Poliomyelitis
Poliomyelitis
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Classification and
external resources
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A man with an atrophied right leg due to
poliomyelitis
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Poliomyelitis /poʊlioʊmaɪəlaɪtɪs/, often called polio or infantile
paralysis, is an acute, viral, infectious disease spread from
person to person, primarily via the fecal-oral route.[1] The term
derives from the Greek poliós (πολιός), meaning "grey", myelós (µυελός“marrow”), referring to the
grey matter of the spinal cord, and the suffix -itis, which denotes inflammation.,[2] i.e.,
inflammation of the spinal cord’s grey matter, although a severe infection can
extend into the brainstem and even higher structures, resulting in polioencephalitis,
producing apnea that requires mechanical
assistance such as an iron
lung.
Although approximately 90% of polio infections cause no symptoms at all, affected individuals can
exhibit a range of symptoms if the virus enters the blood stream.[3] In about 1%
of cases, the virus enters the central nervous system, preferentially infecting and
destroying motor neurons, leading to muscle weakness and acute flaccid paralysis. Different types of paralysis
may occur, depending on the nerves involved. Spinal polio is the most common
form, characterized by asymmetric paralysis that most often involves the legs.
Bulbar polio leads to weakness of muscles innervated by cranial nerves. Bulbospinal polio is a
combination of bulbar and spinal paralysis.[4]
Poliomyelitis was first recognized as a distinct
condition by Jakob Heine in 1840.[5] Its
causative agent, poliovirus, was identified in 1908 by Karl Landsteiner.[5] Although
major polio epidemics were
unknown before the late 19th century, polio was one of the most dreaded childhood
diseases of the 20th century. Polio epidemics have crippled
thousands of people, mostly young children; the disease has caused paralysis
and death for much of human history. Polio had existed for
thousands of years quietly as an endemicpathogen until the
1880s, when major epidemics began to occur in Europe; soon after, widespread
epidemics appeared in the United States.[6]
By 1910, much of the world experienced a dramatic
increase in polio cases and epidemics became regular events, primarily in
cities during the summer months. These epidemics — which left thousands of
children and adults paralyzed — provided the impetus for a "Great
Race" towards the development of a vaccine. Developed in the 1950s, polio
vaccines have reduced the global number of polio cases per year from many
hundreds of thousands to under a thousand today.[7] Enhanced vaccination efforts led
by Rotary International, the World
Health Organization, and UNICEF should result in global eradication of the disease.[8][9][10]
Classification
Outcomes of poliovirus
infection
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Outcome
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Proportion of cases[4]
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Asymptomatic
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90–95%
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Minor illness
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4–8%
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Nonparalytic aseptic
meningitis |
1–2%
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Paralytic poliomyelitis
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0.1–0.5%
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— Spinal polio
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79% of paralytic cases
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— Bulbospinal polio
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19% of paralytic cases
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— Bulbar polio
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2% of paralytic cases
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The term "poliomyelitis" is used to
identify the disease caused by any of the three serotypes of
poliovirus. Two basic patterns of polio infection are described: a minor
illness which does not involve the central nervous system (CNS),
sometimes called abortive poliomyelitis, and a major illness involving the CNS,
which may be paralytic or nonparalytic.[11] In most
people with a normal immune system, a poliovirus infection is asymptomatic. Rarely, the infection
produces minor symptoms; these may include upper respiratory tract infection (sore throat and fever), gastrointestinal disturbances
(nausea, vomiting, abdominal pain, constipation or, rarely,
diarrhea), and influenza-like illness.[4]
The virus enters the central nervous system in
about 3% of infections. Most patients with CNS involvement develop nonparalyticaseptic meningitis, with symptoms of headache,
neck, back, abdominal and extremity pain, fever, vomiting, lethargy, and irritability.[2][12] About one
to five in 1000 cases progress to paralytic disease, in
which the muscles become weak, floppy and poorly controlled, and, finally,
completely paralyzed; this condition is known as acute flaccid
paralysis.[13] Depending
on the site of paralysis, paralytic poliomyelitis is classified as spinal,
bulbar, or bulbospinal. Encephalitis, an infection of the brain
tissue itself, can occur in rare cases, and is usually restricted to infants.
