SGEM#489: Smooth Muscle Relaxator – But does Magnesium Work for Renal Colic?

Date: October 10, 2025 Guest Skeptic: Dr. Sergey Motov is an Emergency Physician in the Department of Emergency Medicine, Maimonides Medical Center in New York City. He is also one of the world’s leading researchers on pain management in the emergency department.  Case: A 37-year-old man presents to the emergency department (ED) with severe right-sided flank pain.  The pain started about eight hours ago as a vague discomfort in his right flank, but it has gotten progressively worse and now is radiating to his groin. Patient reports nausea, an increased urge to urinate and noticing blood in his urine on one occasion. The patient denies prior medical or surgical history. Upon ED arrival, his vital signs are normal. Physical examination revealed a stated age patient in distress due to severe right flank pain, prominent right-sided costovertebral angle tenderness, and absence of abdominal tenderness or guarding. While strongly considering renal colic in differential diagnosis and reaching for the bedside ultrasound, you are wondering if a single dose of a non-steroidal anti-inflammatory (NSAID) will be enough to relieve this patient’s pain, or should you add Magnesium or Lidocaine? Background: Renal colic is a common and extremely painful emergency department (ED) complaint encountered in the ED that frequently recurs. The nonsteroidal anti-inflammatory drugs (NSAIDs) given intravenously or intramuscularly (IM) are frequently used as first-line therapy. However, about 30% of ED patients receiving NSAIDS require rescue analgesia in the form of opioids. Opioid use, though effective, is limited at times due to the potentially dangerous adverse effects. Thus, there might be a role for other non-opioid classes of drugs to be co-administered with NSAIDs for relief of renal colic. Magnesium sulfate (MgSO₄) has been suggested as a possible treatment option. It may blunt ureteral smooth muscle spasm by antagonizing calcium influx in smooth muscle and by N‑methyl‑D‑aspartate (NMDA) receptor antagonism. These are mechanisms that can reduce visceral pain and augment other analgesics. Small ED trials and meta‑analyses suggest MgSO₄ can reduce pain scores and opioid use in renal colic, though the evidence base has been limited and heterogeneous [1]. Another suggested treatment modality for renal colic is intravenous lidocaine. We looked at this treatment on SGEM#202 and were unimpressed with the efficacy. Systemic lidocaine blocks voltage‑gated sodium channels and appears to modulate central sensitization and visceral pain pathways. In ED populations, systematic reviews indicate IV lidocaine offers variable analgesia with a mixed signal for benefit, and renal colic–specific RCTs suggest it may be inferior to ketorolac and best considered (if at all) as part of a multimodal strategy rather than as monotherapy [2]. Clinical Question: In adult ED patients with suspected renal colic receiving IM diclofenac, does adding IV magnesium sulfate or IV lidocaine increase the proportion achieving ≥50% reduction in pain at 30 minutes compared to a saline placebo? Reference: Toumia M, Sassi S, Dhaoui R, et al. Magnesium Sulfate Versus Lidocaine as an Adjunct for Renal Colic in the Emergency Department: A Randomized, Double-Blind Controlled Trial. Ann Emerg Med 2024 Population: The study enrolled adults aged 18 to 65 years with suspected acute renal colic and a pain score of 5 or more on a 10-point numerical rating scale (NRS). Exclusions: Pregnancy/breastfeeding; NSAID, MgSO₄, or lidocaine contraindication or allergy; renal/hepatic dysfunction; analgesic use in prior 6 h; bleeding diathesis or GI hemorrhage; significant CAD/arrhythmia; seizures; peritoneal signs; altered mental status; anticoagulation; hemodynamic instability; morphine allergy. Intervention: All patients received 75 mg IM diclofenac. The intervention groups then received either 1g IV MgSO₄ (10 mL) over 2-4 minutes or 1.5 mg/kg IV lidocaine (10 mL) over 2-4 minutes. Comparison: Placebo(10 mL normal saline) after 75 mg IM diclofenac. Outcome: Primary Outcome: The proportion of participants achieving at least a 50% reduction in the NRS score at 30 minutes after drug administration. Secondary Outcomes: Need for rescue analgesia, time required for 50% pain reduction, proportion of participants with persistent pain (NRS>2) at 90 minutes, frequency of adverse events, and frequency of return visits to the ED for renal colic recurrence. Type of Study: Prospective, multicenter, randomized, double‑blind, placebo‑controlled, 3-arm trial conducted from November 2022 to August 2023 in three academic hospital EDs and one regional hospital ED in Tunisia. Authors’ Conclusions: “Adding intravenous MgSO4, but not lidocaine, to IM diclofenac offered superior pain relief but at levels below accepted thresholds for clinical importance.”  