A 32-year-old woman presents to the neurology clinic with progressive muscle weakness, ptosis, and diplopia that worsens towards the end of the day. Repetitive nerve stimulation shows a decremental response. Serology is positive for anti-acetylcholine receptor (AChR) antibodies. She is started on pyridostigmine but develops severe abdominal cramping and diarrhea within hours. What is the most appropriate immediate next step in management?

Explanation

## Clinical Context This patient has **myasthenia gravis (MG)** confirmed by clinical presentation, decremental response on RNS, and positive anti-AChR antibodies. Pyridostigmine, an acetylcholinesterase inhibitor, is first-line therapy but has dose-limiting muscarinic side effects (abdominal cramping, diarrhea, salivation, bronchospasm). ## Mechanism of Pyridostigmine Side Effects **Key Point:** Pyridostigmine inhibits acetylcholinesterase, causing acetylcholine accumulation at **both nicotinic and muscarinic receptors**. Excess muscarinic stimulation at parasympathetic sites (GI tract, salivary glands, bronchi) produces cholinergic crisis symptoms. ## Management of Muscarinic Toxicity | Intervention | Rationale | Timing | |---|---|---| | **Reduce dose** | Titrate to therapeutic window (symptom relief without toxicity) | Immediate | | **Add anticholinergic** (glycopyrrolate, propantheline) | Blocks muscarinic receptors peripherally; does NOT cross BBB, so preserves CNS nicotinic benefit | Concurrent with dose reduction | | **Discontinue entirely** | Premature; removes therapeutic benefit | Not indicated yet | | **Add atropine** | Crosses BBB; can worsen nicotinic blockade at neuromuscular junction | Contraindicated | **High-Yield:** The standard approach is **dose reduction + peripheral anticholinergic agent**, not discontinuation or atropine. ## Clinical Pearl Anticholinergic agents like glycopyrrolate are preferred over atropine in MG because they are quaternary ammonium compounds (charged, do not cross the blood–brain barrier), so they block only peripheral muscarinic effects without interfering with central nicotinic transmission at the neuromuscular junction. ## Why Not Immediate Plasmapheresis? Plasmapheresis is reserved for **myasthenic crisis** (respiratory failure, bulbar weakness) or when rapid symptom control is needed before immunosuppression takes effect. This patient is stable and has only GI side effects—a pharmacological adjustment is appropriate first.