It is characterized by confusion, changes in mental status, headaches, fever,
and, less commonly, seizures and spastic paralysis.[14]
Cause
Main article: Poliovirus
A TEM micrograph of poliovirus
Poliomyelitis is caused by infection with a
member of the genus Enterovirus known as poliovirus (PV). This
group of RNA virusescolonize the gastrointestinal tract[1] —
specifically the oropharynx and the intestine. The incubation time (to the
first signs and symptoms) ranges from three to 35 days, with a more common span
of six to 20 days.[4] PV infects and causes disease in humans
alone.[3] Its structure is very
simple, composed of a single (+)
sense RNA genome enclosed in a protein shell
called a capsid.[3] In addition
to protecting the virus’s genetic material, the capsid proteins enable
poliovirus to infect certain types of cells. Three serotypes of
poliovirus have been identified—poliovirus type 1 (PV1), type 2 (PV2), and type
3 (PV3)—each with a slightly different capsid protein.[15]All three are extremely virulent and produce
the same disease symptoms.[3] PV1 is the
most commonly encountered form, and the one most closely associated with
paralysis.[16]
Individuals who are exposed to the virus, either
through infection or by immunization with polio
vaccine, develop immunity. In immune individuals, IgA antibodies against
poliovirus are present in the tonsils and gastrointestinal tract,
and are able to block virus replication;IgG and IgM antibodies
against PV can prevent the spread of the virus to motor neurons of the central nervous system.[17] Infection
or vaccination with one serotype of poliovirus does not provide immunity
against the other serotypes, and full immunity requires exposure to each
serotype.[17]
A rare condition with a similar presentation,
nonpoliovirus poliomyelitis, may result from infections with nonpoliovirus enteroviruses.[18]
Transmission
Poliomyelitis is highly contagious via the
oral-oral (oropharyngeal source) and fecal-oral (intestinal source) routes.[17] In endemic
areas, wild polioviruses can infect virtually the entire human population.[19] It is
seasonal in temperate climates, with peak transmission
occurring in summer and autumn.[17] These
seasonal differences are far less pronounced in tropical areas.[19] The time
between first exposure and first symptoms, known as the incubation period, is usually six to
20 days, with a maximum range of three to 35 days.[20] Virus
particles are excreted in the feces for several weeks following
initial infection.[20] The disease
is transmitted primarily
via the fecal-oral route, by ingesting contaminated
food or water. It is occasionally transmitted via the oral-oral route,[16] a mode
especially visible in areas with good sanitation and hygiene.[17] Polio is
most infectious between seven and 10 days before and after the appearance of
symptoms, but transmission is possible as long as the virus remains in the
saliva or feces.[16]
Factors that increase the risk of polio infection
or affect the severity of the disease include immune deficiency,[21] malnutrition,[22] tonsillectomy,[23] physical
activity immediately following the onset of paralysis,[24] skeletal muscle
injury due to injection of vaccines
or therapeutic agents,[25] and pregnancy.[26] Although
the virus can cross the maternal-fetal barrier during
pregnancy, the fetus does not appear to be affected by either maternal
infection or polio vaccination.[27] Maternal
antibodies also cross the placenta, providing passive immunity that
protects the infant from polio infection during the first few months of life.[28]
As a precaution against infection, public swimming pools were often
closed in affected areas during poliomyelitis epidemics.
