–
0.2 ms should be delivered, preferably by a constant
current stimulator.
 In routine recording, stimulus parameters include a
stimulus intensity able to produce a clear but
tolerable sensation.
 Stimulation rate should be 3–5 Hz

7.  The stimulation sites typically used for clinical
diagnostic SEP studies are the median nerve at the
wrist, the common peroneal nerve at the knee,
and/or the posterior tibial nerve at the ankle.
 Recording of SEPs to stimulation of the ulnar nerve
at the wrist is useful for intraoperative monitoring
when the mid cervical spinal cord or parts of the
brachial plexus are at risk.
 Recording electrodes are placed over the scalp,
spine, and peripheral nerves proximal to the
stimulation site

8. Recording
 Recording electrodes are standard EEG disk
electrodes.
 Skin-electrode impedance should be less than
5000 Ω.
 The optimal condition to record clinically-
relevant SEP components is a high pass filter at
less than 3 Hz and a low-pass filter over 2000 Hz

9. Median nerve SEP
 For standard clinical recordings at least four channels
designed to highlight one or more component each:
peripheral (Erb’s point) channel (N9), cervical channel (N13),
parietal channel (P14 and N20) and frontal channel (P14, P20
and N30).
 Peripheral Erb’s point electrodes are designated as EP and
must be placed within the angle formed by the posterior
border of the clavicular head of the sternomastoid muscle
and the clavicle, 2–3 cm above the clavicle (Erb’s point).
 The active electrode is ipsilateral to stimulation (EPi), and the
reference is either the contralateral EP electrode (EPc) or a
scalp electrode (generally Fz).

12. Tibial nerve SEP
 For standard clinical recordings four channels designed to
highlight one component each: peripheral (N8), lumbar (N22),
subcortical (P30) and cortical (P39)
 The peripheral recording electrode is placed in the popliteal
fossa (PF) 4–6 cm above the popliteal crease.
 A reference electrode may be placed either on the medial
surface of the knee, over the medial femoral condyle or 3 cm
above the active electrode.
 Lumbar electrodes should be placed on the skin overlying the
spinous processes of a lumbar vertebra, most often L1.

15.  In the lower limb, posterior tibial SEPs are generally preferred
because of the following:
 In clinical diagnostic use, they are larger and display less
intrasubject variability.
 In intraoperative settings, they produce less patient
movement.
 In intraoperative settings, electrodes at the ankle are more
accessible, and thus more easily replaced should they
malfunction, than those at the knee.
 The peripheral nerve compound action potential (CAP) can be
recorded at the popliteal fossa and can be used to determine
whether the nerve is being adequately stimulated.

16. Wave labeling
 In the nomenclature of SEP waveforms, N or P
followed by an integer are, respectively, used to
indicate the polarity and the nominal post-stimulus
latency or typical peak latency (ms) of the recorded
wave in the healthy population (e.g. N20).
 For example, N20 is a negativity that typically peaks
at 20 milliseconds after the stimulus
 The potentials can be recognized by their typical
distribution, reflecting the activation of their
generators, and can be measured in terms of latency
(ms), amplitude (μV) and intervals between peaks.

17. Clinical uses of SEPs
1. Evaluation of the peripheral nervous system and the large-
fiber sensory tracts in the CNS
2. Localization of the anatomic site of somatosensory pathway
lesions
3. Identification of impaired conduction caused by axonal loss
or demyelination
4. Confirmation of a nonorganic cause of sensory loss
5. To confirm the presence of normal conduction pathways in
patients with conversion disorder, malingering, or other
psychological disturbances.

18. Peripheral disorders
 SEPs have been used to evaluate peripheral nerves that
cannot be studied by conventional nerve conduction studies,
and the proximal portions of peripheral nerves that are
otherwise inaccessible for study.
 To detect proximal involvement in patients with Guillain–
Barre´ syndrome, although F-wave studies are more useful for
this purpose.
 For recognition of a lesion in patients with such proximal
entrapment neuropathies as meralgia paresthetica.
 To assess brachial plexus lesions, but their utility is less than
that of EMG.

