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What is polio?

Visit us (http://www.khanacademy.org/science/healthcare-and-medicine) for health and medicine content or (http://www.khanacademy.org/test-prep/mcat) for MCAT related content. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Ian Mannarino.

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Video transcript

- Poliomyelitis, also known as Polio, is caused by the Polio virus. Now the Polio virus causes a debilitating muscle disorder. And this is caused by damaging neurons that extend from the central nervous system. Now, the central nervous system is composed of the brain and the spinal cord. Now these neurons that extend from the spinal cord innervate, or provide tone and responsiveness, to the muscles of the body, so the muscles of the arms are innervated, and the muscles of the legs are innervated by motor neurons. Now Polio affects these motor neurons, causing damage to them and disability. And as you can see over here, this man with the cane actually was afflicted by polio when he was younger, and has faced the consequences of the motor neuron damage. His muscles in his right leg have atrophied, or in other words they've lost their muscle tone, and they've kind of shriveled up. Because without any input from the motor neurons, they just shrivel up. Now another way to think of neurons are like electrical wires that provide electricity to your home. If you shut off these electrical wires, obviously your home will no longer have any electricity, and so you can't power your home with the energy that you need from these wires. The motor neurons act in the same way, and you can no longer power your muscles from these neurons after the Polio virus damages these motor neurons. And so that's what happened to this man here. Now another prominent figure in history that was diagnosed with the Polio virus was Franklin Delano Roosevelt, FDR. The 32nd President of the United States. He was infected in 1921, which left him permanently damaged from the waist down. But interestingly enough, FDR didn't want to show the general public his ailment, so there are actually very few pictures of FDR in a wheelchair. Now though he was diagnosed with Polio, it's now actually believed that he was misdiagnosed, and that his symptoms were actually caused by something called Guillain-Barré syndrome. This would mean that his diagnosis of Polio may have been one of the most prominent misdiagnoses in history. Now there's a few reasons that we believe he may have been afflicted by Guillain-Barré syndrome, and that's for two distinct reasons. Most commonly, Polio affects children. Particularly children who are less than six months of age. So FDR contracting Polio when he's 39 is not very likely. And second of all, Polio causes something called asymmetric paralysis. Asymmetric paralysis. The muscles are damaged in a somewhat random process. So having both legs damaged equally is unlikely. I mean think about it. The Polio virus might affect the neurons on this side, but the neurons providing energy to his left leg may not be damaged. And so you might actually see what happened to our guy over here, where his right leg is primarily affected. Franklin Delano Roosevelt, both his legs were affected. And this is called symmetric paralysis. And Guillain-Barré syndrome is known to cause this symmetric paralysis. So how does Polio infect a person, or specifically, most likely a child under the age of six months? So the virus usually comes in and enters through the mouth and actually starts, goes down and starts replicating in the throat. So it starts increasing in number. And it can also travel down into the stomach and the intestines. And it might replicate in the intestines as well. Once the Polio virus has increased in number, somehow, and we're not actually 100 percent sure how, it gets into the central nervous system, into the spinal cord, where it can damage the motor neurons that provide energy to the muscles. Now not everybody who contracts Polio has this degredative damage to their motor neurons. In fact, 90 to 95 percent of people who are acutely infected are asymptomatic, so they don't have any symptoms. Asymptomatic. Whereas about five to ten percent of people get only mild symptoms. Mild symptoms. So what are the mild symptoms? Well, these are symptoms that you generally see, or can see, with any virus. You might get fever, headache, fatigue. So this is really from your immune system trying to thwart the virus, trying to fight it off. So a patient might also have some throat pain, and this is due to replication in the throat. The patient may also experience abdominal pain, stomach pain, or nausea, or vomiting. And that's from the replication that occurs in the intestine. Patients that have these mild symptoms generally recover, and don't experience the muscle weakness. In fact, only about a half a percent of people who are infected acutely by the virus experience muscle symptoms. And these muscle symptoms may include weakness, atrophy, or in other words, the muscles shrivel up due to under use, since the motor neurons are damaged by the virus. Low muscle tone, and muscle twitching, which can occur by abnormal firing by some of the neurons that may still be providing innervation to the muscles. Now you might think, wait, only half a percent of people face this motor neuron damage and muscle paralysis? That seems like such a small number. Well, let's put it into a little bit of perspective. If we take a look at a football stadium of about maybe 100,000 people here, about 90,000 people may be asymptomatic. Only about 5,000 to 10,000 people will actually have symptoms. So maybe this group of people right here. And the people that experience muscle weakness, maybe all of the members of the band and all of the members, the players, the coaches, of both teams will be affected. Now still this may not seem like a lot in comparison to all the other people who are asymptomatic, who aren't feeling any symptoms. But the disease can spread very easily from person to person, which leads to more and more people being affected by the Polio virus. So though it's a small amount of people, the damage can be very severe. In fact, we think about maybe the arms and the legs, right, you're not able to walk or move your arms. But what happens if the muscles that are used in respiration, in breathing, are paralyzed? What if these muscles are damaged? Then patients can no longer breathe on their own. So this can be very debilitating, and fatal if it affects the respiratory muscles. But what other muscles do we not commonly think about? Well, there's also muscles in our throat. And our heart is a muscle. There's actually nerves that extend from the lower part of the brain. It's also known as the brain stem. This area has some neurons that control speech as well as the functioning of the heart. So if we go ahead and take a view of the underside of the brain, you can see, right here's the brain stem, and there's all these nerves that extend from this part of the central nervous system. And these are known as the cranial nerves. So nerves that might be affected are the glossopharyngeal, cranial nerve nine, the vagus nerve, cranial nerve 10, the accessory nerve, cranial nerve 11, and the hypoglossal nerve, cranial nerve 12. So these are motor neurons that can be damaged as well. So what are some of the symptoms you might see? The glossopharyngeal nerve assists in allowing you to swallow. So patients may have difficulty swallowing. The vagus nerve controls movement of the throat. So I wouldn't be able to talk to you right now without the vagus nerve. The vagus nerve innervates other parts of the body as well, including the muscle known as the heart. So a patient who has their cranial nerves affected should have close cardio-vascular monitoring. Their heart should be monitored very carefully. And then finally the accessory, this controls shoulder movement, and the hypoglossal controls tongue movement. So you see, not only the muscles of the arms and the legs are affected, but also some patients can get this Bulbar Palsy, which is the name for this cranial nerve defect. So when these cranial nerves are affected, so a patient might have difficulty talking, difficulty swallowing, and have heart problems as well, difficulty actually maintaining their heart's function, this cranial nerve paralysis is known as Bulbar Palsy. And "bulb" just refers to the brain stem, it's the bulb of your brain. And "palsy" means paralysis. So there you go. Nerves from the brain stem are paralyzed, Bulbar Palsy. Finally, after the initial infection, and the patient has had some muscle damage, years later, a patient may get something called Post-Polio syndrome. Post-Polio syndrome occurs years after the initial infection, it's slow, and you get progressive muscle wasting, so these muscles that may already be weak can get weaker and weaker. Now the prevalence of this is not entirely clear. It doesn't occur to all patients who end up getting the muscle weakness. But we do know that some of these patients get this onset of deterioration and dysfunction of the muscles years after the initial infection.