Symptomatic Bradycardia Probably Due to Tizanidine Hydrochloride in a Chronic Hemodialysis Patient

Potassium wasting nephropathy in the setting of tizanidine overdose: a case report

Background

Hypokalemia has been rarely attributed to tizanidine, although the precise mechanism is unclear. Severe hypokalemia is a well-established cause of abnormalities involving cardiac conduction. Given this agent’s well-known cardiac arrhythmogenic potential, awareness of potential concomitant electrolyte abnormalities is important.

Case presentation

Electrolyte disorders, including hypokalemia, are rare complications of the antispasmodic medicine tizanidine when taken in doses as outlined by the manufacturer’s prescribing instructions. Although cases of severe hypokalemia have also been described in the literature in association with this agent, such reports are few. We report a Caucasian case of an intentional overdose involving a very large ingestion of tizanidine. In addition to the characteristic abnormalities on the electrocardiogram, our patient developed electrolyte derangements as well as self-limited acute kidney injury. These biochemical abnormalities included profound hypokalemia that was refractory to aggressive replacement over the ensuing several days, before eventually dissipating. A thorough assessment of the etiology of this hypokalemia was consistent with a defect in renal potassium handling.

Conclusion

In our patient with intentional tizanidine overdose, severe and refractory hypokalemia appears to have been due to a transient potassium wasting nephropathy.

Tizanidine (Zanaflex): a muscle relaxant that may prolong the QT interval by blocking IKr

BACKGROUND

Tizanidine (Zanaflex) is a centrally acting imidazoline muscle relaxant that is structurally similar to clonidine (α(2)-adrenergic agonist) but not to other myorelaxants such as baclofen or benzodiazepines. Interestingly, cardiac arrhythmias and QT interval prolongation have been reported with tizanidine.

OBJECTIVE

To evaluate the effects of tizanidine on cardiac ventricular repolarization.

METHODS

(1) Whole-cell patch-clamp experiments: HERG- or KCNQ1+KCNE1-transfected cells were exposed to tizanidine 0.1-100 µmol/L (n = 29 cells, total) to assess drug effect on the rapid (I(Kr)) and slow (I(Ks)) components of the delayed rectifier potassium current. (2) Langendorff retroperfusion experiments: isolated hearts from male Hartley guinea pigs (n = 6) were exposed to tizanidine 1 µmol/L to assess drug-induced prolongation of monophasic action potential duration measured at 90% repolarization (MAPD(90)). (3) In vivo wireless cardiac telemetry experiments: guinea pigs (n = 6) implanted with radio transmitters were injected a single intraperitoneal (ip) dose of tizanidine 0.25 mg/kg and 24 hours electrocardiography (ECG) recordings were made.

RESULTS

(1) Patch-clamp experiments revealed an estimated IC(50) for tizanidine on I(Kr) above 100 µmol/L. Moreover, tizanidine 1 µmol/L had hardly any effect on I(Ks) (5.23% ± 4.54% inhibition, n = 5 cells). (2) While pacing the hearts at stimulation cycle lengths of 200 or 250 ms, tizanidine 1 µmol/L prolonged MAPD(90) by 8.22 ± 2.03 (6.7%) and 11.70 ± 3.08 ms (8.5%), respectively (both P < .05 vs baseline). (3) Tizanidine 0.25 mg/kg ip caused a maximal 11.93 ± 1.49 ms prolongation of corrected QT interval (QTc), 90 minutes after injection.

CONCLUSION

Tizanidine prolongs the QT interval by blocking I(Kr). Patients could be at risk of cardiac proarrhythmia during impaired drug elimination, such as in case of CYP1A2 inhibition during drug interactions.

Pediatric safety of tizanidine: clinical adverse event database and retrospective chart assessment

BACKGROUND

Tizanidine is an imidazoline with central alpha(2)-adrenoceptor agonist activity at both spinal and supraspinal levels, which is indicated as a short-acting drug for the management of spasticity. Despite being used in pediatric populations, there is no adequate information or well controlled studies to document the safety and efficacy of tizanidine in this group.

OBJECTIVE

To evaluate the safety of tizanidine in the pediatric population. We compared spontaneous adverse event reports in the Acorda Therapeutics worldwide clinical adverse event database for children (< or = 16 years; n = 99) and adults (>16 years; n = 1153) who had received tizanidine and for whom at least one adverse event was reported, and performed a retrospective chart review of the safety of tizanidine in children (< or = 16 years; n = 76) at a large US pediatric neurology practice. Causality of adverse events in our worldwide clinical adverse event database were neither assessed nor assigned by the company.

RESULTS

When adverse events from the clinical adverse event database were collapsed into the 25 Medical Dictionary for Regulatory Activities (MedDRA; version 9.0) organ system classes, five classes were more frequent in adults (general disorders and administration site conditions [p = 0.0006], hepatobiliary disorders [p = 0.0031], nervous system disorders [p = 0.0108], skin and subcutaneous disorders [p = 0.0063], and vascular disorders [p = 0.0029]), while one class was more frequent in children (psychiatric disorders [p < 0.0001]). The most common adverse event classes in children were psychiatric disorders (52.5%) followed by nervous system disorders (29.3%), and gastrointestinal disorders (16.2%), whereas the most common adverse event classes in adults were nervous system disorders (42.4%), general disorders and administration site conditions (28.6%), and gastrointestinal disorders (21.3%). Serious adverse events were substantially less frequent in children than adults (19.2% vs 45.9%) in the clinical adverse event database. In the pediatric practice chart review, the incidence of adverse events in the MedDRA psychiatric disorders class was very similar (52.6%) to that for children in the clinical adverse event database, while the next most common classes were gastrointestinal disorders (14.5%), and nervous system disorders (13.2%). There were three deaths in children across the databases, including one from accidental exposure and two from cardiac events; the relationship of cardiac events in relation to tizanidine or other causes was difficult to assess with the limited available information.The major causes of death in adults were related to suicide or overdose. Minor, transient liver transaminase increases were occasionally reported; the effect of tizanidine could not be ruled out.

CONCLUSION

The overall safety of tizanidine in the pediatric group appeared good; however, the adverse event profile differed from that in adults. This difference most likely reflects the off-label use of tizanidine as adjunctive treatment for attention disorders and autism. The frequency and nature of adverse events in adults were consistent with the tizanidine prescribing information as reported for its approved indication, i.e. management of spasticity.