Spinal anesthesia is a type of neuraxial regional anesthesia which covers for lower limb and lower abdominal surgeries. However, plain local anesthetics associated with cardiac and neurological toxicities [13, 14]. For reduction of this, local anesthetics were mixed with other drugs, to decrease the dose requirement and potentiation of local anesthetics.
This Meta analysis of randomized control trials showed that 50 µg intrathecal clonidine as an adjuvant provided the prolonged postoperative analgesia compared with 25 µg intrathecal fentanyl as a mean difference of [83.57(95 % CI, 29.33 to 137.82), I2 = 97%, P < 0.00001] minutes. Different studies have been found that both clonidine and fentanyl added to spinal block with bupivacaine are effective for the persistence of postoperative analgesic time. [16, 17]
It was also supported by other different studies which gave the evidence of 50 µg clonidine to intrathecal bupivacaine made the lengthening of analgesic duration in comparison to 25 µg fentanyl with bupivacaine[18-20].
In regarding to secondary outcomes of interest Mata analysis study, the sensory and motor duration of block is significantly prolonged in intrathecal clonidine group compared to fentanyl. This was consistent with many different studies [19, 21, 22]. However, there was not considerable inter sub group differences of block duration between the motor and sensory.
The means of clonidine related potentiation of spinal sensory block is reported to be reliant on presynaptic (decrease transmission) and postsynaptic (hyper polarization) action. It activates the α2 receptors with blocking of Aδ and C nerve fibers at substantia gelatinosa of spinal cord to generate analgesia[23, 24]. Fentanyl is the preferred opioid drug for regional anesthesia with action on µ1 and µ2 receptor agonist. It is highly potent drug due to high lipid solubility[25, 26] despite some related complications of nausea, vomiting, and pruritus .
According to some study, even at higher dose of clonidine (450 µg) only in intrathecal, didn’t result muscle weakness and motor block  , but combination to spinal bupivacaine caused significant enhancement of the strength and duration of motor block[29, 30]. Tilkar et al study compared clonidine and fentanyl added to intrathecal bupivacaine and reached as conclusion of clonidine was more helpful than fentanyl in pain-relieving properties.
Even though different studies have been used intrathecal clonidine from dose of 15 µg to150 µg , the 50 µg intrathecal clonidine was provided the extended postoperative analgesia with minimal side effects compared to fentanyl[32-34].
There are a number of limitations to our Meta analysis. It was possible to miss some studies which satisfied the inclusion criteria, and number of studies to be excluded as the full text was unavailable. In addition, there was significant heterogeneity to duration of analgesia, duration of sensory block and duration of motor block with considering different doses of bupivacaine drug and types of surgery.