What happens when Utterson and Enfield see Jekyll in the window?

In Chapter Seven, Mr. Enfield and Mr. Utterson are walking by the mysterious door. Through one of the windows, Utterson spots Dr. Jekyll, whom he has not seen for weeks. Utterson calls to Jekyll and tells him he should get outside more. Jekyll replies that he wishes he could, but doesn't dare. As he finishes his sentence, his smile disappears from his face and a look of utter terror takes over. It appears as though Jekyll suffers some kind of seizure. Enfield and Utterson only briefly saw the pain in Jekyll's face before he quickly shut the window, but are both appalled. They walk on without speaking of the incident.