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Difficulty: MediumDiagnostic Workup and Test Sequencing

A 52-year-old woman is brought to the emergency department by her husband after the sudden onset of an excruciating headache that began 10 hours ago while she was lifting heavy boxes. She describes it as the "worst headache of my life" and reports associated photophobia, nausea, and two episodes of vomiting. She has a history of well-controlled hypertension. Her temperature is 37.1°C (98.8°F), blood pressure is 165/95 mm Hg, pulse is 98/min, and respirations are 16/min. Physical examination reveals a mildly lethargic patient who prefers to keep her eyes closed in the darkened room. She exhibits significant resistance and pain with passive neck flexion. No focal motor or sensory deficits are noted. Laboratory studies show a serum sodium level of 132 mEq/L and a leukocyte count of 11,000/mm³. A non-contrast computed tomography (CT) scan of the head is performed and shows no evidence of acute intracranial hemorrhage, mass effect, or midline shift. Which of the following is the most appropriate next step in the diagnostic workup?

  1. Lumbar punctureAnswer
  2. B
    Computed tomography angiography of the head
  3. C
    Magnetic resonance imaging of the brain
  4. D
    Measurement of serum and urine osmolality
  5. E
    Measurement of erythrocyte sedimentation rate

Answer

The most appropriate next step is a lumbar puncture to rule out subarachnoid hemorrhage in a patient with a negative CT scan performed more than 6 hours after symptom onset.
The patient's presentation of a sudden-onset, severe thunderclap headache with nuchal rigidity is the classic presentation of a subarachnoid hemorrhage (SAH). The initial diagnostic test is a non-contrast CT scan of the head. If performed within 6 hours of symptom onset, a normal CT head effectively rules out an SAH. However, this patient presented 10 hours after symptom onset. Because the sensitivity of a CT scan decreases over time, a negative CT scan >6 hours after onset requires a lumbar puncture to definitively assess for the presence of red blood cells and xanthochromia (bilirubin resulting from red blood cell breakdown).

Step-by-Step Solution

1
Identify the primary clinical syndrome from the patient's presentation.
The patient presents with a 'thunderclap headache' (sudden, worst of life), meningismus, and vomiting following exertion.
Recognizing this classic triad establishes a high pre-test probability for a spontaneous subarachnoid hemorrhage (SAH).
2
Evaluate the results of the initial diagnostic testing in the context of the symptom timeline.
The non-contrast CT head is negative, but it was performed 10 hours after symptom onset.
Non-contrast CT is nearly 100% sensitive for SAH within the first 6 hours, but sensitivity drops significantly as time passes due to cerebrospinal fluid circulation and red blood cell breakdown.
3
Determine the required next diagnostic step based on guidelines for suspected SAH with a negative late CT.
A lumbar puncture must be performed to check for xanthochromia and red blood cells.
When clinical suspicion remains high and CT is non-diagnostic (>6 hours post-onset), cerebrospinal fluid analysis is the definitive non-invasive method to rule out an SAH.

Key Concept

In patients with a suspected subarachnoid hemorrhage, a negative non-contrast CT scan of the head performed more than 6 hours after symptom onset must be followed by a lumbar puncture to definitively rule out the diagnosis.
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Diagnostic Workup and Test Sequencing

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