After injection of the local anaesthetic, the sciatic nerve is more easily visualised due to the contrast between the bright (hyperechoic) sciatic nerve and the background of the black (anechoic) local anaesthetic
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When anaesthetising the upper abdomen (above the umbilicus), the local anaesthetic must be positioned as high and medial as possible
This means behind the posterior rectus sheath (and rectus abdominis muscle) and above the transversus abdominis muscle. This is called the intercostal TAP block
All thoracolumbar nerves that innervate the anterior abdominal wall travel as multiple mixed segmental nerves, which branch and communicate widely within the transversus abdominis plane (TAP) (1) anterolaterally (intercostal TAP plexus = Th6-Th9) (2) in plexuses that run with the deep circumflex iliac artery (DCIA) (classical TAP plexus = Th10-Th12) and (3) the deep inferior epigastric artery (DIEA) (rectus sheath plexus)
Segmental nerves Th6 to Th9 emerge from the costal margin to enter the TAP between the midline and the anterior axillary line
Th6 enters the TAP just lateral to the linea alba
Th7-Th9 emerge from the costal margin at increasingly lateral positions
Branches of Th9 emerge from the costal margin either medial or lateral to the anterior axillary line
When anaesthetising the lower abdomen (below the umbilicus), the needle entry point must be above the iliac crest and below the thoracic cage with the insertion point being in the anterior axillary line
The local anaesthetic must be positioned in the neurovascuar plane between the internal oblique and transversus abdomens muscles
The visibility of the sciatic nerve and its branches are often reduced in the very obese patients
In obese patients it may be required to use a curved array transducer in order to perform a popliteal sciatic nerve block
However, the sciatic branches – the tibial nerve and the common peroneal nerve – are often sonographically visible near the popliteal crease, where they are usually superficially located. Even in obese patients
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It is important not to penetrate the target nerves with the block needle and especially not to inject local anaesthetic intraneurally
Insert the needle tangentially to the branches of the sciatic nerve in the popliteal fossa
Needle is indicated by arrow head
The in-plane approach to block the two branches of the sciatic nerve – the tibial nerve and the peroneal nerve – allows real-time visualisation of the needle tip, the target nerves and the perineural spread of the local anesthetic
It also gives the opportunity to relocate the needle tip, if the spread is not perfect
The real time ultrasound imaging also makes it possible to avoid piercing the peroneal nerve, when the needle is advanced towards the tibial nerve
The endpoint of ultrasound guided peripheral nerve blockade is complete perineural spread of local anaesthetic
This can be obtained by repositioing the tip of the needle during injection
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It is also possible to move the probe proximal to the bifurcation of the sciatic nerve and block the nerve guided by ultrasound at this level
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You can touch the nerve with the needle, but you should not pierce it
And especially you should not inject local anaesthetic intraneurally
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