Corresponding author: Warren M. Rozen, MD, PhD, Discipline of Surgery, School of Medicine, James Cook University, Townsville, Queensland 4814, Australia. Tel.: +61 747961417; E-mail: moc.liamtoh@nezornerraw
Copyright 2015 by the International College of Surgeons
Abstract
Surgery in the prone position is often a necessity when access to posterior anatomic structures is required. However, many complications are known to be associated with this type of surgery, as physiologic changes occur with increased pressure to anterior structures. While several studies have discussed postoperative vision loss, much fewer studies with lower levels of evidence have addressed other complications. A systematic literature review was conducted using 2 different databases, and 53 papers were regarded as appropriate for inclusion. Qualitative and quantitative analysis was performed. Thirteen complications were identified. Postoperative vision loss and cardiovascular complications, including hypovolemia and cardiac arrest, had the most number of studies and highest level of evidence. Careful planning for optimal positioning, padding, timing, as well as increased vigilance are evidence-based recommendations where operative prone positioning is required.
Key words: Prone, Surgery, Trunk, Supine, Retinal, Complications, Evidence based medicine, Qualitative and quantitative methods
Prone positioning is a common position used for access to the posterior head, neck, and spine during spinal surgery, access to the retroperitoneum and upper urinary tracts and access to posterior structures when required during plastic surgery., Prone surgery is associated with a variety of complications many of which are derived from increased pressure on anterior structures. Rates of pressure sores as an intraoperative complication have been reported to be between 5% and 66%. As such, pressure sores incur longer hospital stays and healthcare costs. Postoperative vision loss can result from inappropriate orbital pressure and can also be a permanently debilitating condition. Lateral femoral cutaneous nerve neuropathy in 20% of spinal surgery cases causing pain and dysesthesia in the anterolateral thigh. Inappropriate pressure on vital structures of the abdomen can cause ischemia and organ failure, resulting in prolonged hospitalization, permanent disability, or death. Cardiovascular changes associated with prone surgery increase risk of intraoperative cardiac arrest. At the same time, prone surgery is also associated with airway management and CPR issues as access to anterior structures is limited.
Materials and Methods
We conducted a systematic literature search using Medline and Scopus for literature using the MeSH terms “surgery,” and “prone position.” We limited the search to those in the English language and with a focus on humans. Articles were included or excluded based on the content of the article's title and abstract, particularly if surgical complications were addressed, and if they were tied to prone positioning intraoperatively. Full text articles were reviewed for citations, which had appropriate content or those that could not be excluded indisputably based on abstract content (Table 1).
Table 1
Details of reviewed literature
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Type of complication and surgery were identified and articles were sorted accordingly. Guidelines for level of evidence from the Oxford Centre for Evidence-Based Medicine (OCEBM) were also applied (Table 2). Results were presented qualitatively and quantitatively.
Table 2
Complications and level of evidence
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Results
Fifty-three papers addressing complications during prone surgery were reviewed. In these, 13 complications were identified: increased intra-abdominal pressure and increased bleeding, abdominal compartment syndrome, limb compartment syndrome, nerve palsies, pressure sores, cardiovascular compromise, thrombosis and stroke, hepatic dysfunction, postoperative vision loss, oropharyngeal swelling, venous air embolism, and endotracheal tube dislodgement. The 2 complications with the highest number of studies had the highest level of evidence. Half of the papers found addressed postoperative vision loss despite its low rate of incidence. Cardiovascular complications including hypovolemia and cardiac arrest were discussed in 13 studies. Many other complications had few studies with low levels of evidence. Several studies have discussed recommendations to reduce risk of complication, which will be discussed further.
Discussion
Several studies addressed physiologic changes associated with prone position.
Without proper padding and bolster placement, significant pressure is put on thorax and abdomen. Pelvic and abdominal compression results in increased intra-abdominal pressure causes direct pressure on inferior vena cava and venous pooling and decreased venous return., Increased thoracic pressure causes decreased left ventricular compliance and filling, resulting in reduced ventricular volume, stroke volume and cardiac index, while raising central venous pressure.- Reduced stroke volume and cardiac index results in a drop in blood pressure often seen when a patient is turned from supine to prone.
Respiratory changes include a 30% to 35% decrease in respiratory compliance and increase in peak airway pressure. This in turn decreases venous return and cardiac output.
Increased intra-abdominal pressure may cause a decrease in renal perfusion and impair function, however, this is controversial.
Prone position also increases intraocular pressure as the episcleral venous system is connected to the central venous system by valveless vessels.,,- Thus an increase in central venous pressure will result in an increase in episcleral venous pressure and intraocular pressure.
