Excruciating, burning, stabbing pain in and around one eye, including facial swelling, sweating, nasal congestion or running nose only on the affected side. Attacks begin and increase to maximum intensity within minutes and last 15 minutes to 3 hours, repeating up to 8 times per day. Attacks occur at regular intervals, particularly during sleep. Most patients experience periodic "clusters" of attacks during the same time every year for several weeks.