Retrospective review of Tizanidine (Zanaflex) overdose.

BACKGROUND

Tizanidine is a centrally acting muscle relaxant with a novel mechanism of action and structurally related to clonidine. There are no large case series of tizanidine exposure.

METHODS

Retrospective review of all ingestions involving tizanidine reported to a poison control center from January 2000 through February 2003. Exclusion criteria were polydrug ingestion, no follow-up or lost to follow-up.

RESULTS

There were 121 cases of which 45 patients met entrance criteria. Mean age was 32 years (range 1 to 80). Thirty-seven patients were evaluated in a health care facility of which 27 were admitted for medical care. Clinical effects included lethargy (n = 38), bradycardia (n = 14), hypotension (n = 8), agitation (n = 7), confusion (n = 5), vomiting (n = 3), and coma (n = 2). Mean dose ingested by history was 72 mg (S.D. + 86). The lowest dose associated with hypotension was 28mg, which occurred in a 63-year-old female with a BP of 88/52 and a HR of 54. The lowest dose associated with coma was between 60 mg and 120 mg, which occurred in a 30-year-old female with a HR of 30 and BP of 81/48. There were 6 patients < 6 yrs. The lowest dose with bradycardia and drowsiness in a small child was 16 mg in a 2 YO (weight unknown). All other cases in children < 6 yrs involved ingestion of a single tablet (2 or 4 mg) with only mild drowsiness reported. Therapy in this series was primarily supportive and included pressors in 3 cases and intubation in 3 cases. Naloxone was administered to 7 patients. There was no response to naloxone in 5 patients, poor documentation of response in one, and arousal in one patient. All patients recovered without residual complications.

CONCLUSION

Clinical manifestations of tizanidine overdose include alterations of mental status, bradycardia, and hypotension. In this series, outcome was good with supportive therapy.