The Outcomes Research™ Group, Department of Anaesthesia and Pain Management, Royal Melbourne Hospital, Melbourne, Australia
Presented in part at the National Scientific Congress of the Australian Society of Anaesthetists, Cairns, Australia, October 1999.
Accepted for publication
Address correspondence to Kate Leslie, MD, FANZCA, Department of Anaesthesia and Pain Management, Royal Melbourne Hospital, Parkville, VIC, 3050, Australia. Address e-mail to kate.leslie @mh.org.au.
Functional neurosurgery is increasingly used for the treatment of Parkinson’s Disease (1). Using stereotactic localizing systems and physiological mapping in awake cooperative patients, precise lesions can be made in the basal ganglia with substantial relief of symptoms (1–3). A framed, rather than frameless, stereotactic system is used for functional procedures to accurately target areas deep within the brain and to minimize patient movement. Unfortunately, placement of the frame results in considerable discomfort in awake patients.
In our practice, local anesthesia for stereotactic frame placement is usually provided via subcutaneous infiltration at the pin sites during positioning of the frame. However, injection of the local anesthetic can be quite painful. We therefore compared supraorbital and greater occipital nerve blocks with subcutaneous infiltration for stereotactic frame placement in patients undergoing functional surgery for visual analog scale pain scores associated with injection of local anesthetic and stereotactic pin placement.
Materials and Methods
With ethics committee approval and informed consent, we studied 13 male and 7 female patients, aged 58 ± 14 yr, of Anglo-European descent, undergoing functional stereotactic surgery for Parkinson’s disease (thalamotomy, pallidotomy and deep brain stimulator implantation). A sample size of 20 patients was calculated to provide adequate statistical power to detect a difference in visual analog scale (VAS) pain scores of 20/100 between sides of the head (sd = 20; α = 0.05: β = 0.2).
Patients were instructed on the use of VAS preoperatively. Premedication was limited to temazepam 10 mg PO on the morning of surgery. Patients were randomized to receive subcutaneous infiltration on one side of the head and supraorbital and greater occipital nerve blocks on the other. IV access was established before local anesthetic injection. Bupivacaine 0.5% was injected in 2–3 mL aliquots, using a 25-gauge sharp-bevelled needle, for blocks and subcutaneous infiltration.
The stereotactic system was fitted to each patient’s head just before magnetic resonance imaging. The system consisted of a head ring, with four posts and screws that were in the same position on the ring in all patients (CRW MRI Stereotactic System; Radionics Inc., Burlington, MA).
Nerve blocks were performed by one of the investigators using standard techniques (4). The supraorbital nerve was blocked by subcutaneous infiltration of local anesthetic above the eyebrow, in vertical alignment with the pupil, where the nerve emerges from the supraorbital notch. The greater occipital nerve was blocked on a line drawn between the external occipital protuberance and the mastoid process (the superior nuchal line). If the occipital artery was palpable on this line, then local anesthetic was injected just medial to the pulse. Otherwise the injection was made at the junction of the medial third and the lateral two-thirds of the line. The pins were applied approximately 10 min later.
Subcutaneous infiltration was performed by the neurosurgeon. With the stereotactic frame held in position by an assistant, local anesthetic solution was injected subcutaneously at each of the anticipated pin sites. After approximately 2 min, the pins were applied. Supplementary subcutaneous injections were given at the time of pin placement if the patient volunteered that the level of pain was unacceptable or they requested more analgesia.
VAS pain scores (scale: 0–100) were recorded after each local anesthetic injection, and for each pin site during frame placement and at hourly intervals thereafter until the completion of surgery. The extent of nerve blockade was mapped with ice before frame placement and at the conclusion of surgery.
VAS pain scores were considered parametric data, and consequently were compared using paired, two-tailed t-tests. Changes in VAS pain scores over time were assessed using repeated-measures analysis of variance. The frequency of supplementary local injections during pin placement for the two techniques was assessed using a χ2 test (one-tailed). Results are presented as mean (sd), unless otherwise specified;P < 0.05 was considered statistically significant.
Nerve blocks were less painful than subcutaneous infiltration at both frontal (P = 0.008) and occipital (P = 0.01) sites. However, there were no differences in VAS pain scores for stereotactic pin placement between nerve blocks and subcutaneous infiltration at either site (frontal:P = 0.69; occipital:P = 0.88). VAS pain scores at each site for local anesthetic administration and frame placement were significantly greater than for all subsequent assessments, but were not different from each other (Fig. 1). Supraorbital nerve blocks required more supplementation than frontal subcutaneous infiltration (Table 1).
