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PBLD – 2

Craniotomy in the Sitting Position

 Authors: Renée Young, DO, Federico Bilotta, MD, PhD and Shaun E. Gruenbaum, MD

CASE: A 42 year-old man is brought to the operating room for a craniotomy for posterior fossa tumor resection. He has a 6-month history of headaches, which have progressively worsened over the past 2 months.  He now complains of diplopia and ataxia.  The surgeon informs you that she wants to perform the craniotomy with the patient in the sitting position.

Past Medical History: Former smoker, otherwise healthy
Medications: None

Imaging: Brain MRI shows a 5.3 cm left-sided posterior fossa tumor with mass effect and mild obstructive hydrocephalus
Past Surgical/Anesthetic History: laparoscopic appendectomy 2012, no past anesthetic problems
Physical Examination: 92 kg, well-appearing man
Neurological Examination: left abducens nerve palsy, CN II-V and VII-XII grossly intact. No other focal deficits.

Vitals: BP 138/76, HR 89, RR 15, T 37°C
Labs: Hemoglobin 15 g/dl, Sodium 140 mEq/L, Potassium 3.8 mEq/L, Glucose 110 mg/Dl, ECG – normal

KEY QUESTIONS:

  1. What are the potential benefits to performing a craniotomy in the sitting position?
  2. Are there potential risks that should be considered in this position?
  3. What can be done to minimize these risks?
  4. Are there any contraindications to performing a craniotomy in the sitting position?
  5. Are there any alternative positions to the sitting position when surgical access to the posterior fossa is required?
  6. Would you delay surgery to evaluate for a patent foramen ovale (PFO)?
  7. How would you evaluate for a PFO?
  8. Suppose the patient has a known PFO; would you require that it be corrected prior to surgery?
  9. Is it necessary to place a central venous catheter in this patient?
  10. If you placed a central venous catheter, would a multi-orifice or single-orifice catheter be preferred? Does it matter?
  11. If you placed a central venous catheter, where should the tip of the catheter be located?
  12. How is a venous air embolism best detected?
  13. In the event of venous air embolism that results in hemodynamic compromise, how would you treat it?

DISCUSSION:

  1. BENEFITS OF CRANIOTOMY IN THE SITTING POSITION
  • Traditionally, the sitting position has been the preferred position to surgically access the posterior cranial fossa or posterior cervical spine [1].
  • Although the prone and lateral positions may also be used when operating on the posterior cranial fossa, the sitting position offers several physiological advantages over alternative positions.
  • During craniotomy in the sitting position, the effects of gravity facilitate the drainage cerebral spinal fluid away from the surgical field, thereby lowering intracranial pressure (ICP) more than any other position [2].
  • During craniotomy in the sitting position, cerebral venous drainage is also improved, thereby draining blood away from the surgical field. This results in decreased blood loss, a reduced need for blood transfusion, optimal surgical exposure, and a shorter length of surgery compared with other positions [3].
  • When performing craniotomy in the sitting position, there is direct surgical access to the superficial and deep structures of the posterior cranial fossa. This exposure results in less tissue retraction and a reduced risk of cranial nerve damage compared with other positions.
  • Compared to the prone position, the sitting position offers better access to the patient’s airway.
  • The sitting position results in a decrease in intrathoracic pressure, thereby optimizing respiratory mechanics and allowing for improved ventilation.
  • Compared with craniotomy in the sitting position, the prone position is associated with an increase in intraocular pressure and increased risk of post-operative blindness [4].
  1. RISKS OF CRANIOTOMY IN THE SITTING POSITION
  • Craniotomy in the sitting position is associated with significant and potentially life-threatening risks. Some studies have reported that the sitting position is among the top three factors (in addition to length of surgery and patient co-morbidities) associated with an increased risk of intraoperative complications [5].
  • Due to the effects of gravity on the vascular system, venous pooling can occur with a reduction in cardiac preload, and patients may potentially experience significant systemic hypotension. This drop in blood pressure can lead to reduced cerebral perfusion pressure (CPP), cerebral and/or spinal cord ischemia, and possibly even quadriplegia.
  • To best assess CPP, it is important to position the arterial line transducer at the level of the external auditory meatus during craniotomy in the sitting position.
  • Hypotension may be partially ameliorated by a fluid bolus prior to positioning, by proper positioning of the legs and the placement of thigh high compression stockings or elastic wrap on the lower extremities, and by the administration of vasopressors.
  • The sitting position is associated with an increase in pulmonary and systemic vascular resistance [6], and an increased risk of thromboembolic events [7].
  • Postoperative tension pneumocephalus can occur in up to 3% of posterior cranial fossa surgeries in the sitting position [8]. Tension pneumocephalus may result when air enters the epidural space in large enough volumes to cause a mass effect, which can result in hemiparesis, seizures, or life-threatening brain herniation.  Some authors recommend that minute ventilation be decreased to allow for brain expansion as the dura is closed [9], and nitrous oxide should be avoided in the first 14 days after posterior cranial fossa surgery.  The risk is further decreased by the placement of a ventriculostomy drain.
  • Extreme neck flexion, in which the chin rests on the chest, combined with the use of an oral airway may obstruct venous and lymphatic drainage, resulting in significant macroglossia [9]. This can result in significant airway obstruction and hypoxia after tracheal extubation.  Proper head and neck positioning, along with the placement of a soft bite block may reduce this risk.
  • Rarely, performing surgeries in the sitting position can result in peripheral neuropathies [4], and may result from ischemic compression or excessive stretching of the nerve. The most commonly injured nerve during surgery in the sitting position is the common peroneal nerve, which results in foot drop. Other nerves particularly susceptible to injury include the brachial plexus and sciatic nerves [4]. Another reported injury is mid-cervical quadriplegia, which can result from stretching of the spinal cord from extreme neck flexion in the setting of hypotension and impaired autoregulation in the sitting position [10, 11]. The use of padding as well as proper positioning, in which two or more more fingers’ width is maintained from the chin to sternum, may minimize the risk of nerve damage.
  • In the event of a cardiac arrest, the performance of effective cardiopulmonary resuscitation (CPR) requires that patients in the sitting position be first repositioned to the supine position. Alternatively, CPR may be effectively performed with the patient in the prone position, without any need to reposition the patient [12].
  • Venous air embolism (VAE) is a potentially life-threatening complication of undergoing craniotomy in the sitting position.
  1. VENOUS AIR EMBOLISM
  • VAE can occur when the surgical site is above the level of the heart, creating a pressure gradient between the surgical site and the heart. The risk of VAE varies depending on the type of surgery.  The non-collapsible venous sinuses exposed during posterior cranial fossa surgery in the sitting position make these procedures at particularly high risk of developing a VAE.
  • The two most important factors that determine the morbidity and mortality of VAE are the volume of air entrapment and rate of air accumulation [13]. The amount of air that could result in a fatal embolism is debated, but has been reported to be a 200-300 mL bolus (3-5 mL/kg) [14].
  • In the event of a VAE, hypoxia can quickly ensue. Right heart strain can further result in cardiac ischemia and significant hypotension and cardiac arrest.
  • A paradoxical air embolism can occur when a VAE crosses into the arterial circulation (for example, through a PFO), and can result in a significantly increased risk of stroke and death [15].
  • Patients with VAE can also develop thrombocytopenia, increasing the patient’s risk of bleeding [16].
  • A VAE can be detected by several monitoring modalities, although no single method will accurately predict all cases of VAE [17].
