Lower Extremity SSEP: Obligate peaks and recording montages following stimulation of the posterior tibial nerve. EP = Erb's. Obligate peaks and recording montages following stimulation of the posterior tibial nerve. T12 = 12th thoracic vertebra, CS = Cervical Spine, Fpz = center of frontal pole, CP = midpoint between ...

1. Lower Extremity SSEP
Anurag Tewari MD
Anurag Tewari MD

2. Anurag Tewari MD

3. Lower SSEP
• FASCICULUS GRACILIS or ‘tract of Goll’ is a
bundle of axon fibers in the posterior column-
medial lemniscus pathway of the spinal cord
and carries information from
• Middle thoracic and
• Lower limbs of the body
• Fasciculus gracilis carries information from
vertebral levels T7 and below
• It meets with nucleus cuneatus at T6
Anurag Tewari MD

4. Lower SSEP
• First-order Neurons:
• found on the IPSILATERAL SIDE of the spinal cord
• Ends at the nucleus gracile in medulla oblongata
Anurag Tewari MD

5. Lower SSEP
• Second-order Neurons
• found in the MEDULLA
• Decussate in the medulla as internal arcuate fibers
• with cuneate fasiculus, form MEDIAL LEMNISCUS
• relays information received from first-order
neurons to thalamus
Anurag Tewari MD

6. Lower SSEP
• Third-order Neurons
• fibers which relay the signal received from
THE MEDIAL LEMNISCUS
TO THE THALAMUS (CEREBRAL CORTEX)
Anurag Tewari MD

7. How are Lower SSEPs Generated?
• Ankle: Posterior Tibial Nerve Knee: Peroneal nerve
Anurag Tewari MD

8. Obtaining Lower SSEP
• Posterior Tibial Nerve Stimulation (L4-S3)
• Ideal stimulation site is posterior to or slightly above the level of the medial malleolus
• This stimulates both the medial and lateral plantar terminal nerves
• Distal stimulus will stimulate only one of the above
• Anode is to be 3cm distal to cathode
Stimulation should produce a clearly visible toe/ankle PLANTER FLEXION
Anurag Tewari MD

9. Obtaining Lower SSEP
• Peroneal Nerve Stimulation (L4-S2)
• Superficial over Fibula, immediately distal to the fibular head
• Anode is to be 3cm distal to cathode
• Can cause strong movement of leg on stimulation
Stimulation should produce a clearly visible ankle DORSIFLEXION
Anurag Tewari MD

10. Lumbosacral Plexus
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11. Lumbosacral Plexus
• Lumbar and Sacral plexus are supplied by the branches of
• INTERNAL ILIAC ARTERY
• Iliolumbar artery
• Superior Gluteal Artery
• Inferior Gluteal Artery
• Lateral Sacral Artery
• DEEP CIRCUMFLEX ARTERY
Anurag Tewari MD

12. Conus Medullaris
• The conus medullaris (Latin for "medullary cone") or conical extremity of the
Medulla Spinalis (spinal cord)
• The tapered, lower end of the spinal cord
• It occurs near lumbar vertebral L1 and L2, occasionally lower
• The upper end of the conus medullaris is usually not well defined.
Anurag Tewari MD

13. Cauda Equina
• In humans, the spinal cord stops growing in infancy and the end of the spinal
cord is about the level of the third lumbar vertebra, or L3, at birth.
• Because the bones of the vertebral column continue to grow, by about 12
months of age, the end of the cord reaches its permanent position at the level
of L1 or L2 (closer to the head).
Cauda: Latin for TAIL, Equina: Horse Anurag Tewari MD

14. Filum terminale
• The filum terminale ("terminal thread"), is a delicate strand of fibrous
tissue, about 20 cm in length, proceeding downward from the apex of the
conus medullaris.
• It gives longitudinal support to the spinal cord and consists of two parts:
• The upper part, or filum terminale internum, is about 15 cm long and reaches as far
as the lower border of the second sacral vertebra.
• The lower part, or filum terminale externum, closely adheres to the dura mater. It
extends downward from the apex of the tubular sheath and is attached to the back
of the first segment of the coccyx in a structure sometimes referred to as the
coccygeal ligament.
Anurag Tewari MD