Supraorbital and greater occipital nerve blocks produced several distributions of anesthesia in this study. In three patients, the territory supplied by the supratrochlear nerve was also anesthetized, and in five patients, the supraorbital block was deficient posteriorly. Surgery lasted 6.3 ± 0.8 h. At the end of surgery, no block was demonstrable in seven patients, some frontal blockade remained in two patients, some occipital block remained in five patients, and substantial fronto-occipital block remained in six patients. Head dressings prevented further assessments being made.
Minor complications with the supraorbital nerve blocks occurred in two patients. One patient developed a supraorbital hematoma, and a second described a transient worsening of preexisting diplopia after the nerve block. There were no serious or long-term complications.
Nerve blocks and subcutaneous infiltration provide comparable anesthesia for placement of a stereotactic frame. Although performance of nerve blocks was less painful for the patient than subcutaneous infiltration, no differences between the two techniques were evident at pin placement or during surgery. Nerve blocks are therefore an alternative to subcutaneous infiltration in patients undergoing functional neurosurgery.
The technique and timing of subcutaneous infiltration may have been detrimental to its success at pin placement, relative to nerve blocks. Nerve blocks were performed and tested before frame placement, whereas subcutaneous infiltration was performed during frame placement. Subcutaneous infiltration might have been more successful if more time had been allowed between injection and pin placement, or if a local anesthetic with a more rapid onset of action (i.e., lidocaine) had been used. In addition, local anesthetic was injected at a single point during the subcutaneous infiltration technique, raising a tight bleb. The tissues of the scalp are very dense (5) and this local increase in tissue pressure may be quite painful. For these reasons, our results may not be generalizable to situations where different local anesthetics or different timing of injections are used.
Supraorbital nerve blocks required more supplementation than greater occipital nerve blocks or subcutaneous infiltration. There are several potential reasons for this finding. First, the frontal pin of the frame was often close to the lateral border of the supraorbital nerve territory, particularly in patients with narrow faces, and sometimes may have strayed into the territory of the zygomatico-temporal nerve (a branch of the maxillary nerve), which supplies the lateral part of the forehead and temple. Local anesthetic infiltration in the temporal area may reduce the need for further local supplementation during pin placement, but is not likely to have any advantage over subcutaneous infiltration at the pin site (4). Finally, lack of blinding in this study may have allowed biased administration of supplementary subcutaneous infiltration.
Second, there is considerable interindividual variation in the anatomy of the supraorbital nerve, which may affect the success of local blocks. The supraorbital nerve may exit the skull undivided or its medial and lateral branches may exit separately. Failure to block the lateral branch may account for deficiency of the block posteriorly in some of our patients (6). Occasionally, the supratrochlear nerve may exit at the same point as the supraorbital nerve. This may explain why a block of the supratrochlear nerve territory was occasionally observed in this study. Exits may consist of a notch in the supraorbital ridge or a foramen, which may be situated along the whole orbital rim or even lateral to it. In addition, the exit point may be as much as 1.9 cm superior to the orbital rim (7). These data highlight the need to infiltrate a band of local anesthetic along the orbital rim and to carefully test the block before frame placement.
The greater occipital nerve also varies in its exit from the nuchal muscles, course, and division (8). However, the nerve is frequently intimately related to the occipital artery at the superior nuchal line, potentially making it easier to locate. This may account for fewer supplementary local anesthetic injections posteriorly.
Although cutaneous anesthesia was acceptable with both methods, patients often reported discomfort during final tightening of the pins. This could be because of periosteal pain. However, the periosteum is innervated by the nerves supplying overlying scalp and should be anesthetized along with the scalp (5). An alternative explanation is that pressure was transmitted by the pins to the cranial sutures. The cranial sutures are lined with dura, which is innervated by the same cranial nerve or cervical nerve root as the overlying scalp. However, the dura may receive its innervation from within the cranium rather than via the scalp and therefore will not be anesthetized by nerve blocks or subcutaneous infiltration (5). We speculate that because subsequent analgesic requirements were minimal, pressure transmission to the cranial sutures as the pins were tightened may be an important factor.
Supraorbital nerve blocks were associated with minor complications in 2 of the 20 patients, whereas no complications arose from greater occipital nerve blocks or subcutaneous infiltration. The risk of complications must be balanced against the potential for less pain on injection in each patient.
In conclusion, we found that supraorbital and greater occipital nerve blocks provide comparable analgesia to subcutaneous infiltration for the placement of a stereotactic frame. Blocks are therefore an alternative to subcutaneous infiltration in patients undergoing functional neurosurgery.
We thank Associate Professor Jeffrey V. Rosenfeld and Professor Robert Iansec for allowing us to recruit their patients for this study.
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