The lights at the Metropolitan Opera House were already beginning to dim as the elderly couple made their way past the pumps and wingtips of the mostly seated audience. As the familiar notes of “La Traviata” began to sound from the orchestra, the man leaned over to his wife of 60-some years. “I feel terrible,” he whispered. He struggled to his feet, and the two edged back down the row.

The man leaned heavily on his wife. His right leg was too weak to support him. In the lobby, he gratefully lowered himself into a wheelchair. He felt awful and wondered if he might be dying. Finally, he heard a siren announcing that an ambulance was on its way.

In the emergency room at Roosevelt Hospital, Dr. Barbara Kilian hurried to see the new arrival. He looked younger than 81, trim and, judging from the way he filled the stretcher, quite tall. His angular face was pale and covered with sweat. The young doctor glanced at the monitors. Heart rate was slow and steady; blood pressure was normal. The patient seemed comfortable. She had a little time.

He was a neurosurgeon, the patient told the doctor. He and his wife went to the opera to celebrate her 77th birthday. He felt fine all day until quite suddenly he did not. He was generally healthy despite a little high blood pressure and mild Parkinson’s disease. The terrible feeling he had in the theater was gone, but his right leg was numb, and he couldn’t move it. Could he be having a stroke? “I’m wondering that myself,” the doctor told him. “Then you should give me tPA,” he told her. TPA, short for tissue plasminogen activator, is a clot-busting medicine used to treat strokes and heart attacks. These diseases are caused by a blood clot blocking an artery, leaving the tissues beyond to die. When used early it can prevent this damage, but the powerful drug can also cause life-threatening bleeding. The twin responsibilities of an E.R. physician are to treat diseases that are true emergencies — the ones that can kill you before morning — while simultaneously making sure not to harm the patient in the process. “Let’s see what you’ve got before we start treating you,” she told him.

2. Investigation

A few minutes later the patient-doctor was whisked out of the E.R. to get a CT scan of his head. If this was a stroke, there was a good chance it would show up. The scan was a completely normal. By the time the patient returned to the E.R., his symptoms had changed: he could move his right leg, but it was unbearably painful, especially at the thigh. Kilian quickly examined the leg: no cuts or bruises, no swelling or redness. If anything, the right leg was a little paler than the left. Was it possible that he had a clot, not in his brain where it would cause a stroke, but in his leg? Kilian ordered some morphine for the pain and called the vascular surgeon.

When the surgical resident arrived, the older man was sitting up in bed. His mind was clear, despite the hefty dose of opiate. The resident introduced himself then reviewed the events of the night. Initially, you thought you were having a stroke, the resident stated. Yes, the patient said, but then added thoughtfully that a stroke didn’t really make sense. His leg had been weak, but he had no weakness anywhere else. Usually a stroke devastating enough to paralyze a leg will also affect the arm on the same side, and his arm was fine.

On exam, the surgeon noticed that the right leg was still weaker than the left leg. Was that from a stroke too small to be seen on the scan or from the pain he still had, despite the morphine he’d been given? It wasn’t clear. Or maybe this was a T.I.A. — a transient ischemic attack — a temporary stroke where blood flow is restored before permanent damage is done. But then what about this pain? Strokes and T.I.A.’s are usually painless. Could he have a clot blocking the blood flow to the right leg? And if so, why? Almost all clots like this occur in patients with a chronically irregular heartbeat — a condition known as atrial fibrillation. This patient had no history of that.

He explained his thinking to the patient, who listened, nodding. As he moved toward the door, the doctor-patient couldn’t resist adding one more possibility to the list: “Could I have dissected my aorta?” he asked. The aorta is the thick, muscular blood vessel that delivers oxygenated blood from the heart to the rest of the body. Sometimes the inner lining of the artery can get torn — often from a spike in blood pressure. When that happens, blood pours into the tear, creating a separate channel between the inner layers of the vessel and the outer muscular wall. This new channel can compress the arteries leading off the aorta, starving the tissues they normally feed.

The resident asked the patient whether he’d had chest pain or back pain. These were by far the most common symptoms of a dissection. No, the patient said, he’d had neither of those. That made the diagnosis fairly unlikely, the resident said. Also, he’d seen several dissections in the course of his training. Those patients, all men in their 50s or 60s, were writhing in pain.

The resident found Kilian, and they discussed the possibilities. The resident thought the most likely was that the patient did have a clot blocking blood to the leg and suggested Kilian start him on the blood thinner Heparin.

The blood clot would account for the pallor in his leg, Kilian thought, but it wasn’t a perfect fit. What else could it be? More important, was there any condition she hadn’t yet ruled out that could worsen if she started a blood thinner? Like the doctor-patient, she thought of an aortic dissection. It was unlikely given the patient’s age and the absence of chest pain, but if he had torn his aorta, anticoagulation could cause the patient to bleed to death. She sent the patient back to get a CT scan of his chest and abdomen.

3. Resolution

A few minutes later, Kilian heard her name paged with instructions to call radiology. The radiologist was breathless with excitement. “You can’t believe the size of this dissection,” she told Kilian. “It starts up in his carotid arteries and continues through the heart. I don’t know where it ends, because he’s still getting scanned. Just wanted to give you a heads up.”

Kilian quickly dialed up the CT scan on her desktop computer. She could see the wide mouth of the aorta as well as a new channel filled with the blood that had flowed through the tear in the inner layers of the vessel. His aorta now looked like some strange double-barreled shotgun. This guy needed surgery immediately. In a case like this, the chance of mortality increases 1 percent to 2 percent with every passing hour.

Within the hour, the patient was on his way to the operating room. The next day, Kilian went by the surgical I.C.U. to see how the patient fared. She had seen only two other patients with this condition; both were younger than her patient and one had died. Not this guy, though. Twenty-four hours after his operation, he was sitting up in a chair. He looked great.

An aortic dissection is one of the classic difficult diagnoses in medicine. Far too often it’s not even considered. Or as in the case of John Ritter, who died of a dissection in 2003, it is considered but too late. (That case is now being litigated in a Glendale, Calif., courtroom, with Ritter’s family charging wrongful death.) In the case of this elderly patient, it was the very hardest kind of diagnosis — an unusual presentation of an unusual disease. A dissection without the usual accompanying chest or back pain. In spite of that, two physicians reached this diagnosis coming at it from two different perspectives — that of an E.R. doctor who conscientiously made sure that she first did no harm and that of a patient who couldn’t stop himself from thinking like a doctor.