A 52-year-old man presents to the acute medical intake with a two-week history of severe pain around his right eye. The pain began without any apparent cause and woke him from his sleep. He describes the pain as severe and boring, radiating upwards over the right frontal and temporal region. The pain is not constant but seems to start every evening and persist for periods of 20 minutes to over an hour. The headaches are associated with watering of his right eye and a blocked left nostril. He has no past history of similar episodes or any other significant medical history.
Upon examination, he is alert and oriented with a Glasgow coma scale score of 15/15. His blood pressure is 125/75 mmHg, and he is afebrile with no neck stiffness. The right eye is red with conjunctival injection and mild eyelid edema. There is a partial right-sided ptosis and miosis.
Which therapeutic option is most likely to alleviate his symptoms?