The challenge of pain management in patients with myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disorder of the neuromuscular junction. The complexity of the disease and its treatments make MG patients particularly susceptible to adverse effects of drugs. MG is not a painful condition; however, as pain management armamentarium includes drugs from diverse pharmacological groups and with potential for drug-drug interactions, managing pain in patients with MG can be challenging. The underlying disease and the concomitant medications of each patient must be considered and the analgesic treatment individualized. This review presents an update on the various aspects of pain pharmacotherapy in patients with MG, focusing primarily on medications used to treat chronic pain. Drugs discussed are opioids, nonsteroidal anti-inflammatory drugs, antidepressants, anticonvulsants, muscle relaxants, benzodiazepines, intravenous magnesium, and local anesthetics. Drug interactions with agents used for MG treatment (acethylcholinesterase inhibitors, corticosteroids, immunosuppressants) and plasmapheresis are discussed. The clinical usefulness and limitations of each of the drug classes and agents are described.

Drug removal by plasmapheresis: an evidence-based review

Contrary to the literature about drug removal during hemodialysis, data regarding drug removal during plasmapheresis are sparse. Over the last 40 years, approximately 70 publications-mostly case reports of overdoses-have described the effects of plasmapheresis on pharmaceutical agents. Important issues are drug extraction during plasma exchange with chemotherapy, as well as drug classes such as antiinfectives, anticoagulants, antiepileptics, cardiovascular agents, and immunosuppressants. Other considerations are the merits and pitfalls of the different methods used in published reports and recommendations for future pharmacokinetic studies in this field.

Removal of vancomycin during plasmapheresis

OBJECTIVE

To examine the removal of vancomycin during plasmapheresis, determine whether drug administration should be withheld prior to or a supplemental dose given after the procedure, and determine whether a redistribution phenomenon in vancomycin serum concentrations occurs after plasmapheresis.

DESIGN

Prospective, cohort study.

SETTING

An 800-bed, tertiary-care, teaching hospital.

PATIENTS

Twelve patients receiving vancomycin as prescribed who were also undergoing therapeutic plasmapheresis.

METHODS

Blood samples for determination of vancomycin concentrations were obtained from each patient immediately before, during, immediately after, and 2 hours after plasmapheresis. Vancomycin concentration in plasma removed by plasmapheresis and volume of plasma removed were measured. Patient-specific pharmacokinetic parameters were determined for each patient using serum concentration data and a one-compartment model. Percent of drug removed by plasmapheresis and percent increase in vancomycin total clearance secondary to plasmapheresis were calculated.

RESULTS

A mean of 6.3% of the total body store of vancomycin was removed by plasmapheresis. Vancomycin clearance during plasmapheresis averaged 1.6 L/h, which was an average increase of 285% in the total clearance of vancomycin from the body. Nine of 10 patients had a higher observed vancomycin concentration 2 hours after plasmapheresis than that predicted by degrading the concentration observed immediately after the procedure, suggesting that redistribution in serum concentrations occurs after the procedure.

CONCLUSIONS

A single one-volume plasmapheresis does not remove a clinically important amount of vancomycin; therefore, supplemental dosing after the procedure is not necessary. A redistribution phenomenon in vancomycin concentrations appears to exist after plasmapheresis. Further study is needed to determine how long the redistribution phase lasts and when vancomycin concentrations should be measured after plasmapheresis.

Zidovudine removal during plasma exchange

Pharmacokinetics of zidovudine (azidothymidine, AZT) were investigated after oral administration (300 mg every 8 hours) in a human immunodeficiency virus seropositive patient who was also treated with plasma exchange (60 ml/kg). Plasma AZT concentrations were measured using high performance liquid chromatography assay. Plasma exchange clearances reached 1-3.8% of the total clearance. The fraction of AZT removed from separated plasma accounted for only 1% of the administered dose. The contribution of plasma exchange to the elimination of AZT appears to be negligible. Solely on the basis of AZT pharmacokinetic data, no particular dose adjustment appears to be necessary in patients treated with multiple plasma exchanges.