Article Text

Intra-operative localisation of skull base tumours. A case report using the ISG viewing wand in the management of trigeminal neuroma

Abstract

Deep-seated skull base tumours provide as much a challenge to the surgeons' skills of localisation as to his technical abilities during the resection. These lesions are frequently inaccessible and lie adjacent to vital structures requiring extensive cerebral retraction for adequate exposure and direct visualisation. The ISG viewing wand is a newly developed image guidance system to aid direction of the operative approach and localisation of intracerebral pathology. We discuss its use in the management of a trigeminal neuroma.

  • neuronavigation
  • trigeminal neuroma

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In any modern neurosurgical practice, localisation of cerebral pathology depends on computed tomography (CT) and magnetic resonance imaging (MRI).These two-dimensional (2-D) studies refer to a three-dimensional (3-D) reality and frequently this information is displayed away from the operative field. Localisation of cerebral pathology can be achieved using CT or MRI to locate accurately a point in three-dimensional space based on Cartesian mathematics, in practice the stereotactic frame.

The development of frame-based stereotactic systems was initiated by Clarke and Horsley in 1908.1 This has culminated in the many advanced systems now available (eg, CRW, BRW and Lexsell). However, these frames have the disadvantages of being cumbersome, heavy, restrictive to the operative field, and frequently require the intra-operative transfer of the patient to the radiology department with all its attendant risks.2 This undoubtedly leads to a prolongation of the operative and anaesthetic time. In addition, the surgeon is typically limited to target points on a linear trajectory, and frame-based stereotactic systems do not provide ongoing feedback about anatomical structures encountered in the surgical field. Conventional stereotaxy is little used for posterior fossa or skull base procedures.

The development of the digital computer and user-friendly software has led to the incorporation of interactive image-guided technology into neurosurgical practice. The only system which currently holds Federal Drug Administration approval is the ISG viewing wand.

Case report

We present the case of a 50-year-old man with a 4-month history of altered sensation in the right side of his face, right-sided hearing loss, increasing headaches and unsteadiness in his gait. Clinical examination was unremarkable, the only positive finding being altered sensation to light touch and pin-prick in the right trigeminal nerve distribution in each of the three divisions.

Pure tone audiometry demonstrated a small amount of low frequency loss in the right ear. MRI demonstrated a 2.0 × 2.5 cm mass in the right posterior fossa adjacent to the medial part of the petrous ridge. It consisted of a cystic component markedly compressing the brainstem, and an enhancing solid part lateral to the carotid syphon and extending into Meckel's cave (figure 1). These appearances were consistent with a trigeminal neuroma.

Figure 1

Axial, sagittal and coronal MRI images of a trigeminal neuroma. (A) Axial slice demonstrating a cystic posterior component and solid anterior component of the tumour lateral to the carotid syphon; (B) sagittal slice showing the cystic component confined to the posterior fossa and the solid component expanding anteriorly through Meckel's cave; (C) coronal slice illustrating the marked brainstem compression caused by the cystic tumour

Deep-seated skull base tumours such as this provide as much a challenge to the surgeon's skills of localisation as to his technical abilities during the resection. These lesions are frequently inaccessible and lie adjacent to vital structures requiring extensive cerebral retraction for adequate exposure and direct visualisation. Thus, it was decided to direct the operative approach using the ISG viewing wand.

Pre-operative 3-D reconstruction of the lesion allowed for planning of the favoured approach and accurate flap placement (figure 2). The information can be displayed either as a 3-D picture or as a triplanar display of axial, coronal or sagittal images (figure 3). The 3-D surface objects can be assigned colours or degrees of transparency and windows may be cut into the object in any plane. In 3-D display the object may be turned or rotated in any direction. A low temporo-parietal flap was formed and a sub-temporal approach taken to the tumour. As the wand is moved around within the head so the tip of the wand is shown on the screen relative to the pre-operative image. The ISG wand was continuously used to direct the trajectory towards the tumour and away from vital structures (figure 4). At the medial border of the petrous ridge the tentorium was opened. The cystic part of the tumour was aspirated and its capsule dissected. The solid portion of the tumour was dissected from the lateral wall of the cavernous sinus and the depths of Meckel's cave. The whole tumour was then excised from the trigeminal nerve. Histology revealed a trigeminal neuroma.

Figure 2

Pre-operative 3-D reconstruction of the tumour to direct flap placement and define the operative approach

Figure 3

Operative screen display of the various viewing formats

Figure 4

The exact position of the wand tip with reference to the pre-operative scans can be defined at any point during the procedure and used to provide a safe trajectory to the tumour

The patient made an uneventful post-operative recovery and was discharged 10 days later. There was no facial nerve weakness or hearing loss. The only neurological deficit was partial facial numbness on the right side.