With proper supportive padding to alleviate pressure on abdomen and thorax there is minimal reduction in respiratory compliance. Lung volume and oxygenation increase due to improved ventilation/perfusion ratio in both adults and pediatric demographics.,
Obese patients are at particular risk of increased abdominal pressure, increased venous pressure in the head, and decreased cardiac output and reduced end organ blood flow., However, with proper supportive padding, these patients have better oxygenation in supported prone position versus supine position, as mediastinal contents fall anteriorly facilitating ventilation.
Increased intra-abdominal pressure, abdominal compartment syndrome, and increased bleeding in spinal surgery
An increase of intra-abdominal pressure in the prone position of more than 12 mmHg is high risk for abdominal compartment syndrome, as visceral compression and intra-abdominal hypertension cause dropped perfusion pressure resulting in multi-organ failure. Patients with previous abdominal surgeries are at particularly high risk as tight abdominal closures can reduce abdominal compliance, increasing abdominal pressure. In addition, patients with previous abdominal surgeries are at risk for incisional hernia, particularly if the patient is also obese.
Bleeding from vertebral veins during spinal surgery is increased in the prone position and can obscure the surgical field and prolong surgical time. Multiple thin-walled, low-pressure venous plexuses closely related to the vertebrae are connected to chest and abdominal venous supplies. Pressure on the chest and abdomen therefore increases pressure in the vertebral vessels resulting in increased bleeding in spinal surgery., Venous engorgement in combination with arterial hypotension, can lead to poor spinal cord perfusion and result in ischemia.
Recommendations
Chest rolls provide sufficient supportive padding to the chest to allow the abdomen to hang freely. Similarly, the Jackson table also achieves a freely hanging abdomen, which decreases visceral congestion and pressure on the cardiac and respiratory systems.,,
A study by Akinci et al, showed a decrease in intra-abdominal pressure and bleeding when spinal surgery patients were placed in a jackknife position rather than the prone position, as it uses hip flexion to relax and decrease intra-abdominal pressure while supporting the chest.
Monitoring intra-abdominal pressure by intravesicular transducer should be considered for high-risk patients or high risk procedures. Frequent checks for signs of multi-organ failure during surgery such as drop in blood pressure, rise in PCO2, drop in oxygen saturation and PO2, decrease in urine output, coagulopathy or abnormal base deficit is also suggested. If abdominal compartment syndrome is suspected, delay for emergency abdominal decompression should be avoided.
Nerve palsies and neuropraxia
Surgery in prone position has increased risk of injury of cervical spine and brachial plexus due to focal pressure points or strain in flexion or extension, leading to increased intraneural venous pressure, local edema, and impaired axoplasmic transmission., Risk factors include hypotension, diabetes, hypothermia, anatomic variations, and malnutrition.
Recommendations
For prevention of brachial plexus injury, anterior positioning of the humerus to the thoracic cage along with supportive padding for the arms is recommended., Palpating the pectoralis major muscle tendon along the anterior axilla border can monitor tension on the brachial plexus. Appropriate supportive padding should be used at bony surfaces where known superficial nerves are known to travel to avoid nerve compression. Neutral positioning is encouraged to avoid impingement and excessive stretch forces on nerves., Neuropraxia assessment should be done postoperatively. Evaluation for muscle weakness, impaired sensation, or myalgia can be done as soon as anesthesia has worn off. Electromyography can be used to pick up lesions 2−3 weeks after surgery. Treatment options include physiotherapy, pain management, and repair by nerve graft.
Head and neck pressure sores
During prone surgery, the rate of stage-1 pressure sores is said to be between 5% and 66%., The risk increases with longer duration and increased volume replacement causing increased facial edema. Pressure on the face in prone position is on average 30 mmHg, but can be higher than 50 mmHg in certain areas such as the chin and forehead above the supraorbital ridge. Prolonged pressure on ears, particularly on cartilaginous auricles can result in hematoma, chondritis, ischemia or necrosis.
Recommendations
Supportive padded headrests and bed inclination to elevate the head can be used to minimize inappropriate pressure on the face. However, the latter is not always possible as lumbar flexion is sometimes required for adequate access during spinal surgery, thus increasing pressure on the face. Koreckil et al proposed upward traction on the head to alleviate pressure off the face by way of Gardner-Wells tongs applying 15 lbs of traction at a 45° angle.
Chest pressure injuries
Pressure sores on the chest are also associated with prolonged surgical procedures in the prone position. Large breasts are subject to greater direct pressure. In addition, patients with breast implants have a theoretical risk of rupture and risk of breast necrosis with the direct pressure applied in prone position.
