Posterior fossa surgery

Learning objectives

Describe the posterior fossa
Describe the indications for posterior fossa surgery
Manage patients undergoing posterior fossa surgery

Background

The posterior fossa is the deepest cranial fossa
Surrounded by:
- Anteriorly: The dorsum sellae and basilar portion of the occipital bone (clivus)
- Laterally: The petrosal and mastoid components of the temporal bone
- Superiorly: The dural layer (tentorium cerebelli), and posteriorly and
- Inferiorly and posteriorly: The occipital bone
Contains many important structures: the brainstem, cerebellum and lower cranial nerves
The cerebrospinal fluid pathway is very narrow through the cerebral aqueduct and any obstruction can cause hydrocephalus which can result in a significant increase in intracranial pressure

Pathologies

Tumors are the most common pathologies of the posterior fossa
Pathologies which require surgical intervention:

Tumors	Axial tumors	Medulloblastoma (most common)
		Cerebellar astrocytoma
		Brainstem glioma
		Ependymoma
		Choroid plexus papilloma
		Dermoid tumours
		Hemangioblastoma
		Metastatic tumours
	Cerebellopontine angle tumours	Schwannoma
		Meningioma
		Acoustic neuroma
		Glomus jugulare tumour
Vascular malformations	Posterior cerebellar artery aneurysm
	Vertebral/vertebrobasillar aneurysm
	Basillar tip aneurysm
	AV malformations
	Cerebellar hematoma
	Cerebellar infarction
Cysts	Epidermoid cyst
	Arachnoid cyst
Cranial nerve lesions	Trigeminal neuralgia (cranial nerve V)
	Hemifacial spasm (cranial nerve VII)
	Glossopharyngeal neuralgia (cranial nerve IX)
Craniocervical abnormalities	Atlanto-occipital instability	Congenital
		Acquired
	Atlanto-axial instability	Congenital
		Acquired
	Arnold–Chiarri malformation

Management

posterior fossa surgery, cranial nerve, ICP, intracranial pressure, airway, positioning, oximegty, ecg, capnography, invasive arterial blood pressure, central venous catheter, venous air embolism, precordial doppler, eeg, ssep, baep, emg, cpp, remifentanil, nitrous oxide, aspiration pneumonia, ventilation, edema, tongue swelling, prone, hypertension, antiemetics, analgesia

ICP, intracranial pressure; ECG, electrocardiography; EEG, electroencephalography; SSEP, somatosensory evoked potential; BAEP, brainstem auditory evoked potential; EMG, electromyography; CPP, cerebral perfusion pressure

Keep in mind

Maintain consistent and modest levels of inhalation or IV anesthetic agents to minimize interference during SSEP monitoring
Avoid neuromuscular blocking agents
Use total IV anesthesia during motor evoked potential monitoring
Intraoperative positioning
- The sitting position improves surgical access to the posterior fossa, but is associated with several potential complications:

Complication	Management
Cardiovascular instability	Notify the surgeon of their proximity to vital structures
Venous air embolism	Administer high-concentration oxygen, discontinue nitrous oxide, maintain cardiovascular stability, central venous catheter to aspirate air from right atrium, immediate initiation of chest compression in the event of a massive air embolism with cardiac arrest
Pneumocephalus	High-flow oxygen, burr hole and aspiration of air in severe cases
Macroglossia	Ensure airway clearance
Quadriplegia	Avoid this complication by paying close attention to positioning and avoiding prolonged hypotension