Pathophysiology
Poliovirus enters the body through the mouth,
infecting the first cells with which it comes in contact — the pharynx and intestinal mucosa. It gains entry by binding to
an immunoglobulin-like receptor,
known as the poliovirus receptor or CD155, on the cell membrane.[29] The virus
then hijacks the host cell's own
machinery, and begins to replicate. Poliovirus divides within
gastrointestinal cells for about a week, from where it spreads to the tonsils (specifically
the follicular
dendritic cells residing within the tonsilar germinal centers), the intestinal lymphoid tissue including
the M cells of Peyer's patches, and the deep cervical and mesenteric lymph nodes, where it multiplies
abundantly. The virus is subsequently absorbed into the bloodstream.[30]
Known as viremia, the presence of virus in the
bloodstream enables it to be widely distributed throughout the body. Poliovirus
can survive and multiply within the blood and lymphatics for long periods of
time, sometimes as long as 17 weeks.[31] In a small
percentage of cases, it can spread and replicate in other sites, such as brown fat, the reticuloendothelial tissues,
and muscle.[32] This
sustained replication causes a major viremia, and leads to the development of
minor influenza-like symptoms. Rarely, this may progress and the virus may
invade the central nervous system, provoking a local inflammatory response. In most cases, this causes a
self-limiting inflammation of the meninges, the layers of tissue
surrounding the brain, which is known as nonparalytic
aseptic meningitis.[2]Penetration of the CNS provides
no known benefit to the virus, and is quite possibly an incidental deviation of
a normal gastrointestinal infection.[33] The
mechanisms by which poliovirus spreads to the CNS are poorly understood, but it
appears to be primarily a chance event—largely independent of the age, gender,
or socioeconomic position of
the individual.[33]
Denervation of skeletal muscle tissue secondary to poliovirus infection can lead
to paralysis.
In around 1% of infections, poliovirus spreads
along certain nerve fiber pathways, preferentially replicating in and
destroying motor neurons within the spinal cord, brain stem, or motor cortex. This leads to the development
of paralytic poliomyelitis, the various forms of which (spinal, bulbar, and bulbospinal)
vary only with the amount of neuronal damage and inflammation that occurs, and
the region of the CNS affected.
The destruction of neuronal cells produces lesions within the spinal ganglia; these may also occur in the reticular formation, vestibular nuclei, cerebellar vermis, and deep cerebellar nuclei.[33] Inflammation
associated with nerve cell destruction often alters the
color and appearance of the gray matter in the spinal column, causing it to appear reddish
and swollen.[2] Other
destructive changes associated with paralytic disease occur in the forebrain region,
specifically the hypothalamus and thalamus.[33] The
molecular mechanisms by which poliovirus causes paralytic disease are poorly
understood.
Early symptoms of paralytic polio include high
fever, headache, stiffness in the back and neck, asymmetrical weakness of
various muscles, sensitivity to touch, difficulty swallowing, muscle pain, loss of superficial and deep reflexes, paresthesia (pins and
needles), irritability, constipation, or difficulty urinating. Paralysis
generally develops one to ten days after early symptoms begin, progresses for two
to three days, and is usually complete by the time the fever breaks.[34]
The likelihood of developing paralytic polio
increases with age, as does the extent of paralysis. In children, nonparalytic
meningitis is the most likely consequence of CNS involvement, and paralysis
occurs in only one in 1000 cases. In adults, paralysis occurs in one in 75
cases.[35] In children
under five years of age, paralysis of one leg is most common; in adults,
extensive paralysis of the chest and abdomen also
affecting all four limbs — quadriplegia — is more
likely.[36] Paralysis
rates also vary depending on the serotype of the infecting poliovirus; the
highest rates of paralysis (one in 200) are associated with poliovirus type 1,
the lowest rates (one in 2,000) are associated with type 2.[37]
The location of motor neurons in theanterior horn cells of the spinal column
Spinal polio, the most common form of paralytic
poliomyelitis, results from viral invasion of the motor neurons of the anterior
horn cells, or the ventral (front) grey matter section in
the spinal column, which are responsible for
movement of the muscles, including those of thetrunk, limbs, and the intercostal muscles.[13] Virus
invasion causes inflammation of the nerve cells, leading to damage or destruction