Discussion

The ISG viewing wand (ISG Technologies, Canada) is an interactive image-directed surgical navigation system incorporating a passive robotic pointer, the FARO arm (FARO Medical Technologies, Miami, USA). The arm is constructed with six degrees of freedom by having three joints, based on a torso, shoulder, elbow and wrist configuration, each with two degrees of freedom (figure 5). It has a 60-cm reach and a choice of end effectors. Potentiometers measure the angular position of each joint and the analogue signals are digitalized by a Toshiba T3200 computer which then calculates the position of the end effector tip. The position of the wand tip is updated 30 times per second.

Figure 5

The FARO arm and passive robotic pointer fixed as during surgery to the Mayfield holder

The viewing wand differs fundamentally from other forms of frame-based stereotaxy in that the images used during surgery are acquired pre-operatively negating the need for per-operative scanning. CT or MRI data, for use in image reconstruction, is obtained by acquiring contiguous 3-mm thick scans, 3 mm apart in an axial plane. The gantry angle is tilted, if necessary, to ensure the incorporation of the nose and eyes, which are used as landmarks during patient-to-image registration. Metallic spheres, 1 mm in diameter, can be attached to the scalp surface prior to scanning to allow extra-anatomical points of commonality to be established for use in patient registration. An alternative method is to perform an image-to-patient registration based on anatomical surface points.3 This precludes the necessity to use the spheres, whose position may change or be removed, especially by children. The position of the tip of the digitising device is said to be registered with the imaging data so that any point on the patient's head can be mapped to its corresponding point in the imaging study and vice versa.4

The image is then transferred to the ISG wand via 1/4 inch tape or an ethernet connection. The main computer is a Hewlett Packard Apollo series running UNIX. The software is written in C and Assembly languages. A 3-D reconstruction is obtained by according structures of interest a threshold and window recorded in Hounsfield units. The object of interest then acts as a seed; automatic region contouring routines then encircle the region, based on the original seed, and will search for the same object in subsequent slices. Contours can be drawn or edited manually.

In the operating theatre the patient's head is fixed into the Mayfield head rest. The wand attaches directly to the Mayfield holder. The FARO arm end effector is then calibrated to the wand using the calibration port. Patient-to-image registration is performed by placing the tip of the probe against a point on the patient's skin or against an external skin fiducial and locating the same point on the 3-D reconstruction using a mouse-driven cursor. Thirty to forty surface points are then touched at random, especially those of curvature to refine the fit. The accuracy of the fit is then assessed by the operating surgeon, by placing the end effector against the skin and onto unique points and observing its position and orientation on the image. The wand probe is then autoclaved and the FARO arm draped. Prior to cutting the bone flap, four points outside the proposed flap are drilled to allow for re-registration if movement of the head occurs during fashioning of the flap. The probe can then be used interoperatively to locate the underlying abnormality or point of cortical incision, identifiy vessels and orientate deep within brain parenchyma.

The main contention over the use of frameless stereotaxy relates to the accuracy and reliability with which the equipment can correlate the location of structures within the operative field with the corresponding representation of the structure on the 2-D or 3-D image on the computer screen. However, recent data suggest accuracy equal to, or in some cases better than, traditional frame-based systems.5

The ISG viewing wand affords the first interactive image directed navigation system for the neurosurgeon. It provides real-time anatomical and position information for the surgeon while remaining out of the way when not required. It is simple to use and does not add to the pre-operative preparation time when compared to standard frame-based stereotaxy. Precise localisation, a minimally invasive approach with minimal disruption of the normal brain reduces morbidity and risk, increases speed of procedure and therefore throughput, and reduces time in hospital, which in turn increases the efficiency with which resources can be utilised.2 Its great advantage lies in the fact that it can continually update the surgeon's position throughout the operation and direct the trajectory of approach accurately and safely. It has potential uses in the localisation of infra- and supratentorial mass lesions, intracranial biopsies, the accurate placement of ventricular shunts, and in vascular and epilepsy surgery. In this case it proved invaluable in directing flap placement, the approach along the skull base, safely navigating vital structures and assessing complete resection of the tumour. When the lesion is adjacent to the skull base or other fixed landmarks such as the tentorial edge, as in this case, the system appears even more accurate in judging the extent of tumour resection.

Interactive technology is advancing rapidly. Already it is possible to register MRA and MRI images to increase accuracy further. Efforts are also being made to use the system to identify biopsy sites and correlate these data with metabolic information from MR spectroscopy and positron emission studies. Further refinements can only increase its potential benefits to a modern neurosurgical practice.

References

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