Regional Anesthesia 22(3):223-228, 1997

T h e Parasacral Sciat ic N e r v e B l o c k

Gary F. Morris, M.D., F.R.C.P.(C.),* Scott A. Lang, M.D., F.R.C.P.(C.),* William N. Dust, M.D., F.R.C.S.(C.),t and

Michael Van der Wal, M.B., Ch.B, F.R.C.P.(C.)*

Background and Objectives. The clinical utility of a new parasacral approach for con- duction block of the sciatic nerve was investigated, with critical examination of onset, extent, and success rates when this block was used for surgical procedures below the knee. Methods. Thirty ASA I-III patients presenting for surgery on the lower limb were enrolled. All received 30 mL of 1.5% lidocaine with 1:200,000 epinephrine following nerve stimulator identification of the sciatic nerve at _<0.2 mA or less. Trans-sartorial saphenous nerve blocks were performed to provide anes- thesia to the medial leg. Results. Overall success for surgical anesthesia with this block was 97%. All components of the sacral plexus could be blocked with this approach, and 93% of patients displayed evidence of obturator nerve motor block. However, no patient displayed evidence of obturator sensory anesthesia that could be mapped. Saphenous nerve blocks were 100% effective in providing surgical anesthesia for the procedures performed. Conclusions: The parasacral approach to the sciatic nerve exhibits a high success rate, resulting in anesthesia of the entire sacral plexus and generally in motor block of the obturator nerve was an interesting observation. Reg Anesth 1997: 22: 223-228. Key words: trans-sartorial saphenous nerve block, parasacral sciatic nerve block, sacral plexus, obturator nerve, alfentanil.

Regional anesthetic techniques have long been used to provide surgical anesthesia for procedures on the lower limb (1-3). Sciatic nerve blocks are useful but may be technically demanding (4,5), with vari- able success rates ranging from 33% to 95% (1-3). Surgical anesthesia of the entire lower limb would require anesthesia in the distributions of the fem- oral, lateral femoral cutaneous, obturator, posterior femoral cutaneous, and sciatic nerves. When using peripheral nerve blocks to achieve this, at least two separate injections must be made. Typically, a com- bination of a sciatic nerve block and a femoral three-

From the Department of *Anesthesia and tOrthopedics, Royal University tlospital, Saskatoon, Saskatchewan, Canada.

Presented at the American Society of Regional Anesthesia, San Diego, California, Annual Meeting, March 30, 1996

Accepted for publication December 12, 1996. Reprint requests: Gary E Morris, M.D., Department of Anes-

thcsia, Royal University.Hospital, Saskatoon, Saskatchewan, Canada, S7N 0W8.

in-one nerve block are performed to accomplish this task. However, the sensory distribution of the obturator nerve and its clinical relevance remain unclear (9). In addition, successful anesthesia of the obturator nerve as a component of tile femoral three-in-one nerve block remains controversial (9).

Mansour (10) has described a parasacral approach to the sciatic nerve (Fig. 1). This proximal approach attempts to place the local anesthetic solution within a fascial plane enclosing the fibers of the sacral plexus prior to their coalescence as the sciatic nerve (Fig. 2). This approach has been reported to achieve reliable anesthesia of the sciatic nerve, including the distribution of tile posterior femoral cutaneous nerve. Because of the proximity of the sacral plexus to tile obturator nerve in the pelvis (Fig. 2), we hypothesized that this approach may also, coinci- dentally, result in conduction block of the obturator nerve. Since there were no data in the literature on the expected success rate with this block, its associ-

223

224 Regional Anesthesia Vol. 22 No. 3 May-June 1997

J

Fig. 1. Parasacral sciatic nerve block. At point (B), which is 6 cm along a line constructed between the posterior superior iliac spine (A) and the ischial tuberosity (C), a 22-gauge insulated needle is inserted. The needle is advanced seeking a motor response.

ated side effects, or its clinical utility, we decided to critically evaluate the implemen ta t ion of the para- sacral ne rve block into our regional anesthesia a r m a m e n t a r i u m . We fur ther hypothes ized that this t echn ique w o u l d be easy to learn, w o u l d d e m o n - strate a h igh success rate because of its simplicity, and w o u l d block all c o m p o n e n t s of the sacral plexus. Finally, w e assessed the clinical utility of the trans-sartorial s aphenous nerve block (Fig. 3).

