The neurosurgical operating theatre
Over the last century the neurosurgical operating theatre has evolved with significant improvements in lighting, equipment, air flow, and sterility. Further developments with intraoperative imaging and integrated navigation systems are now beginning to emerge. (Fig. 4.1).
Modern standard theatre complexes are divided into zones in order to minimize bacterial contamination. The operating room is an aseptic zone, the anaesthetic room is a clean zone and other areas are dirty zones. Standard theatres have a unidirectional air flow system to reduce the risk of airborne contamination with a minimum of 20 air changes per hour. Air is filtered before it enters the operating theatre and travels from the cleanest areas to the more contaminated ones, but the air flow is disrupted if theatre doors are left open. Smoke tests may be used to assess how air flows through operating theatres. Ultraclean theatres with laminar flow hoods have been developed for use during orthopaedic implant surgery but their use in neurosurgery is problematic because of the difficulties of positioning equipment, especially the operating microscope.
The temperature and humidity of operating theatres are also regulated. An average temperature of 20–22oC is comfortable for most theatre teams but warming blankets and the infusion of warmed intravenous fluids must be used to prevent patient hypothermia.
Special consideration is needed to ensure and maintain sterility during instrumented spine and implant surgery. This may include minimizing the number and entry of theatre personnel and using a ‘no-touch technique’ skin preparation, equipment sterilization, and intraoperative antibiotic prophylaxis to minimize the risks of infection are considered in detail in Chapter 94.
In 2009 the World Health Organization (WHO) published a landmark study that found that implementing a systemic process of checks can reduced safety incidents by up to a third (Haynes et al., 2009). The National Patient Safety Agency’s (NPSA) launched the ‘five steps to safer surgery’ initiative in 2010 and all NHS healthcare organizations are now obliged to use the WHO Surgical Safety Checklist (Fig. 4.2). The aim of the initiative is to reduce harm associated with perioperative care and to support a change in culture within the theatre environment leading to better communication throughout the team. The five steps are: (1) preoperative team briefing; (2) sign in; (3) time out; (4) sign out; and (5) postoperative team briefing.
Surgical patient positioning
The central piece of equipment in the operating theatre is the operating table. Modern neurosurgical operating tables are radiolucent with interchangeable head and leg sections and are set on an offset column base that allows the table top to be moved cranially or caudally and tilted in all planes. Head-up positioning is used for most neurosurgical procedures as it reduces intracranial pressure and venous haemorrhage. Excessive head-up positioning however should normally be avoided because of the risks of air embolus, subdural postoperative pneumocephalus, and watershed infarction. Skull fixation is indicated in most microsurgical procedures to ensure that the head position is fixed in an optimal position with consideration given to the resulting angulation of the skull base, the desired surgical corridor, and the need to minimize brain retraction. Skull fixation is especially important when intraoperative image guidance systems are used, and it is also required for many cervical spine operations.
Skull fixation is achieved by using a pin-fixation device such as the three-point fixation Mayfield clamp. Tension rings of 20, 40, 60, and 80 lbs are marked on the rocker pin with 60 lbs of tension providing secure fixation in an adult. When positioning the pins it is important to avoid the squamous part of the temporal bone, the supraorbital ridge, implanted shunt devices, and large frontal sinuses. Skull fixation should be used with care in paediatric patients and is generally best avoided as it can result in skull perforation and extradural haemorrhage.
Various surgical positions are used in neurosurgery and each surgeon will develop their preferred position for each operation. Common surgical positions include the supine, prone, park-bench, and sitting positions (Fig. 4.3). Close collaboration with the anaesthetist is essential in ensuring the patient is placed in the optimal surgical position but every member of the surgical team must remain vigilant to the potential complications of improper positioning. Bony prominences are at risk of pressure damage during surgery and must be well padded, while the patient’s eyes must also be covered to protect them from pressure and to avoid injury from alcoholic cleaning solutions. Peripheral nerve injuries may be caused by pressure or traction palsies with the ulnar and common peroneal nerves most at risk, especially when operating in the lateral (park-bench) position. In the sitting position there is an increased risk of air embolism (evidenced most rapidly by a rise in end tidal CO2), and there is a small risk of blindness associated with surgery performed in the prone position as a result of increased intraocular pressure and reduced optic nerve perfusion pressure. Improper skull fixation can also cause scalp necrosis or local haematomas.
Positioning equipment in theatre to optimize surgical performance
The surgeon needs to consider the position of theatre personnel during a procedure. The surgical assistant should not be positioned between the surgeon and the scrub nurse, excess unused equipment should not be in theatre. Drills, ultrasonic aspirators, and bipolars must be positioned so the surgeon can use them with their dominant hand and without wires crossing the operative field. Image-guidance cameras should be positioned in such a way that the microscope and theatre staff do not obstruct the line of site to the patient. Sound levels in theatre should not be at a level where the surgeon, anaesthetist, and other theatre staff cannot communicate effectively.
Hypoglycaemia, coffee, and a full bladder may all affect a surgeon’s ability to conduct microsurgery. Surgeon comfort is not a luxury but a necessity for safe surgery. Some retractor systems and chairs will allow a surgeon to rest their elbows and wrists on a firm surface and this is very useful in minimizing fatigue. Some surgical positions, notably the sitting position require that the surgeon operates with arms elevated in a way that makes fatigue problematic. Long operations may be best performed by surgical teams so that rests can be taken as required.
