What should be done if a patient experiences muscle twitching while taking terbutaline (beta-2 agonist)?

Management of Muscle Twitching with Terbutaline

Muscle twitching (fasciculations) from terbutaline is a common, dose-dependent beta-2 adrenergic side effect that typically does not require discontinuation of the medication unless severe or accompanied by cardiovascular symptoms. 1, 2

Understanding the Mechanism

Terbutaline causes muscle twitching through beta-adrenergic receptor activation and the β-adrenergic receptor/cAMP/PKA pathway, which increases intracellular calcium cycling and produces rhythmic muscle contractions—this is mechanistically distinct from true fasciculations caused by cholinesterase inhibitors. 1 The effect is mediated by alterations in calcium transient amplitude and decay rates in muscle fibers, primarily through phosphorylation of the ryanodine receptor and phospholamban. 3

Clinical Assessment

Evaluate the severity and associated symptoms:

• Mild isolated twitching: Tremor affecting hands or limbs without functional impairment 4
• Moderate twitching: Noticeable tremor with some functional limitation 1
• Severe symptoms requiring intervention: Twitching accompanied by tachycardia >25 beats/minute above baseline, palpitations, chest pain, severe hypertension or hypotension, arrhythmias, or seizures 5, 6

The incidence of tremor with terbutaline can reach 38% at standard doses, making it a very common side effect. 1

Route-Specific Considerations

The route of administration dramatically affects the severity of muscle twitching:

• Inhaled terbutaline produces bronchodilation with minimal systemic exposure and significantly reduced tremor risk compared to parenteral routes 1, 4
• Intravenous terbutaline produces more pronounced tremor and doubles the tremor intensity compared to therapeutic oral doses 4
• Inhaled administration at therapeutic doses (2.5-5 mg nebulized or equivalent) produces equivalent bronchodilation without affecting heart rate, blood pressure, or causing tremor 4

Management Algorithm

For Mild to Moderate Twitching:

Reassure the patient that this is an expected pharmacologic effect that does not indicate toxicity or danger. 1, 2

• Continue terbutaline at the current dose if asthma control is adequate 7
• Consider switching from systemic (oral/IV) to inhaled route if not already using inhalation 1, 4
• Reduce to the minimum effective dose—for acute asthma, this is 2.5-5 mg nebulized salbutamol equivalent or 5-10 mg terbutaline 7
• Avoid supratherapeutic dosing, as doses exceeding recommended levels more than double tremor intensity without proportional bronchodilator benefit 4

For Severe Symptoms or Signs of Overdose:

Discontinue terbutaline immediately if the patient exhibits seizures, angina, severe tachycardia (>200 bpm), arrhythmias, severe hypertension or hypotension, or altered mental status. 5

• Provide supportive symptomatic therapy 5
• Monitor for hypokalemia, which can occur with beta-agonist overdose 5
• Consider judicious use of a cardioselective beta-blocker (e.g., metoprolol) for severe cardiovascular symptoms, but use extreme caution as this can precipitate bronchospasm 5
• There is no specific antidote for terbutaline overdose 5
• Dialysis has insufficient evidence to support its use 5

Transition Strategy:

Switch to short-acting beta-agonists via metered-dose inhaler with spacer once the acute exacerbation stabilizes, as this permits better dose control and reduced systemic effects. 7

• For ongoing asthma control, ensure the patient is on appropriate inhaled corticosteroid therapy rather than relying on frequent beta-agonist use 7
• If beta-agonist use exceeds 2 days per week for symptom relief (excluding exercise-induced bronchospasm prevention), this indicates inadequate asthma control requiring intensification of anti-inflammatory therapy 7

Important Clinical Caveats

Do not confuse beta-agonist-induced tremor with true fasciculations from neuromuscular disease or cholinesterase inhibitor toxicity—the mechanisms are entirely different, with beta-agonists causing rhythmic tremor through adrenergic stimulation rather than spontaneous motor unit depolarization. 1

Avoid regular scheduled daily chronic use of short-acting beta-agonists like terbutaline, as this pattern suggests poor asthma control and necessitates escalation of controller medications rather than increased bronchodilator use. 7

The cardiovascular effects (tachycardia, increased systolic blood pressure, decreased diastolic pressure, decreased serum potassium) occur alongside tremor and should be monitored, particularly with IV administration where heart rate increases of 25+ beats per minute are common. 6