Posterior Fossa Surgery

Question 1

A 48-year-old male patient is undergoing posterior fossa surgery for excision of a metastatic lesion secondary to lung cancer. He has a body mass index (BMI) of 41 and initially presented with seizures. You are asked to review him prior to his procedure.

What added information would you like before proceeding with this case?

Answer 1

1) Patient factors:
* Take an anaesthetic history focusing on pre-existing comorbidities; particularly complications associated with obesity e.g. obstructive sleep apnoea, hypertension and ischaemic heart disease.

• Explore the diagnosis of lung cancer, including investigations and treatment so far.
• Discuss any interventions for the metastatic brain lesion and in particular, symptoms he has developed including the seizures mentioned in the history.
• Ask the patient about previous anaesthetics and conduct an airway assessment (the he may have a difcult airway due to his raised BMI).
• Take a medication and social history.

2) Surgical factors:
* Preferred patient positioning. Te options for posterior fossa surgery include sitting, prone, lateral and park-bench.

• Discuss any potential complications or challenges that may arise peri-operatively with the suggested management in an emergency.

Question 2

What specific signs and symptoms may the patient have due to the tumour?

Answer 2

• Signs suggestive of cerebellar involvement e.g. tremor, ataxic gait and dysarthria.
• Bulbar cranial nerve palsies. The patient may have had episodes of choking or aspiration due to a poor gag reflex. Coughing may also be impaired.
• The patient presented with seizures, which suggests raised intracranial pressure. Other symptoms include a headache, nausea, vomiting and fluctuating conscious levels. The patient should be assessed for these on the day of surgery as he may require preoperative medical or surgical management.
• Fluid and electrolyte imbalance and signs suggestive of hypovolaemia secondary to vomiting, diabetes insipidus, SIADH or poor oral intake.
• Side effects of high-dose steroids and other treatment that may have been initiated following the initial diagnosis.

Question 3

The surgeons would like to proceed in the sitting position. What are the contraindications to surgery in this way?

Answer 3

Absolute (VAE risk)
* Presence of a ventriculo-atrial shunt.
* Presence of a patent foramen ovale.

Relative
* Poorly controlled hypertension (due to the risk associated with hypotension when sitting).
* Very young/old patients.
* Chronic obstructive pulmonary disease.
* Autonomic neuropathy.

Question 4

What specific complications are associated with surgery in the sitting position?

Answer 4

• Perioperative haemodynamic instability.
• Venous air embolism.
• Pneumocephalus.
• Tongue swelling.
• Permanent nerve damage (cervical spine flexion injury).

Question 5

What forms of monitoring would you use when anaesthetising this patient?

Answer 5

• Full AAGBI standard monitoring including ECG, capnography, pulse oximetry and core temperature.
• “Train of four” monitoring if using neuromuscular blocking agents.
• Invasive blood pressure monitoring.
• Central venous pressure monitoring.
• Consider monitoring for venous air embolus:
• Precordial Doppler.
• TOE.
• Somatosensory-evoked potentials if there is a surgical indication.

Question 6

What are the anaesthetic goals for this patient?

Answer 6

• Maintenance of a stable blood pressure and cerebral perfusion pressure.
• Quick ofset of anaesthetic to allow for rapid postoperative neurological monitoring.
• Careful patient positioning and padding to minimise the risk of complications secondary to the procedure and sitting position.