Monitoring during neurosurgery By Dr Mai Mohsen Abdel

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 3. Bispectral index . Visual. Somatosensory. Auditory II. Motor evoked potentials

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 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 (Sj. VO 2) 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: Sjv. O 2 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 O 2 level (PO 2) 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 PO 2 less than 20 mm. Hg should be treated by improving oxygen delivery -ICP <20 mm. Hg -CPP >60 mm. Hg, -increasing Fi. O 2, 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.

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