This patient should undergo lumbar puncture. He reports sudden onset of a severe headache, most likely a thunderclap headache. Thunderclap headache is defined as a severe headache that reaches maximum intensity within 60 seconds of onset. Although classically associated with subarachnoid hemorrhage (SAH), thunderclap headache also may be caused by other conditions, ranging from benign to life-threatening ones. Because approximately 25% of thunderclap headache presentations result from an SAH, this type of headache should be approached as a neurologic emergency. Although other causes are possible, including dural sinus thrombosis, meningitis, and migraine, the presence of a dilated unreactive pupil suggests external compression of the left oculomotor nerve (cranial nerve III). In the presence of a normal mental status, cerebral herniation and increased intracranial pressure are unlikely, but an aneurysm of the left posterior communicating artery is possible that may not be visible on a noncontrast CT scan of the head. Patients with an aneurysmal SAH may first experience less extensive bleeding that also is not visible on a noncontrast head CT before more significant bleeding occurs. After this initial or “sentinel” bleeding, the patient is at high risk for a clinically significant SAH with associated high morbidity and mortality. Therefore, establishing the diagnosis is a priority.

When the suspicion of an SAH is high and the noncontrast CT scan of the head is normal, a lumbar puncture is required to evaluate the cerebrospinal fluid (CSF) for erythrocytes or xanthochromia. Xanthochromia describes a yellow discoloration of the CSF from breakdown of erythrocytes, which may not develop for at least 6 hours after the initial event.

Magnetic resonance angiography (MRA) is premature before SAH is excluded. Once SAH has been ruled out, MRA may be needed to exclude other arterial causes, such as cervicocephalic arterial dissection. Internal carotid artery dissection can cause pupillary abnormalities, but these are typically from a Horner syndrome, with a smaller pupillary diameter in the affected eye.

Similarly, magnetic resonance venography (MRV) of the brain should not be performed until SAH is excluded. MRV is used to diagnose deep venous thrombosis and cerebral venous sinus thrombosis. Although the latter disorder sometimes presents with thunderclap headache, it is unlikely to cause a dilated unreactive left pupil.

The usefulness of MRI for diagnosing SAH remains under investigation. MRI may ultimately be required if the CSF is normal to rule out other disorders.