Magnesium Sulfate Fails to Relieve Cesarean Pain


SCOTTSDALE, ARIZ. — Perioperative use of magnesium sulfate during cesarean delivery did not relieve short- or long-term pain for women in a double-blind, randomized controlled trial of 120 women.

Neither high nor low doses produced any benefit in pain control or satisfaction compared with saline solution in the intent-to-treat analysis.

Other than the expected variation in serum concentrations of magnesium, the only significant differences were slightly greater blood loss and longer time to solid foods for the women given magnesium sulfate, study investigator Everett F. Magann, M.D., reported at the annual meeting of the Central Association of Obstetricians and Gynecologists.

“Given the absence of any apparent analgesic benefit and the apparent increase in blood loss, we do not believe that the perioperative administration is clinically useful,” said Dr. Magann, a captain in the U.S. Naval Reserve who currently practices at Naval Medical Center Portsmouth (Va.).

The investigators wanted to test magnesium sulfate because it has been used successfully to control pain from cancer and some surgical procedures in other fields of medicine, according to Dr. Magann. He said they had found one abstract reporting that preeclamptic women undergoing cesarean deliveries had less pain than did matched controls.

“The use of magnesium is very familiar to obstetricians,” he said. “It is used in pregnancies complicated by preeclampsia and in pregnancies complicated by preterm labor to delay delivery until corticosteroids have been administered to accelerate fetal lung maturity and lessen neonatal morbidity.”

Magnesium is believed to alter pain processing, because it acts as an antagonist to receptors in the spinal cord.

“There is experimental evidence that magnesium may modulate acute pain [and] reduce postsurgical pain intensity and/or the dosage of analgesics,” Dr. Magann said.

He conducted the study at King Edward Memorial Hospital for Women, Perth, Western Australia, with colleagues from the hospital and the University of Western Australia.

From October 2002 to June 2004, they enrolled 131 women aged 18 years or older who were undergoing a planned cesarean delivery of a single infant, had no contraindication to magnesium sulfate, and consented to a combined spinal-epidural anesthetic.

Patients and health care providers were blinded to the randomization of the women. Eleven women withdrew consent or did not proceed to cesarean delivery. Although 15 women did not receive a full 24-hour infusion of magnesium sulfate and 9 did not adhere to analgesic protocol, these patients were included in the intent-to-treat analysis.

The high-dose group of 42 patients was given 50 mg/kg of magnesium sulfate an hour before surgery and 2 g/hour afterward.

A low-dose cohort of 38 women received a 25 mg/kg loading dose followed by 1 g/hour after surgery.

Forty women in a control group received a saline solution.

There were no significant differences between groups in baseline characteristics such as age and weight.

Although none of the women had serious complications requiring transfusions, Dr. Magann reported blood loss as 400 mL for the control group, 475 mL for the low-dose patients, and 500 mL in women at the higher dose.

Time to solid foods was 3 hours in the control group and 6 hours in the two cohorts on magnesium sulfate.

At 6 weeks, none of the women were using analgesic drugs. They also did not have any wound pain with movement or pressure.

Discussant Paul Ogburn, M.D., said the paper answered “fairly definitively no” to the question of whether magnesium sulfate would relieve cesarean pain.

Dr. Ogburn, director of maternal-fetal medicine at the State University of New York at Stony Brook, also suggested that the incidental findings of increased blood loss and delay until solid food is consumed may be important clinically.

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