This patient should undergo CT angiography of the brain to assess for cerebral vasospasm. He is now at day 7 after a subarachnoid hemorrhage due to a left middle cerebral artery aneurysm. In the first 48 hours after a subarachnoid hemorrhage, rebleeding from an unsecured aneurysm and hydrocephalus are the principal causes of neurologic deterioration. This patient's aneurysm has been successfully clipped, and hydrocephalus is being managed with the use of an external ventricular drain. Potential neurologic complications after the first 48 hours include seizures, hydrocephalus, infection, and symptomatic cerebral vasospasm; the incidence of cerebral vasospasm peaks on days 5 to 10 after a hemorrhage. Cerebral vasospasm can manifest as a decline in neurologic function in patients who are awake enough for a neurologic examination. Although transcranial Doppler ultrasonography may reveal a vasospasm, CT angiography is more sensitive at detecting vasospasm that can be treated with the initiation of vasopressors to augment the blood pressure or with endovascular treatment in more refractory cases. CT angiography has the additional benefit of imaging the brain parenchyma for evidence of cerebral edema or infarction and the ventricles for evidence of hydrocephalus that may be amenable to shunting.

Electroencephalography (EEG) is inappropriate as the next diagnostic step in this patient with an aneurysmal subarachnoid hemorrhage. Convulsive and nonconvulsive status epilepticus is common and underdiagnosed after hemorrhagic stroke and is associated with poor neurologic outcome. If imaging does not identify a clear cause of the patient's decline that can be treated with medical or surgical therapy before irreversible damage occurs, then continuous EEG monitoring may help in diagnosing seizures.

Lumbar puncture may be useful for measuring intracranial pressure in this patient with an external ventricular drain. However, repeat imaging would first be required, independent of the presence of the drain, to rule out mass effect that could precipitate cerebral herniation after a lumbar puncture.

MRI requires too long a time to complete in the setting of a neurologic emergency and may not adequately detect arterial narrowing. Vasospasm is more readily detected with CT angiography.