Fig. 2. Pelvic view of the sacral plexus. A, sacral plexus; B, sciatic nerve; C, pudendal nerve; D, obturator nerve; E, sympathetic chain; F, internal iliac artery; G, internal iliac vein.

Fig. 3. Trans-sartorial nerve block. A blunt 20-gauge Tuohy needle was advanced through the sartorius mus- cle by using a loss of resistance technique (similar to that used for identification of the epidural space with saline). The saphenous nerve and relevant anatomy (anterome- dial view of the leg) are (a) sartorius muscle; (b) fascial envelope, containing saphenous nerve (black), superfi- cial femoral artery (white), and femoral vein (black- white stripes); (c) saphenous nerve; (d) fat pad beneath sartorius muscle, within which lies the saphenous nerve; (e) vastus medialis.

Materials and Methods

Approval from our institutional committee on ethics was obtained prior to commencemen t of the study. Following written informed consent, 30 ASA I-III patients, 14 w o m e n and 16 men, pre- senting for surgical procedures on the lower limb were prospectively enrolled in this observational study (Table 1). Patients were transferred to a monitored operating theater or induction room, intravenous access was secured, and supplemental oxygen was administered via nasal prongs. Incre- mental boluses of alfentanil were administered intravenously to facilitate positioning and perfor- mance of the blocks. The total alfentanil dosage was recorded for each patient.

Sensory analgesia to pinprick was assessed in the distributions for each of the nerves of the lower extremity (12) (Fig. 4). Sensory anesthesia was mapped for each patient by means of a dia- gram identical to that depicted in Figure 4. Motor strength was assessed on a four-point grading scale: 0, no muscle power; 1, patient can tense muscles or show minimal resistance; 2, moderate resistance but definitely diminished power; 3, full power. All assessments, sensory and motor, were recorded before and after performance of the sci- atic nerve block. Obturator muscle strength was assessed with the patient supine by determining hip adductor strength using the above scale and a technique previously validated technique (9), defining marked adductor motor weakness arbi- trarily as grades 0 and 1. Patient sedation during the performance of the blocks was recorded by use of four-point sedation scale (0, alert; 1, drowsy but awake; 2, asleep but easily roused; 3, asleep,

Parasacral Sciatic Nerve Block �9 Morris et al. 225

[ ] Femoral

�9 Obturator

[ ] Saphenous

[ ] Sciatic

[ ] Lateral femoral cutaneous

[ ] Posterior femoral cutaneous

[ ] Sacral

Fig. 4. Classical cutaneous sensory distribution to the lower limb.

T a b l e 1. Demographic Data

Variable Mean • SD Range

Age (y) 43.5 • 16.6 19-75 tleight (cm) 169.7 + 8.4 157-182 Weight (kg) 80.8 • 19.6 50-122 Time to completion of parasacral block (rain) 9.0 • 3.0 4.5-15.0 Number of attempts (parasacral) 3.6 • 2.6 1-10 Endpoint (mA) 0.18 • 0.04 0.1-0.2 Alfentanil dose (p.g) 1662 • 987 0--4,000 Sedation* 0.3 • 0.5 0-2 Sensory onset of parasacral block (min) 5.1 • 3.5 1-15 Motor onset of parasacral block (rain) 8.8 • 5.5 1-22 Duration of parasacral block (h) 6.1 • 1.7 3.5-9.0 Pain during parasacral block (VAS cm) !.3 • 1.1 0--4.2 Pain during saphcnous block (VAS cm! 1.3 • 1.5 0--7.0

"Graded on a scale of O, alert; 1, drowsy but awake; 2, asleep but easily roused; 3, asleep, not easily roused. VAS, 10-cm visual analog scale.

