Interventional Radiology for Aneurysms & Stroke

SSEP monitors blood profusion

ASNM Webinar 2016 - LanJun Guo, MD, MSc, DABNM c/o Larry Wiezerbowski

A cerebral aneurysm is a "blister'like" dilation of a brain artery that can become thin and rupture. The resultant bleeding can lead to a stroke, coma, and/or death. 
Aneurysms can occur from anterior circulating arteries (80% - 90%)

• Anterior communicating artery (AcomA)
• Middle Cerebral Artery (MCA)
• Internal Carotid Artery (ICA)
• Posterior Communicating artery (PcomA)

From posterior circulation (10% - 20%)

• Basilar Apex
• Superior Cerebral Artery (SCA)
• Vertebrobasilar Junction (VBJ)
• Posterior Inferior Cerebellar Artery

Surgical clipping of Aneurysm

• In 1937, Walter Dandy, MD, a famous American neurosurgeon, introduced the method of "clipping" an aneurysm when he applied a V-shaped, silver clip to the neck of an ICA aneuryms
• Since the 1960's, using different clips along with operating a microscope have made surgical clipping the gold standard in the treatment of cerebral aneurysms.
• An aneurysm is clipped through a craniotomy, in which the brain and the blood vessels are accessed through an opening in the skull
• A small metal clip is placed across the neck of the aneurysm to stop or prevent an aneurysm from bleeding.

The risk of Brain Ischemia during surgery

• During the surgery, using temporary blood vessel occlusion can effectively reduce the risk of intraoperative aneurysm rupture and improve the outcome of the operation.
• However, temporary occlusion of parent artery may induce cerebral ischemia
• A permanent aneurysm clip can be also occlude unseen small perforating arteries
• Therefore, postoperative neurological deficit with new cerebral infarction can be severe morbidity after cerebral aneurysm surgery

A video clip of MCA aneurysm clipping

• The process of temporary occlusion of MCA and permanent clipping of an un-ruptured MCA aneurysm

Somatosensory Evoked Potentials (SEPs)

• Somatosensory Evoked Potentials (SEPs) have been used for more than 30 years for monitoring brain cortical and subcortical functional to prevent cerebral ischemia or infarction during aneurysm surgery
• The primary goal of SEP monitoring is to detect impairment of cortical perfusion by intended or inadvertent blood vessel occlusion
• SEPs are considered particularly useful during temporary vascular occlusion and to predict cortical ischemia
• SEP changes are expected to also reflect perfusion of non-sensory eloquent areas like motor or speech cortex, as well as local events (pressure, traction from retractor) and systemic influences

Using SEPs to identify brain ischemia during temporary occlusion of parent artery

• There is a strong correlation between cerebral blood flow and the amplitude of SEP generated in the structures of the brain
• If the flow is greater than about 16m./100g/min the SEP cortical response is not affected but at flows less than 12 ml/100g/min the evoked potential is abolished
• Therefore, intraoperative SEP monitoring is considered very sensitive to predict the cortical ischemia, especially during temporary parent artery occlusion

SEPs are sensitive to cortical ischemia

• The blood flow and threshold of SEP change
• Brain location/Cortex/Subcortex/Brainstem
• Blood flow ml/100 g/min/15-20/10-15/<10
• SEP waves following median nerve stimulation at the wrist
  • N20-P30; cortex component
  • N14-N18 brainstem-thalamus
  • N13-N14 reflects activity within the cervicomedulla
  • N9 (ERB's point); peripheral nerve CAP traverses the brachial plexus

A case shows that SEPs are sensitive to cortical ischemia, especially during temporary occlusion

• This 60 year old female underwent right craniotomy for clipping of a middle cerebral artery aneurysm
• Anesthesia was maintained at 125 ug/kg/min of Propofol, 0.3 MAC of Desflurane, No additional muscle relaxants were given after intubation

SEPs drop during temporal clip on an artery, not MEPs

• During the procedure, left ulnar nerve SEPs were almost flat after placing temporary clip on MCA and SEPs recovered after removing the clip
• MEPs remained stable during the procedure and the patient had no post-operative neurological deficit

SEPs may not be sensitive to subcortical ischemia

• EXCEPT in instances where there is concomitant cortical and subcortical ischemia caused by temporary parent (large) artery occlusion as we just discussed, there is also a risk of accidentally clamping small perforating vessels in intracranial aneurysm surgery
• The occlusion of the small artery can cause focal ischemia, lacunar infarction, which may result pure motor deficit.
• SEP monitoring may not effectively detect the deficit

The most common perforating arteries causing subcortical infarction during aneurysm surgery

