Monitoring during
neurosurgery
By
Dr. Mai Mohsen Abdel Aziz
Lecturer of anesthesia , Ain Shams University
The CNS can sometimes be
insufficiently monitored leading to
grave postoperative complications
Why do we need it ?
• Monitor functional integrity of neural structures (nerves,
spinal cord, certain brain areas)
• Reduce the risk of iatrogenic damage to the nervous
system
• Provide functional guidance to the surgeon and
anesthiologist
• Locate neural structures (eg. Locate cranial n. during
skull base surgery)
• Reduce morbidity and mortality without introducing
additional risks
What do we want to monitor?
A. Monitor of brain electrical activity
1.
2.
Electroencephalography
Evoked potentials:
I. Sensory evoked potentials
. Visual
. Somatosensory
. Auditory
II. Motor evoked potentials
3. Bispectral index
B. Monitor of intracranial pressure and
blood flow dynamics:
1.
2.
3.
4.
5.
Intracranial pressure monitoring
Jugular venous oximetry
Transcranial doppler sonography
Brain tissue oxygen tension monitor
Near-infrared spectroscopy
1. Electroencephalography
Value:
1.Measures electrical function of brain
2.Indirectly measures blood flow
3.Measures anesthetic effects
4.Guide reduction of cerebral metabolism prior to
induced reduction of blood flow
5.Predict neurologic outcome after brain insult
6.During cortical mapping in surgery for epilepsy .
7. In cardiac surgery : to determine the end point for
hypothermia during circulatory arrest by EEG
isoelectricity ( EEG burst suppression).
• 3 parameters of the signal:
– Amplitude – size or voltage of signal
– Frequency – number of times signal oscillates
– Time – duration of the sampling of the signal
Abnormal EEG detect
Regional
.
Epilepsy
. Focal ischemia
Global
. Global ischemia
. Hypoxemia
Anesthetic Agents and EEG
• Light anesthesia : Larger voltage, slower
frequency
• General anesthesia : Irregular slow activity
• Deeper anesthesia : Alternating activity
• Very deep anesthesia : Burst suppression 
eventually isoelectric
•Anesthetic drugs affect frequency and amplitude of
EEG waveforms
eg.barbiturates produce initial activation, then doserelated depression while ketamine activates EEG at
low doses and cannot achieve electrocortical silence.
Non-anesthetic Factors Affecting
EEG
• Surgical
• Cardiopulmonary bypass
• Occlusion of major
cerebral vessel (carotid
cross-clamping,
aneurysm clipping)
• Retraction on cerebral
cortex
• Surgically induced emboli
to brain
•
•
•
•
•
Pathophysiologic Factors
Hypoxemia
Hypotension
Hypothermia
Hypercarbia and
hypocarbia
• What to do if EEG technician
indicates a possible problem?
– Check to see if anesthetic milieu is stable
– Rule out hypoxemia, hypotension,
hypothermia, hypercarbia and hypocarbia
• Raise the MAP, obtain ABG
– See if there is a surgical reason
2. Evoked potentials
I. Somatosensory evoked potentials
(SSEP)
What does it monitor?
• Monitor dorsal column-medial lemniscus pathway to
assess spinal cord function
• Stimulus is applied to a nerve distal to surgical site and
recording is made from the cerebral cortex or other
locations rostral to surgery
• Baseline is obtained, significant changes
amplitude
>50%
Latency >10%
When do we use it?
spinal surgery…
deliberate hypotension…
Thoracic aortic aneurysm…
Carotid endarterectomy…
Cerebral aneurysm…
Is it specific?
• The response is non-specific.
• Can be affected by hypoperfusion,
temperature changes, changes in anesthetic drugs
II. Visual evoked potential (VEP)
• Visual stimuli from flashing diodes in goggles measures
intactness of visual pathways from optic nerve to
occipital cortex
• Very sensitive to anesthetic drugs and variable signals
When can we use it?
Trans-sphenoidal …
anterior fossa surgeries…
III. Auditory evoked potentials
(AEP)
• Auditory signal transmitted to patient follows auditory
pathways CN VIII, cochlear nucleus, rostral brain stem,
inferior colliculus, auditory cortex
• It is resistant to anesthetic drugs
When do we use it?
Decompression of CN VIII…
Resection of acoustic neuroma..
