Anesthesia for Posterior Cranial Fossa Surgery
Anatomy
Three cranial fossae
Anterior
Middle
posterior
Located between tentorium cerebelli and foramen magnum
Boundaries Anterior:
– Superior border of the petrous temporal bone
– Clivus “slope” of occipital,Sphenoid bone
• Posterior:– Squamous part of
occipital bone
• Laterally:– Mastoid part of
temporal bone
CONTENTS
Cerebellar hemispheres, large portion of the brainstem (lower midbrain, pons and upper medulla) 3rd to 12th cranial nerves nuclei and many efferent and afferent fiber tracts that connect the brain with the rest of the body
Blood supply :Through vertebrobasilar system ,located mostly anteriorly
Foramina/outlets
• Foramen magnum (medulla, ascending portion of spinal accessory nerve, vertebral arteries)
• Internal accoustic meatus (cranial nerves VII, VIII)
• Jugular foramen (IJV, cranial nerves IX, X, XI)
• Hypoglossal canal (cranial nerve XII)
Pathology
Tumors:(quite of concern,small space with very vital structures) Astrocytomas Medulloblastoma Hemangioblastoma CP angle tumors Metastatic disease
Vascular pathologyhematoma, aneurysm
Infection/ inflammatory
Anesthetic Considerations
Overall goal
• facilitate surgical access
• maintain respiratory and cardiovascular stability
• minimise nervous tissue trauma
Pre-op evaluation
• Complete medical history with diagnostic procedures & review of medicines
• Thorough physical/neurologic examination
Clinical features
Hydrocephalus: features of raised ICP Nausea Vomiting Headache Altered consciousness
Range of other symptoms Dysphagia, laryngeal dysfunction Visual disturbances Hearing impairment Weakness or numbness in face Difficulties with balance and walking
Investigations
• CBC
• RFTs
• Coagulation profile
• Random blood sugar
• CXR
• ECG
• CT/MRI
Pre-op preparation
• Thorough neurologic assessment
• Adequate units of blood should be arranged and cross matched
• Extra, large bore IV cannula
Intra-op Monitoring:
• HR
• ECG
• NIBP
• IBP
• SpO2
• CVP
• Urine output: Foley’s catheter
• ETCO2
• Precordial Doppler
• TEE
Positioning
• Positions
– Sitting
– Prone
– Lateral/ park bench position
semirecumbent in the standard sitting position :the back is elevated to 60°, and the legs are elevated with the knees flexed. The head is fixed in a three-point holder with the neck flexed; the arms remain at the sides with the hands resting on the lap.
Sitting position
– Used for some posterior fossa and cervical spine surgery
– Advantages• Better venous and CSF drainage (↓ICP)
• Better surgical access
• Decreased blood loss
• Better facial view for cranial evoked responses
Disadvantages
– Possibly greater dangers than alternative positions
• Hypotension, cerebral ischemia (decreased CPP)
– Volume loading and pressors to maintain CPP ≥60 mmHg
– Perfusion pressure should be measured at ear level
– TED stockings or calf compression devices
• Tongue and pharynx compression or spinal injury from neck flexion
• Pressure areas: buttocks, potential brachial plexus distraction
• Venous air embolism ± paradoxical embolism
• Pneumocephalus– May be worsened by N2O diffusion after dural closure
– Cease N2O with dural closure
Prone position
• facial skin ulcerations can occur from uneven pressure distribution when the horseshoe headrest is used
• Post operative Visual Loss (POVL)eye compression and retinal ischaemia
• Other pressure areas: elbows, breasts, iliac crests, genitalia, knees, toes
• Abdominal pressure: increased PAW, IVC obstruction
• Neck flexion may cause compression of base of tongue and pharynx
– Especially with instrumentation: ETT, TEE
Park-bench position
• Can be used for post parietal & occipital lobes & lat. post fossa, including tumors at the cerebellopntine angle & aneurysms of the vertebral & basilar arteries.
