MAPPING THE BRAINSTEM NUCLEI:
The mistery of the Floor of the Fourth Ventricle
Dr. Isabel Fernández-Conejero University Hospital of Bellvitge
Winter Symposium ASNM, february 27th 2021 “Surgery and intraoperative neuromonitoring related to lesions within the brainstem represents the most challenging procedures in neurosurgery” • Mapping: – Cranial motor nerve nuclei – Intracranial part of cranial nerves – Corticospinal tract • Monitoring: – Lateral lemniscus (Brainstem auditory evoked potentials) – Medial lemnicus (Somatosensory evoked potentials) – Corticospinal tract (Motor evoked potentials) – Cranial nerves • Corticobulbar motor evoked potentials • Free running EMG • Brainstem reflexes: – Blink reflex – Masseteric reflex – Laryngeal adductor reflex
Brainstem mapping: DEFINITION
Brainstem mapping (BSM) is an intraoperative neurophysiological method to localize cranial motor nuclei (CMN) on the floor of the fourth ventricle.
BSM enables the neurosurgeon to understand functional anatomy on the distorted floor of the fourth ventricle, thus, it is emerging as an indispensable tool for challenging brainstem surgery.
Anatomical landmarks on the floor of the fourth ventricle cannot be easily recognized even in non distorted brainstem. Acta Neurochir (1991) 113:84-90
J Neurosurg (1999) 91:768-775
Acta Neurochir (2006) 148:499-509
Lang et al, Acta Neurochir (Wien) (1991) 113:84-90 Safe entry to the brainstem
1. Substancia ferruginea 2. Facial colliculus 3. Striae medullaris 4. Vestibular area 5. Hypoglossal triangle 6. Vagal triangle 7. Calamus scriptorius Strauss. J Neurosurg 91:768-775;1999
BRAINSTEM MAPPING: Methodology STIMULATION PARAMETERS
• Cathode: Hand-held monopolar (diameter: 0.75mm), The tip of the probe is round and of moderate size top prevent damaging the floor of the fourth ventricle during the stimulation.
• Anode: Cervical muscles in the operative field or Fz
• Wave form: square wave, single pulse
• Duration of stimulation: 0.2 ms
• Frequency: 1.0 – 4.0 Hz
• Intensity: 2.0 mA for screening, then squeeze intensity to detect threshold. RECORDING PARAMETERS
• Hook wire electrodes: Teflon coated wire 76 µm in diameter passing though 27 gauge needles.
• Filter: 20 – 3000 Hz
• Epoch length: 20 ms
• Amplification: 10.000 times CMN VII: orbicularis oculi and oris (nasalis and mentalis) CMN IX: posterior pharyngeal wall CMN X: vocal cord or cricothyroid CMN XI: trapezius CMN XII: lateral wall of the intrinsic tongue muscle ANESTHESIA and brainstem mapping
• Any type of anesthesia is compatible with BSM except long-lasting muscle relaxant
• But since it is also needed monitoring mMEPs, SEPs and CoMEPs the ideal anesthesia regime will be TIVA:
Propofol 100-150 mg/Kg/min + Fentanyl 1mg/Kg/h + No bolus + Avoid halogenated agents and muscle relaxant after intubation APPLICATION OF BRAINSTEM MAPPING DURING SURGERY
• The threshold intensity depends on: – Pathology • Brainstem hematoma 0.2-0.4 mA • Brainstem tumor up to 2.0 mA – Degree of brainstem compression – Distance to the cranial motor nuclei
• Surgeon starts to stimulate the floor 2 mA, once the response is recorded the intensity is decreased up to the threshold FACIAL NUCLEUS LOWER CRANIAL NUCLEI
N. Morota and V. Deletis (2006) Displacement of cranial motor nuclei due to brainstem tumor
N. Morota and V. Deletis (2006) Surgical anatomy of the floor of the fourth ventricle during brainstem surgery
N. Morota and V. Deletis (2006) Limitations of brainstem mapping
– It is unable to monitor the functional integrity of the corticobulbar tract and the reflex circuits of the lower CMN (swallowing or coughing)
– Preserved EMG responses following BSM does not always promise preserved function of lower CMN.
– It is unable to monitor functional integrity of the CMN during tumor resection
– BSM is not a monitoring technique, Corticobulbar motor evoked potentials should be done Practical concepts of brainstem mapping
• Understanding typical patterns for the CMN displacement by brainstem lesions will help planning the surgical strategy
• Any retraction of the floor of the fourth ventricle or mielotomy should be directed away from the mapped CMN
• Threedimensional anatomical relationship between brainstem and the displaced CMN is the key for safe brainstem surgery
• The silent area demonstrated by BSM is the key approach route to the brainstem while preserving the function of CMN.
The true safe entry zone to the brainstem can be revealed only by brainstem mapping Case 1
• Patient 34 years old, female
• MRI: Cervicomedullary junction ependymoma
• Physical exam: Cranial nerve examination was normal
Intraoperative Neuromonitoring strategy
• Mapping: – Cranial motor nerve nuclei – Intracranial part of cranial nerves – Corticospinal tract • Monitoring: – Lateral lemniscus (Brainstem auditory evoked potentials) – Medial lemnicus (Somatosensory evoked potentials) – Corticospinal tract (Motor evoked potentials) – Cranial nerves • Corticobulbar motor evoked potentials • Free running EMG • Brainstem reflexes: – Blink reflex – Masseteric reflex – Laryngeal adductor reflex
Bilateral hypoglossal corticobulbar motor evoked potentials
RH Opening C3/Z 70 mA
RH Closing
LH Opening
C4/Z 70 mA LH Closing BRAINSTEM MAPPING
Outcome: Bilateral hypoglossal nuclei palsy Case 2
Patient: RCD, 42 years old
Clinical syntoms: Sudden headache, left hemiparesis and hemihipoesthesia, nausea Physical exam: drowsy, severe dysartria, bilateral facial paresis (D>I), Left hemiparesis; brachial proximal 2/5 and distal4/5, crural 3/5
MRI: Right pons lesion with recent hemorrage compatible with brainstem cavernoma Baselines
R Med SEP L Med SEP R PTN SEP L PTN SEP
R side MEP L side MEP R CoMEPs L CoMEPs
L R
VII VII
X CLINICAL OUTCOME 1 WEEK AFTER SURGERY
• Left Hemiparesis 4/5
• Improvement of right facial paresis (total oclusion of the eye)
• Improvement of disartria CONCLUSIONS
• There is a great correlation between clinical status of patients and intraoperative neurophysiology.
• Mapping the floor of the fourth ventricle helps to avoid lesions to the cranial motor nerves nuclei.
• The optimal strategy in order to prevent neurological deficits during brainstem surgery requires the combination of mapping and monitoring techniques. Jay Shils, Host
Isabel Fernández-Conejero ISIN President