A 45-year-old female presented to the ED after an intentional overdose of naproxen and an unknown amount of a muscle relaxant, experiencing CNS depression and bradycardia. She remains bradycardic and has developed hypotension despite fluid boluses.
Vital Signs:
Initial BP: 93/70 mmHg, HR: 45 bpm, O2 sat 99% on room air
Latest BP: 81/49 mmHg, HR: 35 bpm, Resp Rate: 16 bpm, Pulse Ox 98% on room air
ECG:
Rate: 35 bpm, QRS: 82 msec, QTc: 561 msec (prolonged), Sinus Bradycardia
Labs:
Na+: 135 mEq/L, K+: 4.3 mEq/L, Cl-: 114 mEq/L, CO₂: 17 mEq/L, BUN: 28 mg/dL, Cr: 0.83 mg/dL, AG: 4 mEq/L, Ca++: 8.1 mg/dL
Which of the following muscle relaxants could potentially explain her symptoms? Select ONE answer that best explains her symptoms.
A) Tizanidine
B) Cyclobenzaprine
C) Methocarbamol
D) Carisoprodol
Reveal the answer
Correct Answer: A) Tizanidine
In cases of muscle relaxant overdose presenting with CNS depression and bradycardia, along with hypotension in overdose, the effects align most closely with Tizanidine overdose. Here’s an in-depth look at why Tizanidine is the most plausible explanation for this patient’s symptoms and why the other options are less likely to be responsible.
Why Tizanidine is the Likely Cause of CNS Depression and Bradycardia?
Tizanidine Overdose Symptoms and Clinical Presentation
Tizanidine is an α-2 adrenergic agonist primarily acting through stimulation of α-2 adrenergic receptors in the central nervous system. This mechanism decreases sympathetic outflow, suppresses CNS activity, and reduces heart rate and blood pressure. Tizanidine overdose symptoms are well-documented to include sedation (CNS depression), bradycardia, and hypotension, all of which are present in this patient’s clinical presentation, making Tizanidine the most plausible explanation.
Overdose Profile
In overdose cases, Tizanidine can cause the following:
Profound CNS depression: Leads to drowsiness, sedation, and even respiratory depression.
Bradycardia: Reduced sympathetic tone results in a slower heart rate.
Hypotension: Caused by decreased sympathetic output and peripheral vasodilation due to α-2 receptor stimulation.
These effects align closely with the patient’s presentation of CNS depression, bradycardia, and hypotension. This clinical presentation of Tizanidine overdose is consistent with the symptoms observed, supporting Tizanidine as the likely muscle relaxant involved.
Why Other Muscle Relaxants Are Unlikely to Cause This Presentation?
1. Cyclobenzaprine: Overdose Profile and Differences from Tizanidine
Mechanism: Cyclobenzaprine is similar to tricyclic antidepressants (TCAs) and has both anticholinergic and serotonergic effects. It primarily works by reducing muscle spasms via action on the brainstem.
Overdose Profile: Cyclobenzaprine overdose often leads to CNS depression, but it typically causes tachycardia (not bradycardia) and other anticholinergic effects, such as dry mouth, urinary retention, and constipation. Its anticholinergic properties usually counteract bradycardia.
Conclusion: Cyclobenzaprine is unlikely to be responsible for this presentation because it typically produces tachycardia, not bradycardia.
2. Methocarbamol: Mechanism and Overdose Symptoms
1. Mechanism: Methocarbamol is a CNS depressant with sedative properties, though its exact mechanism is not fully understood.
It primarily affects CNS pathways that relax muscles, with minimal direct cardiovascular effects.
2. Overdose Profile: In overdose, methocarbamol generally causes CNS depression (drowsiness, dizziness) but rarely results in significant cardiovascular effects such as bradycardia or hypotension.
Conclusion: Methocarbamol is less likely because it does not typically cause bradycardia or hypotension, making it a poor match for the patient’s symptoms.
3. Carisoprodol Overdose: CNS Depression Without Bradycardia
1. Mechanism: Carisoprodol is metabolized into meprobamate, which has sedative and anxiolytic properties similar to barbiturates, causing muscle relaxation through CNS depression.
Overdose Profile: Carisoprodol overdose can lead to severe CNS and respiratory depression. However, bradycardia and hypotension are not commonly associated with its toxic effects.
Conclusion: While Carisoprodol could explain CNS depression, it is unlikely to cause the combination of bradycardia and hypotension seen in this patient.
Conclusion: Identifying the Muscle Relaxant Responsible for CNS Depression and Bradycardia
The clinical presentation of CNS depression, bradycardia, and hypotension is most consistent with Tizanidine overdose due to its α-2 adrenergic agonist activity, which directly reduces sympathetic tone and affects both heart rate and blood pressure. Other muscle relaxants do not typically produce this specific combination of symptoms based on their pharmacologic profiles and common overdose effects, making Tizanidine the best answer for identifying the muscle relaxant responsible for the patient’s clinical presentation.
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Authors:
Bio:
Dr. Omid Mehrpour is a distinguished medical toxicologist known for his extensive clinical and research expertise. He focuses on understanding and treating toxic exposures. Renowned for his ability to diagnose and manage poisoning cases, Dr. Mehrpour has authored numerous impactful publications and is dedicated to educating future medical toxicologists. His innovative approach and commitment to patient care make him a leading figure in medical toxicology.
References:
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