Quality Checklist for Randomized Clinical Trials: The study population included or focused on those in the emergency department. Yes The patients were adequately randomized. Yes  The randomization process was concealed. Yes The patients were analyzed in the groups to which they were randomized. Yes The study patients were recruited consecutively (i.e. no selection bias). Unsure The patients in both groups were similar with respect to prognostic factors. Yes  All participants (patients, clinicians, outcome assessors) were unaware of group allocation. Yes All groups were treated equally except for the intervention. Yes Follow-up was complete (i.e. at least 80% for both groups). Yes All patient-important outcomes were considered. Yes The treatment effect was large enough and precise enough to be clinically significant. No  Financial conflicts of interest. This study was conducted without financial support. The authors have no conflict of interest relevant to this article to disclose. Results: They screened 1,321 patients and included 840 who were randomized (280 per arm). The mean age is in the mid 40s with a fairly even male/female split. The mean baseline NRS ~8.5–8.7. Ultrasound showed stones in ~20% and pyelocaliceal dilation in over one-third of patients. Key Result: Adding IV MgSO₄ to IM diclofenac increased the proportion of patients achieving ≥50% pain reduction at 30 min and reduced rescue analgesia use, but pain score differences never reached the 1.3‑point MCID, while IV lidocaine was not superior to diclofenac alone. Primary Outcome: ≥50% NRS reduction at 30 minutes MgSO₄ + diclofenac: 227/280 (81.7%) Lidocaine + diclofenac: 204/280 (72.9%) Placebo + diclofenac: 201/280 (71.8%) MgSO₄ vs lidocaine +8.8% (95% CI 1.89–15.7, p=0.013) MgSO₄ vs control +9.9% (95% CI 2.95–16.84, p=0.004). Secondary Outcomes: Rescue analgesia: Lower need in MgSO₄ group (17.1%) versus lidocaine (22.5%) and control (28.9%). Time to 50% pain reduction: No significant differences between groups (approximately 21 minutes for all groups). Persistent pain at 90 minutes: Similar across groups (20-22.1% with NRS>2). Return ED visits: No differences (22-26%) Adverse events: Significantly more in MgSO₄ group (57.1%) versus control (22.5%), primarily facial flushing (48% MgSO₄ vs 10% Lido vs 14% placebo) Statistical vs. Clinical Significance: This study exemplifies a critical issue in clinical research: achieving statistical significance without clinical importance. While the primary outcome showed statistical significance for MgSO₄ lidocaine and even more so for control, the actual NRS differences were all below 0.5 points at every time point. In addition, no between-group differences reached the generally accepted reduction of 1.3 for being clinically important [3,4]. There are concerns in the literature about what is considered a minimally important difference (MID) or minimally clinically important difference (MCID) [5, 6]. Choice of Primary Outcome Metric: The primary outcome was the proportion of patients achieving ≥50% pain reduction rather than the actual change in pain scores. This dichotomization of a continuous variable has several problems: Loss of Information: Converting continuous NRS data to a binary outcome discards valuable information about the magnitude of pain relief. For example, a patient going from NRS 10 to 5 (50% reduction) is counted the same as a patient going from 10 to 2 (80% reduction), yet these represent vastly different clinical scenarios. Arbitrary Threshold: The 50% reduction threshold, while commonly used, may not represent meaningful relief for all patients. For example, some patients with NRS 10→5 may still require rescue analgesia, while others with NRS 10→6 (40% reduction) might be satisfied. Diagnostic Verification of Kidney Stone: The diagnosis of renal colic was based on emergency physician clinical judgment or imaging with ultrasound showing direct or indirect signs of urinary tract stones in only 58% of participants when performed. This is consistent with current radiology, emergency medicine and urology guidelines not to get imaging on all patients [7].  We cover this on the SGEM Xtra: Come Together, Right Now...Over Renal Colic. While this approach may increase real-world applicability (generalizability), the “suspected renal colic” inclusion without universal confirmatory imaging may reduce internal validity and introduce misclassification bias. A confirmed stone sensitivity analysis would have helped strengthen the findings. Adjunct to IM Rather Than IV NSAID: While acknowledging and respecting the authors’ standard of care practice of using IM Diclofenac, there are several important implications: Delayed NSAID Absorption: IM administration results in slower and more variable absorption compared to IV administration.