Why didn’t Dr. Mortimer recognize Sir Charles at first?

Dr. Mortimer didn't recognize Sir Charles because he had an odd facial distortion.

"No signs of violence were to be discovered upon Sir Charles’s person, and though the doctor’s evidence pointed to an almost incredible facial distortion—so great that Dr. Mortimer refused at first to believe that it was indeed his friend and patient who lay before him—it was explained that that is a symptom which is not unusual in cases of dyspnoea and death from cardiac exhaustion."