SSEP

Overview of SSEPs

Somatosensory Evoked Potentials (SSEPs) are electrical responses recorded from the nervous system following electrical stimulation of a peripheral nerve.  For example, stimulation of the median nerve at the wrist produces electrical activity that travels along the sensory pathway on its way to the brain. This activity can be recorded with electrodes positioned along that pathway.

SSEPs recorded from various locations along the sensory pathway following electrical stimulation of the ulnar nerve at the wrist.

SSEPs recorded from various locations along the sensory pathway following electrical stimulation of the ulnar nerve at the wrist.

Nerves Commonly Stimulated:

  • Median Nerve (wrist).
  • Radial Nerve (spiral groove).
  • Ulnar Nerve (wrist).
  • Saphenous Nerve (distal border between the vastus medialis and sartorius muscles)
  • Common Peroneal (popliteal fossa).
  • Deep Peroneal (ankle).
  • Posterior Tibial Nerve (medial malleolus).

Common Recording Sites for Upper Extremity Stimulation:

  • Erb’s Point
  • Cervical Spine (C5, C2)
  • Cerebral Cortex

Common Recording Sites for Lower Extremity Stimulation:

  • Popliteal Fossa
  • Thoracic Spine (T10)
  • Cervical Spine (C5, C2)
  • Cerebral Cortex

SSEPs in IONM

The SSEP is ultimately an average of all responses recorded after many successive stimulus presentations. The number of presentations required to record a well-formed, reliable SSEP ranges from dozens to hundreds. This is because the SSEP is a relatively small response recorded in an electrically-noisy environment. Because many trials are required to record SSEPs, it can take seconds to minutes to record an SSEP. Thus, the response is delayed and not a real-time indicator of neural function.

How we view it:

Each peak and trough is designated by a letter/number combination.  The letter P for positive, N for negative describes the polarity of the response. A Negative signal results in upward deflection, and a Positive signal results in downward deflection). The number indicates the latency in milliseconds after stimulus presentation.

Upper Extremity SSEP:

Obligate peaks and recording montages following stimulation of the median or ulnar nerve.

Obligate peaks and recording montages following stimulation of the median or ulnar nerve. EP = Erb’s Point, CS = Cervical Spine, CP = midpoint between central and parietal electrodes, i = ipsilateral to stimulation site, c = contralateral to stimulation site.

Lower Extremity SSEP:

Obligate peaks and recording montages following stimulation of the posterior tibial nerve. EP = Erb's Point, CS = Cervical Spine, CP = midpoint between central and parietal electrodes, i = ipsilateral to stimulation site, c = contralateral to stimulation site.

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 central and parietal electrodes, i = ipsilateral to stimulation site, c = contralateral to stimulation site.

Interpretation:

Changes in the SSEP may be indicative of evolving injury somewhere along the pathway between stimulating and recording electrodes. In order to detect changes in SSEPs, the responses must be quantified.  Thus, when we record SSEPs, we look at the following variables:

Latency:

Nerve conduction times are fairly well established.  If we stimulate a nerve at the wrist, we know that it takes approximately 9 msec for that activity to reach the shoulder, 13 msec to reach the cervical spine, and 2o msec to reach the cerebral cortex. Thus, if we record at those points, we should expect to see responses at 9, 13, and 20 msecs after stimulus onset, with individual variability. Conduction time is slowed by low temperatures, nerve compression, nerve injury, etc.  So, increases in these latencies, can be indicative of evolving injury.

Amplitude:

SSEP amplitude varies by recording location and patient (individual variability, pathology, etc). Neural transmission is altered by low temperatures, nerve compression, nerve injury, etc.  So, decreases in amplitude, can be indicative of evolving injury.

Why we use it:

1.  Monitoring Peripheral Nerves (positioning):

Compression or stretch of peripheral nerve, usually as a result of patient malpositionng during surgery, can result in attenuation of SSEPs. The most common sites of compression or stretch include the ulnar notch, brachial plexus, spiral groove, femoral nerve and fibular head. When the appropriate nerve is selected, SSEPs are particularly sensitive to detecting evolving injuries, allowing the surgical team to intervene by optimizing the patient’s position.  Left untreated, malpositioning can result in permanent neurologic deficit.

2. Monitoring Peripheral Nerves (vascular):

Vascular occlusion can result in decreased perfusion to a limb, and ultimately permanent neurologic injury.  Perhaps the most common situation in which this occurs is during anterior lumber interbody fusion (ALIF) in which 1) retractors are place on the iliac artery/vein, or 2) there is risk to injury of the iliac artery/vein. While surgeons usually place a pulse oximitry monitor on the great toe, SSEPs  are particularly sensitive to detecting vascular insufficiency as well. Thus, they serve as a useful adjunct.

3. Monitoring Spinal Cord Conduction:

SSEPs activate the dorsal column-medial lemniscus system. While they cannot be expected to evaluate motor function, they serve as a useful adjunct to motor-evoked potentials for monitoring spinal cord conduction during spine surgery. SSEPs are not particularly useful for monitoring spinal nerve roots.

4. Monitoring Long-Tract Sensory Conduction and Subcortical Blood Flow.

Sensory signals pass through the brainstem and internal capsule on the way to cerebral cortex. SSEPs are particularly useful for detecting ischemia and iatrogenic injury to this pathway during vascular and intracranial tumor surgeries. EEG is unreliable for detecting these types of injuries.

