Myasthenic crisis with bulbar weakness — FFICM MCQ

Correct answer: C — Myasthenic crisis

The correct answer is C, Myasthenic crisis. The combination of fluctuating ptosis, dysphagia and proximal weakness, triggered by an intercurrent chest infection, with preserved reflexes and normal pupils, is classic for a neuromuscular junction disorder rather than a peripheral nerve, brainstem or spinal cord lesion. Infection is the commonest precipitant of decompensation in myasthenia gravis, and a vital capacity of 12 ml/kg (falling towards the 15 to 20 ml/kg threshold for concern) with a weak cough signals impending ventilatory failure requiring escalation before saturations fall, since hypoxia is a late and unreliable sign in myasthenic crisis. A myasthenic crisis is defined as a severe worsening of MG symptoms causing extreme weakness and severe breathing or swallowing problems, potentially life-threatening and requiring emergency treatment, exactly as described here. Fluctuation (fatigability) plus fully preserved reflexes and pupillary responses excludes the alternative diagnoses below. Why the other options are wrong: B. Guillain-Barre syndrome: typically produces an ascending, symmetrical, non-fluctuating weakness with early hyporeflexia or areflexia, which is absent here as reflexes are normal. A. Botulism: causes descending paralysis with prominent early autonomic and pupillary involvement (fixed dilated pupils, dry mouth), which is explicitly excluded by pupil sparing in this stem. E. Opioid toxicity: causes miosis, sedation and reduced respiratory rate/depth rather than fluctuating fatigable ptosis, dysphagia and proximal limb weakness. D. Cervical cord compression: would produce upper motor neurone signs, altered reflexes (often hyperreflexia below the lesion) and a sensory level, none of which are present. Key point: Fluctuating (fatigable) ptosis, dysphagia and proximal weakness with normal reflexes and pupils, precipitated by infection, points to myasthenic crisis, and respiratory failure here often precedes any fall in oxygen saturation, so vital capacity and cough strength must guide escalation.

Reference: Muscular Dystrophy UK, Myasthenia gravis (MG): information on myasthenic crisis, https://www.musculardystrophyuk.org/conditions/a-z/myasthenia-gravis/