Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.

A 56-year-old woman is evaluated in the emergency department for sudden onset of a severe generalized headache that began 36 hours ago and has not responded to over-the-counter medications. The patient has a history of hypertension treated with lifestyle modifications. She has a 30-pack-year smoking history.

On physical examination, blood pressure is 148/68 mm Hg, pulse rate is 96/min and regular, and respiration rate is 16/min. Nuchal rigidity is noted. Other general examination findings are normal.

Results of laboratory studies are notable for a platelet count of 190,000/µL (190 × 10⁹/L), an INR of 0.9, and a serum creatinine level of 0.9 mg/dL (79.6 µmol/L).

A CT scan of the head without contrast is normal.

Which of the following is the most appropriate next diagnostic test?

A: CT of the head with contrast
B: Lumbar puncture
C: Magnetic resonance angiography of the head and neck
D: MRI of the brain without contrast

MKSAP Answer and Critique

The correct answer is B: Lumbar puncture.

This patient should have a lumbar puncture. At presentation, she has sudden onset of a severe headache, which is most concerning for subarachnoid hemorrhage. In a minority of patients with a small amount of blood in the subarachnoid space, a head CT may initially be normal. When this occurs, a lumbar puncture is required to detect erythrocytes or xanthochromia (a yellowish discoloration caused by the breakdown of erythrocytes) in the cerebrospinal fluid (CSF). Because xanthochromia may not develop for 6 hours or longer after the initial event, the presence of erythrocytes in the CSF should prompt consideration of a subarachnoid hemorrhage. A lumbar puncture is also helpful for excluding other diagnoses, such as meningitis, and for measuring the opening pressure.

Because the initial CT of the head without contrast was normal, a CT with contrast is unlikely to show a mass lesion sufficient in size to cause headache. CT angiography or venography may eventually be used to rule out aneurysms or dural sinus thrombosis, but subarachnoid hemorrhage first needs to be ruled out in the acute setting.

Although magnetic resonance angiography (MRA) eventually may be necessary to exclude dissection or aneurysms as the cause of this patient’s symptoms, it is inappropriate at this time. Unless the presence of a subarachnoid hemorrhage is first established, an aneurysm detected on MRA or other vascular imaging may be an incidental finding that does not require surgical intervention.

The effectiveness of MRI for diagnosing subarachnoid hemorrhage remains under investigation. Additionally, MRI is time consuming and may not differentiate subarachnoid hemorrhage from other diagnoses well enough. Unlike CSF analysis, MRI does not afford the additional benefit of measuring the CSF opening pressure.

Of note, clinical examination findings, including those from funduscopy, have insufficient sensitivity and specificity to establish the diagnosis of subarachnoid hemorrhage.

Key Point

• In a patient with a suspected subarachnoid hemorrhage and normal results on a head CT scan, a lumbar puncture is the most appropriate next step in evaluation.

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