Parasacral sciatic (sacral plexus) block

Advanced. The target is deep but sits on a clear bony landmark (the ilium at the sciatic notch). The main risks are the gluteal and pelvic vessels, and the pelvic organs if the needle goes too far medially or deep.

The sacral plexus forms from L4–S3 on the front of the piriformis muscle inside the pelvis. It leaves the pelvis through the greater sciatic foramen, below piriformis, as the sciatic nerve with the posterior femoral cutaneous nerve, the gluteal nerves, the pudendal nerve (medially) and the nerves to the hip rotators.

At the notch the plexus lies on the bone of the ilium, beneath gluteus maximus and piriformis, with the inferior gluteal artery medial to it. The superior gluteal artery leaves above piriformis.

Blocking here, before the branches separate, gives the widest posterior cover of any sciatic approach, including the back of the thigh and the hip joint's posterior capsule.

• Posterior superior iliac spine (PSIS).
• Ischial tuberosity.
• Mansour line: a line from the PSIS to the ischial tuberosity; the needle entry is 6 cm along it from the PSIS.

• Hip surgery, with a lumbar plexus block (complete hip anaesthesia).
• Above-knee amputation and surgery on the back of the thigh.
• Knee and below-knee surgery where a thigh tourniquet is used (covers the posterior femoral cutaneous nerve).

• Sciatic nerve (tibial and common peroneal parts): everything below the knee except the saphenous area; hamstrings.
• Posterior femoral cutaneous nerve: back of the thigh.
• Superior and inferior gluteal nerves: gluteal muscles.
• Often the obturator nerve (spread along the pelvic wall).

Surface anatomy, relevant structures and landmarks

Mansour approach: draw a line from the PSIS to the ischial tuberosity. The entry point is 6 cm from the PSIS along this line.

Positioning

Lateral (Sims) position, operative side up, upper hip and knee flexed.

Block technique

Insert the needle perpendicular to the skin in a sagittal plane. If bone is contacted (ilium), walk the needle caudally and slightly medially off the bone into the notch. The sciatic response is usually found at 6–8 cm.

Needle choice

21–22 G, 100 mm insulated needle.

Neurostimulation technique

Accept plantar flexion or inversion of the foot (tibial) or dorsiflexion and eversion (peroneal) at 0.3–0.5 mA. Gluteal or hamstring twitch alone is not enough.

Choice of local anaesthetic (including volume)

15–20 mL of long-acting local anaesthetic.

Show scan

GMm Gluteus maximus muscleSacrum SacrumPi.m Piriformis muscleGme.m Gluteus medius muscle

Positioning

Prone, or lateral (Sims) with the operative side up.

Scanning technique

• Place the curved probe in a transverse oblique plane on the PSIS, then slide caudally and medially along the line to the ischial tuberosity.
• The continuous bright line of the ilium breaks where the greater sciatic notch begins. At this point, look for the bright, oval, flattened sacral plexus lying deep to gluteus maximus and piriformis, on the edge of the bone.
• Use colour Doppler to identify the inferior gluteal artery, which lies medial to the plexus.

Structures to identify

• Ilium (bony line) and the notch
• Gluteus maximus
• Piriformis
• Sacral plexus / sciatic nerve
• Inferior gluteal artery

Position / ergonomics

Stand behind the patient with the screen on the opposite side.

Probe

Curved low-frequency probe (2–5 MHz); a linear probe may work in thin patients.

Settings

Abdominal or musculoskeletal preset, depth 8–10 cm, colour Doppler on.

Depth

5–8 cm.

Needle size

21–22 G, 100 mm.

Needling technique

In-plane from lateral to medial, or out-of-plane. Aim for the plexus on the edge of the ilium. Avoid going medial into the pelvis.

Optimal needle-tip position

Next to the plexus, under piriformis, at the lateral edge of the notch, away from the inferior gluteal artery.

Drug choice

0.5% ropivacaine or 0.375–0.5% levobupivacaine for long blocks; 1.5–2% mepivacaine or lidocaine for shorter surgery.

Volume

15–20 mL.

Optimal current if nerve stimulation is used

0.3–0.5 mA.

Muscle responses acceptable

• Plantar flexion or inversion of the foot
• Dorsiflexion or eversion of the foot

Testing block success

• Loss of sensation on the back of the thigh and the sole of the foot
• Weak plantar flexion and dorsiflexion

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Indication

Possible after hip surgery or above-knee amputation, but catheters here are uncommon because of depth.

Kit

Insulated 100 mm Tuohy needle and catheter.

Catheter

Advanced 3–5 cm next to the plexus.

Regime

0.2% ropivacaine at 5–8 mL/h.

• Follow the bone: the plexus appears where the ilium line ends.
• Use Doppler to find the inferior gluteal artery before you advance.
• Combine with a lumbar plexus block for complete hip or leg anaesthesia.
• Covers the posterior femoral cutaneous nerve, so a thigh tourniquet is better tolerated than with distal sciatic blocks.

• Mansour (1993) described the parasacral landmark approach using the PSIS-to-ischial-tuberosity line.
• Ben-Ari and colleagues (2009) described ultrasound localisation of the sacral plexus with a parasacral approach.

• Mansour NY. Reevaluating the sciatic nerve block: another landmark for consideration. Reg Anesth 1993;18:322–323. PubMed
• Ben-Ari AY, Joshi R, Uskova A, Chelly JE. Ultrasound localization of the sacral plexus using a parasacral approach. Anesth Analg 2009;108:1977–1980. PubMed