Positioning injuries associated with craniotomy are best considered by the position the patient will be in for the surgery. The risks of injury associated with each of the following patient positions are not necessarily exclusive to neurosurgical procedures.
The sitting position for cranial surgery is associated with a high incidence of air embolism (25%–45%). This situation occurs because noncollapsible venous channels, such as diploic and emissary veins, can be violated during craniotomy; this, in turn, leads to air entrainment in the sitting position because of the negative pressure gradient between the surgical site and the heart. The sitting position can result in hypotension, especially if not attained gradually and not preceded by adequate hydration. Hypotension seemed to be particularly an issue in patients who are classified as ASA III or IV.
Cardiac output decreases because of a reduction in venous return and the inability for cardiovascular reflexes to compensate. Peripheral and pulmonary vascular resistance increases but only incompletely compensates for the decrease in cardiac output, and cerebral perfusion pressure decreases. In fact, orthostatic blood pressure decreases are common (76%) in the first 60 minutes after induction of anesthesia. Hence, direct arterial pressure must be followed closely on attaining the sitting position, with the transducer leveled at the head.
Other reported complications include midcervical quadriplegia (thought to be attributable to a combination of preexisting cervical spine disease and low perfusion pressure); airway, tongue, and facial swelling (exacerbated by placement of oral airways for lengthy surgery); obstruction of the endotracheal tube (ETT; usually the result of overzealous flexion in the absence of a bite-block); brachial plexus stretch injuries (caused by inadequate arm support); and cardiovascular instability during tumor dissection. Airway obstructing supraglottic edema has also been reported after prolonged posterior fossa surgery conducted with the head in forced flexion. Although 100% of patients undergoing sitting craniotomies develop pneumocephalus, the contributory role of nitrous oxide (N2O) is in doubt. Its elimination from the anesthetic regimen at the time of dural closure has not been found to be efficacious. To date, experts are aware of at least 20 cases of paraplegia after acoustic neuroma resection in the sitting position.
Beach Chair Position
This position is frequently used for shoulder arthroscopic surgery. Patients are positioned 30° to 60° head-up. This technique has recently been associated with hypotensive episodes and consequent severe neurologic dysfunction, including brain stem infarction from cerebral hypoperfusion and visual loss. The incidence of hypotensive or bradycardic events has been reported to be 5.7% under propofol target-controlled anesthesia and up to 28% with an interscalene brachial plexus block. Prophylactically administered metoprolol (up to 10 mg) but not glycopyrrolate decreases the incidence of hypotensive or bradycardic events.
In rare cases, what is thought to be an activation of the Bezold-Jarisch reflex (Hypopnea and bradycardia due to diminished intracardiac volume) has resulted in full cardiac arrest. Cerebral perfusion pressure should be maintained at a level of 70 to 80 mm Hg. It is important to realize that the blood pressure cuff or arterial catheter is situated substantially below the level of the head and that cerebral perfusion pressure calculations need to be adjusted for the blood pressure readings obtained from these devices. For example, if mean arterial pressure (MAP) by cuff is 70 mm Hg and the cuff is positioned 12.5 cm below the foramen magnum, cerebral perfusion pressure is only 60 mm Hg, because there is a decrease in 1 mm Hg for each 1.25-cm gradient in height between the cuff and head. As is true for other sitting position variants, arm support is important to avert stretch injury to the brachial plexus. An elevated contralateral arm position was associated with fewer episodes of venous thrombophlebitis from intravenous catheter placement for arthroscopic surgery.
The Prone Position
Important position injuries in the prone position include eye injury (eg, corneal abrasion, retinal ischemia); brachial plexus stretch injuries; and pressure injury to the face, elbows, knees, breasts, and male genitalia. Brachial plexus injury can occur when the head is severely rotated toward the contralateral side or extended, stretching nerve roots; excessive pressure on the clavicle can compress the neurovascular bundle against the first rib; and the head of the humerus can likewise press into the brachial plexus when the arms are hyperabducted and the shoulder is not sufficiently mobile and relaxed. The arms should be abducted no more than 90°. Pressure on the femoral nerve or lateral femoral cutaneous nerve may be experienced, especially with post-type positioning devices (eg, the Relton-Hall or Jackson frame). In addition, ulnar nerve damage may occur from pressure against hard OR table edges or leaning by the surgical team.
Postoperative visual loss in the prone position
Faulty positioning only accounts for a small fraction of postoperative visual loss (POVL). An example is the occurrence of central retinal artery occlusion (CRAO), which is known to be a consequence of direct or indirect pressure on the globe. There is periorbital and scleral edema; funduscopic examination reveals the hallmark “cherry red spot” of CRAO. An orbital compartment syndrome has also been reported in which increased orbital venous pressure and external compression from the silicone headrest were implicated as potential causes. Intraocular pressure increases progressively with time in the prone position, although a causal relation between this phenomenon and postoperative blindness has not been established. Such anatomic features as exophthalmos and a small nasal bridge may predispose to excess pressure on the eyes during prone positioning.
Still, most POVL (89% in the ASA POVL registry) is diagnosed as ischemic optic neuropathy (ION), which, to date, remains an enigmatic idiopathic disease. ION is associated with the prone position, Mayfield pin headholder use, anesthetic duration greater than 6 hours, or high blood loss greater than 1000 mL. Association with other conditions, such as diabetes, hypertension, smoking, atherosclerosis, anemia, ulcerative colitis, and preexisting retinal disease, has been reported. It is not thought to be the result of optic globe compression or specific prone position-related factors. ION occurred in patients who were positioned on the Wilson frame , the Jackson table, or chest rolls; Mayfield head pinning was not protective, and ION occurred in patients positioned on foam or gel pads whether or not regular eye checks were documented during the case.