Myasthenia gravis and neuromuscular junction disorders: SAQ

This girl presents the classic picture of autoimmune myasthenia gravis: fluctuating, fatigable weakness of the ocular, bulbar, and proximal muscles that worsens through the day and recovers with rest, with preserved reflexes and pupils, and a positive ice pack test. Her falling forced vital capacity and bulbar weakness are the immediate threats, and they drive the decision to escalate to intensive care. The diagnosis is supported by the clinical picture and confirmed by the antibody panel and neurophysiology, and the symptomatic treatment is pyridostigmine with disease-modifying immunotherapy. [1]

Question 1 (10 marks)

Outline your immediate assessment and investigations, and your initial management over the first 24 hours. [1]

I would treat this as probable myasthenia gravis with a threatened crisis and escalate immediately to intensive care, because the forced vital capacity of 17 mL per kilogram is under the 20 mL per kilogram watch threshold and has fallen by more than thirty percent from baseline, and the weak cough signals bulbar weakness and aspiration risk. The decision to intubate is driven by the numbers, not by how the child looks, and a child who is still talking can have a falling vital capacity. I would secure the airway, preoxygenate, and arrange a rapid sequence induction with the intensive care team, choosing sedation that avoids prolonged neuromuscular blockade, and I would place cardiac and respiratory monitoring. [7]

The workup runs in parallel. I would send the acetylcholine receptor antibody as binding, blocking, and modulating assays and the muscle-specific kinase antibody, knowing that the acetylcholine receptor antibody is positive in around eighty percent of generalised disease and a positive result is essentially diagnostic. I would request repetitive nerve stimulation at three hertz, looking for a decremental response of greater than ten percent in a clinically weak muscle, and single-fibre electromyography for its sensitivity if available. I would obtain a contrast computed tomography or magnetic resonance imaging of the chest to exclude a thymoma, which is rare in children but mandates resection, and a thyroid function test to exclude an associated thyroid disease. [11]

I would start symptomatic therapy with pyridostigmine around one milligram per kilogram per dose four to five times daily, which is around 50 mg four to five times daily for this 52 kg girl, titrated to the response and limited by the cholinergic side effects. I would start crisis-specific therapy with intravenous immunoglobulin 2 g per kg over two to five days, which is 104 grams in total, or plasma exchange three to five sessions over one to two weeks, which the Cochrane review found equally effective for an exacerbation. I would begin prednisolone low at around half a milligram per kiligram per day and titrate upward over weeks, because a high starting dose can transiently worsen the disease, and add a steroid-sparing agent early. I would search for and reverse a precipitant such as infection or a worsening drug. [5]