Anesthetic Agents and SEPs
• Most anesthetic drugs increase latency and
decrease amplitude
– Volatile agents: increase latency, decrease amplitude
– Barbituates: increase in latency, decrease amplitude
• Exceptions:
–
–
–
–
–
Nitrous oxide: latency stable, decrease amplitude
Etomidate: increases latency, increase in amplitude
Ketamine: increases amplitude
Opiods: no clinically significant changes
Muscle relaxants: no changes
Physiologic Factors and SEP’s
•
•
•
•
•
•
Hypotension
Hyperthermia and hypothermia
Hypoxemia
Hypercapnea
Significant anemia (HCT <15%)
Technical factor: poor electode-to skin-contact and high
electrical impedence (eg electrocautery)
How do we manage?
. First rule out systemic factors:
– improve neural tissue blood flow and nutrient delivery
– Intravascular volume and cardiac performance optimized
(crystalloid/colloid or blood) to increase oxygen-carrying
capacity – optimal HCT 30% or higher
– Elevate MAP
– Blood gas – assure oxygenation, normocarbia to help improve
collateral blood supply if hypocarbic
– Consider steroids (shown to work with traumatic spinal cord
injury)
– Mannitol – improve microcirculatory flow and reducing
interstitial cord edema
. Second rule out neurological factors: ischemia or nerve
compression
IV. Motor evoked potentials
• Spinal cord monitoring esp. motor tracts not covered by
SSEP monitoring
• Substituted old-fashioned Stagnara wake-up test
• It is the reverse of SSEP, motor cortex is stimulated
transcranially
• Recordings are made from muscles in the limb or from
spinal cord caudal to surgical site
When can we use it?
Spine surgeries
Is it sensitive?
• More sensitive in detecting post-operative motor deficits
• Intravenous agents produce significantly less depression
• Muscle relaxants interfere
3. Bispectral index (BIS)
• integrates single-channel EEG
• The BIS value of 100 indicate the patient is fully awake.
The BIS value of 0 indicated absence of brain activity.
• used to monitor depth of anesthesia (with a target of
less than 60)
Value?
• reduce the overall anesthetic dose and its related side
effects
• Decreases the incidence of postoperative awareness
• shorten emergence time
• and allow early postoperative recovery
Monitors of blood flow dynamics
1.Transcranial doppler
• Direct, noninvasive measurement of CBF
• Sound waves transmitted through
thin temporal bone contact blood,
are reflected, and detected
• Most easily monitor middle
cerebral artery
When is it used?
• evaluate integrity of cerebral vasculature in carotid
endarterectomy surgery.
• In cardiac surgery: can be used as a tool to detect and
quantify embolic phenomena in MCA.
• Detection of vasospasm
2. Cerebral Oximetry (Near
infrared spectroscopy)
•
determine cerebral saturation
• uses a similar principle to pulse oximetry
by using multiple wave lengths of near infrared light ,
the absorption of this light by oxygenated and
deoxygenated haemoglobin determines
the overall saturation
of the blood present within
the brain tissues.
Is it sensitive?
– High intersubject variability
– Low specificity
When is it used ?
– cardiac surgery to detect the cerebral desaturation
(20% from the base cerebral saturation) with CPB .
– carotid endarterectomy
3. Jugular venous oxygen
saturation (SjVO2)
The jugular bulb is the dilated portion of the jugular vein
just below the base of the skull which contain blood with
little extra cerebral contamination.
Measurement of oxygen saturation of the jugular bulb
provide information about the global oxygenation state
of the brain.
• Analysis of the bulb saturation can be performed by:
-Intermittent blood sampling via standard
intravascular catheters.
-Continuous fiberoptic jugular probes placed
retrograde via the internal jugular vein in the jugular
bulb.
• Was primarily employed in cardiac surgery: SjvO2 was
used to study the cerebral desaturation with the CPB
• relatively invasive and insensitive technique.
• has been replaced by noninvasive cerebral oximetery.
4.Direct tissue oxygen monitoring
• Direct measurement of brain O2 level (PO2) with small
oxygen electrodes placed through a burrhole.
• It is used mainly in traumatic brain injury (TBI) to prevent
secondary cerebral ischemia .
• decrease in direct brain PO2 less than 20 mmHg should
be treated by improving oxygen delivery
-ICP <20mmHg
-CPP >60mmHg,
-increasing FiO2, hemoglobin and decreasing oxygen
demand with sedation
Conclusion
Multimodality in monitoring the brain is
important as no single monitor provides
definite information regarding brain
function.
However integrating the information from
several monitors may provide a therapeutic
guide for intervention and reducing
morbidity and mortality.
If we care, we can
Thank You