What is the Tensilon (edrophonium chloride) test used to diagnose in patients with suspected myasthenia gravis?

The Tensilon Test for Myasthenia Gravis Diagnosis

The Tensilon (edrophonium chloride) test is a rapid diagnostic procedure used to confirm myasthenia gravis by temporarily improving muscle weakness through acetylcholinesterase inhibition, with 95% sensitivity for generalized MG and 86% for ocular MG, though it requires experienced practitioners with atropine immediately available due to potentially life-threatening muscarinic side effects. 1, 2

What the Test Measures

The Tensilon test exploits the pathophysiology of myasthenia gravis by temporarily increasing acetylcholine availability at the neuromuscular junction. 2 Edrophonium is a short-acting reversible acetylcholinesterase inhibitor that prevents breakdown of acetylcholine, allowing more neurotransmitter to bind to the reduced number of available receptors in myasthenic patients. 2, 3

Standard Testing Protocol

Adult Dosing (Intravenous)

• Prepare a tuberculin syringe with 1 mL (10 mg) of edrophonium with an intravenous needle in place. 2
• Inject 0.2 mL (2 mg) intravenously within 15-30 seconds and leave the needle in situ. 2
• Wait 45 seconds to observe for response or adverse reactions. 2
• If no reaction occurs after 45 seconds, inject the remaining 0.8 mL (8 mg). 2
• If cholinergic reaction (muscarinic side effects, fasciculations, increased weakness) occurs after the initial 2 mg dose, discontinue the test immediately and administer atropine sulfate 0.4-0.5 mg intravenously. 2

Pediatric Dosing

• Children ≤75 lbs: Initial dose 0.1 mL (1 mg) IV, may titrate up to 0.5 mL (5 mg) in 0.1 mL increments every 30-45 seconds. 2
• Children >75 lbs: Initial dose 0.2 mL (2 mg) IV, may titrate up to 1 mL (10 mg). 2
• Infants: 0.05 mL (0.5 mg). 2
• Intramuscular route may be used due to technical difficulty with IV access in children, though response is delayed 2-10 minutes. 2

What Constitutes a Positive Test

Myasthenic Response (Positive for MG)

• Increased muscle strength including improvement in ptosis, diplopia, dysphonia, dysphagia, dysarthria, respiration, and limb strength. 2
• Fasciculations may be absent or present. 2
• Minimal to absent muscarinic side effects (lacrimation, diaphoresis, salivation, abdominal cramps, nausea, vomiting, diarrhea). 2
• Improvement typically occurs within 30 seconds to 5 minutes and lasts only 5-10 minutes due to edrophonium's brief duration of action. 2, 4

Adequate Response (Normal/Treated Patient)

• No change in muscle strength. 2
• Fasciculations may be present or absent. 2
• Minimal muscarinic side effects. 2

Cholinergic Response (Overtreated Patient)

• Decreased muscle strength indicating excessive anticholinesterase medication. 2
• Fasciculations present or absent. 2
• Severe muscarinic side effects requiring immediate atropine administration. 2

Critical Safety Requirements

Mandatory Pre-Test Preparation

• Atropine must be drawn up and immediately available before administering edrophonium, as cardiopulmonary arrest has been reported. 5, 3
• The test should only be performed by practitioners experienced with proper administration in a monitored setting. 5
• Obtain a careful medication history, as certain drugs increase risk of adverse reactions. 3

Monitoring During Test

• Continuously observe heart action throughout the procedure. 2
• Monitor for muscarinic side effects including bradycardia, bronchospasm, increased salivation, lacrimation, and gastrointestinal symptoms. 5, 2
• Have resuscitation equipment immediately available. 3

Contraindications and Cautions

• Avoid in patients currently taking cholinesterase inhibitors like pyridostigmine, as they have reduced plasma cholinesterase activity and risk prolonged effects. 5
• Use extreme caution in patients with cardiac arrhythmias or respiratory compromise. 3
• In myasthenic crisis with apnea, secure controlled ventilation before attempting the test, and use only 0.1 mL (1 mg) initially, waiting one minute before considering an additional 0.1 mL (1 mg). 2

Clinical Applications Beyond Initial Diagnosis

Evaluating Treatment Adequacy

• Administer 0.1-0.2 mL (1-2 mg) IV one hour after oral intake of the patient's current anticholinesterase medication. 2
• Myasthenic response indicates undertreatment. 2
• Adequate response indicates appropriate therapy. 2
• Cholinergic response indicates overtreatment requiring medication reduction. 2

Differentiating Myasthenic vs. Cholinergic Crisis

• When a patient is in respiratory crisis, the test helps distinguish whether weakness is from inadequate treatment (myasthenic crisis) or excessive anticholinesterase medication (cholinergic crisis). 2
• Use only 0.2 mL (2 mg) maximum in crisis situations, given in 0.1 mL increments. 2
• If no clear improvement occurs after 0.2 mL, discontinue all anticholinesterase therapy and secure controlled ventilation by tracheostomy with assisted respiration. 2

Comparison to Alternative Diagnostic Methods

Advantages of Tensilon Test

• Rapid results within seconds to minutes. 2, 4
• High sensitivity: 95% for generalized MG and 86% for ocular MG. 1
• Can be performed at bedside in emergency settings. 6

Disadvantages and When to Consider Alternatives

• Requires experienced practitioner and monitoring capability. 5, 3
• Risk of serious adverse effects including cardiopulmonary complications. 3
• Brief duration limits utility to diagnostic purposes only, not maintenance therapy. 2

Safer Alternative: Neostigmine Challenge Test

• Neostigmine offers higher sensitivity, particularly in ocular myasthenia, compared to edrophonium. 5
• Longer duration of action allows more thorough assessment. 5
• Similar safety precautions required with atropine availability. 5

Non-Pharmacologic Alternative: Sleep Test

• The sleep test is a safe alternative based on the principle that myasthenic symptoms improve after rest. 4
• Observe for resolution of ptosis or ophthalmoparesis immediately after 30 minutes of sleep, with reappearance of signs over 30 seconds to 5 minutes. 4
• Moderately sensitive and specific without risk of cholinergic side effects. 4

Ice Pack Test

• Highly specific for myasthenia gravis, particularly for ocular symptoms. 7, 1
• Apply ice pack over closed eyes for 2 minutes and observe for symptom reduction. 7, 1
• Completely safe with no adverse effects. 7

Current Role in Diagnostic Algorithm

Given the availability of safer alternatives and the risks associated with Tensilon, many practitioners now reserve this test for specific situations where rapid bedside confirmation is essential and other methods are unavailable or inconclusive. 3 The diagnostic workup should prioritize anti-AChR antibodies (80% sensitivity in generalized MG), anti-MuSK antibodies if seronegative, and single-fiber EMG (>90% sensitivity for ocular MG) as first-line confirmatory tests. 1 The ice pack test provides a safe bedside alternative for ocular symptoms. 7, 1