CT of the head without contrast is the most appropriate diagnostic test in this patient. She had sudden-onset severe headache followed by impaired consciousness, symptoms that are most concerning for hemorrhagic stroke caused by an aneurysmal subarachnoid hemorrhage or intracerebral hemorrhage. A neurologic examination by itself lacks sufficient predictive value to evaluate the source of impaired consciousness. Rapid imaging is required to initiate rapid treatment. CT of the head is readily available, can be performed quickly, and is the test of choice to rule out intracerebral hemorrhage, subarachnoid bleeding, and hydrocephalus, all of which may necessitate rapid neurosurgical intervention.

Catheter-based angiography is ultimately indicated in most patients with subarachnoid hemorrhage to determine the source of bleeding. However, CT of the head without contrast, which allows for more rapid imaging, should be performed first to evaluate for any condition (such as hydrocephalus from increased intracranial pressure) that can be rapidly reversed with emergency neurosurgical treatment.

Lumbar puncture should not be performed in a patient with stroke symptoms until the presence of a mass lesion has been excluded. If the patient has elevated intracranial pressure from mass effect, particularly in the cerebellum, lumbar puncture may worsen any cerebral herniation.

In the evaluation of a patient with symptoms of a stroke, MRI is time consuming, not readily available, and not cost-effective; this test also leaves the patient in a less monitored setting during the scanning than does CT. In the acute setting, stroke that requires rapid neurosurgical intervention should be diagnosed as quickly as possible.