The neurosurgeon needs to be familiar with many types of surgical equipment and to cover all these is beyond the scope of this chapter, but the following equipment merits special consideration.
There remains considerable controversy regarding who invented the first true microscope, but it is thought to have been developed in the late sixteenth century. It was Carl Zeiss, a German machinist working with the physicist Ernst Abbé who revolutionized lens making and microscope manufacturing. By the early twentieth century, the microscope was an integral part of laboratory medical research but was not used in the operating room until 1921 when Carl Nylén, a Swedish otolaryngologist, used a microscope to operate on a patient with chronic otitis media. Zeiss introduced their first series operating microscope in 1953 and in 1957 Theodore Kurze became the first neurosurgeon to use the microscope on a patient, removing a neurilemmoma of the seventh nerve in a 5-year-old child. The first microneurovascular case was performed in 1960 by Raymond Donaghy. He also established a microsurgical training laboratory in Vermont where a young Gazi Yasgaril was sent to learn this emerging technique. More than anyone else, Yasgaril publicized the advantages of microscopic neurosurgery and made the operating microscope an integral part of modern neurosurgery (Kriss and Kriss, 1998).
The modern microscope enables the operating surgeon to see three dimensional images at high magnification. The main components of a modern microscope are the objective lens, the binocular tube, the light source, a motorized zoom system, and the suspension system. The design of the operating microscope ensures that high-quality lighting runs exactly parallel and very close to the optical path. The two eye fields are also close together, permitting binocular vision can be achieved at the bottom of a deep, narrow wound. The more a microscope is zoomed in the less depth of field the surgeon has so the surgeon should attempt to keep the microscope as close as possible to the patient. However, a good working distance between the microscope and the wound is required to allow instruments to be used without clashing with each other or the microscope. Modern microscopes have electric variable objective lenses with focal lengths from 200 to 500 mm and the focal length is virtually identical to the working distance. Eyepieces are used to re-magnify the intermediate image 10-fold and can be used to compensate for ametropia. In order to allow free movement of the microscope preoperative balancing is essential, and drapes must not restrict microscope movements.
Modern microscopes are also commonly fitted with two additional light sources; an infrared 800 nm light allows visualization of blood flow using intraoperative fluorescence and blue 400 nm light is used for fluorescence-guided tumour surgery.
Neuroendoscopy was first performed in 1910 when urologist Victor Lespinasse used a paediatric cystoscope to treat paediatric hydrocephalus by attempting endoscopic coagulation of the choroid plexus (Abd-El-Barr and Cohen, 2013). Following this, Dandy and Mixter attempted endoscopic fenestration of the third ventricle for the treatment of hydrocephalus in the 1920s but true advances in neuroendoscopy came in the 1970s with technological developments in optics and electronics (Liu et al., 2004).
The amount of intraoperative illumination and vision under the microscope is determined by the surgical approach and may be significantly limited in cases where the surgical corridor is long and narrow. Modern endoscopes are able to bring increased light intensity into the surgical field and provide an extended viewing angle with better magnification in close-up positions. Other potential advantages of endoscopic surgery include minimal tissue disruption and brain retraction, improved cosmetic results, shorter hospital stay, and reduced surgical morbidity.
Two main types of endoscopes are available; namely, fibreoptic flexible and rigid rod lens systems. Both types of endoscope use a xenon light source that is transmitted via a fibreoptic cable to the endoscope. Fibreoptic flexible endoscopes transmit the image through a group of tightly packed fibreoptic threads and are smaller and more malleable than rigid solid lens endoscopes. Rigid endoscopes are more expensive than fibreoptic endoscopes, but they provide superior image resolution and light transmission through a series of lenses. Some endoscopes have a working channel that allows the insertion of compatible instruments such as microforceps, endoscopic scissors, monopolar diathermy, or Fogarty balloon catheters. Irrigation channels allow the constant irrigation of fluid to clear debris from the lens and bleeding from the surgical field. Endoscopes come with a variety of lens angles (0°, 30°, 70°, and 120°) to allow visualization of hidden parts of the surgical field although intervention is usually only performed using 0° and 30° scopes.
Neuroendoscopic procedures were initially limited to the ventricles of the brain but the endoscope is now used in treating a wide spectrum of neurosurgical pathology. H.D. Jho pioneered the endonasal endoscopic approach for pituitary surgery (Jho and Alfieri, 2001) and its application to other skull base pathologies continues to expand (Paluzzi et al., 2012). The endoscope is also utilized in spinal surgery, particularly for thoracic procedures. It is important to select an appropriate endoscope with the desired characteristics for the procedure being performed. For instance, a non-channelled fibreoptic scope may be appropriate for a diagnostic ventriculostomy but a rigid solid multichannelled lens endoscope is preferable when performing endoscopic endonasal surgery. Despite endoscopy being a rapidly expanding field endoscopic surgery is still not practised routinely by all neurosurgeons and there is consequently a steep learning curve. Appropriate training is essential and collaboration with consultants from other specialties, such as otolaryngology, who are experienced in endoscopy, may shorten the difficult learning phase and reduce operative time and surgical morbidity.
