FRCA Notes

Posterior Fossa Surgery

The curriculum asks for 'understanding of anaesthesia for neurosurgical procedures including posterior fossa surgery posterior fossa surgery [including vascular disease and tumours]'.

Resources

  • Pathology within the brainstem or cerebellum typically has a profound physiological impact as important structures are concentrated:

    • Category Structures
    CNS Respiratory and cardiovascular control centres
    Nerve tracts Ascending and descending spinal tracts
    Cranial nerves
    Vasculature Transverse, occipital and sigmoid sinuses
    CSF CSF outflow through the aqueduct of Sylvius

  • The posterior (infratentorial) fossa is compact and rigid, with poor compliance
  • Small additional volumes e.g. SOL or blood can result in significant rises in intra-compartmental pressure and life-threatening brainstem compression
  • This makes surgery hazardous and poses anaesthetic challenges

  • Tumours are the commonest posterior fossa pathology
    • 60% of paediatric brain tumours are in the posterior fossa
    • In adults, metastatic lesions are more prevalent than primary lesions
    • Intra-axial tumours (i.e. rooted to brain parnechyma); medulloblastoma is the commonest, although astrocytoma, glioma and ependymoma are others
    • Extra-axial tumours commonly arise from the cerebellopontine angle e.g. meningioma, schwannoma, acoustic neuroma

  • Vascular lesions
    • 15% of intracranial aneurysms occur in the posterior fossa vasculature
    • Others include AVMs or cerebellar haematoma/infarct

  • Epidermoid or arachnoid cysts
  • Resection around cranial nerves e.g.
    • Trigeminal neuralgia
    • Glossopharyngeal neuralgia
    • Hemifacial spasm from CN VII lesion
  • Arnold-Chiari malformation decompression
  • Correction of congenital or acquired craniovertebral abnormalities (e.g. atlanto-occipital or atlanto-axial instability)

  • A benign tumour of the Schwann cells of the vestibular division of CN VIII
  • Symptoms present when the tumour is relatively small owing to the tightness of the posterior fossa, although headaches are uncommon unless the tumour is large

Clinical features

  • Unilateral sensorineural hearing loss
  • Unilateral tinnitus
  • Vertigo or other balance issues
  • Trigeminal nerve (CN V) palsy
    • Facial numbness or paraesthesia
    • Decreased corneal reflex

Facial nerve palsy

  • A rare complication of surgical excision of acoustic neuroma
  • Leads to unilateral symptoms from loss of innervation of various CN VII branches:
Unilateral symptom Branch affected
Facial muscle weakness Multiple
Loss of taste/metallic taste (anterior 2/3rds) Chorda tympani
Altered hearing Nerve to stapedius
Decreased salivation Greater petrosal nerve
Decreased tear production Greater petrosal nerve
Inability to close eye / ptosis Temporal and zygomatic branches


Perioperative management of the patient undergoing posterior fossa surgery


History and examination

  • Conscious level
  • Cranial and cerebellar nerve function
  • Strength of airway (gag, cough) reflexes ± potential chronic respiratory changes from aspiration pneumonitis
  • ICP; hydrocephalus and elevated ICP are common in patients with posterior fossa pathology; may need EVD or shunt prior to definitive surgery
  • Fluid balance; patients may be dehydrated from obtunded consciousness, vomiting, DI or use of IV contrast
  • Electrolyte status
  • Cervical spine assessment if surgery is for craniovertebral junction anomalies; there may be a difficult airway

  • A proportion of patients will be paediatric owing to the higher incidence of certain pathologies in this group e.g. medulloblastoma, Arnold-Chiari malformation

Monitoring and access

  • AAGBI
  • Arterial line with transducer at level of external auditory meatus to assess cerebral perfusion
  • Neuromuscular blockade monitoring
  • + RIJCVC for patients undergoing surgery in sitting position
  • + VAE monitoring for patients undergoing surgery in sitting position e.g. transoesophageal echo, pre-cordial doppler

