You have been referred a 40-year-old man by the Accident and Emergency doctors with a severe headache. The headache woke him up at 3am, and he describes it as the worst headache he’s ever had, (although he admits he’s not a regular headache sufferer). He has had seven episodes like this over the past two weeks that have followed a very similar pattern, with the other two headaches lasting around 60 minutes before going.
The pain is mainly around the left eye and temple and is sharp in nature. You have to ask him to sit down to examine him because he is up and pacing around his room, clearly very agitated. On examining him you note that his left eye is watering and swollen, and there’s some redness and mild bruising just above the eye. When you ask him about this bruising he says that the pain was so bad he bashed his head against the fridge door to try and help take it away.
On examination he has normal power, sensation and reflexes in all four limbs. On examining his cranial nerves you notice that he has a mild left sided ptosis and miosis, and there is conjunctival injection and lacrimation on that side too.
What is the most likely cause of his headache?