1. Anaesthetic considerations for Posterior Fossa Surgery Padmaja Durga, Prof, Department of Anesthesiology and Intensive Care, Nizam’s Institute of Medical Sciences, Hyderabad. A.P.

2. Posterior Fossa Surgery- Challenges for the Anesthesiologist Confined Space Neuronal and vascular structures ▫Possible injury to vital brain stem centers ▫Obstructive hydrocephalus Unusual positioning Venous air embolism Pneumocephalus

3. Functional anatomy of posterior fossa Brain Stem ▫Cranial Nerves ▫Intrinsic control nuclei  Consciousness  Respiratory Function  Cardiovascular control Cerebellum

4. Anaesthetic Goals Provide optimal operating conditions Pharmacologically improve exposure Minimize/ Prevention of complications by providing monitoring Early Awakening ▫Early neurological Assessment Prevention of Post-operative Pain

5. Preoperative Evaluation Neurological Status ▫Midline cerebellar ▫Lateral cerebellar ▫Cerebello-pontine ▫Brain stem syndromes  ↑ ICP  Lower Cranial nerve palsy → Pulmonary Aspiration Medulloblastoma

6. Preoperative Evaluation Patient physical status ▫Cardiovascular and pulmonary stability Nutritional deficiency Coagulation Abnormality Airway manageability RA catheter access

7. Types of Lesions Tumor ▫Medulloblastoma ▫Cerebellar Astrocytoma ▫Ependymoma ▫Brain Stem Glioma ▫Vestibular Schwannoma ▫Meningioma ▫Haemangioblastoma ▫Choroid Plexus Papilloma ▫Chordoma ▫Metastatic Vascular AVM Vein of Galen Malformations Cavernous lesions Aneurysm Spontaneous Haemorrhage Cerebellar Infarction Developmental Disorders Dandy-Walker’s Complex Chiari Malformations Arachnoid Cyst Cranial Nerve Dysfunction Microvascular Decompresion

8. TumorClinical Syndrome Standard Approach Anaesthetic concerns MedulloblastomaMidline-cerbellarMidline-sub occipital Haemodynamic Instability, ICP EpendymomaMidline-cerbellarMidline-sub occipital Haemodynamic Instability, ICP Cerebellar Astrocytoma Lateral-cerbellarLateral -sub occipital Brain Stem gliomaBrain stem syndrome Midline orLateral - sub occipital Haemodynamic Instability, Cranial nerve palsy, leave intubated? HaemangioblastomaCerebellar or brain stem syndrome Lateral -sub occipital Polycythemia, occult pheocromocytoma, hypertension Pineal regionHydrocephalus, Parinaud’s syndrome Suupracerbellar infratentorial Sitting often used MetastasisLateral-cerbellarLateral sub-occipitalICP Clival CordomaBrain stem syndrome ComplexLower cranial nerve palsy

10. Monitoring of Central nervous System Area or Cranial Nerve Monitor Classical Activity description Physical signs CN V EMG Arterial line, ECG Motor: Jaw jerk Sensory: Hypertension, Bradycardia CN VIIEMGFacial twitch CN VIIIBAEPIncrease latency CN XArterial line, ECGHypotension, Bradycardia CN XIEMGShoulder jerk PonsBAEP, SSEP ECG Arterial Line ECG Respirator trigger Increase latency Ectopic cardiac foci Hypertension, hypotension Tachycardia/ Bradycardia Gasp, irregular respiration

11. Induction of Anaesthesia Adequate depth Haemodynamic Stability Smooth induction ▫No coughing and gagging  Intravenous induction with thiopental or propofol

12. Maintenance Low-dose (4 to 6 μg/kg fentanyl), narcotic- based, muscle relaxant technique with 0.5 to 1.0 MAC volatile inhalational anaesthetic → Adequate analgesia and amnesia → Preservation of autonomic nervous system activity → Rapid awakening Propofol infusion (50-100 μg/kg/min) → better surgical access

13. Choice of Anaesthetics Intravenous anaesthetics pentobarbital, fentanyl, and ketamine → Maintain a higher threshold for trapping air bubbles in the pulmonary circulation than halothane → Reduces transpulmonary air passage → Decreases the risk and severity of air emboli if they occur.

