Definition
Acute flaccid paralysis is a sudden loss of strength in the arms or legs over a few days, where the muscles go limp like a deflated balloon instead of stiffening up. It is not the name of one disease. It is a group of symptoms that can come from many causes. The world counts these cases to check whether polio is still around.
Why do the muscles go limp instead of stiff?
When the strings of a puppet are cut, its arms and legs drop to the floor. Acute flaccid paralysis is much like that. A healthy person's arm or leg muscles suddenly lose strength and go limp within a few days. Unlike spastic paralysis, where muscles turn rigid, the muscles here become floppy like a deflated balloon.
The key point is that this is not a single disease, but a collection of symptoms (a syndrome) that can have many causes. It can happen when something goes wrong with the nerve network, such as Guillain-Barre syndrome, where the nerves become inflamed, other viral infections, or an injury to the spine.
Movement commands start in the brain and travel through the spinal cord and peripheral nerves to the muscles all over the body. If the peripheral motor nerves or spinal cord cells that connect directly to the muscles are damaged, the muscles lose their tension and go slack.
The sharpest radar in the polio watch network
Health experts pay close attention to this symptom because of polio. Of the people infected with the polio virus, only a small share ever develop paralysis. Most of the rest pass through without anyone noticing.
Most infected people have no symptoms at all, or just a mild illness like a cold. That makes it very hard to tell where the virus is hiding just by looking at people. So disease-control systems around the world act fast when a child develops a floppy, limp paralysis in the legs. They collect a stool sample and test it carefully for the virus.
Even where polio is gone, this symptom still shows up in some people every year because of other nerve disorders. So the people doing the watching check two things together: are we finding enough paralysis cases, and did we test every case we found? The World Health Organization uses a benchmark for this. It asks whether a certain number of non-polio flaccid paralysis cases are reported each year per 100,000 children under 15, as a sign that the watch network is working. Only when the virus does not turn up in the paralysis cases we find can we say the disease is not there.
To be more precise: one system, two sets of eyes
Let's look at why counting this symptom counts as surveillance. Starting from one confirmed patient and tracing the people they met is contact tracing. Continuously watching where and how often this symptom appears across a whole population is surveillance. They work at different levels, and surveillance is aiming not to count a disease that is there, but to confirm that it is not there.
To drive out a disease like polio, health workers do more than find and test every patient with paralyzed legs. They also collect water from city sewers and search it for the virus. Finding paralysis cases at hospitals and finding the virus in sewage run together as two branches of one surveillance system. That is because people who are infected but have no symptoms can shed the virus, and it may be flowing through the sewers.
Clinical surveillance, which looks for patients, and environmental surveillance, which checks sewage, are the two sets of eyes. Only when both work together is the net free of gaps. When these two paths lead to a world where the virus has completely disappeared, we can finally reach the end goal: eradication of the disease.
Common misconceptions
Acute flaccid paralysis is a specific disease that you only get when infected with the polio virus.
It is a 'syndrome' (a group of symptoms) in which muscles lose strength. Polio is one cause, but so are Guillain-Barre syndrome, spinal nerve injuries, and other problems.
If someone has acute flaccid paralysis, polio must be circulating in that area.
Other nerve disorders can cause this symptom too, so the paralysis alone does not tell you. What settles it is whether testing finds the polio virus. In fact, even places without polio see some of these paralysis cases every year, and testing every one of them is exactly what surveillance does.
Where you meet it
Even in countries where polio has completely disappeared, a small number of acute flaccid paralysis cases still occur naturally each year because of nerve damage and other causes.
When a child suddenly loses strength in the legs and cannot walk, it is reported to health authorities, and a stool sample is tested for a virus that could be the cause.
In one sentence
It is a symptom in which muscles go limp and lose strength, and it is the measure counted across a whole population to check whether polio is still around.