15. Anurag Tewari MD
Innermost?
Outermost?
Spinal Cord and its Meningeal Coverage
PAD

16. The segmental control of leg movements and reflexes
Anurag Tewari MD

17. Posterior Tibial Nerve
Nerve roots: L4-S3
• Sciatic Nerve bifurcates in to
• Tibial Nerve
• Common Peroneal Nerve
• Sensory: Innervates the skin of the
• Posterolateral side of the leg
• Lateral side of the foot
• Sole of the foot
• Motor: Innervates the posterior
compartment of the leg.
Anurag Tewari MD

18. Posterior Tibial Nerve
• Motor Functions
• The tibial nerve innervates all the muscles in the posterior compartment of the
leg.
• They are divided into a deep and superficial compartment:
• Deep
• Popliteus – Laterally rotates the femur on the tibia to unlock the knee.
• Flexor Hallucis Longus – Flexes the big toe and plantar flexes the ankle.
• Flexor digitorum Longus – Flexes the other digits and plantar flexes the ankle.
• Tibialis Posterior – Inverts the foot and plantar flexes the ankle.
• Superficial
• Plantaris – Plantar flexes the ankle.
• Soleus – Plantar flexes the ankle.
• Gastrocnemius – Plantar flexes the ankle and flexes the knee.
Anurag Tewari MD

19. Common Peroneal Nerve
Nerve roots: L4-S2
• Sciatic Nerve bifurcates in to
• Tibial Nerve
• Common Peroneal Nerve
• Sensory: Innervates the skin over the
• upper lateral and lower posterolateral leg
• the skin of the anterolateral leg
• the dorsum of the foot
• Motor: Innervates the
• Short head of the biceps femoris directly.
• The muscles in the lateral and anterior
compartments of the leg.
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20. Common Peroneal Nerve
MOTOR FUNCTIONS
• Superficial Peroneal Nerve
• Foot Eversion
• Deep peroneal nerve
• Foot Dorsiflexion
• Toe Extension
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21. Anurag Tewari MD

22. Break
Questions?
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23. Recording of Lower SSEPs
• Recording of cortical responses elicited by lower limb stimulation might be
done by electrodes placed on
• the midline scalp (at Cz 1) level (or, better, 3–4 cm posterior to Cz) using Fpz
• or the ipsilateral mastoid as reference
• An electrode location 3–4 cm posterior to the Cz with a noncephalic
reference placed on the upper neck is also often used.
Anurag Tewari MD

24. Neural Generators of Lower SSEP
• The electrical stimulation of the PTN has a
• main positive peak with a latency of approximately 40 ms (P40)
• followed by a large negative peak at a latency of 45 ms (N45)
• This negative peak is generally assumed to be generated by cortical
structures
• Best recorded with
• Active electrode at the midline, 3–4 cm behind the Cz
• A frontal reference (Fz)
Anurag Tewari MD

25. Recordings of Lower SSEPs
• Active electrode placed on Cz (or 2 cm behind)
• Reference electrode either at
• Frontal scalp position or
• Non-cephalic location
• (shoulder or upper neck)
Anurag Tewari MD

26. Anurag Tewari MD
• The cortical components (P40 or P37) and subcortical components (N34 and
N21) of the lower SSEP have value for IONM
• Recordings of the P40 or P37 components of the cortical components of the
lower limb SSEP are usually made with
• the active electrode placed 2 cm posterior to the vertex (Cz′)
• and the reference electrode placed at the forehead

27. Lower SSEPs Recording Parameters
• Pulse Width = 100-300microseconds
• Stimulation Intensity = 30-40 mA (up to 100mA)
• Till PF amplitude no longer increases in size
• Twice motor threshold (around 6-10mA)
• Impedance < 5 Kilo Ohms
• for both Stimulating and Recording electrodes
• Monophasic rectangular pulses
Anurag Tewari MD

28. Lower SSEPs Recording Parameters
• Number of Trials as per ACNS = 250-1000
• Number of Trials as per ASNM = 500-2000
• Analysis Time as per ACNS = 75-150ms
• Analysis Time as per ASNM = 100ms
• Stimulation Rate as per ACNS = 2-8 per second
• Stimulation Rate as per ASNM = 2-5 per second
Anurag Tewari MD