14. Nitrous Oxide Risk of VAE Enhance depressant effect of volatile agents on SSEP Need for High FiO 2

15. Role of Positive Pressure Ventilation Less likelihood of patient movement Maintenance of lighter levels of anaesthesia Less cardiovascular depression Hyperventilation ↓ PaCO 2 → ↓ S ympathetic stimulation → ↓ Blood pressure → Cerebral vasoconstriction ↓ ICP ↓ Bleeding

16. Special techniques Deliberate hypotension ▫Maintain MAP- 60-70mmHg  NTG, SNP, Esmolol Electrophysiological Monitoring ▫Volatile agents > 1 MAC –effect amplitude of SSEP ▫N2O enhance depression ▫Avoid muscle relaxants if MEP, CN monitored

17. Patient Positioning Supine Prone 3/4 prone Park-Bench Lateral positions Sitting position

18. Practical Reasons for use of Sitting Position Major Drawbacks Better Surgical Exposure  More complete resection  Less tissue retraction  Less cranial nerve Damage  Less bleeding Ready Access to Airway, chest and extremities Haemodynamic Changes Higher incidence of Venous Air embolism Justification Modern monitoring systems provide early warning Serious problems due to VAE uncommon Brain Stem compromise, ischemia Pneumocephalus

19. Relative Contraindications for operative sitting Position  Ventriculoperitneal shunt in place and open, severe hydrocephalus  Cerebral ischemia in awake upright  Intracardiac R →L shunt, pulmonary AV fistula  Preoperative Demonstration of PFO  Hypovolemia, myocardial depression  May require CPR-Cardiac instability, ?extremes of ages ( but chest compression not really better in prone or lateral position

20. ComplicationsPosition SittingProne Lateral, ¾ prone Park bench, lounge Airway Edema of face, tongue, neck Endotracheal tube migration + + + + + + 0+ 0+ Pulmonary Airway pressure ↑ V/Q abnormality 0+ 0+ ++ + 0/+ + 0+ 0+ Cardiovascular Hypotension Dysrhythmias Blood Loss ++ + ++ 0±± 0±± + ++ +

21. Complications Position SittingProne Lateral, ¾ prone Park bench, lounge Nervous System  Cerebral Ischemia  Tension Pneumocephalus  Cervical Spine Ischemia  Palsies Cranial nerve Brachial plexus Sciatic nerve Peroneal nerve ++ ++ + ++ + ++ 0 0 0/+ 0 ++ 0 ? + 0 + ++ 0 + 0 Eye Compression0++++++ VAE++++++ PAE+++??

22. ComplicationPrevention Position -Skin pressure -Nerve Pressure Venous obstruction Proper position and padding Avoid pressure points, watch for nerves and plexuses Avoid excessive neck rotation and flexion Skeletal fixation -laceration -Air embolism -Hypertension Secure placement Antibiotic paste at pin site Local infiltration or scalp nerve block, deepen anaesthesia, attenuate Airway -Obstuction -Dislocation of airway Aromoured tube Secure tube to skull fixation and face Eye injury -Eye abrasion -Blindness Closure of lids, secure pin placement

24. Anterior Transoral- transclival, transpalatine, Extended maxillotomy (trans maxillary, trans clival) Implications: ▫Oral and soft palate retraction-lingual edema, airway compromise, infection, dysphagia ▫Pharngeal incision- Infection, aspiration, dysphagia ▫Hard palate dissection-haemorrhage, infection ▫Bone/ligament dissection- vertebral artery injury, clival removal-carotid injury, haemorrhage ▫CN palsy(VI to XII) ▫CV Junction instability ▫Dural opening-CSF leak, meningitis, death

25. Anterolateral Pterional, Sub-temporal preauricular- infratemporal, infratemporal Implications ▫Haemorrhage,  carotid injury  superior petrous sinus injury ▫Sylvian dissection- stroke ▫Temporal retraction, herniation ▫II, IV, V, VII nerve injury ▫Infection, CSf leakage, meningitis ▫Sigmoid sinus ligation-cerebellar swelling

26. Lateral Sub-temporal- transtentorial Middle cranial fossa- transpetrous Trans cochlear, Retrolabyrinthine Sub-temporal dissection ▫middle meningeal haemorrhage ▫superior and inferior petrosal sinus injury Temporal lobe retraction Injury to vein of labbe → Edema, haemorrhage,Infarction, Seizures Petrous drilling → Carotid laceration, V, VII CN palsy, CSF leak, superior petrosal vein injury, deafness

27. Posterolateral Retrosigmoid,Petrosal,Extreme lateral Emissary vein haemorrhage Cerebellar swelling Haemorrhage Infarction Cranial neuropathies(V-XII) Venous sinus injury CSF leak Extreme lateral-verterbral dissection- haemorrhage, spinal cord injury, instability

28. Posterior Sub-occipital, Occipital- transtentorial Cerebellar swelling, infarction, herniation Occipital retraction-infarction Incisural dissection-deep venous injury, diencephalic infarction CSF leak