Recommendations
Patients with larger breasts should have medial placement of the breasts, as lateral placement can be painful.,
Cardiovascular collapse, arrest
As mentioned above, prone position during surgery is associated with reduced stroke volume, cardiac index, raised central venous pressure and low blood pressure. This, when combined with other factors, is associated with an increased risk for cardiovascular collapse and arrest. Risk factors include massive blood loss, hypothermia, fluid shifts, cardiac comorbidities, venous air embolism, and anatomic deformities such as thoracic lordosis or pectus excavatum, which can aggravate hypotension.,,,
Recommendations
It is important to maintain euvolemia and monitor fluid responsiveness by pulse pressure variation and stroke volume variation, both of which remain reliable indicators for fluid status monitoring, despite both increasing in the prone position. Systolic pressure variation may also be used for fluid status monitoring as it increases during hypovolemia. High-risk patients should be identified so that positioning can be optimized, and adhesive defibrillator pads can be applied prior to surgery. Patients in prone position on the Jackson spine table showed the least change in cardiac function (relative to supine position) compared with other prone position types. In this position, the abdomen hangs free and the chest and pelvis are supported, with the legs supported at heart level.
An arterial line is also recommended as a means to monitor real-time blood pressure and to allow immediate access to arterial blood samples to detect acid-base and electrolyte abnormalities., Central venous pressure monitoring can be used to monitor for trends indicating changes in right ventricular preload and intravascular volume. As central venous pressure is raised in the prone position, trends should be monitored for indication of change, not absolute values.
For patients with significant chest deformity and scoliosis, transesophageal echocardiography can be used to detect flow abnormalities in the prone position and find optimal supportive padding configuration to minimize the impact on cardiovascular function., In certain cases, pectus excavatum may need surgical correction prior to prone surgery.
During cardiac arrest, cardiac compressions may need to be commenced while the patient is still in prone position as turning the patient to supine position in a timely manner may not be feasible due to open wounds, ongoing blood loss, protruding metal work, or exposed or unstable structures such as the spine.
Hepatic dysfunction
Also liver congestion can be a consequence of raised central venous pressure. This paired with hypotension could lead to hypoperfusion and ischemic hepatitis.
Thrombosis and stroke
Thrombosis risk increases with patient age, malignancy, obesity and recent major surgery. Position of the patient contributes significantly to risk of thrombosis, as position-related occlusion of blood flow results in stasis and clotting. Increased risk of carotid and vertebro-basilar artery dissection is associated with non-neutral neck positioning and movements. Such movements can lead to kinking of blood vessels, intimal injury and thrombosis. The highest risk is at the atlanto-axial joint, which is the site of maximal head rotation., Significant neck flexion or rotation can also lead to reduced cerebral perfusion.,
In addition, foreign bodies introduced into the blood stream such as central venous catheters can act as sites of thrombosis, as foreign materials can activate the clotting cascade.
Recommendations
Reduction of excessive pressure and stabilization of the head and neck are important to avoid changes in blood supply and thrombosis during prolonged surgeries. Shermak et al suggests maintaining the neck in a neutral nonextended position to reduce pressure on neurovasculature., A subclavian venous catheter follows a less thrombogenic route than an internal jugular venous catheter. Nicol et al recommends kneeling or seated position to prevent venous thromboembolism during spinal surgery, as this reduces venous kinking in the lower extremities.
Limb compartment syndrome
Lower limb compartment syndrome and rhabdomyolysis are common complications associated with positioning in nonsupine positions., Elevated intracompartmental pressure can result in reduced local perfusion. Inadequate blood supply to the limbs can result in ischemia, which in turn can lead to edema and further increase in intracompartmental pressure. Ischemia can lead to necrosis of local tissue leading to neuropathy, rhabdomyolysis, raised myoglobin, and renal failure. In severe cases, permanent injury, disability, and death can result.,
Risk factors include obesity, increased muscularity, peripheral vascular disease, and surgeries with long duration. In addition, the risk for anterior shoulder dislocation may be higher in the prone position, and may lead to ischemia and compartment syndrome.
Recommendations
Monitoring of arterial supply to the limb by way of pulse oximetry can be a useful in detecting poor limb perfusion. Further, neutral positioning and padding can reduce the risk of thrombosis and the risk of limb compartment syndrome.