226 Regional Anesthesia Voh 22 No. 3 May-June 1997

roused with difficulty). Three independent 10 cm visual analog scale (VAS) assessments evaluated patient discomfort during the performance of the saphenous and sciatic nerve blocks as well as an overall assessment of procedural discomfort. The time to completion of the sciatic block was recorded as the time from landmarking (ie, identification of the site of planned needle insertion for the parasacral sciatic nerve block) to injection of the local anesthetic. Sensory and motor functions were assessed every 5 minutes for a total of 30 minutes following completion of the local anes- thetic injection. The needs for intraoperative use of intravenous sedation/analgesia, local anesthetic supplementation, and conversion to general anes- thesia were all recorded.

The parasacral sciatic nerve block was performed as described by Mansour (10). The posterior supe- rior iliac spine was identified and a line was con- structed between that point and the ischial tuberosity. Three fingerbreadths (6 cm) from the posterior superior iliac spine along this line, a 100- mm 22-gauge insulated needle was introduced and advanced in a sagittal plane (Fig. 1). If bone was contacted, the needle was removed and reinserted 1-2 cm more caudally along the previously described line. An attempt was defined as removal of the needle to the skin surface after contacting bone or after passing the needle beyond bone without elic- iting an acceptable motor endpoint. With the aid of a Digistim II peripheral nerve stimulator (Neu- rotechnology, Houston, TX; pulse width 100 ms, frequency 1 Hz), a brisk motor response at the ankle or foot was sought. Motor responses were graded 1 (minimal), 2 (brisk), or 3 (violent). Once a motor response had been elicited at 0.2 mA or lower, 30 mL of lidocaine 1.5% with 1:200,000 epinephrine was then injected. In order to mini- mize the possibility of interference with the assess- ment of the obturator nerve, blocks were not used. Instead, trans-sartorial saphenous nerve blocks were performed by a trans-sartorial loss of resis- tance technique previously described, injecting 10 mL of 1.5% lidocaine with 1:200,000 epinephrine (11) (Fig. 4). Success of saphenous and sciatic nerve blocks was defined as sensory or sensory and motor anesthesia in the distribution of the nerves tested, as well as completion of the surgery with- out conversion to general anesthesia. In addition, a complete sensory assessment was performed on each patient to map any sensory changes that could be attributed to anesthesia of elements of the sacral plexus or the obturator nerve. Side. effects and complications of the nerve block techniques were recorded.

Results

A variety of surgical procedures were performed on the 30 patients with this combination of parasacral sciatic and saphenous nerve blocks (Table 2). Time to completion of the block averaged 9 minutes (range, 4.5-15 minutes), and an average of 3.6 attempts (range, 1-10) were made. The doses of alfentanil that facilitated performance of the blocks, patient sedation levels, and procedural VAS pain scores are shown in Table 1.

This approach was successful in 29 of the 30 patients enrolled, for a success rate of 97%. In one patient, a suitable endpoint was attained, but only a small amount of local anesthetic could be injected owing to obstruction from blood clotted within the shaft of the needle. It was decided that further attempts at localizing the sciatic nerve with this approach would be ill advised because of partial anesthesia of the nerve. This patient's surgery was performed uneventfully under regional anesthesia with a combination of a trans-sartorial saphenous nerve block and a popliteal sciatic nerve block.

Of the 29 parasacral sciatic nerve blocks in which local anesthetic was injected, all resulted in success- ful sensory and motor anesthesia in the distribution of the sciatic nerve. Excellent surgical anesthesia was provided for all the procedures in Table 2. Perineal anesthesia on the ipsilateral side of the injection was demonstrated in 24 of the 30 patients (80%).