19. Central nervous system
 In patients with possible multiple sclerosis (MS) who do not
have clinical involvement of the central somatosensory
pathways, the tibial-derived SEP may be abnormal in about
one-third of cases.
 However, multifocal involvement of central white matter
either clinically or electrophysiologically is not specific to
multiple sclerosis but may occur in patients with human HIV
infection, vitamin B12 or vitamin E deficiency ,neurosyphilis,
hereditary ataxic syndromes, hereditary spastic paraplegia ,
and other neurological disorders.

20.  Evoked potential studies and MRI are complementary
techniques for detecting lesions in patients with MS, but at
the present time SEPs are not recommended for the
detection of subclinical lesions unless imaging facilities are
unavailable.
 SEPs may be useful, however, to test the integrity of
pathways in MS patients with vague symptoms of uncertain
significance.
 SEPs have also been used for monitoring disease progression
and evaluating novel therapeutic agents in patients with
suspected or definite MS

21.  In patients with spinal injury, SEPs may be helpful in showing
the completeness of the lesion.
 An incomplete lesion is suggested by preserved responses or
their early recovery after injury and thus a good prognosis.
 In patients with spinal cord tumors or other structural lesions
involving the dorsal column, SEPs may be abnormal and help
to localize the lesion; but they are usually unnecessary
because imaging studies are more useful in this regard.

22.  In patients with intractable pain being considered for spinal
cord stimulation, good functional status of the dorsal columns
is mandatory if a good clinical result is to be achieved.
 Accordingly, the finding of abnormal preoperative SEPs may
be taken to reliably predict a lack of clinical effect and is thus
a contraindication to spinal cord stimulation

23. Clinical applications of SEPs in the ICU
Diagnosis
 Diagnosis is usually not the primary aim of SEPs in ICU, except
in some circumstances:
 the identification of a possible structural brainstem lesion in a
coma of unknown aetiology (when MRI is unavailable)
 as a contributory tool for the diagnosis of de-efferented
states and psychogenic unresponsiveness
 and together with other neurophysiologic and/or
angiographic tools, to confirm a clinically suspected brain
death.
 Brain death is associated with the loss of all cortical and
subcortical SEP components, including P14, with preserved
sensory nerve action potential, spinal N13 and P13.

24. Prognosis
 The prognostic value of SEPs differs in anoxic and
traumatic coma.
 Briefly speaking, SEPs are the most powerful tool
to provide bad (but not good) news in brain
anoxia, and an excellent tool to provide good
(but not bad) news in head trauma.

25. Anoxic coma
 The bilateral absence of N20 (with P14 preservation) more
than 12 (and, probably six) hours after coma onset in anoxic
coma has always been associated with death or permanent
vegetative state.
 This makes SEPs the most powerful tool for an unfavorable
prognosis in anoxic coma.
 Conversely, mildly altered SEPs (CA Level 1) do not allow
drawing any conclusion in terms of prognosis.

26. Head trauma
 One major difference between brain anoxia and head trauma
is that, in the latter situation, the bilateral absence of N20 has
been associated with recovery in about 15% of cases.
 The most likely explanation is that, in head trauma, a
transient N20 disappearance may be consecutive to focal
midbrain dysfunction due to edema.
 Conversely, mildly altered SEPs in the absence of brainstem
dysfunction has been associated with a good recovery in
more than 80% (Level 2 CA) to 90% (Level 1 CA) of cases.

27. Use of SEPs for intraoperative monitoring
 Like other neurophysiologic tools, SEPs can be
used in the operating room (OR) for three main
purposes:
 to prevent neurological damage
 to follow-up induced physiological changes
 and to locate the central sulcus

28. Basic principles of SEP analysis for
intra-operative monitoring
Structure Tested Clinical applications
Peripheral nerve Large sensory fibres Prevention of peripheral-nerve
lesions in peripheral surgery
Spinal cord Posterior columns Scoliosis surgery
Thoracic and thoracoabdominal
aorta surgery
Brainstem Lemniscal pathways Brainstem surgery (in association
with BAEPs and motor EPs)
Thalamo cortical
pathways
Cerebral cortex MCA territory(median
nerve)
ACA territory(tibial
nerve)
Carotid endarterectomy
Intracranial aneurysm surgery
Cardiac surgery

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