Indocyanine green (ICG)
Indocyanine green (ICG) is the commonest fluorescent agent used for intraoperative angiography. It has a peak spectral absorption at around 800 nm following intraoperative intravenous injection and due to its tight binding to plasma proteins, becomes confined to the vascular system, permitting excellent intraoperative angiography.
5-aminolevulinic acid (5-ALA)
Fluorescence-guided tumour surgery is performed following the oral administration of 5-aminolevulinic acid (5-ALA) approximately 2–4 hours before anaesthesia. Once administered patients should avoid direct sunlight due to the risk of developing a photosensitivity skin reaction. 5-ALA is a natural precursor of protoporphyrin IX (PpIX) in the haem biosynthesis pathway and excess 5-ALA provides selective and abundant accumulation of PpIX in malignant glioma cells but only slight or no accumulation in the normal brain. 5-ALA in itself is not fluorescent but PpIX is a highly fluorescent substance with maximal absorption at 440 nm, thus enabling the surgeon to differentiate between pathological tumour and normal brain intraoperatively.
Fluorescein is a synthetic organic compound available as a dark orange/red powder. It is soluble in water and is widely used as a fluorescent tracer for many applications. In neurosurgery, intrathecal fluorescein administration has become an important tool in the diagnosis and localization of cerebrospinal fluid (CSF) leak and is used intraoperatively to assist in localizing the site of skull base defects. Current recommendations of fluorescein administration suggest removing 10 ml of the patient’s CSF and replacing it with an equivalent injection of a 10 ml dilute solution of fluorescein into the intrathecal space. The dilute solution is bright green in colour and does not usually require visual augmentation; however, a blue light filter can assist in localization of fluorescein-coloured CSF at the skull base in equivocal cases.
Modern high-speed surgical bone drills may be pneumatically or electrically powered. The rotational speed of a drill is measured in revolutions per minute (rpm) and pneumatic drills can generate up to 100 000 rpm. Electric drills have adjustable speeds from 200 to 75 000 rpm and are smaller, lighter, and more easily manoeuvrable, although they do not provide the same level of power as pneumatic drills.
Various drill bits may be attached to the drill depending on the surgical need. Table 4.1 illustrates some of the commonly used drill attachments and how they are used in cranial and spinal neurosurgery.
Table 4.1 Common drill bits and attachments used in cranial and spinal neurosurgery
Match stick/match head
Straight short/large bore attachment
Straight variable exposure attachment (10 mm adjustment)
Angled short/large bore attachment
Medtronic, Midas Rex.
The brain retractor was first introduced by Harvey Cushing and Victor Horsley in the early twentieth century and has since been a key neurosurgical instrument. Brain spatulas are either rectangular or tapered and are available in a variety of sizes. Articulated arms, such as the Yasargil Leyla retractor, were designed to stabilize the spatula while in use and various retractor systems have since been developed to provide a solid anchoring platform for retractors (Dujovny et al., 2010).
Inappropriate or prolonged brain retraction has the potential to cause cortical contusions, vessel injury, brain swelling, and neurological deficit and the incidence of brain retraction injury has been observed to be as high as 10% in some series (Spetzler et al., 1992). Other surgeons advocate the avoidance of retractors altogether—and certainly careful patient positioning, so that gravity assists rather than hinders the surgeon, draining cerebral spinal fluid, mannitol infusion, skilled anaesthetics, and surgical patience all contribute to make excessive retraction unnecessary. Judicious and careful use of brain retraction can minimize retraction related complications and such techniques include the placing of surgical linteens under the retractors, avoiding firm retraction, and the use of silicon coated retractor blades.
Diathermy is used for cutting tissues and haemostasis. Heat is generated by the passage of high-frequency alternating current through body tissues. Currents of up to 500 mA are safe at frequencies of 400 kHz to 10 MHz and locally concentrated high-density currents can generate temperatures of up to 1000°C. There is no stimulation of neuromuscular tissue at frequencies above 50 kHz.
There are two distinct types of diathermy.
Monopolar diathermy uses a high-power unit (400 W) to generate high-frequency alternating current that passes from a tipped electrode (high current density), via a ground plate (with a low current density) back to the generator. The voltage used changes the mode of diathermy. Cutting diathermy uses continuous low voltage output (500–1000 V) to generate high local temperature that causes superficial tissue disruption and water vaporization but minimal vessel coagulation and haemostasis. The alternative coagulation setting generates pulsed high voltage output (up to 6000 V) of high-frequency current at short intervals resulting in tissue water vaporization and vessel coagulation. Many diathermy machines will also allow a blend of cutting and coagulation.
Monopolar diathermy does not work in a wet field and can cause heat to spread, damaging the surrounding (1–2 cm) tissues. Incorrect placement of the patient electrode plate is the most common cause of accidental diathermy burns. To minimize the risk of complications the patient electrode plate should have a surface area of at least 70 cm2 and be placed with good contact on dry, shaved skin away from bony prominences, metal prostheses (e.g. hip replacements) and scar tissue.
Monopolar diathermy should be used with caution in patients with a history of arrhythmias and cannot be used in patients with a cardiac pacemaker due to the risk of causing cardiac arrest. There are case reports of alcoholic skin prep catching fire if not left to dry before diathermy is used.