• Anterior choroidal artery (AChA) - Aneurysm location ICA
• Lenticulostriate artery - MCA or ICA

Lesser common artery causing infarction - (perforating artery causing infarction/aneurysm location)

• Anterior thalamo-perforating artery/ICA
• Recurrent artery of Heubner/ACoA
• Hypothalamic artery/ACoA
• Perforating arteris of BA (Posterior thalamo-perforating artery)/Basilar bifurcation
• Perforating artery of the VA/Vertebral artery
• Superficial branch (anterior branch) of MCA/MCA

Subcortical (Lacunar) infarction / pure motor deficit

• Lacunar infarct is small subcortical infarct that results from occlusion of one of the small penetrating arteries, branches of the large arteries
• Patient may appear as hemiparesis characterized by preservation of cortical function
  • Pure motor infarction/hemiparesis is a most common lacunar syndrome
  • The infarction can be located at
    • Posterior limb of the internal capsule (most common)
    • Basis pontis
    • Corona radiata

Lacunar infarct and pure motor

• Pure motor deficit (hemiplegia) refers to paresis involving the face, arm and leg on one side in the absence of sensory deficit, homonymous hemianopia, aphasia, agnosia, and apraxia
• It is caused by destruction of the pyramidal tract fibers from corona radiata descending into the brainstem
• Therefore, trans-cranial motor evoked potentials (MEPs) monitoring has been introduced as a supplementary technique to detect perforating vessel compromise, which may lead to motor impairment not reflected by any SEPs/change

A case with post-operative internal capsule ischemia

MEPs signals transfer through the corticospinal tract

• MEPs are recorded following stimulation of motor cortex
• After electrical stimulation, multiple volleys descend along the corticospinal tracts
• The signals travel along nerve axons from the cortex through the posterior limb of internal capsule, the cerebral peduncle and into the brainstem and anterior medulla
• Then it is recorded in contralateral muscles of the limbs

MEP stimulation; MEPs can be stimulated through

• transcranial stimulation of motor cortex
• corkscrew-like stimulation electrodes are commonly placed subcutaneously at C3/C4 sites
• Anode serving as the active electrode
• The stimlation parameters are different depending on the system used
  • constant voltage with 6-9 short train, 50-75 us pulse width
  • constant current train with 5 train 500 us pulse width
• There is no standard for maximum stimulation, although, one study reported that the maximum stimulation intensity did not exceed 240 mA

For dcMEP

• A strip electrode with four or eight contacts was placed subdural over the motor cortex
• One of the contacts services as the anode
• A electrode placed at Fpz services as cathode
• Same montages used for tcMEP can be used for direct cortical stimulation
• The stimulation intensity normally do not exceed 25mA at 500 us pulse width

dcMEP recording

• From upper limbs or lower limbs with the low stimulation voltage, depending where to place the strip electrode
• The contact requiring the lowest stimulation intensity to elicit a contralateral muscle response when stimulating the vascular territory of interest:
  • Muscles of the upper extremity are chosen to continue monitoring for ICA, MCA or posterior circulation aneurysms
  • Muscles of  lower extremity are chosen for ACA aneurysms

tcMEP recording

• From the muscles in upper extremity only, or upper and lower limbs
• One study reported that MEPs were recorded from the upper extremity in all cases and from the lower extremity as well in cases of ACA aneurysms

tcMEPs vs dcMEPs

• tcMEPs advantages
  • tcMEPs relatively easy to perform
  • it allows rapid assessment of motor system integrity from anesthetic induction to skin closure
• tcMEPs disadvantages
  • tMEPs may result in patient movement interfering with micro-dissection
  • There is also concern that tcMEPs may not detect subcortical motor pathway ischemia by directly stimulating deeper subcortical structures which may bypass the ischemic area
• dcMEPs advantages
  • This technique produces focal muscle activation and has less movement
    
  • The focal and low stimulation may activate brain at superficial level that could detect cortical and subcortical ischemia and avoid false negatives
    
• dcMEPs disadvantages
  
  • There is about a 2% incidence of bridging vein rupture with subdural bleeding during the blind electrode insertion
    
  • The strip electrode can be hard to reach the leg area
    
  • Monitoring can not start until the strip electrode is placed under the dura and must end before the dura is closed
    
  • In patients with subarachnoid hemorrhage, it can be dangerous to push the electrode strip over the edematous and vulnerable cortex toward the motor cortex
    

Study comparing

• One study compared the tcMEP and dcMEP in aneurysm surgery
  
• Not different in their capacity to detect an impending lesion of the motor cortex or its efferent pathways, if using close-to-motor-threshold stimulation and the most focal stimulating electrode montage
• They could be alternatively used within the same monitoring surgery to maximize the advantages of each