29. Venous Air Embolism ETCO 2 monitoring 9%-28% Doppler monitoring - 43% Higher in sitting position

30. Paradoxical Air Embolism Mechanism Right-to-left shunting ▫Intracardiac defect ▫Patent formen ovale(incidence of PFO 20-30%). likelihood increased  Right atrial pressure exceeds left atrial pressure  Positive End-Expiratory Pressure

31. Causes for VAE with posterior fossa procedures Facilitation of air entry by sub-atmospheric pressure in an opened vein Presence of non-collapsible venous channels, such as diploic veins and dural sinuses Air may enter the venous circulation via burr holes or wounds from the skull head holder when the head was elevated Sites of central venous access- Air can be entrained around the site of catheter entry, from open catheters and may also occur when central catheters are removed with the patient’s head up

32. Monitoring for Venous Air Embolism MonitorAdvantagesDisadvantages Precordial Doppler ultrasonography Non-Invasive Very Sensitive Early Detection(before air enters PA) Technical Difficulty in obese, positions like prone False negative and false positive (air may not pass under the beam, mannitol, microaggregates of platelets mimic VAE) Not quantitative TEE Most sensitive detector of air Can detect air in left heart/ aorta Expensive, cumbersome, invasive, not quantitative, needs constant observation, interferes with doppler PA catheter Widely available, minimal difficulty in experienced hands Quantitative, more sensitive than ETCO 2 Small lumen, less air aspirated than RA cath Rise in PA pressure not specific for air ETCO 2 Widely available, Non-invasive, sensitive, quantitative Non-specific for air, Accuracy affected by low CO, COPD, tachypnoea less sensitive than PA cath ETN 2 Specific for air Detects earlier than ETCO 2 Less sensitive Affected by hypotension Others-Doppler Monitoring of Carotid and Middle Cerebral Arteries, Brain Electrical Activity: acute decrements in the bispectral index as a consequence of air embolism

33. Prevention of Air Embolism Controlled positive-pressure ventilation Adequate hydration Proper wrapping of the lower extremities Minimum required head elevation Meticulous surgical technique with careful dissection and liberal use of bone wax Avoidance of N2O in patients with known Intracardiac defects Avoidance of drugs that may increase venous capacitance (e.g., nitroglycerin).

34. Treatment of Venous Air Embolism Inform surgeon immediately. Discontinue N2O, increase O2 flows. Modify the anaesthetic. Have the surgeon flood the surgical field with fluids. Provide jugular vein compression. Aspirate the right atrial catheter. Provide cardiovascular support. Change the patient’s position.

35. Complication Prevention Homeostasis -Hypothermia -Hypotension - Hypoxia Warming Blanket, warm fluids, monitor temperature Monitor blood pressure, fluid replacement, Titrate with Swan-Ganz catheter if required slow elevation if sitting used, Leg stockings, Control Airway, adjust ventilation, RA catheter Exposure -Cerebellar swelling -Neurologic Deficit -Cerebral ischemia Avoid venous obstruction, good airway, Proper exposure, CSF drainage, gentle retraction, brain relaxation, lower CO 2 Good exposure, maintain gentle traction, sharp dissection, laser, ultrasound Proper clip placement, avoid perforators, prevent vasospasm Closure CSF leak- meningitis Pneumocephalus Haemorrhage Infection Proper closure, Drain CSF Replace air with CSF/ saline, close all sinuses Haemostasis, Avoid CSF loss, dent in dura, watch sitting position

36. Emergence from Anaesthesia Determinants feasibility of immediate postoperative extubation ▫Nature and extent of surgery  Extensive brainstem manipulation → Postoperative Brainstem Edema  Difficult tumor resection → Brainstem injury

37. Emergence from Anaesthesia Extensive manipulation of the medullary structures or significant edema ▫Maintain secured airway until  Awake  Following commands  Demonstrating return of protective airway reflexes

38. Anaesthetic Goals during Emergence Prevent abrupt rises in blood pressure Effect rapid ▫Awakening ▫Return motor strength Minimize coughing and straining on the endotracheal tube Post-operative Pain Management Persistent postoperative hypertension ▫Brainstem compression ▫Ischemia ▫Hematoma

39. Post- operative complications Neurological ▫Haematoma ▫Tension Pneumocephalus ▫Hydrocephalus ▫Edema ▫Vascular Injury ▫Cranial nerve Injury ▫Spinal Cord Injury ▫CSF leakage ▫Infection ▫Meningitis ▫Seizures

40. Post- operative complications Respiratory ▫Failure to regain spontaneous ventilation ▫Aspiration ▫Pulmonary emboli ▫Sleep apnoea ▫Neurogenic pulmonary edema

41. Post- operative complications Others ▫Metabolic ▫GI bleeding ▫DVT

42. Understanding of pathophysiology Requirements of Surgery Anticipate/ Prevent/ Manage Complication Team Work