Bipolar diathermy uses lower power (50 W) to generate high-frequency alternating current that passes between the two tips of diathermy forceps. It uses a 1 MHz waveform for both cutting and coagulation and because it only uses 140 V it can be used in patients with pacemakers and defibrillators. There is no need to use a patient electrode plate and bipolar diathermy can be used effectively in a wet environment.
Forceps vary in length and tip sharpness. Coagulated, charred tissue can stick to the forceps and they must be kept clean to avoid this risk. Modifications available include irrigating bipolar forceps and cooler tipped forceps that make this risk less likely.
Ultrasonic aspirators are vibrating surgical instruments which utilize ultrasound as a physical energy to fragment (cavitate), emulsify, and remove unwanted tissue. Examples of ultrasonic aspirators used in neurosurgery include the Cavitron Ultrasonic Surgical Aspirator (CUSA) (Integra) and Sonopet (Stryker). The hollow titanium tip vibrates along its longitudinal axis generating an ultrasound frequency of 20–60 kHz and all models allow the surgeon to specify the level of vibration, irrigation, and suction in order to achieve maximal control during tissue dissection. The CUSA includes a supplementary Tissue Select function that further regulates the relative tissue fragmentation rate and some aspirators (e.g. Sonopet) permit fine bone dissection by coupling longitudinal vibration with torsional vibration.
Ultrasonic surgical aspirators provide ultrasonic cavitation, irrigation and aspiration in a single hand-held unit which helps to limit instrument traffic within the surgical field and improve the surgeon’s view. The selective fragmentation of target neural tissues also reduces the risk of vessel injury, but one must be mindful of causing collateral damage from thermal injury and the risk of developing indistinct ‘false’ tissue planes.
Achieving and maintaining haemostasis in neurosurgery is critical to the outcome. Various haemostatic agents have been developed and target different elements of the clotting cascade (Fig. 4.4 and Table 4.2) (Grant, 2007). Some of these agents may swell in the postoperative period and care should be taken to avoid leaving haemostatic agents in places where swelling will cause neurological compression. This is especially relevant in the spine. Some haemostatic agents may result in MR artefacts making postoperative assessment of residual tumour difficult.
Table 4.2 Haemostatic adjuncts and their mechanism of action
Topical haemostatic agents
Oxidized regenerated cellulose
Microcrystalline collagen (reacts with platelets)
Tisseel Haemaseel APR
Fibrinogen/factor XIII concentrate (cryoprecipitate) and thrombin mixture
Gelatine matrix and thrombin mixture
Fibrinolytic inhibitor of plasmin (inhibits fibrinolysis)
Adapted from Surgery, Volume 142, issue 4, Gerald A. Grant, Update on hemostasis: neurosurgery, pp. S55–S60, Copyright (2007), with permission from Elsevier.
Problematic bleeding can also be controlled by changing the patient position, tranexamic acid, haemostatic clips, or muscle patches. Bees wax was developed for use in neurosurgery by Victor Horsley, and is now widely used to stop bleeding from bone edges.
Imaging techniques have revolutionized the practice of neurosurgery and have become essential in guiding diagnosis, preoperative planning, and intraoperative navigation. In particular, image-guided surgery has become an indispensable modern neurosurgical technique and is applicable to numerous neurosurgical procedures including neuro-oncology, vascular, and functional neurosurgery.
The aim of stereotactic surgery is to register the anatomical or imaging space to the surgical or physical space in which the patient is located and in which surgery is to take place. Image guidance allows for smaller more precisely positioned incisions and accurate localization of lesions. Stereotactic neurosurgery may be frame-based or frameless.
Frame-based stereotactic neurosurgery
The concept of stereotactic localization was first applied in 1908 in the device of Victor Horsley and Robert Clark although the apparatus was only developed for work on small to medium sized experimental animals (Fig. 4.5A). The first successful cranial application of stereotactic surgery in humans is credited to the team of Ernest Speigel and Henry Wycis in 1947 but it was Lars Leksell, a Swedish neurosurgeon, who developed the arc-centred stereotactic frame system following a visit to Wycis in Philadelphia in 1949. Leksell’s frame consisted of a rectangular base ring attached to the skull with four pins and used three polar coordinates (angle, depth, and anterior–posterior location). The three coordinates always indicate the centre of the arc, representing the target, and the arc-centred device allows maximum flexibility in choosing the probe entry point and trajectory. The frame has been modified over the ensuing years, but remarkably remains very similar in function and appearance to the original 1949 device (Fig. 4.5B).
Another commonly used frame system is the Cosman-Roberts-Wells (CRW) Precision™Arc System (Fig. 4.5C). It has an N-shaped ‘picket-fence’ localizer ring and the lesion is placed at the centre of a stereotactic sphere with a fixed radius that allows free selection of an unlimited number of entry points. The CRW Precision™ Arc System also has a phantom base that can be used to confirm the target before applying the settings to the patient. Computed tomography (CT) and MRI compatible models of both Leksell and CRW Precision™ Arc Systems are available.
