This study aim to compare efficacy of transperineal pudendal nerve block between 2 techniques, by dual guidance (ultrasound and neurostimulation) vs by ultrasound-guided only, in pediatric patients who undergo circumcision.

Primary outcome : Block efficacy will be assessed from ratio of patients who receive opioids in immediate postoperative period.

Secondary outcomes :

• This study also measure other aspects of pain which can reflect the block efficacy including pain score in 24 hours, postoperative analgesic requirement in 24 hours, time to first analgesia, block success rate.
• Block performance : imaging time, needling time, total performance time,
• Block safety : collect complications from block including vascular/rectal puncture, bleeding (hematoma), local anesthetic systemic toxicity, voiding difficulty
• Parental satisfaction will be also collected which scoring use 1-5 metric.

Pain after circumcision is moderate to high level. Inadequate pain control leads to severe agitation, swelling of the wound, neurobehavioral change, lower parental satisfaction. Pain control for circumcision including topical, Dorsal penile nerve block, Caudal block. Topical is inadequate to control post-circumcision pain. Dorsal penile nerve block is not completely cover every areas of penis and also has 4-6.7% failure rate. For Caudal block provides good pain control but it is not proper for ambulatory surgery because of common adverse effects such as motor weakness, urinary retension, moreover it has short duration only 6 hours after block.

In the last 15 years, large-scale studies from many countries studied the safety of regional anesthesia in pediatric patients. It was found that the central neuraxial and peripheral nerve blockage can be done safely, especially using ultrasound-guided to increase safety. Therefore, regional anesthesia is extensive performed in pediatric patients with the main purpose of controlling postoperative pain after surgery, reducing the use of opioids analgesics which may cause respiratory depression, especially in young children.Pudendal nerve block for postoperative pain relief for circumcision, hypospadias has been shown to be effective.

Pudendal nerve derives from anterior rami of spinal nerve root S2-S4 then runs into Alcock's canal and enters the ischiorectal fossa. In this area, pudendal nerve will separate to 3 branches 1) Dorsal penile nerve 2) Perineal branch 3) Rectal branch, therefore pudendal nerve block can provide effective postoperative pain control for surgery around perineal area such as circumcision, hypospadias, anoplasty.

There were several studies on the effectiveness of pudendal nerve block for treating pudendal neuralgia and to control pain after hemorrhoidectomy in adult patients. For pediatric patients, the effectiveness of pudendal nerve block in circumcision surgery compared to the dorsal nerve block was found that in the group receiving the pudendal nerve block, the pain score is lower, with less painkiller and longer pain control. In addition, the study of the pudendal nerve block in pediatric patients who underwent hypospadias in comparison to caudal block, the group that received the pudendal nerve block had a longer duration of pain relief.It can be concluded that the pudendal nerve block is more effective in pain control than the traditional dorsal penile nerve block for circumcision and more effective than caudal block for hypospadias. In addition, it can avoid side effects from caudal block, such as motor weakness, urinary retension, and can be a good alternative pain relief for patients who have contraindication to neuraxial block.

However, the effectiveness of pain control by the pudendal nerve block method is also different, which may be the technical difference of the block. In principle, for the performing peripheral nerve block, the more injection of local anesthetics approach the nerve, the more efficacy will also be followed, and the duration of action will be longer. But the injection near the nerve will increase the chance of causing nerve damage.Therefore, the ideal method of the peripheral nerve block is to inject as close to the nerve as possible without causing injury to the nerves, which is currently a device that can be used to identify the position of the nerve. 1) Using Nerve Stimulator (NS), stimulator by looking at the response of muscles when electrically stimulated, the position used to identify the proximity of the needles to the nerve is the response of the muscle at 0.5 milliampere and no response when using electricity 0.2 milliampere,which indicates that the needle tip is in the right position, not too close or too far 2) Using an ultrasound-guided, which allows to see the position of nerves, blood vessels, and adjacent organs, which can be injected at the point without harm. 3) Using dual-guidance (both ultrasound together with nerve stimulator, which is expected to benefit both of these methods.

The study conducted by Naja and Kendigelen supported that the use of the Nerve Stimulator-guided could provide 100% success rate with patients requiring extra painkillers within the first 24 hours only 7.5-20%. The performance time of block is 2-8 minutes. Later, the study by Gaudet-Ferrand that performed pudendal nerve block under U/S-guided only in pediatric patients who underwent perineum surgery, they reported the success rate only 88% and 60% of patients required opioids in recovery room. Which , compared to previous studies, there was no patient who wanted opioid in recovery room.That show the use of U/S guided pudendal nerve block only is less effective than performing by Nerve Stimulator-guided, which possible cause is U/S could not identify pudendal nerve in every patients.That makes the injection too far from nerve or can injure the nerve unintentionally.

However, using only the Nerve Stimulator still has a risk of injury to important organs such as rectum or adjacent vessels, especially in small children or performing by less expertise. So using U/S will increase safety of block.

This study will find out the effectiveness of performing transperineal pudendal nerve block by dual guidance which use both of NS and U/S guided compare with using U/S guided only. Assuming that the dual guidance technique should increase the effectiveness of the pudendal nerve block compare to the use of U/S guided alone. The primary outcome is the efficacy of postoperative pain control by evaluating the proportion of patients who need opioids during immediate postoperative period.

Inclusion Criteria:

• Age between 8 months to 7 years
• American Society of Anesthesiologists (ASA) class 1-3
• Undergoing elective circumcision
• Consent to participate in research by the parents signing the letter of intent to participate in the research project

Exclusion Criteria:

• Refuse to receive regional nerve block
• Allergic to Bupivacaine or Paracetamol
• Coagulopathy
• Severe coexisting liver or kidney disease
• There is a wound or infection in the pelvis area
• Underlying neurodeficit including attention deficit, cerebral palsy, Down's syndrome
• Parents cannot evaluate pain scores
• Patients or parents have willing to withdraw from research.