Our Experiences

• SEPs and tcMEPs are routinely used during aneurysm surgery
• Slightly higher than threshold stimulation for tcMEP are used to reduce patient movement and avoid stimulation on deeper level to bypass the ischemia
• MEP may change along or accompanied with SEP change depending on the cause and ischemia location

Some concerns of using MEPs

• The motor deficit caused by ischemia can occur on deferent depths in the brain, such as motor cortex, corona radiata, internal capsule, or brain stem, which depends on involved perforator arteries
• There is possibility that MEPs may not be able to detect the ischemia by activating corticospinal tract distally to the targe vascular territory if larger stimulation intensity is given.

A case showing increased MEP stimulating voltages can cause false negative MEPs

• a case with SEP reduction after temporary clip on proximal ICA, SEPs did not recover to baseline at the end of the surgery
• Loss of MEPs on the left side during temporary clip on the ICA
• MEPs returned when stimulating voltage was increased
• Postoperatively, patient had left side hemiplegia

MEP stimulus can activate different parts of the brain

• An anode stimulation to the surface of the cortex induced electrical current flow, which entered the dendritic tree of pyramidal cells and flowed through the soma
• It left the cell at the initial segment region of axon, at where excitation took place, resulting in a desending volley (D wave) in the corticospinal tract
• Therefore, it is axons rather than soma and dendrites of motor cortical neuron that may be the primary tissue to be excited in a stimulus field during surgery

Excitation of the pyramidal tract axons on different level

• A series of descending volleys in the spinal cord (D waves) were recorded after stimulating on motor cortex
• Gradually increasing intensities of cortical stimulation recruited three D-wave componenets (D1, D2 and D3) with a latency shift from D1 to D3 of 1.9ms
• D3 component resulting from cortical stimulation had the same latency as the volley evoked by brainstem stimulation

Excitation of the pyramidal tract axons on different level

• the studies suggested that the site of impulse initiation with electrical stimulation of the motor cortex shifts from superficial cortex to deep structures
• approximately 5-10 cm below the cortex while increasing stimulus intensity
• These deep structures are probably the internal capsule and the cerebral peduncle (brainstem)
• On an animal study, D waves still survived with injured or removed cortex
• which could be activated selectively by stimulation of underlying white matter

The favorable points of MEP activation

• there are three favorable points that are susceptible to depolarization of the corticospinal tract:
  • cortex/subcortex (weak electrical stimulation)
  • internal capsule (moderate stimulation)
  • brainstem/foramen magnum (strong stimulation)
• therefore, relatively lower stimulus intensity should be used to avoid initiating motor axons on deeper level, which may bypass ischemia area

Infarction on different depths due to MCA perforators

• Occlusion of MCA perforating branches can cause subcortical infarction on different level
• Perforating arteries arising from the distal M1 segment are related to infarcts involving the upper part of corticospinal tract, corona radiata
• Perforators from proximal M1 segment are related to lower lesions, internal capsule

The favorable points of MEP activation

• The target lesion can not be detected by MEPs if ischemia is at superficial subcortical motor pathway (such as corona radiata) and the activation occurs at the internal capsule
• Low stimulation voltages should be used to avoid movement and stimulation on deeper part of the brain

Using lower stimulating voltage

• Lower voltage stimulation may be used to record MEPs from the hand and muscle only, rather than hand and foot muscles in most cases, especially for MCA aneurysms
• Since much higher voltage is needed to evoke MEPs from lower limbs

No partial paralysis should be used

• Patient's movement may also be reduced by using partial paralysis. However, this requires a much higher stimulation intensity.
• Use the lowest stimulation intensity possible to avoid deeper activation and this also reduces patient movement
• MEPs can be recorded from the muscles of the upper extremities only and this can usually be achieved with a very low voltage
• This can be done without patient movement in our experience and therefore does not necessitate paralyzing the patient to avoid movement

Summary

• Temporary occlusion of parent artery and accidentally clamping the perforating branches are the two major causes of brain ischemia during aneurysm clipping procedure
• SEPs and MEPs provide complementary information to the surgical team
• SEP monitoring is a standard method to predict brain ischemia, which is more sensitive to cortical and subcortical ischemia caused by large artery occlusion
• MEP monitoring is effective indetecting ischemia of the corticospinal tract, especially subcortical ischemia caused by perforator occlusion
• Be aware that MEPs can stimulate on deeper level which may bypass the ischemia area
• Lower MEP stimulating voltages should be used