To the Editor:
Thearticle by Chen et al1 is important as it advances our understanding of the extent of cutaneous sensory blockade fol lowing subcostal transversus abdominis plane (TAP) block. It also highlights the clinical limitations of this technique when a surgical incision involves the upper lat eral abdomen.

In the original description of the oblique subcostal TAP block, Hebbard2 proposed that oblique subcostal TAP block wouldprovide effective analgesia of the ab dominal wall above the umbilicus. Since then, the oblique subcostal TAP block has been used for major upper abdominal sur gery 3 and in conjunction with bilateral TAP block for laparoscopic surgery covering the entire abdominal wall.4 Recently, Ma et al5 mapped the analgesic efficacy of subcostal TAP block, observing that the technique produces effective analgesia in the anterior abdominal wall with the exception of the upper lateral abdomen.

Chen et al1 have now confirmed the usefulness of oblique subcostal TAP block in providing analgesia for incisions in the midabdominal area, simultaneously dem onstrating that this block appears unsuit able for incisions extending into the lateral abdominal wall. In addition, the distribution of cutaneous sensory block appears to show there is no advantage of bilateral oblique subcostal TAP block over bilateral rectus sheath blocks. Indeed, the multiple needle insertions and hydrodissection required to achieve effective blockade of the midabdominal region using the oblique subcostal TAP block may represent a disadvantage over the rectus sheath block.

Interestingly, these studies1,2,5 confirm that the oblique subcostal TAP block pro duces sensory blockade that extends below the umbilicus, contrary to common belief.

The findings of Chen et al1 can be explained by an understanding of the anatomical course of the thoracoabdominal nerves and their branches. The thoracoabdominal nerves originate from the anterior rami of spinal nerves T7–T11. About the middle of their course through the intercostal space, they give off a lateral cutaneous branch, which pierces the external intercostal and external oblique muscle to subsequently di vide into anterior and posterior branches that innervate the skin of the lateral thorax and abdominal wall as far as the margin of the rectus abdominis muscle anteriorly.

The terminal part of the thoraco abdominal nerves pass behind the costal cartilage and continue anteriorly in the plane between the internal oblique and transversus abdominis muscle, before piercing the rectus abdominis sheath as anterior cutaneous branches supplying the skin of the midabdomen.

It is the terminal branches of the thoracoabdominal nerves that are blocked by both oblique subcostal TAP block and the rectus sheath block, which explains the similarity in cutaneous sensory block distribution.

We therefore propose that effective analgesia for incisions involving the upper lateral abdomen requires blockade of the lateral cutaneous branches of T7–T11, which can never beachieved using a TAPblock performed inferior to the costal margin, but could potentially be achieved by instilling local anesthetic solution above the costal margin in the thoracic fascial plane either deep or superficial to the external oblique muscle.

Duncan L. Hamilton, BMedSci, BMBS, EDRA, FRCA, FFICM, FRCSEd

Department of Anaesthesia
James Cook University Hospital
Middlesbrough, United Kingdom
and Hull York Medical School
York, United Kingdom

Baskar P. Manickam, MD, DA, FRCA
Department of Anaesthesia
County Durham & Darlington
NHSFoundation Trust, Darlington
United Kingdom
The authors declare no conflict of interest.

1. Chen Y, Shi K, Xia Y, et al. Sensory assessment
and regression rate of bilateral oblique subcostal
transversus abdominis plane block in volunteers.
Reg Anesth Pain Med. 2018;43:174–179.
2. Hebbard P. Subcostal transversus abdominis
plane block under ultrasound guidance. Anesth
Analg. 2008;106:674–675.
3. Niraj G,Kelkar A,Jeyapalan I, etal. Comparison
of analgesic efficacy of subcostal transversus
abdominis plane blocks with epidural analgesia
following upper abdominal surgery. Anaesthesia.
2011;66:465–471.
4. Tanggaard K, Jensen K, Lenz K, et al. A
randomised controlled trial of bilateral dual
transversus abdominis plane blockade for
laparoscopic appendicectomy. Anaesthesia.
2015;70:1395–1400.
5. MaJ,JiangY,TangS,etal.Analgesicefficacyof
ultrasound-guided subcostal transversus
abdominis plane block. Medicine (Baltimore).
2017;96:e6309.