Evidence of anesthesia of the obturator nerve was demonstrated in 28 of 30 patients (93%) as obturator motor weakness. Of these patients, 11 had grade 0 adductor motor power, 11 had grade 1 adductor motor power, while only 2 had grade 3 motor power (as expected, the one patient in whom the parasacral sciatic nerve block "failed" had grade 3 motor power). Therefore, as defined,

T a b l e 2. Surg ica l P r o c e d u r e s

Procedure No. Performed

ORIF ankle 16 Dwyer's forefoot reconstruction 1 Achilles tendon repair 2 Posterior tibial tendon transfer 1 Reduction forefoot fracture 1 Closed reduction, ankle 1 Closed reduction, talonavicular dislocation 1 Metatarsectomy 1 Debridement, ulcer, base of foot 1 Debridement, auger injury, forefoot 1 Debridement, abscess, talonavicular joint 1 Irrigation septic ankle 1 Below-knee amputation 1

ORIE open reduction and internal fixation.

22 of 30 patients (73%) had marked adductor motor weakness following their parasacral sciatic nerve blocks. Despite the evidence of loss of hip adductor motor power, only 4 of 30 patients (13%) noted sensory changes in the cutaneous distribu- tion of the obturator nerve. These patients noted "vague" (ie, not reliably reproducible) sensory changes in the region of the medial thigh, but in no patient could a discrete area of sensory anesthe- sia be mapped. In all 30 patients, the saphenous nerve block at the knee provided dense anesthesia to the medial portion of the lower leg in the distri- bution of the saphenous nerve (Fig. 3).

Aside from the one failed attempt, there were no complications from performance of the para- sacral sciatic nerve blocks. No patients developed urinary retention or required postoperative catheterization, and there was no evidence of local anesthetic toxicity.

Discussion

A great variety of approaches for conduction block of the sciatic nerve have been described. Man- sour (10) has described a parasacral approach to the sciatic nerve which may possess unique advantages and disadvantages over more traditional techniques.

The bony landmarks with ttle parasacral approach are readily identified, and this seems to lead to a steep learning curve with a high success rate. A novice (GM) quickly gained the skills necessary to efficiently perform this block with a high degree of success (29 of 30 patients, 97%). With the excep- tion of the one parasacral failure, no patient required supplementation because of an ineffective block, and patient satisfaction remained high, with low VAS pain scores recorded for performance of the block. No neurologic complications were seen, although few would be expected in a study this small.

The sacral plexus (Fig. 2) consists of nerve fibers originating from L4 to $3 nerve roots. These nerve roots travel within the pelvis anterior to the ischial bone and the piriformis muscle. They then coalesce to form the common peroneal, tibial, posterior femoral cutaneous, inferior gluteal, and superior gluteal nerves just prior to exiting the pelvis through the obturator foramen via the greater sci- atic notch immediately caudal to the piriformis muscle. In addition to the components of the sci- atic nerve, the sacral plexus also gives rise to the pudendal nerve. Of the 30 patients, 24 (80%) experienced unilateral anesthesia of the perineum. These observations are consistent with the fact that the parasacral approach represents a sacral plexus block rather than an isolated sciatic nelme block.

Parasacral Sciatic Nerve Block �9 Morris et al. 227

The incidence of urinary retention was exam- ined as an index of autonomic neural block to tile pelvic viscera. The pelvic splanchnic nerves ($2-4), the terminal portion of tile sympathetic trunk, and the inferior hypogastric plexus all lie in close prox- imity to the elements of the sacral plexus. Of note, despite sacral plexus anesthesia and the close prox- imity of the sympathetic nerve supply to the blad- der, no patient experienced difficulty with voiding and none required postoperative catheterization.

