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Difficulty: MediumDiagnostic Workup and Test Sequencing

A 54-year-old female teacher is brought to the emergency department by her spouse due to the sudden onset of a severe, diffuse headache that began roughly 8 hours ago. She describes it as unlike any headache she has ever experienced, reaching maximum intensity within seconds. The patient also reports significant nausea and sensitivity to light. Her past medical history is notable for well-controlled essential hypertension and hypothyroidism. Current medications include lisinopril and levothyroxine. On arrival, her temperature is 37.2°C (99.0°F), blood pressure is 162/94 mmHg, pulse is 92/min, and respiratory rate is 16/min. Physical examination demonstrates moderate resistance to passive neck flexion, but cranial nerves, motor strength, and sensory functions are fully intact. A non-contrast computed tomography (CT) scan of the head is obtained immediately and reveals no evidence of intracranial hemorrhage, mass effect, or midline shift. Which of the following is the most appropriate next step in the diagnostic workup?

  1. A
    Computed tomography angiography of the head
  2. B
    Magnetic resonance imaging of the brain
  3. Lumbar punctureAnswer
  4. D
    Administration of intravenous ceftriaxone and vancomycin
  5. E
    Discharge home with oral naproxen and neurology follow-up

Answer

Lumbar puncture
The correct answer is a lumbar puncture. The patient presents with classic signs of a subarachnoid hemorrhage (SAH), including a 'thunderclap' headache and nuchal rigidity. While a non-contrast head CT is the best initial screening test, its sensitivity approaches 100% only if performed within 6 hours of symptom onset. Because this patient's symptoms began 8 hours prior to the scan, a negative CT cannot definitively rule out SAH. In this scenario, a lumbar puncture is required to evaluate the cerebrospinal fluid for red blood cells and xanthochromia (bilirubin resulting from hemoglobin breakdown), which would confirm the diagnosis.

Step-by-Step Solution

1
Evaluate the clinical presentation.
The patient has a sudden-onset 'thunderclap' headache with meningismus, which is highly suspicious for a subarachnoid hemorrhage (SAH).
Identifying the most life-threatening condition in the differential diagnosis is critical for guiding the initial and subsequent diagnostic workup.
2
Assess the results and timing of the initial diagnostic study.
A non-contrast head CT is negative, but it was performed 8 hours after symptom onset.
The sensitivity of a non-contrast CT for detecting SAH is nearly 100% within the first 6 hours, but drops significantly thereafter. Thus, a negative scan outside this window does not definitively rule out the condition.
3
Determine the most appropriate next step in the testing sequence.
A lumbar puncture must be performed.
When clinical suspicion for SAH remains high and the head CT is performed outside the highly sensitive time window, examining the cerebrospinal fluid for red blood cells and xanthochromia is the mandatory next step.

Key Concept

Diagnostic test sequencing for suspected subarachnoid hemorrhage based on symptom onset timing.
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Topics

Diagnostic Workup and Test Sequencing

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