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Difficulty: MediumDiagnostic Workup and Test Sequencing

A 4545-year-old woman presents to the emergency department reporting the sudden onset of a severe, diffuse headache that began 1818 hours ago. She describes it as the "worst headache of my life." The headache was accompanied by two episodes of vomiting and sensitivity to light. Her past medical history is significant for hypertension and tobacco use. Her temperature is 37.1∘C37.1^\circ\text{C} (98.8∘F98.8^\circ\text{F}), pulse is 9292/min, blood pressure is 168/95168/95 mmHg, and respirations are 1616/min. On physical examination, the patient appears uncomfortable and prefers to keep the room lights off. Neurologic examination shows intact cranial nerves, 5/55/5 motor strength in all extremities, and normal deep tendon reflexes. Neck flexion produces mild discomfort. A non-contrast computed tomography (CT) scan of the head 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. Lumbar punctureAnswer
  2. B
    Computed tomography angiography of the brain
  3. C
    Magnetic resonance imaging of the brain
  4. D
    Discharge home with outpatient neurology follow-up
  5. E
    Subcutaneous administration of sumatriptan

Answer

The most appropriate next step in the diagnostic workup is a lumbar puncture to evaluate for red blood cells or xanthochromia.
A non-contrast head CT is the initial test of choice for suspected subarachnoid hemorrhage (SAH). While it is highly sensitive within the first 66 hours of symptom onset, the sensitivity drops significantly over time. In a patient presenting 1818 hours after the onset of a classic thunderclap headache, a negative CT scan does not definitively rule out an SAH. Therefore, the diagnostic sequence mandates a lumbar puncture to evaluate for red blood cells or xanthochromia (yellowish discoloration due to bilirubin from broken-down red blood cells) in the cerebrospinal fluid. Only after a normal lumbar puncture can SAH be safely excluded.

Step-by-Step Solution

1
Evaluate the clinical presentation for high-risk features.
The patient describes a sudden-onset, thunderclap headache accompanied by vomiting and photophobia.
These are classic red-flag symptoms for an acute subarachnoid hemorrhage (SAH).
2
Interpret the initial diagnostic test (non-contrast head CT) in the context of symptom timeline.
The CT scan is negative for hemorrhage, but the symptom onset was 1818 hours prior to the scan.
Non-contrast CT approaches 100%100\% sensitivity for SAH if performed within 66 hours of onset, but sensitivity steadily declines thereafter due to cerebrospinal fluid circulating and clearing the blood.
3
Determine the appropriate sequential step to rule out the life-threatening condition.
A lumbar puncture (LP) must be performed.
An LP is highly sensitive for detecting red blood cells or xanthochromia in the CSF, which definitively diagnoses SAH in cases where the delayed CT is falsely negative.

Key Concept

In patients with a suspected subarachnoid hemorrhage who have a normal non-contrast head CT performed more than 66 hours after symptom onset, a lumbar puncture is required to definitively rule out the diagnosis.
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Topics

Diagnostic Workup and Test Sequencing

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