Are muscle relaxants safe to use during pregnancy?

Safety of Muscle Relaxants During Pregnancy

Muscle relaxants should generally be avoided during pregnancy unless the potential benefits clearly outweigh the possible risks to the fetus, as their safety has not been well established. 1

Types of Muscle Relaxants and Their Safety Profiles

Skeletal Muscle Relaxants

• Metaxalone has not shown evidence of fetal harm in animal studies, but human data are limited. The FDA states it "should not be used in women who are or may become pregnant and particularly during early pregnancy unless, in the judgment of the physician, the potential benefits outweigh the possible hazards." 1
• Non-depolarizing neuromuscular blocking agents (like atracurium, vecuronium, and pancuronium) can cross the placenta in small amounts (7-26% of maternal concentration), potentially causing partial residual curarization in neonates. 2
• Succinylcholine (a depolarizing muscle relaxant) is contraindicated during pregnancy except in specific emergency anesthesia situations. 3

Anesthetic Muscle Relaxants

• Streptomycin, which can interact with muscle relaxants, is explicitly contraindicated in pregnancy due to the risk of fetal hearing loss. 4
• Suxamethonium (succinylcholine) remains the expert choice for rapid-sequence induction in emergency situations during pregnancy, but should be used with caution. 4
• Rocuronium may be an alternative when suxamethonium is contraindicated. 4

Physiological Considerations

• Pregnancy causes significant physiological changes that can affect the pharmacokinetics of muscle relaxants: 3, 2

  • 45% increase in plasma volume
  • 35% increase in blood volume
  • Changes in drug distribution, metabolism, and elimination

• Despite these changes, the clinical duration of atracurium remains unchanged during pregnancy, while vecuronium's duration may be doubled in term and postpartum women. 2

Non-Pharmacological Alternatives

• Physical therapy and appropriate exercise programs should be considered as first-line approaches for musculoskeletal pain during pregnancy. 5
• Activities such as stationary cycling, swimming, or aquatic exercise are safer options with less risk of falling. 5
• Proper body mechanics and ergonomic adjustments can help reduce musculoskeletal strain without medication. 5

Special Considerations

• If muscle relaxation is needed, certain beta-blockers may be safer options in the third trimester compared to dedicated muscle relaxants. 5
• Metoprolol is preferred over atenolol during pregnancy as it has a lower incidence of fetal growth retardation. 5, 6
• Women with active preterm labor, severe pre-eclampsia, or intrauterine growth restriction should avoid both muscle relaxants and strenuous physical activity. 5

Clinical Approach

• Assess the severity and cause of muscle pain to determine if pharmacological intervention is truly necessary. 5
• If medication is deemed necessary, consult with both obstetric and maternal-fetal medicine specialists to weigh risks and benefits. 5
• For pregnant women requiring anesthesia, special attention must be paid to the choice and dosing of muscle relaxants due to altered pharmacokinetics and potential fetal effects. 3

Warning Signs

• If muscle relaxants are used, patients should immediately seek medical attention if they experience: 5
  • Regular and painful uterine contractions
  • Vaginal bleeding
  • Persistent dizziness or faintness
  • Severe chest pain or shortness of breath

Monitoring Recommendations

• If muscle relaxants must be used during pregnancy, careful monitoring of both maternal and fetal status is essential. 3
• For anesthetic muscle relaxants, intraoperative monitoring of neuromuscular blockade is strongly recommended. 4
• Liver function tests should be obtained at baseline and at monthly intervals if there is underlying liver disease. 4