Anal fistula is an abnormal tract communicating an external opening in the perianal skin with an internal opening in the anal canal. Anal fistula is treated by fistulotomy . sphincter sparing procedures are usually done under general anesthesia omitting neuromuscular blocking agents in order to preserve sphincter tone intraoperatively. The aim of this study is to evaluate the effect of caudal block analgesia versus perianal block analgesia combined with general anesthesia on the postoperative analgesic profile and the tone of external anal sphincter in sphincter sparing surgery.

Anal fistula is an abnormal tract communicating an external opening in the perianal skin with an internal opening in the anal canal. Anal fistula is treated by fistulotomy . sphincter sparing procedures are usually done under general anesthesia omitting neuromuscular blocking agents in order to preserve sphincter tone intraoperatively.

The study will be designed to recruit 46 patients who have complex anal fistula and will be scheduled for sphincter sparing surgery.

All patients will receive routine preanesthetic evaluation. They will be kept fasting for2 hours for clear fluids and at least 8 hours for solid food prior to performing the block. On arrival in the operating room, heart rate (HR), non invasive blood pressure, oxygen saturation (SPO2) and electrocardiography monitors will be applied and the baseline values will be noted. Intravenous access (IV) will be secured and an appropriate IV fluid will be started.

Patients will receive general anesthesia with induction of 2mg/kg IV propofol, 1mcg/kg IV fentanyl and laryngeal mask insertion in supine positionand maintenance of anesthesia with isoflurane inhalation.

Patients will be randomly allocated into two study groups;

• 1st group (caudal block group): will receive general anesthesia with ultra sound (u/s) guided caudal block with 40 ml 0.125% bupivacaine (10 ml bupivacaine + 30 ml normal saline 0.9%).
• 2nd group (perianal block group): will receive general anesthesia with perianal block analgesia with 40 ml 0.125% bupivacaine (10 ml bupivacaine + 30 ml normal saline 0.9%).

Intraoperative, optimal surgical conditions and persistence of muscle tone will be determined by the general surgeon according to patient's anal sphincter relaxation using Digital Rectal Examination Scoring System (DRESS) which ranges from 0 to 5 but with usage of resting pressure only without squeezing pressure as the patient is anesthetized.

Postoperativepain control regimen will be used for the two groups as Paracetamol 15mg/kg IV /6 hours, Ketorlac 30 mg IV /8hours. A rescue analgesic dose of opioid in the form ofMorphine 0.05mg/kg as needed on patient demand if numeric pain score is more than three to be repeated every 20 min. till maximum dose 0.2mg/kg. if numerical rating score persists more than three.

Inclusion Criteria:

• 18-60 years old patients
• ASA I-IIIpatients
• Patients with complex anal fistula

Exclusion Criteria:

• known hypersensitivity to amide type local anesthetics
• Patients with contraindications to caudal block or perianal block: use of anticoagulant medication, local infection in the intervention site, increased intracranial pressure and severe aortic and / or mitral valve stenosis.
• Patients who do not accept regional anesthesia
• BMI (Body mass index) > 35 kg/m2
• Anatomical abnormalities and previous surgeries involving the sacrum

Double blinded(the surgeon and the outcome assessor)