of motor neuron ganglia. When spinal neurons die, Wallerian degeneration takes
place, leading to weakness of those muscles formerlyinnervated by the
now-dead neurons.[38] With the
destruction of nerve cells, the muscles no longer receive signals from the
brain or spinal cord; without nerve stimulation, the muscles atrophy, becoming weak, floppy and
poorly controlled, and finally completely paralyzed.[13]Progression to maximum
paralysis is rapid (two to four days), and is usually associated with fever and
muscle pain.[38] Deep tendonreflexes are also affected, and are
usually absent or diminished; sensation (the
ability to feel) in the paralyzed limbs, however, is not affected.[38]
The extent of spinal paralysis depends on the
region of the cord affected, which may be cervical, thoracic, or lumbar.[39] The virus
may affect muscles on both sides of the body, but more often the paralysis is asymmetrical.[30] Any limb or
combination of limbs may be affected—one leg, one arm, or both legs and both
arms. Paralysis is often more severe proximally (where the
limb joins the body) thandistally (the fingertips and toes).[30]
Bulbar polio
The location and anatomy of the
bulbar region (in orange)
Making up about 2% of cases of paralytic polio,
bulbar polio occurs when poliovirus invades and destroys nerves within the bulbar region of the brain stem.[4] The bulbar
region is a white matter pathway
that connects the cerebral cortex to the
brain stem. The destruction of these nerves weakens the muscles supplied by the cranial nerves, producing symptoms of encephalitis, and causes difficulty breathing, speaking and swallowing.[12] Critical
nerves affected are the glossopharyngeal nerve (which
partially controls swallowing and functions in the throat, tongue movement, and
taste), the vagus nerve (which
sends signals to the heart, intestines, and lungs), and the accessory nerve (which
controls upper neck movement). Due to the effect on swallowing, secretions of mucus may build
up in the airway, causing suffocation.[34] Other signs
and symptoms include facial weakness (caused by
destruction of the trigeminal nerve and facial nerve, which innervate the cheeks, tear ducts, gums, and muscles of the
face, among other structures), double vision, difficulty in chewing, and
abnormal respiratory rate, depth, and rhythm (which may
lead to respiratory arrest). Pulmonary edema and shock are also
possible and may be fatal.[39]
Bulbospinal polio
Approximately 19% of all paralytic polio cases
have both bulbar and spinal symptoms; this subtype is called respiratory or
bulbospinal polio.[4] Here, the
virus affects the upper part of the cervical spinal cord (cervical vertebrae C3 through
C5), and paralysis of the diaphragmoccurs. The critical nerves
affected are the phrenic nerve (which
drives the diaphragm to inflate the lungs) and those that drive the
muscles needed for swallowing. By destroying these nerves, this form of polio
affects breathing, making it difficult or impossible for the patient to breathe
without the support of a ventilator. It can lead to paralysis of
the arms and legs and may also affect swallowing and heart functions.[40]
Diagnosis
Paralytic poliomyelitis may be clinically
suspected in individuals experiencing acute onset of flaccid paralysis in one
or more limbs with decreased or absent tendon reflexes in the affected limbs
that cannot be attributed to another apparent cause, and without sensory or cognitive loss.[41]
A laboratory diagnosis is usually made based on
recovery of poliovirus from a stool sample or a swab of the pharynx. Antibodies to
poliovirus can be diagnostic, and are generally detected in the blood of
infected patients early in the course of infection.[4] Analysis of
the patient's cerebrospinal fluid (CSF),
which is collected by a lumbar puncture ("spinal
tap"), reveals an increased number of white blood cells (primarily lymphocytes) and a mildly elevated protein
level. Detection of virus in the CSF is diagnostic of paralytic polio, but
rarely occurs.[4]
If poliovirus is isolated from a patient
experiencing acute flaccid paralysis, it is further tested through oligonucleotide mapping (genetic fingerprinting), or more recently by PCRamplification, to determine
whether it is "wild type" (that is, the virus
encountered in nature) or "vaccine type" (derived from a strain of
poliovirus used to produce polio vaccine).[42] It is
important to determine the source of the virus because for each reported case
of paralytic polio caused by wild poliovirus, an estimated 200 to 3,000 other
contagiousasymptomatic carriers exist.[43]
Prevention
Passive immunization
In 1950, William Hammon at the University
