A Weakness That Comes and Goes

A vague, frustrating symptom gets a precise explanation

By Pamela Grim|Friday, November 01, 2002
The patient was 29, previously healthy, well built, and male. I gazed around at the faces of the second-year medical students and said, "So, Mr. Bulinski, what brings you here today?"

"I get weak," he said. With his muscular arms and forearms, he looked as though he could flatten me without a second thought.

"OK ... " I said. I had warned my students to give the patient time to tell his story. But he said nothing more.

"How do you mean weak?"

"Well, I mean weak, like my muscles get weak."

"How long has this been going on?"

"Oh, two, three years now."

"You've been weak for two or three years?"

"Yeah," he said. We all stood waiting until he finally added, "It comes and goes."

"Have you seen a doctor for it?"

"I'm a truck driver. I've seen doctors all over America."

"What did they say you had?"

"They didn't know."

"Did you have any tests done?"

"Yeah, lots of tests." I waited. Nothing. "And what did the tests show?"

"They never told me."

He would have been a terrible teaching case, except that millions of patients are just like him. The dogma in medicine is that 80 percent of the time, you can make the diagnosis with the history. Chalk this guy up to the other 20 percent.

Maybe the physical exam would give us some clues. I walked the students through a focused exam—the most we can do in the emergency room. We found nothing. So I moved on to my own personal method of evaluation: If you don't know what's going on with a patient and you are certain that he won't die tonight, then just do a test. If it's normal, send the patient home. I like to do electrocardiograms. You can drill the students on what a normal ECG looks like and how it reflects the electrical activity in the heart, and, once in a while, you can even pick up something unexpected.

"Show me how to do an ECG," I said to the students.

Four brilliant kids, tops in almost everything they ever tried, became instant klutzes. Physical medicine confounded them. The sticky tabs and alligator clips of ECG leads had never appeared on a written exam. When they finally brought the ECG back to me, I was in the conference room drawing the chief characteristics of a normal test result on the chalkboard. I looked down at the ECG, and it stopped me cold. It was so weirdly abnormal that I turned the paper around to see if it was upside down.

For a moment, I wasn't even sure why it was abnormal. After all, the rhythm was a simple sinus tachycardia. That means the heart rate is normal but rapid—more than 100 beats per minute. The QRS complexes, which demonstrate the electrical function of the ventricles, looked all right. But the ST segment, the point in the cardiogram when the heart returns to its electrical baseline in order to gear up for the next beat, looked flattened. And there was a squiggle after the T wave—a big squiggle that didn't belong there. About the only time you see a squiggle like that is when you have a big U wave—and about the only time you see a U wave is when there is hypokalemia, low potassium. But I had never seen U waves quite that big. And it occurred to me that this man really did get weak. He had a real disease, and I knew what that disease was.

Only one disease that I knew of was associated with both weakness and low potassium. I had learned about it as a first-year student and would have forgotten it, except that I once had a professor who was the world's expert on "channelopathies," diseases of cell-membrane channels.

"OK," I told the students. "We have a patient here who gets weak intermittently but who otherwise appears healthy. His cardiogram shows large U waves, which suggest . . ."

"Low potassium," one of the students said immediately.

"Good. Now the question is, what disease causes episodic weakness and low potassium?"

"Hypokalemic periodic paralysis," one of the other students said. He sounded almost bored.

"Oh," I said, trying not to look disappointed. The disease is rare, very rare. I thought these students would have never heard of it.

"What causes the disease?"

The third student chimed in. "It's a genetic disease caused by a mutation in the gene coding for a calcium channel in skeletal muscles. Abnormalities in calcium movement are associated with problems in potassium concentrations. This inhibits muscle cell function and causes weakness." The kid smiled at me smugly.

What all that means is that muscles, like batteries, rely on electrically charged ions. When a nerve tells a muscle to flex, it is actually stimulating a massive flux of ions across the membrane of the muscle cell. These ions—sodium, potassium, and calcium—are shuttled from one side to the other through channels, each one specific to one type of ion. Given the right combination of energy and ions, the muscle contracts. Sometimes, though, these channels are flawed—often because of a genetic mutation—and under certain circumstances they can malfunction. When channels malfunction, it's more difficult to move electrolytes around. And if they don't move, muscles don't have enough oomph, and the person gets weak.

"OK," I said. "Can you tell me what the clinical characteristics of periodic paralysis are?"

"Intermittent episodes of severe weakness, lasting from three hours to two days," student number one told me. "Usually attacks are precipitated by meals that are high in carbohydrates or salt. Or they can occur after strenuous exercise." How could they have known this? I looked around at their faces. Then I realized that for these students, this was everyday medicine. All they had ever done was to study rare diseases—"good teaching cases"—that illustrate some key point in basic science but are seen only in textbooks. They didn't see that medicine is, in fact, a long series of common disease, routine stuff, colds and sore throats, ovarian cysts, and bad bruises.

I had one last chance to play the instructor. "If this is a genetic disease, it must be present at birth. If so, why didn't this patient have symptoms until the last two or three years?"

The student hung his head. He didn't know that the reason remains a medical mystery. What doctors do know is that patients with this condition can usually manage their symptoms either with medication or a low-carbohydrate diet.

The patient's blood test showed a potassium level of 2.7, which is low, low, low. Then his wife showed up with copies of charts from previous admissions. Doctors at different hospitals had documented the weakness and the hypokalemia, but they had never put the two of them together.

One of the students broke the news to Mr. Bulinski. "You have hypokalemic periodic paralysis."

"Yeah," the patient said skeptically.

"It's a channelopathy," the student explained. "It's very rare," the student added in a "you should be proud" voice.

Well, I thought, these guys had just spent the better part of two years learning nothing but facts. They would never know more about the science of medicine. Now they had to learn the art.
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