This study will be conducted to compare the efficacy of fentanyl and Dexmedetomidine as an additive to hyperbaric bupivacaine 0.5% in saddle block anesthesia on duration of analgesia in patients undergoing anorectal surgeries.

Pilonidal sinus, hemorrhoidectomy, anal fistula or fissure repair are the main types of anorectal surgery. Various surgical and anesthetic techniques have been introduced to improve analgesia level and limit the motor blockade.Regional anesthesia is preferred for anorectal surgeries to avoid the risks of general anesthesia. Besides, providing effective post-operative analgesia, regional techniques reduce the opioid use.

Intrathecal administration of drugs to obtain anesthesia has been in use since the turn of the twelfth century. Saddle block anesthesia is a kind of low spinal block that provides anesthesia over the saddle area, i.e., perineum, perianal area, medial aspect of legs and thigh.

Saddle block anesthesia is performed by injecting a small dose of hyperbaric local anaesthetic at the level of L3, L4 after which the patient is maintained in sitting position for a few minutes to facilitate preferential impregnation of sacred roots (S1 to S5), which is responsible for innervation of perineum, external genitalia and anus. The saddle block causes a parasympathetic blockade at the bladder level which may result in bladder and rectal atony which is advantageous because of sphincteric relaxation needed for the operation.

Hyperbaric bupivacaine 0.5% is the most common local anaesthetic used for spinal anaesthesia for lower abdominal and lower limb surgeries. One disadvantage with using hyperbaric bupivacaine alone is a relatively shorter duration of action that means that early analgesic intervention is needed in postoperative period. Adjuvants are drugs that increase the efficacy or potency of other drugs when given concurrently. Neuraxial adjuvants are used to increase the speed of the onset of neural blockade (reduce latency), improve the quality and prolong the duration of neural blockade.

Fentanyl is a lipophilic short-acting opioid that exhibits close structural similarities to local anesthesia and has demonstrable local anesthetic efficacy on sensory C primary afferent nerve fibers, which may facilitate analgesic effects. It has improved spinal anesthesia and reduced the anesthetic drug related side effects including pruritus, nausea and vomiting.

Dexmedetomidine, a highly selective alpha_2 agonist, is being introduced as an adjuvant to local anesthetics with significant analgesic, sympatholytic, and sedative properties. Compared to clonidine; Dexmedetomidine is approximately eight times more selective towards α2-adrenergic receptors , which is associated with sedative and analgesic effects in supraspinal and spinal sites and also has an antinociceptive impact on both visceral and somatic pain.

This study will be conducted to compare the efficacy of fentanyl and Dexmedetomidine as an additive to hyperbaric bupivacaine 0.5% in saddle block anesthesia on duration of analgesia in patients undergoing anorectal surgeries.

Inclusion Criteria:

• aged from 18 to 65 years
• American Society of Anesthesiologists' physical I or II of both sexes
• Patients who will be scheduled to surgery for their uncomplicated hemorrhoids, anal fissure, or anal fistula in lithotomy position will be included in the study.

Exclusion Criteria:

• Infection focus at back, history of spine surgery, coagulopathy, mental disturbance, and neurological disease.
• drug allergy.
• intake of experimental or analgesic medication within last 24 hours.
• patients receiving alpha-adrenergic antagonist will be excluded from the study.

Patients, anesthesia administrator, and outcome assessors will be blinded to the group allocation.