Poliomyelitis – causes, symptoms, diagnosis, treatment, pathology

Poliomyelitis – causes, symptoms, diagnosis, treatment, pathology

In poliomyelitis, also called polio, “polio”
refers to the poliovirus, which is an enterovirus that invades the intestines, “myel” refers
to the spinal cord which is affected in the disease, and -itis refers to inflammation. So poliomyelitis is an enteroviral disease
first enters the body through the intestines, but then spreads and causes nerve injury in
the spinal cord. Former US president Franklin D. Roosevelt
contracted polio when he was a baby, and it left him wheelchair-bound. Broadly speaking, the nervous system consists
of two parts. The central nervous system consists of the
brain and the spinal cord. So the peripheral nervous system includes
the nerves that fan out from the central nervous system to reach the skin, muscles, and organs. Now looking at the cross-section of the brain,
there’s gray matter at the periphery of the brain. This is called the cerebral cortex and it
consists of nerve cell bodies. Just inside the gray matter of the brain,
is the white matter, and it consists of nerve axons. In contrast, if you look at the cross-section
of the spinal cord, the white matter is on the outside and the gray matter is on the
inside, and overall it kinda looks like a butterfly. If we draw a horizontal line through the spinal
cord, the front half is the anterior or ventral half, and the back half is the posterior or
dorsal half. And the butterfly wings are sometimes referred
to as horns; so we have two dorsal horns that contain cell bodies for sensory neurons and
two ventral horns that contain cell bodies for motor neurons. So for example, if you step on a lego in your
living room, the sensation of discomfort is carried from the nerves in your foot, through
the peripheral nervous system to reach the dorsal horn in the spinal cord. It then travels up the spinal cord to the
brain, letting you know that there’s tissue damage. In response, your brain sends a message through
the upper motor neurons, which are part of the cerebral cortex, and down the spinal cord
to a lower motor neuron which is located in the anterior horn of the spinal cord. From there, the signal finally gets delivered
to the leg muscles and allows you to lift your foot. In addition to sending signals from the brain,
these lower motor neurons also release trophic or growth factors that promote muscle growth
in the muscle that they innervate. Polio is a viral infection caused by poliovirus
which is a single-stranded RNA virus that’s protected by a capsid, which is a protein
coat. Polio mainly affects children under the age
of 5 and it’s spread by fecal-oral transmission, which means that the virus usually enters
the body through contaminated food and water that goes in the mouth. It’s also transmitted when an infected person
sneezes or coughs, which spreads thousands of virus-containing droplets into the local
area. Once the virus enters the body, it binds to
mucosal cells of the oropharynx and small intestine, and gets inside those cells, and
then releases its RNA. The viral RNA uses the cell’s RNA polymerase
enzyme to make copies of itself, and then the new RNA copies hijack the ribosomes that
normally make proteins for the cell, and forces the ribosomes to make viral proteins instead. The viral protein and RNA self-assemble into
lots of new polioviruses, and within days, they cause the mucosal cell to lyse, releasing
the newly formed viruses which make their way to nearby lymph nodes and eventually into
the bloodstream. Poliovirus prefers to infect motor nerves,
so oftentimes it will leave a blood vessel and get into the interstitial tissue of muscle
tissue. From there, poliovirus invades the motor neuron
and travels retrograde – meaning backwards up through the axon – to the anterior horn
of the spinal cord. Infected motor neurons attract immune cells
like neutrophils and macrophages which cause inflammation and damage to the spinal cord. As infected motor neurons die, the muscles
of the trunk and limbs no longer receive signals from the brain or trophic factors which causes
the muscles to start to atrophy and become weak. On rare occasions, poliovirus can cause bulbar
polio, which is when it affects a part of the brain stem that has motor nerves involved
in speaking and swallowing. This part of the brain stem also sends motor
nerves to the diaphragm and so if they get damaged, it can impair breathing. Some people develop post-polio syndrome many
decades after the initial infection. To understand this, it’s important to know
that when polio damages some motor neurons, other nearby healthy motor neurons form collateral
branches to innervate the muscles that have lost their innervation. Now, over time, the natural process of aging
causes motor neurons to die. So when a motor neuron dies from the aging
process, a person doesn’t just lose function in the muscle cells that were normally innervated,
but also in the other muscle cells that were innervated by collateral branches of that
motor neuron. Most individuals infected by poliovirus have
no symptoms, in fact, polio only causes paralytic disease about 1% of the time. Symptoms of paralytic disease from polio include
high fevers, intense muscle pain from spasms and weakness, loss of muscle reflexes, and
eventually paralysis. Usually the paralysis develops over a few
days and is asymmetric; most often affecting larger proximal muscles especially in the
legs – like the thighs. Infants with poliomyelitis often develop acute
flaccid paralysis where they go limp, kinda like a rag doll – it’s sometimes called
floppy baby syndrome. Ultimately, if the motor nerves to the diaphragm
are affected, it can cause difficulty breathing and death. Diagnosis of polio is based on the recovery
of poliovirus from a stool sample or a throat swab. Poliomyelitis can also be diagnosed by a lumbar
puncture where cerebrospinal fluid is removed from the lumbar sac with a needle. The cerebrospinal fluid might have an increased
number of white blood cells or poliovirus RNA. There’s no specific treatment for polio,
but the key is to support individuals as they deal with the acute infection. Typically that means pain medication to help
with muscle spasms and decompression of the bladder. Finally, individuals might need respiratory
support if they have trouble breathing, something that historically required a device called
the iron lung. Fortunately, it’s possible to prevent infection
with a vaccine, and humans are the only natural host for the virus. There’s inactivated poliovirus vaccine or
IPV, which is where dead or inactive virus is injected into the muscle and there’s
oral poliovirus vaccine or OPV, which is a weakened strain of the live virus. Very rarely, about one time in a million doses
of vaccine, the weakened virus in OPV mutates within the intestines of the person getting
vaccinated, and it reverts to a form of the poliovirus that can cause paralytic polio–called
vaccine-associated paralytic polio or VAPP for short. VAPP usually affects close contacts like someone
that lives in the same home. But sometimes the weakened virus reverts to
a form that can cause vaccine-derived poliovirus or VDPV, which not only affects close contacts,
but can cause outbreaks of poliomyelitis in communities that have low rates of vaccine
coverage. VAPP and VDPV are both worrisome, but the
events are relatively rare and mostly affect unvaccinated individuals, so the benefits
of polio vaccine outweigh the risks, and polio vaccine is recommended. In fact, because of IPV and OPV, poliovirus
is nearly eliminated worldwide. It only circulates endemically in a few areas
within countries like Afghanistan, Pakistan, and Nigeria – where there’s poor public
health and limited vaccine coverage. Alright, as a quick recap. Poliomyelitis or polio, is a viral disease
caused by the poliovirus. It’s transmitted by fecal-oral transmission
or by aerosol droplets. The virus replicates in the small intestine
and oropharynx before it spreads to the central nervous system and causes nerve injury. Diagnosis is done by the recovery of the poliovirus
from a stool or throat, or by analysis of the cerebrospinal fluid. Lastly, there is no cure for polio, but it
can be prevented by polio vaccines.


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