Postoperative vision loss (POVL) and other ophthalmic injuries
Several studies have discussed postoperative vision loss due to prone position. The rate of this complication is estimated to be between 0.05% and 1%.,,,
Ischemic optic neuropathy (ION) and orbital compartment syndrome (OCS)
Direct pressure on orbits can cause trauma resulting in conjunctival edema, hemorrhage, chemosis, pain, and vision loss.,
ION is caused by damage to optic nerve by increased intraocular pressure and orbital venous pressure. ION can be classified as either anterior ION, anterior to lamina cribrosa, or posterior ION. Posterior ION is more common postoperatively and more severe than anterior.
Direct pressure on the orbits causes an increase in intraocular pressure. Raised intraocular pressure in combination with raised intra-abdominal pressure reduces venous return in the orbit increasing venous pressure and edema.,, Edema can be aggravated by high-volume fluid replacement aggravating intraocular pressure. Congestion at the orbital apex can result in OCS and an ischemic orbit., Decreased perfusion pressure on the optic nerve results in ION., ION and OCS are well-established risk even with sufficient supportive facial padding.
Central retinal artery occlusion (CRAO)
CROA is caused by vasospasm, emboli, compression, or hypotension.,,, In particular, it can be due to displacement of plaques from the carotid arteries and can obstruct the central retinal artery. CROA is almost always irreversible.
Cortical blindness
Cortical blindness can be a result of emboli to posterior cerebral artery territory or significant hypotension and bilateral watershed infarctions affecting the visual cortex., It can improve over first few weeks, but is not likely to completely recover sight.,
Acute angle-closure glaucoma
The prone position can shift the lens-iris diaphragm forward, impinging on the drainage angle recess and obstructing the aqueous humor outflow and increased intraocular pressure and optic nerve injury. Patients predisposed to acute angle-closure glaucoma are at high risk with even short surgical procedures.
Other ophthalmic and extraocular injuries
Orbital pressure can also result in extraocular muscle impingement leading to swelling and impaired function. Corneal abrasion and scleral injuries are usually self-limiting. General anesthesia decreases tear production, and in combination with incomplete eye closure can be high risk for increased dryness, irritation, abrasion, laceration and subsequent inflammation and infection.
Risk factors
Venous congestion of the head which can be exacerbated when the head is placed lower than the heart seen when patients are positioned in the Trendelenburg position or in a Wilson's frame.,,,
Anemia, hemodilution, blood loss (>1000 mL) and hypotension, in combination with the increased ocular pressure in the prone position can reduce perfusion pressure to the optic nerve and cause ION.,-,- Surgical duration longer than 6 hours and small cup-to-disc ratio are associated with POVL.,,
Preoperative risk factors include obesity, peripheral vascular disease, hypertension, smoking, atherosclerosis, hyperlipidemia, diabetes hypercoagulability, sleep apnea, polycythemia, renal failure, male gender, and middle age group.,,,−, Direct pressure from the head rest on orbits may be higher in patients with low nasal bridges or exophthalmos.,
Narrow-angle glaucoma may be a risk factor for CRAO. Risk factors for cortical blindness include cardiac arrest, significant hypotension, air embolism, prolonged hypoxia, and cyanosis if intubation is difficult and prolonged.
Acute angle-closure glaucoma may be associated with hyperopia, female gender, advanced age and thickened lenses, certain ethnicities, medications, personal history, or family history of acute angle-closure glaucoma.
Recommendations
Several studies advocate elevating the head above the heart in a neutral position will reduce the risk of venous stasis.,,,−,, The reverse Trendelenburg position at 10° has been suggested to ameliorate raised intraocular pressure in prone position.,,,,,, However, in surgeries involving the head or neck, this reverse Trendelenburg position should not be overdone, as this will increase the risk of venous air embolism. If the Trendelenburg position is required during surgery, minimize time with the head below heart level.
Direct pressure on the orbits should be avoided and use of headrests that ensure pressure is off the orbits and ears is advised.,,, Asok et al recommends the Mayfield head clamp to prevent external pressure on the orbit. Roth et al suggests using a square foam headrest and avoiding using a horseshoe head rest during cervical spine surgery, as frequent movement of the head increases risk of position change and increased orbital pressure. Sherman et al used skull traction to suspend the head and reducing direct orbital pressure from the headrest.
Frequent monitoring of eye position every 20 minutes with a mirror attachment to the headrest ensures that prolonged inappropriate external orbital pressure is avoided. Woodruff et al has developed a computer video streaming system to monitor eye position throughout the prone procedure. Trethowan et al suggests using loose endotracheal tube ties, however, care should be taken to not increase risk of endotracheal tube dislodgement.