The advent of high-resolution MR imaging has transformed stereotactic surgical planning. Stereotaxy is no longer reliant on anatomical atlases and is usually performed on a computer workstation that allows the planning of targets and trajectories on a computer-generated 3D model. Frame-based stereotaxy has the highest degree of precision and, although largely superseded in general intracranial practice by frameless stereotactic techniques, continues to be used for cases where target placement is critical including, for example, functional lesioning and electrode or catheter placement, biopsy of small or deep lesions, and gamma knife stereotactic radiosurgery.
Frameless stereotactic neurosurgery
Frameless stereotaxy involves the use of bony landmarks, facial features, or fiducial markers to coregister the patient’s skull relative to a 3D volumetric image obtained prior to surgery. Fiducials are adhesive markers, visible on both CT and MRI, and are placed on a patient’s head, covering as much of the geometry of the head as possible but centred on the target lesion. For optical registration, the patient’s head is fixed using a pin-fixation device and infrared reflective spheres arranged in a predetermined geometrical configuration and anchored to the same fixation device. An infrared camera detects light reflected from the spheres.
Registration is performed by the surgeon and computer software correlates the imaging studies with the defined anatomical landmarks of the patient’s head. Registration remains accurate so long as the patient’s head remains in same position relative to the reflectors. Registration accuracy can be enhanced by collecting as many registration points as possible, by using fiducials and by spreading anatomical registration points around the head as much as possible.
Electromagnetic (EM) registration involves generating an electromagnetic field around the patient’s head in order to triangulate the position of instruments and intraoperative tools. EM navigation does not require the head to be fixed and is especially useful when the head cannot be fixed easily (e.g. shunt or paediatric surgery). EM systems track the tip of the surgical instrument and can therefore be used to track the tip of a flexible endoscope.
A registration error is a computer-generated number that denotes the degree of deviation from the ideal image-to-patient correlation. Acceptable registration accuracy depends on the size of the target lesion. Volumetric registration enables the surgeon to view images in three orthogonal planes and once registered, instruments fitted with reflective spheres may be used by the surgeon to relate the position of the instrument to the images in real time. Additional image sequences such as functional imaging and tractography can also be merged with the volumetric data enabling the surgeon to view these intraoperatively. Image-guidance systems have also been adapted for use in spinal instrumentation surgery and can be linked to operating microscopes so that the focal point of the microscope is used as the image-guidance pointer and so that image-guidance data can be overlaid in the operators view down the microscope.
Frameless image-guidance systems are a significant technological advance and an essential piece of modern neurosurgical equipment however one must be aware of their limitations. Inaccuracies in registration may occur as a result of intraoperative brain shift following loss of cerebrospinal fluid or tumour resection, because of movement of the patient, the reference frame, or fiducials, and because of a loss of line of sight between the camera and infrared reflectors. In cases where submillimetre accuracy is required and a fixed trajectory is acceptable, a frame-based system is preferable.
Mobile X-ray image intensifiers are commonly used in the neurosurgical operating theatre for a variety of applications, including localization of the correct anatomical level during spinal surgery, imaging during spinal instrumentation, and for localizing other bony anatomy such as skull base foraminae or the pituitary fossa.
Mobile image intensifiers typically consist of a single unit with a low intensity X-ray source and an image intensifier positioned directly opposite each other on a mount. The mount, often referred to as a C-arm, may be rotated and translated around the patient as required. Mobile image intensifiers can provide good spatial resolution of bony anatomy and are quick and cheap to use, although the image quality is dependent on operator experience and the patient’s body habitus.
Imaging during spinal instrumentation helps to ensure that the desired anatomical alignment is achieved and assists in the accurate placement of implanted metalwork, such as pedicle screws. A radiolucent operating table should be used if fluoroscopic imaging is to be used intraoperatively to permit anterior–posterior views in addition to lateral views. Ensuring accurate screw positioning from 2D imaging can be difficult, particularly where there is abnormal anatomy or where complex instrumentation is used. Newer imaging techniques are being introduced to overcome this difficulty including dedicated 3D equipment such as ‘3D C-arms’ or the ‘O-arm’, and neuronavigation techniques incorporating registered fluoroscopic images with preoperative 3D CT scans.
Intraoperative ultrasound is a relatively inexpensive technique that has the potential to provide surgeons with fast, real-time imaging. Ultrasound imaging relies on the use of high-frequency sound waves to image tissues. Sound is defined as mechanical energy travelling through matter as longitudinal waves producing alternating zones of compression and rarefaction. Intraoperative ultrasound uses a transducer to transmit ultrasonic waves in the range of 5–10 MHz into tissues.
Where there are interfaces of different tissue density, energy is reflected back by the tissue towards the transducer as ‘echoes’. The amount of energy reflected depends on a tissue’s acoustic impedance, which is proportional to the density of the tissue propagating the sound wave. Echo vibrations are then converted to electrical pulses and used to produce representative images. Greyscale or Brightness (B-mode) is the mainstay of anatomical ultrasound imaging and provides 2D greyscale images of tissues based on their relative echogenicity. Doppler ultrasound can also be incorporated into intraoperative ultrasound. The doppler effect describes a change in the frequency of sound when a sound wave encounters a moving interface, such as blood within an artery and blood flow may be mapped in colour on real-time ultrasound images.
