Parasacral sciatic block
The parasacral approach is a relatively uncommon approach for sciatic block. We generally recommend performing sciatic block at the distal hamstrings level, because at this level, hamstrings motor block is minimised and the nerve has a higher proportion of connective tissue – theoretically increasing safety in the event of intraneural injection. However, the parasacral sciatic block approach may be necessitated because of the requirement for more proximal anesthesia/analgesia: the two most common indications being hip surgery, and when distal lower limb surgery under thigh tourniquet is performed under peripheral nerve block alone. The parasacral block approach described here relies on the sciatic nerve's consistent proximity to the posterior (acetabular) border of the ischium (PBI).1 Our own experience suggests that this approach has a rapid onset and high success rate. 2,3
Skill level: Moderate due to the sciatic nerve depth at this location.
Procedure time: 3-5 mins.
Sedation: e.g. midazolam 3 mg.
The parasacral sciatic nerve arises from the sacral roots of the lumosacral plexus (L4-S3). It exits the pelvis through the greater sciatic foramen just medial to the posterior (acetabular) border of the ischial bone (PBI) (Fig. 1, 2)(NB.The ischium or ischial bone is the lower/back part of the pelvic/hip bone). It then passes through the buttock and down the lower limb. It is the longest and widest single nerve in the human body. It supplies the hip and knee joints, the hamstring muscle and the entire leg and foot (except the saphenous nerve innervated medial cutaneous areas).
Fig. 1. Sciatic nerve path in the parasacral area relevant to parasacral sciatic block: the sciatic nerve lies deep to piriformis muscle and immediately medial to the posterior (acetabular) border of the ischium (PBI)(labelled "acetabulum" here).
Fig 2. Parasacral sciatic block: within the parasacral area, the sciatic nerve (1) lies medial to the PBI (2) and deep to the piriformis muscle (3). Numerous vessels (4) lie medial and deep to the nerve. The nerve lies close to the peritoneum and intrapelvic structures (5). Note the gluteus maximus (6) and minimus (7) muscles.
Surface landmarks: Upper limit of the gluteal cleft (approx. level for traditional caudal block).
Needle: long large calibre insulated needle e.g. 10 cm 18G insulated Tuohy needle.
1. Curved probe e.g. Sonosite C60.
2. 10-12 cm large calibre insulated needle.
3. Nerve stimulator.
4. 20 mL LA filled syringe.
Procedure (Video 1)
1. First place patient in the Sim's/lateral recumbent position (on the side with lower leg straight and upper hip/knee both flexed).
2. Probe orientatation is in the axial/transverse plane, and approx. 8 cm lateral to the midline at the level of the upper limit of the gluteal cleft.
3. Identify the PBI, which appears as a characteristic curved hyper-echoic structure (Fig 3).
Fig 3. Parasacral sciatic block: probe placement is in the axial plane 8 cm lateral to the upper limit of the gluteal cleft. At this level, the PBI (1), a curved hyperechoic structure can be identified.
3. If the PBI cannot initially be identified:
a) Slide the probe in a cephalad direction to identify the ilium, which appears as continuous hyperechoic oblique structure.
b) Follow the ilium caudally until a gap appears in the bone.
c) At this point, tilt the probe slightly caudally to indentify the PBI (Fig 4).
Fig 4. Parasacral sciatic block: if the PBI cannot be identified at the upper limit of the gluteal cleft: a) Slide the probe in cephalad direction to identify the ilium (1), then b) follow the ilium caudally until a gap in the bone appears (2). At this point, tilt the probe slightly caudally to indentify the PBI (3).
4. The sciatic nerve lies just medial to the PBI (Fig 1,2).
6. Advance the needle (out-of-plane or in-plane) towards the nerve,
7. A single-point injection technique is acceptable (Fig 5).
Fig 5. Single-point LA injection for parasacral sciatic nerve block.
Local anesthetic regimen: 20 mL short acting LA (e.g. lidocaine 1.5 % - prolonged sciatic block is rarely indicated for analgesia after hip surgery). Consider long acting LA (e.g. ropivacaien 0.5%) if used for knee surgery.
Specific Complications: Theoretically, the needle may cause trauma to intra-pelvic structures. Urinary incontinence has been reported.
1. Electrical stimulation is particularly useful to confirm the target nerve and minimise the risk of intra-neural needle placement. In the obese where the nerve often cannot be visualised, sciatic mediated motor responses can be elicited by advancing the needle to a point just medial the PBI.
2. The inferior gluteal and pudendal vessels lie medial and deep to the nerve respectively, therefore, ensure aspiration precautions (Fig 2).
Video 1. Parasacral sciatic block (with sonography).
1. Mansour NY. Reevaluating the sciatic nerve block: another landmark for consideration. Reg Anesth 1993; 18: 322-3.
2. Taha AM. A simple and successful sonographic technique to identify the sciatic nerve in the parasacral area. Can J Anesth 2012;59, 263-7
3. Bendtsen TF, Lönqvist PA, Jepsen KV, Petersen M, & Børglum J: Preliminary results of a new ultrasound-guided approach to block the sacral plexus: the Parasacral Parallel Shift. Br J Anaesth 2011; 107(2): 278-80