29. Lower SSEPs Recording Parameters
• Bandpass as per ACNS = 30 - 1000Hz (-3dB)
• Low Cut = 30Hz and
• High Cut = 1000Hz (up to 3000Hz if trying to record P14)
• Bandpass as per ASNM = 30 - 1000Hz (-3dB)
• 30 -100Hz to 250-3000Hz for Corticals
• 30-100Hz to 1000-3000Hz for Subcorticals
• Notch Filter OFF
Anurag Tewari MD

30. Neural Generators of Lower SSEP
• Origins of the components of the lower limb SSEP are incompletely known
• The N17 peak is assumed to be generated near the hip joint
• The P24 peak is assumed to be generated at the 12th thoracic vertebra
N17
P24
Anurag Tewari MD

31. Neural Generators of Lower SSEP
• The P31 peak is probably generated where the spinal cord passes through
the foramen magnum
• Along with the P34 peak, these potentials might correspond to the P14–P16
complex of the upper limb SSEP.
P31
Anurag Tewari MD

32. Neural Generators of Lower SSEP
• The P34 peak is assumed to be generated in the brainstem (medial lemniscus)
• The large negative deflection (N34) following these positive peaks are
generated in the thalamus and brainstem structures.
• This peak could also be analogous to the N18 peak of the upper limb SSEP
P34
Anurag Tewari MD

33. Neural Generators for Tibial Nerve SSEP
LABEL GENERATOR MONTAGE USED ALTERNATE LABEL
POLITIAL TIBIAL NERVE ACTION POTENTIAL POLITIAL
N23 DORSAL HORN INTERNEURONS T12 – ILLIAC CREST LUMBAR POINT
P31 MEDULLA CSp – Fpz, Mast - Fpz Cervical, Subcortical
N34 Primary Sensory Cortex Cc – Fpz N37
P38 Primary Sensory Cortex Ci – Fpz, Cz’ – Fpz, Ci – Cc, Cz’ – Cc P39, P40, Cortical
N38 Primary Sensory Cortex Cc - Fpz
Cc: C3’ or C4’, which ever is contralateral Ci: ipsilateral C3’ or C4’, which ever is ipsilateral
Anurag Tewari MD

34. Lumbar Potential
(T12 – Iliac Crest)
• Biphasic Wave
• Absolute latency range: 18 to 25ms
• Maximal in amplitude at T12 level
• Represents postsynaptic potentials generated in the dorsal horn of the
spinal cord at the level of the conus
Anurag Tewari MD

35. Neural Generators for Tibial Nerve SSEP
LABEL GENERATOR MONTAGE USED ALTERNATE LABEL
POLITIAL TIBIAL NERVE ACTION POTENTIAL POLITIAL
N23 DORSAL HORN INTERNEURONS T12 –ILLIAC CREST LUMBAR POINT
P31 MEDULLA CSp – Fpz, Mast - Fpz Cervical, Subcortical
N34 Primary Sensory Cortex Cc – Fpz N37
P38 Primary Sensory Cortex Ci – Fpz, Cz’ – Fpz, Ci – Cc, Cz’ – Cc P39, P40, Cortical
N38 Primary Sensory Cortex Cc - Fpz
Cc: C3’ or C4’, which ever is contralateral Ci: ipsilateral C3’ or C4’, which ever is ipsilateral
Anurag Tewari MD

36. Neural Generators for Tibial Nerve SSEP
LABEL GENERATOR MONTAGE USED ALTERNATE LABEL
POLITIAL TIBIAL NERVE ACTION POTENTIAL POLITIAL
N23 DORSAL HORN INTERNEURONS T12 –ILLIAC CREST LUMBAR POINT
P31 MEDULLA CSp – Fpz, Mast - Fpz Cervical, Subcortical
N34 Primary Sensory Cortex Cc – Fpz N37
P38 Primary Sensory Cortex Ci – Fpz, Cz’ – Fpz, Ci – Cc, Cz’ – Cc P39, P40, Cortical
N38 Primary Sensory Cortex Cc - Fpz
Cc: C3’ or C4’, which ever is contralateral Ci: ipsilateral C3’ or C4’, which ever is ipsilateral
Anurag Tewari MD

