A 68-year-old man with myasthenia gravis presents with progressive muscle weakness and ptosis. He requires a drug that reversibly inhibits acetylcholinesterase and can be used both diagnostively and therapeutically. Which of the following is the drug of choice for acute management and diagnostic confirmation in this patient?

Explanation

## Drug of Choice for Diagnostic Confirmation in Myasthenia Gravis **Key Point:** Edrophonium (Tensilon) is the classic drug used for **diagnostic confirmation** of myasthenia gravis due to its ultra-short onset (30–60 seconds) and brief duration (5–10 minutes), making it ideal for the Tensilon test. ### Mechanism & Pharmacokinetics Edrophonium reversibly inhibits acetylcholinesterase by electrostatically binding to the anionic site of the enzyme (without carbamylation). This prolongs acetylcholine action at the neuromuscular junction. Its extremely rapid onset and short duration allow a clear diagnostic window — transient improvement in ptosis and muscle strength confirms myasthenia gravis. ### Comparison of Anticholinesterases in Myasthenia Gravis | Feature | Edrophonium | Neostigmine | Pyridostigmine | | --- | --- | --- | --- | | **Route** | IV only | IV/IM/PO | PO only | | **Onset** | 30–60 sec | 7–15 min (IV) | 30–60 min | | **Duration** | 5–10 min | 2–4 hours | 3–8 hours | | **Primary Use** | **Diagnostic** | Acute + chronic Rx | Chronic maintenance | | **Muscarinic effects** | Minimal | Marked | Mild | ### Why Edrophonium for Diagnostic Confirmation 1. **Ultra-rapid onset** — improvement in ptosis/diplopia within 30–60 seconds of IV administration 2. **Very short duration** — effect wears off in 5–10 minutes, limiting adverse events 3. **Minimal muscarinic side effects** — safer for a brief diagnostic test compared to neostigmine 4. **Classic Tensilon test** — the stem explicitly asks for a drug used **both diagnostically and therapeutically**; edrophonium uniquely fulfills the diagnostic role described **High-Yield:** The **Tensilon (edrophonium) test** is the pharmacological gold standard for diagnosing myasthenia gravis. A positive test (transient improvement in muscle strength/ptosis within 60 seconds) strongly supports the diagnosis. Atropine should be kept at bedside to manage potential bradycardia. **Clinical Pearl:** While edrophonium is primarily diagnostic, neostigmine and pyridostigmine are used for **long-term therapeutic management**. The stem's emphasis on "diagnostic confirmation" and "acute management" with a drug of very rapid onset points to edrophonium as the answer. **Warning:** Edrophonium can precipitate a **cholinergic crisis** in overdose or in patients already on anticholinesterase therapy — distinguish from myasthenic crisis by the presence of muscarinic signs (bradycardia, miosis, salivation) and response to atropine. [cite: KD Tripathi 8e Ch 7; Katzung Basic & Clinical Pharmacology 14e Ch 7]