  • Neurophysiological monitoring
    • EEG to monitor cortical hypoperfusion/ischaemia
    • Continuous EMG monitoring of CN VII
    • SSEP to monitor spinal cord ischaemia
    • BAEP to monitor CN VIII

  • Lumbar CSF drainage may be required to improve surgical conditions e.g. drainage of 10ml aliquots of CSF

Positioning

  • Meticulous positioning is necessary
    • The sitting position is associated with various complications and is generally avoided
      • If it will provide major surgical advantage, pre-operatively screen for a PFO (incidence 10 - 35%) with bubble-contrast TTE as paradoxical embolus can be devastating
    • The lateral/park bench or prone positions are becoming more common, but still carry complications of their own
Area Steps to reduce pressure injury
Head & neck Carefully secure endotracheal tube to avoid pressure on lips
Head & neck Tape eyelids closed and pad the eyes
Upper limb Avoid extreme head rotation due to risk of brachial plexus injury
Upper limb Pad elbows and forearms in supination to avoid ulnar nerve compression injury
Trunk Avoid tension on urinary catheter, which can cause bladder neck pressure injury
Lower limb Avoid straight legs; put pillows under and/or between knees
Lower limb Gel pads under heels

Cardiovascular changes

  • Monitoring cardiovascular variables can help assess the adequacy of medullary perfusion
    • Rapid changes may occur with surgical traction owing to the proximity of the medullary cardiovascular centre (brainstem-cardiac reflex)
    • Respiratory activity is only a marker of adequate medullary perfusion if ventilation is spontaneous
      • If used it is a more sensitive indicator of brain stem dysfunction than cardiovascular changes

  • Stimulation of the fifth cranial nerve intra-operatively may:
    • Increase circulating catecholamines, leading to tachycardia and hypertension
    • Conversely trigger the trigemino-cardiac reflex with profound bradycardia ± asystole
    • Also manifest as jaw jerking (masseter)

  • Patients typically undergo neuromonitoring in a high dependency area or on ICU
  • ICP monitoring is considered if there is a high-risk of hydrocephalus

Post-operative ventilation

  • Post-operative ventilation may be required if there is impaired respiratory drive or lower cranial nerve dysfunction
  • Patients with pre-existing bulbar palsy may not be able to adequate protect their airway immediately and may require controlled ventilation post-operatively
  • Equally, patients with surgery close to the brainstem or lower cranial nerves may increase risk of aspiration post-operatively
    • Extensive intra-operative dissection around the cranial nerve nuclei or floor of 4th ventricle may cause airway compromise post-extubation

  • Initiation of surgical drain suction may initiate cardiovascular changes such as bradycardia

  • PONV is common (45-70%) due to the proximity of the vomiting centre, compounded by opioid use to control post-operative pain; multi-modal anti-emesis is required

  • Pain can be significant, especially if occipital or infratentorial approaches are used; there is extensive muscle cutting, reflection and therefore spasm

  • Pneumocephalus can occur if surgery has occurred in the sitting position

  • Posterior fossa syndrome can occur in children
    • Temporary, complete loss of speech following posterior fossa surgery, with associated ataxia, hypotonia and behavioural-affective symptoms
    • More common if midline tumour and/or medulloblastoma
    • Early diagnosis facilitates parental understanding, coping and SALT

  • Obstructive hydrocephalus; due to local cerebral oedema, cerebral haemorrhage, mass effect from residual tumours

  • Cranial nerve dysfunction (up to 4.8%)
    • Depending on the extent of pathology and surgery, CN III - XII are at risk
    • Typically causes by mechanical and thermal disruption to the cranial nerves
    • Risk is reduced by intra-operative neurophysiologic monitoring
    • Can be temporary or permanent

  • Failure to wake, e.g. due to:
    • Haematoma e.g. SDH, extradural haematoma, intra-parenchymal
    • Direct injury to the brainstem during surgery
    • Hydrocephalus