Intraoperative ultrasound is very operator dependent and image interpretation is associated with a learning curve as most surgeons are unfamiliar with this imaging modality. Ultrasound is unable to penetrate through bone and so cannot be used for planning a craniotomy and it has limited penetration through to deeper structures meaning that careful case selection is required. Intraoperative ultrasound has been used in treating a variety of pathologies but is most frequently used during glioma surgery. Some navigation machines are now also capable of coregistering the ultrasound pictures with a reconstructed plane from the pre-op MR scan. Intraoperative doppler ultrasound has a role in neurovascular surgery and some surgeons also use ultrasound to demonstrate adequate CSF flow following foramen magnum decompression.
Intraoperative MRI (iMRI)
Neurosurgeons have become increasingly dependent on image guidance to perform optimal, safe, and efficient surgery. Routine neuronavigation systems do not allow the neurosurgeon to adjust for intraoperative brain shift and this consideration led to the development of intraoperative MRI systems (Fig. 4.1D) (Hall and Truwit, 2008). The principal feature of iMRI systems is the ability to acquire contemporaneous imaging during surgery to inform surgeons on the extent of resection achieved in order to guide further surgical decision making and most iMRI systems also allow the new images to be registered onto the neuronavigation system.
iMRI systems may be vertically or horizontally open. In vertically open systems the patient remains within a fixed position and the surgeon works within the confines of the scanner. All surgical instruments must be MR compatible and various levels of shielding of other equipment must be maintained. Images can be easily and quickly acquired at any point during the procedure without moving the patient or the magnet however vertically open systems are typically low field scanners (0.3–0.5 T) so the standard of the images obtained are not of the same quality as those generated by standard diagnostic imaging techniques.
Horizontally open iMRI systems include a high-field magnet (1.5–3.0 T) located within an integrated operating theatre (Fig. 4.1D). The table can slide or pivot out of a marked magnetic field line (5-Gauss line), allowing surgery to be performed in the usual manner using standard instruments and equipment until a scan is required. New high-field horizontally open iMRI scanners produce higher quality images allowing greater versatility and acquisition of functional MRI, MR angiography, MR venography, MR spectroscopy, and tractography. The disadvantages of this system include the significant cost required to build the intraoperative suite and the increased time spent intraoperatively manoeuvring the patient in and out of the scanner.
Intraoperative neurophysiological monitoring
Intraoperative neurophysiological monitoring offers a real-time, continuous assessment of the nervous system integrity and is considered essential for various types of neurosurgical procedure (Gonzalez et al., 2009). It is vital to consult with an expert neurophysiologist before the procedure to ensure that the most appropriate modality is selected and results must always be interpreted in the context of the surgical procedure being performed. For this reason, it is helpful to have a neurophysiologist in the operating theatre during surgery. Intraoperative monitoring can be significantly affected by anaesthetic agents so close collaboration with the anaesthetist is also essential.
Sensory evoked potentials
The visual, auditory, and somatosensory systems can be stimulated and evoked potentials can record the biofeedback response of the nervous system to these external stimuli. The recordings give information on the sensory pathway from the peripheral nerve to the level of the cortex.
Somatosensory evoked potentials (SSEP)
Somatosensory evoked potentials (SSEPs) are produced by short (0.1–1.0 msec) electrical stimulation (1–5 Hz) of a peripheral nerve and the generated responses are recorded over a proximal part of the nerve or sensory pathway. Recording electrodes are placed in named, reproducible locations. Each location is designated by a letter; N (negative deflection) or P (positive deflection) and a number that corresponds to the latency of wave. The median or tibial nerve is usually stimulated and SSEPs are monitored in terms of latency and amplitude.
The SSEP waveform is often quite variable and should therefore be monitored before surgery begins and throughout the procedure. To separate the SSEP signal from background noise a technique called signal averaging is used to combine a large number of SSEP signals. The time required to perform signal averaging means that SSEP changes may not be seen until a few minutes after the neurological injury has occurred. A decrease of about 50% in amplitude or a 10% increase in latency of the SSEP waveform is considered to be significant. Inhalational anaesthetic agents, fluctuations in blood pressure, hypothermia, and oxygenation can all affect SSEPs to varying degrees so close consultation with the anaesthetist is needed beforehand to ensure stable anaesthesia is achieved. In thoracic spinal surgery, the upper limb SSEP signal can be used as a control. Continuous SSEP monitoring of the dorsal columns is commonly used during spinal cord and complex spine surgery and is considered essential for scoliosis surgery (Thirumala et al., 2014). Cortical mapping with SSEPs can also be used in epilepsy surgery or sensorimotor and insular cortex tumour surgery.
Visual evoked potentials (VEP) and brainstem auditory evoked potentials (BAEP)
Intraoperative visual evoked potentials (VEPs) and brainstem auditory evoked potentials (BAEPs) are less commonly used. VEPs can be evoked by patterned or unpatterned stimuli and may be used during surgery around the optic chiasm or optic tract however results under anaesthesia are not reliable. BAEPs record a waveform composed of seven discrete waves after an auditory stimulus is delivered to the external auditory canal. Each wave corresponds to a specific locus of the auditory system from the organ of Corti to the auditory cortex. BAEPs may be used during hearing preservation vestibular schwannoma surgery or during microvascular decompression for hemifacial spasm.
