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