Conduct of Anesthesia
• Induction/Airway: Standard induction, ETT
• Surgical Course: Head Pinning by surgeon (very painful, deepen Anesthesia just prior)
• Significant blood loss possible from Scalp
• careful positioning
Complications
• Hypotension• Measures to avoid hypotension:
– Prepositioning hydration
– Wrapping of the legs with elastic bandages to counteract gravitational shifts of blood
– Slow, incremental adjustment of table position.
– Aggressive volume loading and the G suit (also known as pneumatic antishock trousers) attenuate the effects of assuming the sitting position.
Intra operative complications
CVS reflexes: Brain stem injury Bradycardia:
Stimulation of vagus nerve
Changes in BP: Hypertension: stimulation of floor of 4th ventricle, medullary
reticular formation or trigeminal nerve
Mgmt: Inform surgeon Pharmacological Rx: if recurrent
Injury to cranial nerves
Usu during work on cerebro-pontine area
Intra operative stimulation of cranial nerves :
V, VII, VIII, XI, XII
?Use of muscle relaxants
Venous Air Embolism
Occur when the pressure within an open vein is subatmospheric.
Can occur in any position/procedure whenever the surgical above heart(but usu significant > 20cm)
Incidence: highest during sitting craniotomies (40-45%
Massive VAE Vs Gradual air entrainment
Massive VAE
• Less common
• Abrupt,catastrophic hemodynamic response
Slow air Entrainment
• More common
• Slow air entrainment over longer period
• Little respi/hemodynamic compromise
• But↑PVR & ↑ PAP & RAP
• ↑ dead space,↓EtCO2,↑PaCO2
Findings
• Decrease in EtCO2
• Increase in PaCO2
• Decrease in PaO2
• Decreased CO
• ?EtN2
• Mill wheel murmur
Prevention:
• Positioning (prone vs sitting)
• Adequate hydration (avoid hypovolemia)
• ?Use of PEEP
Monitoring for VAE
• Precordial Doppler sonography: • Interruption of the regular swishing of the Doppler
signal by sporadic roaring sounds indicates venous air embolism
• TEE
• ETCO2
• ETN2
• PA catheter
Precordial doppler
• Sensitive, can detect 1 mL of air or less (but NOT quantitative!)
• usu positioned @ middle 3rd sternum on the right
• Position be confirmed by injecting 0.5-1ml air
Transesophageal echocardiography (TEE)
• More sensitive than Doppler ultrasound
• Specific, because air bubbles are visualized directly
• The only monitor that can detect PA
• Expensive, requires special expertise, and demands near constant attention
Management:
Prevent further air entry Notify surgeon (flood or pack surgical field) Jugular compression Lower the head Durants’ position
Treat intravascular Air Aspirate right heart catheter Discontinue N2O FIO2: 1.0 Pressors/inotropes Chest compression
Paradoxical air embolism
• In patients with Patent foramen ovale (PFO)
• In patients with probe PFO, when normal transatrial pressure is reversed :
• Hypovolemia
• PEEP
• TRANSPULMONARY PASSAGE OF AIR:
– Large volumes (>20ml/kg oR > 0.3ml/kg/min)
Post op complications
• Ventilation/airway abnormalities
– Macroglossia
• CVS complications: HTN
• Neurologic complications:
• Pneumocephalus
• Quadriplegia
Macroglossia
• Venous obstruction due to sustained neck flexion
• Also attributed to prolonged ischemia due to FB: Airways
Pneumocephalus:
• Cause delayed awakening and continued impairment of neurological status
• Related with N2O use
• The treatment is a twist-drill hole followed by needle puncture of the dura.
Quadriplegia
• Due to compression on the cervical spinal cord
• Caution with degenerative diseases of the cervical spine
References:
• http://www.frca.co.uk/article.aspx?articleid=100662
• Handbook of Neuroanesthesia, 4th edition: Philippa Newfield, James E. Cottrell
• Millers’ anesthesia:7th edition
• Clinical Anesthesiology: 4th edition: Morgan, Mikhail, Murray