We were frequently able to demonstrate anes- -thesia of the obturator nerve (29 of 30 patients). As illustrated in Figure 2, the obturator nerve courses along the pelvic brim close to and in the same plane as the lumbosacral trunk. Spread of local anesthetic along a common fascial plane may account for adductor weakness in such a large pro- portion of our patients. Interesting observations included a paucity of sensory changes in the upper leg despite marked motor weakness in the distribu- tion of the obturator nerve. At best, a few patients described patchy sensory differences, although there were no patients in whom a dicrete area of cutaneous analgesia to pinprick could be mapped. This obturator nerve sensory/motor dichotomy brings into question the classical description of the sensory distribution of the obturator nerve (12).

Obturator anesthesia is reported to be a neces- sary component of regional anesthesia for major surgery on the knee. Total joint replacement may be impossible despite excellent sciatic and femoral nerve blocks should the obturator nerve be spared (9). Traditional approaches to the obturator nerve can be painful and may be technically difficult for the anesthesiologist to perform (9). In view of ttle controversy surrounding the ability of the femoral three-in-one nerve block to achieve conduction block of the obturator nerve (9), the parasacral sci- atic nerve block may offer a more reliable method of producing obturator nerve anesthesia.

The pelvis is also rich in vascular structures, which fortunately, lie in a different plane, more anteriorly than the neural structures. No episodes of local anesthetic toxicity or vascular puncture were identified in this study. The bladder, uterus, and rec- tum all lie within the pelvis. There were no infec- tious complications seen in this study, and there were no clinical findings in any patient compatible with visceral puncture (hematuria, melena, etc.).

Patients with significant limb trauma have been thought to be poor candidates for regional anesthe- sia with nerve stimulator techniques. The marked pain that may be elicited with movement of tile fractured limb is believed to require extremely deep sedation and/or analgesia prior to perfor-

228 Regional Anesthesia Vol. 22 No. 3 May-June 1997

mance of the block. We have found that modes t sedation wi th alfentanil can provide the anesthesi- ologist with an alert, cooperat ive pat ient wi th min- imal exacerbat ion of pain despite marked m o v e - m e n t of the fractured extremity.

There are a n u m b e r of limitations of our study. First, we did not measure plasma levels of local anesthetic following per formance of our blocks. Absorption characteristics f rom a "parasacral" depot have not been described, and a l though we did not experience any complications with the ra ther large total doses of local anesthetic employed in this study, fur ther investigation in this area is war- ranted. Although dissection in the ana tomy labora- tory suggests that the sacral plexus is contained within a distinct fascial plane, we have not been able to demonst ra te this wi th imaging techniques available to us (radiography and contrast). Fur ther investigation in this area would be interesting, as it m a y be possible to thread a catheter into this "fas- cial" plane for cont inuous infusion techniques. Although a novice had impressive success with his first 30 patients, as m a n y as 10 a t tempts were required in some patients. However, pat ient dis- comfort during the procedure was minimal , and there were no complications.

In summary, we have found the parasacral approach to the sciatic nerve to provide "excellent surgical anesthesia for surgical procedures be low the knee w h e n combined with a trans-sartorial saphenous nerve block. The parasacral sciatic ne rve block has had a h igh success rate in the hands of a novice and has been associated with excel lent patient satisfaction. Coincidentally, it f requently pro- vides anesthesia in the distribution of the obtura tor nerve. The addition of this approach to the sciat ic nerve and the sacral plexus represents a we lcome addition to regional :anesthet ic techniques for the lower limb and m a y also allow reliable anesthesia of the obtura tor nerve. We have also conf i rmed the clinical usefulness of the trans-sartorial saphenous nerve block, which has achieved 100% success for surgical anesthesia in the distribution of the saphe- nous nerve in 30 patients.

Acknowledgments

The authors thank the Depar tments of Or thope- dics and Anesthesia of the Royal Universi ty Hospi- tal, Saskatoon, Saskatchewan, for their coopera t ion with this study. They also express their apprecia- tion to Ms. Beth Lozanoff for the creation of origi- nal illustrations.

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