Motor evoked potentials
Motor evoked potentials (MEPs) are generated by stimulating the motor cortex, spinal cord, or a peripheral nerve with an electrical or magnetic stimulus. Transcranial magnetic stimulation is preferred in preoperative mapping because it is less invasive but transcranial electric stimulation using corkscrew or needle electrodes is usually performed intraoperatively. Neurogenic MEPs are recorded directly from the spinal cord or from peripheral nerves. Neurogenic MEPs generate two types of wave: direct propagation of the stimulus along the corticospinal tract generates direct waves (D waves), while indirect waves (I waves) are caused by activation of adjacent cortex. Myogenic MEPs record the muscle response (M response) of a specified muscle by using electromyography (EMG) recording. The M response reflects the motor unit action potential (MUAP) and represents the summated action potentials of the muscle fibres of one motor unit.
Inhalation anaesthetics have a significant effect on MEPs, in particular I wave responses. D waves are not affected by inhalation anaesthetics, but the α-motor neurone is blocked at higher doses, even when stimulated by transcranial electric stimulation. Intravenous anaesthetics only have a mild effect on MEPs and total IV anaesthetic using propofol and etomidate is typically preferred when using MEPs as they are less suppressive than gases at similar depths of anaesthesia. However, excessively high doses of propofol may be similarly suppressive to inhalation agents. Benzodiazepines are usually avoided because they eliminate MEP responses. Neuromuscular blocking agents do not affect neurophysiological monitoring and can actually enhance I wave recordings but MUAPs cannot be recorded when the neuromuscular transmission of acetylcholine is completely blocked. The anaesthetist observes the M-responses to evaluate the degree of neuromuscular blockade.
MEPs can monitor the dorsolateral and ventral spinal cord tracts during spinal cord and complex spine surgery and complements SSEP monitoring of the dorsal columns (Schwartz et al., 2007). Monitoring of pudendal MEPs and sacral EMG recording from the anal sphincter is used when performing selective dorsal rhizotomy for the treatment of spasticity and helps identify nerve rootlets that can be selectively sectioned.
Unlike SSEPs, MEPs are not usually monitored continuously as the patient may move following a stimulation. The surgeon and neurophysiologist also need to work together to ensure frequent MEPs are obtained without disrupting the surgery.
A recording electrode is placed inside a needle and inserted into a muscle to record the MUAP of that muscle. Signals are then converted into sound to demonstrate muscle activity. Evoked EMG uses electrical stimulation to identify specific nerves whereas free-running EMG is used to identify any interruptions to the nerve’s normal electrical discharge. Spontaneous benign discharges produce short non-sustained discharges (likened to the sound of ‘popcorn’) and indicate proximity of the stimulus to the nerve whereas a neurotonic discharge is prolonged and indicates ongoing nerve injury.
Facial nerve monitoring is the commonest type of intraoperative EMG used in cranial neurosurgery whereby recording electrodes are placed in the orbicularis oris and orbicularis oculis muscles. Muscle activity is recorded either during a surgeon’s intentional stimulation of the nerve or by inadvertent nerve stimulation during manipulation, mechanical, or thermal injury. Optimal facial nerve EMG recordings are obtained when neuromuscular blockade is avoided but facial muscles appear relatively resistant to the effects of muscle relaxants. Hartman’s solution irrigation should be used instead of saline irrigation to avoid stimulation of the facial nerve. EMGs are also used to monitor muscle group activity during selective dorsal root rhizotomy for spasticity and spinal root monitoring is considered a key component of multimodality spinal monitoring used during spinal instrumentation.
Intraoperative cortical mapping (brain mapping)
Intraoperative brain mapping is typically used to locate the motor, sensory, or speech cortex during epilepsy or tumour surgery in and around eloquent areas. Phase reversal SSEPs may be utilized in anaesthetised patients to localize the primary sensory or motor cortex whereby a strip grid is placed on the surface of the brain perpendicular to the anticipated orientation of the central sulcus. A phase reversal of the N20/P20 peak indicates that those electrodes straddle the central sulcus. In awake patients, direct cortical stimulation may be used to map the motor cortex.
For speech mapping in the temporal lobe, a recording electrode strip is placed on the brain surface and the cortex is then stimulated using a bipolar cortical stimulator. The stimulating current is increased in small 2 mA increments, up to a maximum of 10 mA, while observing for any after discharges (akin to focal seizures). Once the patient’s threshold for after discharges has been established, patients are asked to name objects shown on picture cards while the cortex continues to be stimulated and any paraphasic errors or speech arrest are noted. The aforementioned steps are then repeated so that the patient’s speech area may be mapped out on the brain’s surface. Awake craniotomy is usually required for brain mapping, especially for the speech areas.
The key to a successful awake procedure is patient preparation. Good patient cooperation is required, and careful patient selection is needed to ensure the patient fully understands what will happen during surgery. Various anaesthetic and surgical techniques have been described in the context of performing awake craniotomy but a successful procedure mandates anticipation of specific problems and clinical vigilance (Erickson and Cole, 2012). Fully awake surgery is performed after the infiltration of local anaesthetic into the scalp or following regional nerve block of the scalp. The asleep-awake-asleep (AAA) technique uses general anaesthesia, with or without the use of an airway, during the opening and closing portions with emergence of patients from anaesthesia in the interim. However, the more commonly advocated anaesthesia technique for the opening and closing portions is termed monitored anaesthesia care (also called conscious sedation). The same medications used in the AAA technique are given (propofol and fentanyl) but they are given in pulses and at lower doses with the goal of providing a smooth transition to alertness and obviating the difficulties of airway intervention (Erickson and Cole, 2012).
