A 55-year-old woman presents to the emergency department for evaluation of headache. She had a headache that was present for a few hours before subsiding 2 days ago, and then today she developed a headache that was sudden in onset and at maximal intensity within minutes. She has never had a headache like this in the past. Her temperature is 99.0°F (37.2°C), blood pressure is 157/99 mmHg, pulse is 90/min, respirations are 16/min, and oxygen saturation is 99% on room air. On exam, she has pain with passive and active movement of the neck. No focal neurologic deficits are appreciated. A non-contrast head CT is obtained as shown in Figure A. Which of the following is the most likely underlying etiology of this patient's headache?
Dehydration and stress
Hypersensitivity of the trigeminal nerve
Muscular pain and strain
Rupture of a berry aneurysm
Rupture of a berry aneurysm
This patient with sudden-onset headache, neck pain, and a CT scan showing blood in the basal cisterns likely has a subarachnoid hemorrhage. The most common cause of non-traumatic subarachnoid hemorrhage (SAH) is rupture of a saccular (berry) aneurysm.

SAH typically presents with a sudden onset "thunderclap" headache that is often described as the "worst headache of my life." Hypertension is a common risk factor. The diagnosis can be confirmed with a non-contrast head CT. Non-contrast CT scan of the head is sufficient to rule out subarachnoid hemorrhage if performed within 6 hours of symptom onset. If the head CT is negative but SAH is still suspected, a lumbar puncture can help confirm the diagnosis (demonstrating xanthochromia) or CT angiography can be performed. Patients should be admitted for neurosurgery and neuro-IR consultation and may require calcium channel blockers and possible intervention.

Macdonald et. al review spontaneous subarachnoid hemorrhage. They discuss the pathophysiology, clinical manifestations, and diagnosis. They note the typical finding of accumulation of blood in the basal cisterns and sylvian fissures on CT imaging.

Figure A shows a non-contrast head CT demonstrating blood in the subarachnoid space supporting the diagnosis of SAH. Note the hyperdensity in the sylvian fissures and basal cisterns.

Incorrect Answers:
Answer A: CNS vascular endothelial dysfunction results in posterior reversible encephalopathy syndrome, which typically presents with hypertension and encephalopathy.

Answer B: Dehydration and stress could explain a migraine headache or a tension headache. Migraine headaches are often pulsating and may be associated with aura.

Answer C: Hypersensitivity of the trigeminal nerve is thought to be involved in the pathophysiology of migraine headache, and also could describe trigeminal neuralgia which presents with sudden and severe electrical pain in the distribution of the trigeminal nerve with minor stimulation.

Answer D: Muscular pain and strain describes a tension headache which presents with diffuse head pain that is mild and not localized. CT imaging is normal in tension headache.

Bullet Summary:
Spontaneous subarachnoid hemorrhages are most commonly caused by rupture of a berry aneurysm.