Anatomy and Pathophysiology of Posterior Fossa
The posterior fossa, or infratentorial fossa, is a small but critical compartment in the skull that houses essential structures like the brainstem, cerebellum, and lower cranial nerves. Because of the confined space and its poor compliance, even small increases in volume due to tumors, hematomas, or other pathologies can cause a significant rise in intracranial pressure (ICP) and cause brainstem compression.
The foramen magnum, located in the occipital bone, is the largest opening of the posterior fossa and serves as a conduit for the brainstem to connect with the spinal cord. Due to the narrow cerebrospinal fluid (CSF) pathway through the cerebral aqueduct, obstructions in this area can result in hydrocephalus and ICP elevation. Tumors are the most common pathology in the posterior fossa. Cranial nerve lesions and vascular abnormalities, such as aneurysms and arteriovenous malformations, also require surgical intervention.
Surgical Positioning for Posterior Fossa Surgery
Various surgical positions are employed depending on the location of the pathology and the surgical approach required:
- Sitting Position: This position offers several advantages, such as improved surgical orientation, decreased need for retraction, and gravity-assisted drainage of blood and CSF. It is commonly used for midline structures, but it is associated with complications like cardiovascular instability, venous air embolism, and venous pooling.
- Supine and Prone Positions: These positions are used for surgeries involving the cerebellopontine angle or lateralized structures. The supine position with the head turned to the side can minimize stretching of the brachial plexus, while the prone position offers direct access to midline lesions but requires careful management to avoid abdominal compression and excessive bleeding.
- Lateral and Park Bench Positions: These positions are often used for unilateral procedures. The park bench position, a modified lateral position, allows better access to midline structures without the challenges of the prone position.
Complications of Posterior Fossa Surgery
- Venous Air Embolism is associated with surgeries performed in the sitting position due to the negative venous pressure at the surgical site. Pneumocephalus: During surgery, techniques that reduce brain volume may encourage air to enter the cranial cavity, which can lead to tension pneumocephalus as the brain re-expands postoperatively. Nitrous oxide can exacerbate this problem, and its use should be avoided in patients at risk of pneumocephalus.
- Macroglossia: Prolonged surgeries in the sitting position, where the neck is acutely flexed, can lead to venous and lymphatic obstruction, causing tongue swelling and postoperative airway obstruction.
- Quadriplegia: Extreme neck flexion in the sitting position can cause focal pressure on the spinal cord, which leads to ischemic damage and quadriplegia, especially if there are episodes of significant hypotension.
Monitoring and Management of Air Embolism
Precordial Doppler ultrasound is the most commonly used tool to detect air as small as 0.015 mL/kg/min. Transesophageal echocardiography (TEE) is more sensitive but invasive. End-tidal CO₂ monitoring can detect a drop in CO₂ levels due to increased dead space ventilation. Monitoring end-tidal nitrogen is even more sensitive, as it can detect air embolism by measuring the nitrogen expelled from the lungs when air is entrained.
Immediate repositioning the patient to the Trendelenburg or left lateral decubitus position (Durant maneuver) can help trap air in the right atrium and reduce the risk of further embolism. If a central venous catheter is in place, air can be aspirated from the right atrium or right ventricle. Administering 100% oxygen helps reduce the size of air bubbles by promoting nitrogen absorption and improving the oxygenation of hypoxic tissues.
After posterior fossa surgery, the decision to extubate depends on the patient’s preoperative condition and intraoperative course. Postoperative ventilation may be required in patients with lower cranial nerve dysfunction or significant intraoperative dissection near vital centers. Special attention should be given to the risk of postoperative airway edema, especially in cases involving sitting or prolonged prone positioning.