Abd-El-Barr, M.M. & Cohen, A.R. (2013). The origin and evolution of neuroendoscopy. Childs Nerv Syst, 29, 727–37.Find this resource:
Gonzalez, A.A., Jeyanandarajsan, D., Hansen, C., Zada, G. &, Hsieh, P.C. (2009). Intraoperative neurophysiological monitoring during spine surgery: a review. Neurosurg Focus, 27(4), E6.Find this resource:
Hall, W.A. & Truwit, C.L. (2008). Intraoperative MR-guided neurosurgery. J Magn Reson Imaging, 27, 368–75.Find this resource:
Haynes, A.B., et al.; Safe Surgery Saves Lives Study Group (2009). A surgical safety checklist to reduce morbidity and mortality in a global population. N Engl J Med, 360(5), 491–9.Find this resource:
Kriss, T.C. & Kriss, V.M. (1998). History of the operating microscope: from magnifying glass to microneurosurgery. Neurosurgery, 42(4), 899–907.Find this resource:
Abd-El-Barr, M.M. & Cohen, A.R. (2013). The origin and evolution of neuroendoscopy. Childs Nerv Syst, 29, 727–37.Find this resource:
Dujovny, M., Ibe, O., Perlin, A., & Ryder, T. (2010). Brain retractor systems. Neurol Res, 32(7), 675–83.Find this resource:
Erickson, K.M. & Cole, D.J. (2012). Anesthetic considerations for awake craniotomy for epilepsy and functional neurosurgery. Anesthesiol Clin, 30(2), 241–68.Find this resource:
Gonzalez, A.A., Jeyanandarajan, D., Hansen, C., Zada, G., & Hsieh, P.C. (2009). Intraoperative neurophysiological monitoring during spine surgery: a review. Neurosurg Focus, 27(4), E6.Find this resource:
Grant, A.G. (2007). Update on hemostasis: neurosurgery. Surgery, 142(4 Suppl), S55–60.Find this resource:
Hall, W.A. & Truwit, C.L. (2008). Intraoperative MR-guided neurosurgery. J Magn Reson Imaging, 27, 368–75.Find this resource:
Haynes A.B., et al.; Safe Surgery Saves Lives Study Group. (2009). A surgical safety checklist to reduce morbidity and mortality in a global population. N Engl J Med, 360(5), 491–9.Find this resource:
Jho, H.D. & Alfieri, A. (2001). Endoscopic endonasal pituitary surgery: evolution of surgical technique and equipment. Minim Invasive Neurosurg, 44(1), 1–12.Find this resource:
Kriss, T.C. & Kriss, V.M. (1998). History of the operating microscope: from magnifying glass to microneurosurgery. Neurosurgery, 42(4), 899–907.Find this resource:
Liu, C.Y., Wang, M.Y., & Apuzzo, M.L.J. (2004). The physics of image formation in the neuroendoscope. Childs Nerv Syst, 20, 777–82.Find this resource:
Paluzzi, A., Gardner, P., Fernandez-Miranda, J.C., & Snyderman C. (2012). The expanding role of endoscopic skull base surgery. Br J Neurosurg, 26(5), 649–61.Find this resource:
Schwartz, D.M., Auerbach, J.D., Dormans, J.P., et al. (2007). Neurophysiological detection of impending spinal cord injury during scoliosis surgery. J Bone Joint Surg Am, 89(11), 2440–9.Find this resource:
Spetzler, R.F., Daspit, C.P., & Pappas, C.T. (1992). The combined supra- and infratentorial approach for lesions of the petrous and clival regions: experience with 46 cases. J Neurosurg, 76, 588–99.Find this resource:
Thirumala, P.D., Bodily, L., Tint, D., et al. (2014). Somatosensory-evoked potential monitoring during instrumented scoliosis corrective procedures: validity revisited. Spine J, 14(8), 1572–80.Find this resource:
Related links to Ebrain
Advanced Microsurgical Skills. https://learning.ebrain.net/course/view.php?id=93
Image-guided Neurosurgery. https://learning.ebrain.net/course/view.php?id=92
Intraoperative Imaging Techniques. https://learning.ebrain.net/course/view.php?id=89
Intraoperative Neurosurgical Electrophysiological Monitoring. https://learning.ebrain.net/course/view.php?id=489
Preparing the Operative Site. https://learning.ebrain.net/course/view.php?id=96
Principles of Awake Craniotomy. https://learning.ebrain.net/course/view.php?id=496
Surgery in Children. https://learning.ebrain.net/course/view.php?id=73
The Modern Theatre Environment. https://learning.ebrain.net/course/view.php?id=97
The Operating Microscope. https://learning.ebrain.net/course/view.php?id=94
Use of an Endoscope. https://learning.ebrain.net/course/view.php?id=91
Use of Standard Neurosurgical Equipment. https://learning.ebrain.net/course/view.php?id=70