The obligatory positions required for spine surgery subject patients to significant risk above and beyond the inherent risk of the procedure itself. These risks include, but are not limited to, injury to the eyes, ears, nose, breasts, penis, extremities, and peripheral nerves. Peripheral nerve injury is a well-recognized anesthetic complication and is the second largest cause of malpractice actions in anesthesiology, accounting for 16% of claims. Minimizing potential injury to peripheral nerves through proper positioning and monitoring is extremely important.

Monitoring of somatosensory evoked potentials (SSEP) is frequently utilized during spine surgery (75%) and is available in most institutions in the United States (94%). In addition to monitoring spinal cord function, SSEP can detect peripheral nerve injury during spine surgery. Conduction changes, such as a decrease in amplitude or an increase in latency of the signal, are believed to indicate impending upper-extremity nerve injury. Modification of the arm position by the anesthesiologist often improves the SSEP signal and may return it to baseline. Reversal of position-related SSEP changes can influence impending nerve injury and prevent postoperative peripheral nerve injury.

The prone position is a dangerous position for patients. Attention to securing the airway is very important, as it is difficult to re-establish an airway in the prone position. Though we must always be cautious regarding protection of the eyes, this position raises additional concerns. Both ophthalmic ointment and occlusive eye tape may be appropriate if the procedure involves the cervical spine; prep solution that comes into direct contact with the eyes can lead to corneal injury. Additionally, we must assure that there is no direct pressure on the eye, ears, or nose. Pressure could lead to loss of function and/or a disfiguring ischemic injury. For the same reason, it is important when positioning chest rolls to assure yourself that the breasts in women and the genitalia in men are free from compression of any sort.

With regard to the patients’ extremities, the mechanisms of nerve injury associated with surgical positioning and anesthesia are not completely understood. Peripheral nerve injury may be due to direct trauma or more commonly due to ischemia of the intraneural capillaries. Compression of
peripheral nerves or stretch beyond 15% of original length may also cause peripheral nerve injury. Diabetes, hypertension, and uremia are known to affect peripheral nerves and predispose them to injury; intraoperative conditions such as prolonged hypotension and anemia are believed to facilitate ischemic injury. Finally, certain operative positions are known to place patients at increased risk for nerve damage. During spine surgery the overall incidence of impending upper-extremity peripheral nerve injury, as defined by changes in the SSEP, is >6%.

The ulnar nerve and the brachial plexus are the most commonly injured neural structures during this type of surgery. Risk factors for ulnar nerve injury include male gender, very thin or very obese patients, and prolonged hospitalization for >14 days. Risk factors for brachial plexus injury include the use of shoulder braces, the prone head-down position, and some regional anesthetic techniques such as interscalene and axillary blocks.

There are five positions commonly employed during spine surgery: supine, arms out; supine, arms tucked; lateral decubitus; prone “Superman” position; and prone, arms tucked. The prone “Superman” position and the lateral decubitus position have been identified as high-risk positions for upper-extremity nerve injury; especially if the surgical procedure is prolonged. Techniques for minimizing injury in these positions are reviewed below.

The incidence of impending upper-extremity nerve injury during spine surgery in the supine arms-out position is 3.2%. Overstretch of the brachial plexus can occur as a result of abduction of the shoulder >90 degrees and should be consistently avoided. Direct compression of the ulnar nerve against the medial epicondyle may occur, especially if the forearm is in the prone position. Placing the forearm in the supine or neutral position decreases pressure over the ulnar nerve at the elbow. Elbows should be padded to further protect the nerve from compression. Direct compression of the radial nerve in the spiral groove of the humerus can be avoided with proper padding. Overextension of the elbow should be avoided because it may stretch the median nerve. The patient’s head and neck should ideally be maintained in a midline position. Tilting the head and neck laterally may stretch the contralateral brachial plexus.