37. P31 (Fz – CSp5)
• Far Field potential
• Obligate Peak
• Linked with P34
• Iso-potential at Fpz, Fz, Cpz, C3 and C4
• Must be referentially recorded because of In-Phase Cancellation of both
P31 and P34
• Analogous to the median nerve stimulated P13
• Not much affected by anesthetic agents
• Blood Supply: Vertebrobasilar artery
Anurag Tewari MD

38. Neural Generators for Tibial Nerve SSEP
LABEL GENERATOR MONTAGE USED ALTERNATE LABEL
POLITIAL TIBIAL NERVE ACTION POTENTIAL POLITIAL
N23 DORSAL HORN INTERNEURONS T12 –ILLIAC CREST LUMBAR POINT
P31 MEDULLA CSp – Fpz, Mast - Fpz Cervical, Subcortical
N34 Primary Sensory Cortex Cc – Fpz N37
P38 Primary Sensory Cortex Ci – Fpz, Cz’ – Fpz, Ci – Cc, Cz’ – Cc P39, P40, Cortical
N38 Primary Sensory Cortex Cc - Fpz
Cc: C3’ or C4’, which ever is contralateral Ci: ipsilateral C3’ or C4’, which ever is ipsilateral
Anurag Tewari MD

39. N34 (Fz-CSp5)
• Far Field
• Subcortical response
• Resistant to anesthetic agents
• Closely related to P31
• Analogous to N18 (Median Nerve SSEP)
• Represents postsynaptic activity from brainstem and thalamus
Anurag Tewari MD

40. P37 (CPi-FPz, CPz – FPz)
• Near Field
• Obligate peak
• Markedly affected by anesthetic agents
• Affected by High Stimulation Repetition rate
• Blood Supply: Anterior Cerebral Artery
• Bipolar subtracts widespread far field signals
• End of the central conduction time in lower limbs
• Foot area in primary somatosensory cortex
Anurag Tewari MD

41. Neural Generators for Tibial Nerve SSEP
LABEL GENERATOR MONTAGE USED ALTERNATE LABEL
POLITIAL TIBIAL NERVE ACTION POTENTIAL POLITIAL
P31 MEDULLA CSp – Fpz, Mast - Fpz Cervical, Subcortical
N34 Primary Sensory Cortex Cc – Fpz N37
P38 Primary Sensory Cortex Ci – Fpz, Cz’ – Fpz, Ci – Cc, Cz’ – Cc P39, P40, Cortical
Cc: C3’ or C4’, which ever is contralateral Ci: ipsilateral C3’ or C4’, which ever is ipsilateral
Anurag Tewari MD

42. Blood Supply
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43. Break
Questions?
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44. Lower Limb SSEP vs. Upper Limb SSEP
• The nerve tracts involved in lower limb stimulation are much longer than
those involved in median nerve stimulation
• The latencies of the peaks in the lower SSEP are much longer vs. the peaks in
the upper SSEP
Anurag Tewari MD

45. Lower Limb SSEP vs. Upper Limb SSEP
• The early peaks in the SSEP evoked by lower limb stimulation are less
distinct than those evoked by upper limb stimulation
• Because
• greater temporal dispersion of the neural activity that arrives at the brain from the
lower portion of the body because of the longer pathway than those of the upper
limb SSEP.
Anurag Tewari MD

46. Lower Limb SSEP vs. Upper Limb SSEP
• When nerve fibers have different conduction velocities, the temporal
coherence of neural activity will decrease along such nerves.
• Therefore, long nerves tend to deliver less temporally coherent neural
activity to central neural structures, than shorter pathways.
Anurag Tewari MD

47. Lower Limb SSEP vs. Upper Limb SSEP
• The amplitudes of the various peaks in the far-field response depend on the
degree of synchronization of neural activity (temporal coherence)
• Temporal dispersion results in the peaks becoming broader and smaller in
amplitude compared to similar peaks in systems that have shorter
pathways—such as the upper limb SSEP
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48. Simplified Lower Extremity SSEP Obligate Worksheet
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49. Break
Questions?
Anurag Tewari MD