FUNCTIONAL STEREOTAXY

Despite advances in the pharmacological treatment of functional disorders, stereotactic lesioning or stimulation can be of great benefit as an adjunct to control refractory symptoms or as a treatment in itself in those patients who are resistant to medication.

Indications for fimctional stereotactic interventions in movement disorders include Parkinsonian tremor, essential tremor, cerebellar tremor, the post traumatic movement disorders, tortion dystonia torticollis spastica and hemidystonia. Patients with intractable pain, particularly head and neck pain from cancer may show considerable benefit from stereotactic mesencephalotomy or pontine tractotomy. In chronic ’benign’ intractable pain syndrome such as thalamic syndrome, the phantom limb and other denervation pain syndromes the stereotactic implantation of stimulating electrodes in the peri-aqueductal grey matter can achieve satisfactory control of symptoms. Other indications for functional stereotactic intervention are in the placement of chronic depth recording electrodes to elucidate the site of an epileptic focus, or to create lesions in patients with otherwise intractable epilepsy that interrupt the conduction pathways.

Otherwise intractable psychiatric disorders, particularly severe obsessive - compulsive neuroses and phobias may be influenced effectively by stereotactic lesioning in the anterior cingulum or in the anterior part of the internal capsule. The ethics of performing such psychosurgical procedures however remains contentious.

The relocatable frame would have application in all these areas of functional neurosurgery, but to date its use has been limited to thalamotomy for the control of Parkinsonian tremor that is resistant to medication. Four cases have been treated of which 3 were in a personal series. In these 3 patients the diencephalic target selected in each case was the nucleus ventralis intermedius (Vim).

Prior to image acquisition the relocatable frame was secured to the head in a horizontal plane which was approximately parallel to the intercommisural line.

Figure 50 Reformatted image through the third ventricle. AC-PC line drawn on the image

The BRW localiser was then attached to the frame and CT images were acquired in a GE8800 scanner with the frame fixed to the scanner couch via an interface bracket so that it was parallel to and isocentric with the imaging plane. Fifteen axial slices were taken starting just above the level of the posterior clinoid

STEREOTACTIC PROCEDURES - Fimctional 122 process.

1.5mm thick slices were taken with 1.5mm slice separations on a medium body format at a magnification factor o f 1.4. These slices were then reformatted to provide a mid sagittal reconstruction o f the 3rd ventricle.

The anterior and posterior commissures are identified and using computer graphics the AC-PC line is drawn and measured (fig.49).

The coordinates for the nucleus VIM are about 2/10 of the AC-PC distance in front o f PC and 2mm above the AC-PC line with a lateral coordinate of approximately half the AC-PC distance. The target is marked with a cursor on the appropriate axial slice and its BRW coordinates recorded.

In each case Thalamotomy was performed in the Relocatable Frame within 48 hours of target identification. Following refixation of the frame the patient is positioned on the operating table with the frame fixed to the Mayfield head holder by an interface bracket.

The scalp is prepared and draped and the stereoguide, which has been pre set to the target coordinates is secured to the frame. After infiltrating the scalp with local anaesthetic a frontal burr hole is made just anterior to the coronal suture 2cm from the mid line on the appropriate side. An electrode is passed through the burr hole to the thalamic target, guided by the stereotactic instrument.

#

Figure 50 Assessment of motor response in a patient undergoing thalamic stimulation studies prior to the creation of a lesion to control Paridnsonian trem our

Stimulation studies are then performed in an attempt to block the tremour (lOOHz) and exclude capsular motor involvement (2Hz). During these stimulation studies motor and sensory responses are assessed and

STEREOTACTIC PROCEDURES - Functioiial 123 the patient asked to perform a number of motor tasks (fig 51). According to the response the position of the electrode can be adjusted prior to creating a temperature controlled radiofrequency lesion.

Post thalamotomy the three patients in my series had good control of upper limb tremour without neurological deficit.

The position and extent of the radiofrequency lesion was quantified the following day by reimaging the patient in the relocatable frame using an identical set up in the CT scanner. Usually four axial slices through the target were sufficient. In each case the lesion was exactly positioned at the predetermined target coordinates. See (fig. 52)

Fig.52 P la n n in g CT indicating thalamic target in VIM. Post thalamotomy scan shows position of thalamic lesion

These images were obtained, not only to confirm the correct placement of the lesion but as a reference should the patients symptoms recur.

If an inadequate lesion is made recurrent tremor is often evident within a few days of thalamotomy as local oedema settles. In these circumstances a radiofrequency lesion could be extended or a second lesion made at different coordinates by repositioning the frame and carrying out the procedure using existing coordinates.

Conventionally CT guided thalamotomy has been performed in a single stage or in three stages. In single stage procedure a stereotactic frame is invasively fixed to the head under local anaesthesia. The patient is imaged and the coordinates of the target are established with reference to the frame. The patient is then transferred to the operating room and the guided thalamotomy performed after appropriate stimulation studies. The whole procedure can take a number of hours which monopolises the surgeons time and therefore theatre time and subjects the patient to a prolonged period of discomfort in the frame. If the lesion is subsequently found to have inadequately controlled the tremor then to repeat or extend the lesion requires

STEREOTACTIC PROCEDURES - Functional 124 reapplication of the frame, reimaging and replanning prior to repeating it.

In the first stage of a three staged thalamotomy the stereotactic frame is invasively fixed to the skull under general anaesthesia prior to image acquisition and planning. The patient is transferred to the operating theatre and an entry point burr hole made. A Bennet ball is then fixed over the burr hole using screws driven into the outer table o f the skull. The Bennet ball has multiple guide holes that pass in parallel through it. The ball is orientated so that a central guide hole is in the axis o f the pre-planned trajectory. It is locked in position with locking plates. The scalp is then closed over this assembly and the stereotactic frame is removed. The second stage is performed a few days later under local anaesthesia. Here the wound is reopened and an electrode passed to a measured depth to the target and the lesion made after stimulation studies. The third stage is the removal of the Bennet ball which is performed one week later under general anaesthesia allowing time to repeat or extend the lesion if necessary.

The described technique of thalamotomy using the relocatable frame has a number of advantages over these conventional methods. Image acquisition and planning can be performed without anaesthesia and at a time prior to surgery that is convenient for the clinician. Valuable theatre time is therefore not monopolised. The procedure can be performed on a routine operating list and would not normally take more than 45 minutes. Finally the position and size of the lesion can be quantified post operatively and a lesion repeated if necessary by simply relocating the frame and redirecting the electrode to the same coordinates.

A further and perhaps more significant advantage of using the relocatable frame in the management of fimctional disorders is that it facilitates the acquisition of functional information from PET,SPECT or MRS in stereotactic space. Correlation of this data with structural information derived from stereotactic CT or MRI images may allow new and more specific functional targets to be accurately defined for lesioning, stimulation or tissue implantation. With the use of such methods the indications for stereotactic intervention may well increase, particularly for the treatment of intractable epilepsy and psychiatric disease.

STEREOTACTIC PROCEDURES - Brachythenipy 1 2 5