Rhabdomyolysis: a manifestation of cyclobenzaprine toxicity

Cyclobenzaprine (flexeril) after its synthesis in 1961 was found to have limited antidepressant action with no significant advantage over other tricyclic antidepressants[1]. However it was found to act as a centrally acting muscle relaxant and has been widely used ever since. Muscle relaxants account for approximately 18.5% of all prescriptions written for chronic back pain in the United States[2].We present this case as an example of the range of cyclobenzaprine toxicity and underline the treatment protocols used.A 33 year old male was brought into the emergency room after reportedly ingesting approximately 30 pills of 10 mg each of cyclobenzaprine (flexeril) about 2 hours prior in attempt to commit suicide. Vital signs on admission were 148/88 mm Hg 110/min 37.5 C On initial presentation he was disoriented to time and place and person, the patient was extremely agitated and confused. Airway was well maintained and he did not require intubation. In the emergency room gastric decontamination was achieved with administration of ipecac with resultant emesis but with only 4 pills returning. Thereafter the insertion of orogastric tube was performed and gastric lavage with normal saline was performed till clear return occurred and approximately 7 more pills returned. Thereafter 30 gm of activated charcoal was administered through the tube.Diagnostic studies were as follows: Hb, 14.6 gm%, Hct, 43.8% WBC 15, 300 with 78% polymorphonuclear leucocytes. Arterial blood gas was as follows PH 7.32 PO2 130 mmHg PCO2 45 mmHg. Chem 10 was unremarkable on presentation, liver function testing revealed elevated ALT of 39 U/L and AST of 30 U/L both normal. The AST peaked to 298 U/L about 12 hours later and trended towards normal over the course of the next few days. Urine toxicity screen was negative, a qualitative screen was positive for cyclobenzaprine a quantitative screen was not available. Creatine Kinase was elevated on admission at 11,963 U/L peaked to 29,840 U/L in 24 hours and