It’s technically impossible to “officially” diagnose migraine, cluster, or tension headache during the first-ever episode. The latter isn’t as problematic, because there are no red flags. But a new migraine or cluster headache, possibly severe, with nausea and/or eye symptoms, might easily suggest an entity in the DO NOT MISS column.
Careful delineation of symptom chronology is the key. Migraines build to a maximal intensity over 30-60 minutes, not 1-2 like an SAH. An episode commonly lasts 8-72 hours. Cluster headaches begin abruptly, but last less than 3 hours, remit, and recur. There’s no fever, neuro exam is normal, and the patient doesn’t look ill. Patients who actually look sick probably need a same-day E.D. work-up.
See posting Recurrent Headaches for discussion of migraine & cluster.