Subarachnoid block with hyperbaric bupivacaine is a safe, inexpensive technique and popular mode of anesthesia for lower limb orthopedic surgery. One of the major drawbacks of spinal anesthesia using local anesthetics alone is an early postoperative need for analgesia.

For postoperative pain relief, there are multimodal analgesic methods including systemic opioids or NSAIDS or NMDA receptor antagonist (magnesium) and regional techniques in the form of spinal, epidural, or peripheral nerve blocks, etc. It diminishes autonomic, somatic, and endocrine reflexes that finally result in a remarkable reduction in perioperative morbidity (Shukla et al. 2011).

NMDA receptor antagonist has an important role in the prevention of central sensitization of pain. Magnesium sulphate being a non-competitive NMDA receptor antagonist stimulates peripheral nociceptive and helps in the prevention of central sensitization (Tramer and Glynn 2007; Woolf and Thompson 1991; Woolf and Chong 1993).

The present study was designed to examine the onset and duration of sensory and motor block and the analgesic effects of intrathecal magnesium sulphate (50 mg) as an adjuvant to bupivacaine (15 mg) in spinal anesthesia for lower limb orthopedic surgery.

In our study, we observed prolonged onset and duration of sensory and motor blockade in the BM group with better postoperative analgesia without any untoward effects in comparison to the B group. Haubold and Meltzer (1906) utilized intrathecal magnesium sulphate (1000–2000 mg) in human-produced spinal anesthesia, including motor and sensory blocks for 3.27 h without neurological damage.

Faiz et al. (2012) found that intrathecal MgSO4 when combined with local anesthetic agents is known to potentiate the analgesic effect.

In a meta-analysis published by Ramirez et al. (2013), intrathecal magnesium sulphate doses range from 50 to 100 mg. The most commonly used dose is 50 mg.

Paul et al. (2009) showed that adding 50 mg MgSO4 intrathecally in patients undergoing lower limb surgeries delayed the onset of the sensory block as well as the time to reach peak sensory block in the magnesium group (6.65 ± 1.08 min, 19.26 ± 4.41 min) than the control group (5.2 ± 1.21 min, 14.83 ± 3.46 min) significantly, p < 0.001.

Shukla et al. (2011) concluded that the addition of 50 mg intrathecal MgSO4 prolonged the onset of sensory block and motor block in the magnesium group (6.46 ± 1.33 and 7.18 ± 1.38 min) as compared to the control group 4.14 ± 1.06 and 4.81 ± 1.03 min) in lower abdominal surgeries.

The authors of these studies suggested that delayed onset may be due to differences in the pH and baricity of the solution containing MgSO4 (Shukla et al. 2011; Paul et al. 2009). Another study has provided the reason for delayed onset due to the activation of cytochrome P450 (CYP) by magnesium which increases the metabolism of bupivacaine (Morrison et al. 2013). In some studies, it is observed that sensory and motor blockade onset was directly dependent on the dose of magnesium sulphate used (Chaudhary et al. 2016; Prabhavathi et al. 2017).

However, Limbu et al. (2017) concluded that the addition of 75 mg MgSO4 had no effect onset of sensory or motor block which is contradictory to our study results.

In addition, intrathecal magnesium sulphate (50–100 mg) along with bupivacaine provided the extended duration of the sensory and motor blockade to a significant level (Prabhavathi et al. 2017; Hemalatha et al. 2017; Kathuria et al. 2014).

Shah et al. (2016) and Kahraman et al. (2014) found that intravenous infusion of magnesium sulphate under spinal anesthesia prolongs sensory block duration.

However, Khezri et al. (2012) and Limbu et al. (2017) concluded that the addition of 50 mg MgSO4 intrathecally was no difference in sensory and motor block duration which is not in accordance with our study.

In some studies, it is found that the addition of 50 mg magnesium to hyperbaric bupivacaine (12.5 mg) with fentanyl (25 μg) for spinal anesthesia for infraumbilical surgeries results in a significantly delay of the onset of the sensory and motor block with prolonged analgesia, less score of VAS, without significant hemodynamic changes and side effects (Sen et al. 2020; Singh and Sen 2019).

Our results were partially comparable with Rashad and El-Hefnawy (2015) found that the use of magnesium sulphate as an adjunct in spinal anesthesia was associated with increased duration of postoperative analgesia and decreased amounts of postoperative opioid use (6.95 ± 1.19, 6.25 ± 2.22 vs. 5.60 ± 0.94, 9.50 ± 2.76, respectively).

In our study, the addition of intrathecal magnesium to bupivacaine decreased the VAS scores of the group in which magnesium was not used.

Hemalatha et al. (2017) found that higher doses of intrathecal magnesium reported bradycardia and Morrison et al. (2013) found the risk of respiratory depression.

However, Kathuria et al. (2014) showed that 75 mg of intrathecal magnesium was used, without any hemodynamic variability and any adverse effects.

There is a study of the use of intrathecal magnesium sulphate in rats observing neurodegeneration based on electron microscopic examination (Ozdogan et al. 2013).

In our study, we used 50 mg of intrathecal magnesium for better perioperative hemodynamic stability and no adverse effects.

Our findings are limited to bupivacaine only, and future studies are needed for some other local anesthetic drugs.