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Difficulty: HardDiagnostic Workup and Test Sequencing

A 32-year-old woman is evaluated in the neurology clinic for a three-month history of intermittent double vision and bilateral drooping of her eyelids. She notes that her symptoms are typically absent upon awakening but progressively worsen throughout the day, particularly after prolonged reading. Recently, she has also experienced transient difficulty chewing tough foods during the evening. Her past medical history is unremarkable, and she takes no medications. Vital signs are within normal limits. Neurologic examination reveals bilateral ptosis that worsens after sustained upward gaze for two minutes. Extraocular movements are restricted in multiple directions without a clear single cranial nerve pattern. Pupillary responses are brisk and symmetric. Muscle strength in the extremities is 5/5, and deep tendon reflexes are 2+ throughout. Which of the following is the most appropriate next step in the diagnostic workup?

  1. Acetylcholine receptor antibody assayAnswer
  2. B
    Computed tomography of the anterior mediastinum
  3. C
    Intravenous administration of edrophonium
  4. D
    Magnetic resonance imaging of the brain with contrast
  5. E
    Repetitive nerve stimulation study

Answer

The most appropriate initial diagnostic step is serologic testing with an acetylcholine receptor antibody assay.
The correct answer is the acetylcholine receptor antibody assay. This patient's presentation of fluctuating, fatigable weakness (worsening ptosis, diplopia, and mastication fatigue later in the day) is the classic hallmark of myasthenia gravis. The diagnosis is confirmed by identifying autoantibodies directed against components of the neuromuscular junction. The initial, most cost-effective, and highly specific test is the acetylcholine receptor (AChR) antibody assay, which is positive in approximately 85% of patients with generalized myasthenia gravis. Proper test sequencing dictates that non-invasive, highly specific biochemical markers should be evaluated prior to advanced imaging or electrodiagnostic testing.

Step-by-Step Solution

1
Analyze the clinical presentation and physical examination findings.
The patient is a young woman presenting with fatigable weakness involving the extraocular, bulbar, and facial muscles (ptosis, diplopia, chewing difficulty), with normal pupillary responses and intact deep tendon reflexes.
Identifying the specific pattern of motor weakness and absence of sensory or reflex changes characterizes the anatomic location of the lesion.
2
Localize the lesion to formulate a preliminary diagnosis.
Fluctuating, fatigable weakness without sensory or autonomic deficits points precisely to a neuromuscular junction disorder, specifically myasthenia gravis (MG).
Central or peripheral nerve lesions typically present with constant deficits, reflex changes, or sensory loss, distinguishing them from neuromuscular junction pathologies.
3
Determine the optimal sequence of diagnostic tests for suspected myasthenia gravis.
The initial standard-of-care step is to detect disease-specific autoantibodies (AChR). Imaging of the thymus and invasive electrophysiologic studies are deferred until after serologic screening.
Serologic testing is highly specific, non-invasive, and confirms the diagnosis in the vast majority of patients, which safely dictates the subsequent test sequencing.

Key Concept

The initial diagnostic step for suspected myasthenia gravis is serologic testing for acetylcholine receptor (AChR) antibodies; chest imaging to screen for thymoma and electrodiagnostic testing are indicated only later in the diagnostic algorithm.
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Diagnostic Workup and Test Sequencing

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