Most people will have a headache from time to time. Others may suffer from tension, cluster or migraine headaches. But a headache that happens suddenly, causes visual disturbances, confusion or seizures is considered high-risk, as is a headache that is more intense than any the patient has previously experienced. If it's the worst headache of your life, you'll have priority over a patient who is known to have, for example, migraine headaches.
Severe headaches can be caused by serious, sometimes life-threatening, reasons. For example a subarachnoid hemorrhage -- bleeding between the brain and its surrounding tissue -- is sometimes caused by a head injury or a ruptured cerebral aneurysm and is a critical condition. Severe headaches may also be indicative of a stroke or meningitis, two serious conditions that are given priority in the emergency room.