BE SENSIBLE ENOUGH TO SENSE SSEP (SOMATO SENSORY EVOKED POTENTIAL)!

SSEP reflect the ability of a specific neural pathway to conduct an electrical signal from the periphery to the cerebral cortex.

THIS IS WHAT WE DO:

A skin surface electrode is placed near a major peripheral mixed function (motor and sensory) nerve such as the median --> a square-wave electrical stimulus of 0.2 to 2ms is applied at a rate of 1 to 2Hz. --> The stimulus intensity is adjusted to produce minimal muscle contraction (usually 10 to 60mA) --> The resulting electrical potential is recorded at various points along the neural pathway from the peripheral nerve to the cerebral cortex.

COMMON SITES OF STIMULATION:

#Upper extremity : median and ulnar nerves at the wrist. 

#Lower extremity : the common peroneal nerve at the popliteal fossa and the posterior tibial nerve at the ankle 

#Less commonly the tongue, trigeminal nerve, and pudendal nerve have been studied.

RECORDING: 

After upper limb stimulation, potentials are recorded at the brachial plexus (Erb’s point, 2 cm superior to the clavicular head of the sternocleidomastoid muscle), the cervicomedullary junction (posterior midline of the neck at the second cervical vertebra), and the scalp overlying the somatosensory cortex on the contralateral side. 

After stimulation of the lower extremity, potentials are recorded at the popliteal fossa, lumbar and cervical spinal cord, and somatosensory cortex. It is important to record nerve and subcortical potentials to verify adequate stimulation and delineate anesthetic effects.

PLOTTING:

The SSEP is plotted as a waveform of voltage vs. time.

It is characterized by: 

#Amplitude (A), which is measured in microvolts from baseline to peak or peak to peak 

#Latency (L), which is the time, measured in milliseconds, from onset of stimulus to occurrence of a peak or the time from one peak to another

MORPHOLOGY:

described as positive (P, below the baseline) or negative (N, above the baseline) 

A waveform is identified by the letter describing its deflection above or below the baseline followed by a number indicating its latency (e.g., N20) 

INTRAOPERATIVE SSEP's, INDICATIVE OF SURGICAL TRESSPASS / ISCHEMIA INCLUDE,

a . increased latency 

b . decreased amplitude 

c . complete loss 

Any decrease in amplitude greater than 50% or increase in latency greater than 10% may indicate a disruption of the sensory nerve pathways. The spinal cord can tolerate ischemia for about 20 minutes before SSEPs are lost.

ANESTHETIC DRUGS AND SSEP

 All of the halogenated inhaled anesthetics probably cause roughly equivalent dose-dependent decreases in amplitude and increases in latency that are further worsened by the addition of 60% nitrous oxide. It is best to restrict the use of volatile anesthetics and nitrous oxide to levels below 1 minimum alveolar concentration (MAC) and not to combine the two. n If possible, bolus injections of drugs should be avoided, especially during critical stages of the surgery. Continuous infusions are preferable.

CONDITIONS ALTERING SSEP

#Hypothermia : increases latency, whereas amplitude is either decreased or unchanged. For each decrease of 1 degree C, latency is increased by 1ms. 

#Hyperthermia (4 degree C) : decreases amplitude to 15% of the normothermic value. 

#Hypotension: With a decrease of the mean arterial blood pressure (MAP < 40mm Hg), progressive decreases in amplitude are seen. The same change is also seen with a rapid decline in MAP to levels within the limits of cerebral autoregulation. 

#Hypoxia: ?Decreased amplitude 

#Hypocarbia: Increased latency has been described at an end-tidal CO 2 < 25mm Hg. 

#Isovolumic hemodilution: Latency is not increased until the hematocrit is < 15%, and amplitude is not decreased until the hematocrit is < 7%. This effect is likely caused by tissue hypoxia.

INTRAOPERATIVE USES

#scoliosis surgery & Harrington rod placement 

#spinal cord decompression and stabilisation after acute SCI spinal fusion 

#brachial plexus exploration following acute injury 

resection of spinal cord tumours, cysts & vascular anomalies 

correction of cervical spondylosis 

#resection of 4 th  ventricular cysts 

release of tethered spinal cord 

#resection of acoustic neuroma 

resection of intracranial lesions involving the sensory cortex 

resection of thalamic tumours 

abdominal and thoracic aneurysm repair

IF SSEP CHANGES SIGNIFICANTLY,  WHAT THE SURGEON AND ANAESTHESIOLOGIST CAN DO  TO DECREASE THE INSULT?

The anesthesiologist can: 

>Increase mean arterial blood pressure, especially if induced hypotension is used. 

>Correct anemia, if present. 

>Correct hypovolemia, if present. 

>Improve oxygen tension. 

>Correct hypothermia, if present. 

The surgeon can:

>Reduce excessive retractor pressure. 

>Reduce surgical dissection in the affected area. 

>Decrease Harrington rod distraction, if indicated. 

>Check positioning of associated instrumentation (e.g., screws, hooks). 

If changes in the SSEPs persist despite corrective measures, a wake-up test may be performed to confirm or refute the SSEP findings. The patient’s anesthetic level is lightened, and a clinical assessment of neurologic function is performed. The monitoring of motor-evoked potentials along with SSEPs provides a more complete assessment of neural pathway integrity. As the sensory pathways are supplied predominantly from the posterior spinal artery & the motor tracts from the anterior,  a significant motor deficit can develop without significant change in SSEP's.