Question: what is cluster headache really?

Cluster headache. Cluster headache belongs to a group of idiopathic (unknown cause) headache entities known as the trigeminal autonomic cephalalgias, all of which involve one-sided, short-lived, severe headache attacks. There are associated autonomic symptoms, such as ptosis (eyelid drooping), miosis (constricted pupils), lacrimation (tearing in the eyes), conjunctival injection, runny nose, and nasal congestion.
Cluster headaches. As the name indicates, cluster headaches are characterized by excrutiating stabbing and penetrating pain usually around one eye. The are usually unilateral, occursuddenly and without warning with pain peaking within 15 minutes. Symptos include forehead and facial sweating, stuffy or runny nose, swolllen or droopy eyelid, watery tearing eyes, intolerance to noise and light.