Maintaining tight control of hemostasis, monitoring blood pressure by arterial line or central venous pressure line, and timely correction of anemia and hypotension is paramount.,,,, Lower transfusion threshold and maintaining hematocrit above 30% in high risk patients is recommended., A safe lower limit for hypotension for each patient should be established based on an assessment of risk-benefit ratio using the patient's pre-operative blood pressure. However, generally, maintaining systolic pressure above 90 mmHg with mean arterial pressure above 70 mmHg has also been suggested. Colloids along with crystalloids should be used for fluid management as this may theoretically reduce edema as high volumes of crystalloids are avoided.,,
Changes in perfusion-related medications prior to surgery should be avoided and preoperatively withholding medications that may reduce blood pressure or increase blood coagulability should be taken into consideration.
Ophthalmologic evaluation pre-operatively should be considered if the patient has personal or family history of acute angle-closure glaucoma.
Staging prone surgeries if duration is likely to be prolonged.,,,, However, risk of thromboembolism, infection and additional anesthetic risks increase with staging.
Early postoperative assessment of vision and early ophthalmologic intervention if visual disturbance is present is essential after prolonged surgery in the prone position and can be vital to prognosis of POVL.,−,
Oropharyngeal swelling, macroglossia, sublingual hematoma
Oropharyngeal swelling including macroglossia, sublingual hematoma, and salivary gland swelling is a complication that may be seen in prolonged prone surgery, particularly if the head and neck position are in non-neutral, full flexion. Kinking and stretching of the salivary ducts blood vessels and lymphatics when the neck is in full flexion may lead to obstruction, and if prolonged, leads to swelling of local structures., In addition, the endotracheal tube may cause increased compression in the fully flexed neck, affecting venous and lymphatic drainage of the oropharynx and tongue causing swelling. Venous thrombosis may also cause local swelling as a result of ischemia of the oropharynx, reperfusion hyperemia, and subsequent capillary leak. Salivary gland swelling post operatively is a complication that may result from general or spinal anesthetic. Repeated laryngoscopy and certain medications, including angiotensin-converting enzyme inhibitors, can also contribute to oropharyngeal swelling. Luxation of the temporomandicular joint during intubation, poor oral hygiene, dehydration, and malnutrition may also increase risk of salivary gland swelling.
Recommendations
Full neck flexion can be reduced with proper head and neck support. A chin bar can be used to reduce soft tissue compression, and a bite block prevents tongue compression. Also, oral airways can put increased pressure on the tongue, which should be taken into consideration.
Hourly monitoring of head neck and tongue is advocated if significant neck flexion is required during the surgical procedure. Visualization of the oral cavity with laryngoscope for swelling should be done prior to extubation. Lack of air leaking after deflation of the endotracheal tube cuff may be a sign of oropharyngeal swelling and should be checked on extubation. Extubation should be postponed if swelling is suspected and patients should be monitored in the first few hours for postextubation swelling.
Venous air embolism
Air embolisms can be a product of either air entrainment through the surgical wound or accumulation of gas when using hydrogen peroxide for wound irrigation., Risk of air entrainment into the venous system is higher if venous pressure is low, such as when the surgical site is elevated above the heart or hypotensive episodes, as is often the case in the prone position.
Recommendations
Capnography and transesophageal echocardiography can be used to detect gas embolisms, while flooding the surgical field with normal saline or closure of open veins can be used to prevent air embolism. Central venous catheter can be used to remove air from the right heart and should be placed in patients with high risk of blood loss and air embolism.
Endotracheal tube and LMA displacement
Patients in prone position may be at higher risk for endotracheal tube and LMA displacement due to gravitational effects on instruments designed for intubation and airway management in supine position.,, LMAs can be used to maintain the airway if the endotracheal tube becomes dislodged as it can be rapidly inserted without layngoscopy., However, it should be taken into consideration that prone as well as lateral position causes dilation in the airway due to gravitational effects of local anatomic structures. Therefore there is a higher risk of LMA dislodgement in the prone position if intracuff pressure is too low.
Conclusions
Several issues arise when a patient is put in the prone position for surgical procedures. Increased pressure on anterior structures results in complications associated with reduced abdominal and respiratory compliance, compression of the orbits and vital organs, and hemodynamic changes that require close monitoring. There is sufficient evidence in the literature to support several key interventions. Carefully planned positioning and supportive padding, which cater specifically to alleviating pressure off key structures, are essential for prone positioning. Shorter duration or staged surgeries aimed at shortening time in the prone position are also advised. Additionally, interventions that require access to anterior structures are further complicated when the patient is prone and so require increased vigilance.