  • Although rarely used, the most sensitive method for detecting VAE is transvenous intracardiac echocardiography, which has been shown to detect volumes of air as small as 0.15 mL [18]. Other modalities include transesophageal echocardiography, precordial Doppler, pulmonary artery pressure, end-expiratory carbon dioxide (ETCO2), end-expiratory nitrogen, esophageal stethoscope, and electrocardiogram (EKG) [19].
  • In the event of a VAE, early signs in the anesthetized patient include hypotension, tachycardia, sudden decrease in end tidal CO2, increase in arterial pCO2, hypoxemia and increased peak airway pressures.
  • An increase in pulmonary arterial vascular resistance can rapidly result in right heart failure, jugular venous distension, myocardial ischemia and cardiovascular collapse.
  • In the event of a VAE, the anesthesiologist should immediately notify the surgeon, who should subsequently flood the surgical field, compress the veins, and identify and close the source of air entrainment when possible.
  • To abolish the gradient between the surgical site and the heart, the patient should be immediately placed in the Trendelenburg position if tolerated, or in the left lateral decubitus position.
  • If a central venous catheter is in place, the anesthesiologist should attempt to aspirate the entrained air from the heart.
  • It is essential to administer vasopressors such as norepinephrine to maintain adequate cardiac output and end-organ perfusion.
  1. PATIENT WITH A PFO
  • Because of the increased risk of sequelae from VAE in patients with a PFO, many clinicians advocate for routine preoperative screening with contrast enhanced transesophageal or transthoracic echocardiography prior undergoing a craniotomy in the sitting position.
  • The risks of delaying the procedure to evaluate for PFO should always be weighed against the risks of VAE. In our case, the patient has worsening symptoms and evidence of mass effect and hydrocephalus, which should be taken into consideration when deciding whether to delay the procedure and evaluate for PFO.
  • In recent years, transcranial echocardiography has emerged as an inexpensive, noninvasive, and easy method of preoperative screening for PFO [20]. During transcranial echocardiography, a contrast agent with a small amount of air is injected into the antecubital vein.  If the presence of a right to left shunt, the contrast will bypass the pulmonary circulation and result in microembolic signals in the basal cerebral arteries.  The amount of microbubbles detected and the conditions necessary to elicit the shunt (spontaneous versus with valsalva) indicates the severity of PFO.
  • When a PFO is identified prior to performing craniotomy in the sitting position, some authors recommend that either the PFO be surgically closed prior to surgery [15] or that the surgical position be changed to prone.
  • However, recent studies have suggested that even in patients with a PFO, craniotomy in the sitting position can be safely performed with minimal risk of VAE [21-23].
  1. PLACEMENT OF A CENTRAL VENOUS CATHETER
  • A central venous catheter enables entrained air to be aspirated from the right atrium, and should be considered in patients undergoing craniotomy in the sitting position.
  • Compared with a single-orifice catheter, placing a multi-orifice catheter increases the probability that an orifice will be located where the air-fluid interface exists. Moreover, a multi-orifice catheter is thought to facilitate the aspiration of a larger volume of air compared with a single-orifice catheter, and is therefore preferable when placed in patients undergoing craniotomy in the sitting position.
  • When placing a single-orifice catheter, the tip should be placed 3 cm above the junction of the superior vena cave and right atrium. In contrast, the tip of a multi-orifice catheter should be placed 2 cm below this junction (in the right atrium).
  1. CONTRAINDICATIONS AND ALTERNATIVE POSITIONS
  • The presence of a right to left intracardiac shunt has generally been considered an absolute contraindication to surgery in the sitting position, although this idea has been challenged in recent years. Limited data suggests that even patients with a PFO can be safely operated on in the sitting position, with a minimal risk of VAE [21-23].
  • Other relative contraindications to the sitting position include patients with severe spinal stenosis, uncontrolled and severe hypo or hypertension, and severe atherosclerotic disease [4].
  • Alternative positions for posterior cranial fossa surgery include lateral decubitus, supine with extra padding under ipsilateral shoulder, supine with head rotation or prone.
  1. SAFETY OF CRANIOTOMY IN THE SITTING POSITION
  • The risk to benefit ratio of neurosurgical procedures in the sitting position has been considerably debated [15, 24].
  • In recent years, the sitting position has largely fallen out of favor in the United States and many other countries due to fears of its associated complications [25]. Today, the most common procedure performed in the sitting position in the United States is an insertion of a deep-brain stimulator [26].
  • It has never been firmly established that neurosurgical procedures in the lateral or prone position are safer than the sitting position. Although the incidence of air embolism is higher in the sitting position compared with the prone position, recent studies have failed to demonstrate any differences in morbidity and mortality [27].
  • Many experts have recently argued that the fear of catastrophic complications related to the sitting position seems unwarranted [28]. With an experienced surgical and anesthesia team, neurosurgery in the sitting position can likely be done safely, and may be advantageous to the prone position [17, 29, 30].
  • In a recent retrospective study in 85 pediatric patients undergoing craniotomy for posterior fossa tumor in either the sitting or prone position, patients in the sitting position experienced fewer intraoperative and postoperative complications, with a shorter intensive care and hospital stay [31].
  • Most importantly, when craniotomy is done in the sitting position, communication between the surgical and anesthesia team is paramount, especially with regards to the early detection of VAE.

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