5. Monitoring Cerebral Blood Flow (CBF).

Sometimes surgery carries risk for cerebral ischemia, which can result in permanent loss of brain function due to anoxia-related cell death.  While multi-channel scalp EEG is sensitive to more global reduction in blood flow, SSEPs tell you about a specific brain function by monitoring blood flow to eloquent sensory areas. The brain can tolerate minor reductions in cerebral blood flow fairly well; however, when CBF falls below a functional threshold in eloquent sensory areas, changes in SSEPs are observed.

6. Identifying Central Sulcus of the Brain:

When eloquent cortex is at risk for injury during tumor resection surgery, identification of the central sulcus with SSEP is the first step toward preserving neurologic function. A grid of recording electrodes is placed directly on the brain, and SSEPs are recorded from regions both anterior and posterior to the central sulcus. SSEP waveforms invert when recordings are made anterior to the central sulcus. This is called SSEP Phase Reversal. For more information, see the section on Mapping Techniques.

SSEPs to median nerve stimulation recorded from cortical surface electrodes. Inversion of the N20/P20 component (arrowheads) across the central sulcus. The amplitude is largest over the postcentral gyrus, where the component is negative in polarity.

SSEPs to median nerve stimulation recorded from cortical surface electrodes. Inversion of the N20/P20 component (arrowheads) across the central sulcus. The amplitude is largest over the postcentral gyrus, where the component is negative in polarity.

7. Identifying Dorsomedian Raphe of the Spinal Cord:

When dorsal column function is at risk for injury during intramedullary spinal cord tumor resection, identification of the physiologically-silent region of the dorsal spinal cord is the first step toward preserving neurologic function when the surgeon must cut into the spinal cord (myelotomy). Direct electrical stimulation of the dorsal spinal cord will result in an SSEP recorded over the scalp.  Using the CP3-Fpz montage, and moving from left to right across the spinal cord, the SSEP will invert (phase reversal).  Prior to inversion, the SSEP will be virtually flat.  This is the physiologically-silent midline, and is usually an optimal place to perform a myelotomy.  Other mapping techniques should be used in concert with this technique to confirm midline prior to myelotomy.

8. Assist in determining the probability of nerve root avulsion (preganglionic injury vs. postganglionic).

SSEPs can be recorded in response to direct electrical stimulation of a nerve root or rootlet, and this can be helpful in determining the functional integrity.

Benefits of SSEPs in Surgery:

  • Relatively noninvasive.
  • Consistent standards and protocols for use.
  • Well established criteria for alert.
  • High specificity in spine surgery.
  • High sensitivity and specificity for cerebral ischemia during vascular surgery.
  • High sensitivity and specificity for subcortical ischemia, which cannot be detected by EEG during vascular surgery.
  • Can be used with paralytics.
  • Useful for detecting limb ischemia.
  • Useful for detecting peripheral nerve compression due to malpositioning.
  • Useful for determining whether sensory root(let) is intact during root avulsion surgery.
  • Useful for identifying the central sulcus when eloquent cortex is at risk.
  • Useful for identifying the dorsomedian raphe of the spinal cord when dorsal columns are at risk due to myelotomy.

Limitations of SSEPs in Surgery:

  • May remain unchanged in the face of motor deficit.
  • Low sensitivity to spinal cord injury.
  • Monitors only sensory division of the nervous system served by the dorsal column – medial lemniscus pathway.
  • Low sensitivity to nerve root injury.
  • Averaged response requires many trials, delayed analysis (10-30 sec).
  • Can be difficult/impossible to record in presence of peripheral neuropathies.
  • Cortical SSEPs are particularly susceptible to inhalational anesthetic agents.
  • Saturated by electrocautery.

Surgical Procedures for which SSEPs are Indicated:

Spine (extradural, decompression/fusion):

  • Cervical.
  • Thoracic.
  • Lumbosacral.
  • Spinal cord stimulator placement for intractable pain.

Spinal Cord (intradural):

  • Extramedullary tumor.
  • Intramedullary tumor, syrinx.
  • Cauda equina tumor, tethered cord.

Intracranial (supratentorial):

  • Long tract motor/sensory monitoring.
  • Motor/sensory cortex and internal capsule mapping.

Intracranial (infratentorial):

  • Brain stem (CPA, acoustic neuroma, cranial nerves, etc).

Intracranial Vascular:

  • Cerebral aneurysm clipping.
  • Repair of arteriovenous malformation (AVM).
  • Cerebral aneurysm coiling (interventional radiology; IR).

Vascular:

  • Carotid endarterectomy (CEA).
  • Open heart total/partial bypass.
  • Aorta (ascending, descending, arch aneurysm/repair).

Peripheral Nervous System – Neurosurgery:

  • Brachial Plexus.
  • Lumbosacral Plexus.
  • Individual Peripheral Nerves.

Peripheral Nervous System – Orthopedic Surgery:

  • Shoulder arthroplasty.
  • First rib resection, thoracic outlet syndrome.
  • Hip arthroplasty.
  • Pelvis fixation.
  • Other extremities.

Diagnostics (non-surgical):

  • Nerve conduction studies.

References:

  1. Bhalodia VM, Sestokas AK, Tomak PR, Schwartz DM. Transcranial electric motor evoked potential detection of compressional peroneal nerve injury in the lateral decubitus position. J Clin Monit Comput. 2008;22(4):319-326.
  2. Nuwer Chapter 32 Crum BA, Strommen JA. (2008) Nerve Root Assessment with SSEP and MEP. In MR Nuwer (Ed.), Intraoperative Monitoring of Neural Function: Handbook of Clinical Neurophysiology, Vol. 8 (pp. 455-463). Elsevier.
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