of Pittsburgh purified the gamma globulin component
of the blood plasma of polio
survivors.[44] Hammon
proposed the gamma globulin, which contained antibodies to poliovirus, could be
used to halt poliovirus infection, prevent disease, and reduce the severity of disease
in other patients who had contracted polio. The results of a large clinical trial were
promising; the gamma globulin was shown to be about 80% effective in preventing
the development of paralytic poliomyelitis.[45] It was also
shown to reduce the severity of the disease in patients who developed polio.[44] The gamma
globulin approach was later deemed impractical for widespread use, however, due
in large part to the limited supply of blood plasma, so the medical community
turned its focus to the development of a polio vaccine.[46]
Vaccine
Main article: Polio vaccine
A child receiving an oral polio
vaccine
Two types of vaccine are used throughout the
world to combat polio. Both types induce immunity to polio, efficiently
blocking person-to-person transmission of wild poliovirus, thereby protecting
both individual vaccine recipients and the wider community (so-called herd immunity).[47]
The first candidate polio vaccine, based on one serotype of a
live but attenuated (weakened)
virus, was
developed by the virologist Hilary Koprowski. Koprowski's prototype vaccine
was given to an eight-year-old boy on February 27, 1950.[48] Koprowski
continued to work on the vaccine throughout the 1950s, leading to large-scale
trials in the then Belgian Congo and the
vaccination of seven million children in Poland against serotypes PV1 and PV3
between 1958 and 1960.[49]
The second inactivated virus vaccine was
developed in 1952 by Jonas Salk at the
University of Pittsburgh, and announced to the world on April 12, 1955.[50] The Salk
vaccine, or inactivated poliovirus vaccine (IPV), is based on poliovirus grown
in a type of monkey kidneytissue culture (vero cell line), which is chemically
inactivated with formalin.[17] After two
doses of IPV (given by injection), 90% or more of individuals
develop protective antibody to all three serotypes of
poliovirus, and at least 99% are immune to poliovirus following three doses.[4]
Subsequently, Albert Sabin developed
another live, oral polio vaccine (OPV). It was produced by the repeated passage
of the virus through nonhuman cells at subphysiological temperatures.[51] The
attenuated poliovirus in the Sabin vaccine replicates very efficiently in the
gut, the primary site of wild poliovirus infection and replication, but the
vaccine strain is unable to replicate efficiently within nervous system tissue.[52] A single
dose of Sabin's oral polio vaccine produces immunity to all three poliovirus
serotypes in about 50% of recipients. Three doses of live-attenuated OPV
produce protective antibody to all three poliovirus types in more than 95% of
recipients.[4] Human trials of Sabin's
vaccine began in 1957,[53] and in 1958 it was selected, in competition with the live vaccines of
Koprowski and other researchers, by the US National Institutes of Health.[49] Licensed in
1962,[53] it rapidly became the only polio vaccine used worldwide.[49]
Because OPV is inexpensive, easy to administer,
and produces excellent immunity in the intestine (which helps prevent infection
with wild virus in areas where it is endemic), it has been the vaccine of
choice for controlling poliomyelitis in many countries.[54] On very
rare occasions (about one case per 750,000 vaccine recipients), the attenuated
virus in OPV reverts into a form that can paralyze.[20] Most industrialized
countries have switched to IPV, which cannot revert, either as
the sole vaccine against poliomyelitis or in combination with oral polio
vaccine.[55]
Treatment
There is no cure for polio. The focus of
modern treatment has been on providing relief of symptoms, speeding recovery
and preventing complications. Supportive measures includeantibiotics to prevent
infections in weakened muscles, analgesics for pain,
moderate exercise and a nutritious diet.[56] Treatment
of polio often requires long-term rehabilitation, including occupational therapy, physical therapy, braces, corrective shoes and,
in some cases, orthopedic surgery.[39]
Portable ventilators may be
required to support breathing. Historically, a noninvasive, negative-pressure
ventilator, more commonly called an iron
lung, was
used to artificially maintain respiration during an acute polio infection until
a person could breathe independently (generally about one to two weeks). Today,
many polio survivors with permanent respiratory paralysis use modern jacket-type negative-pressure
ventilators worn over the chest and abdomen.[57]
Other historical
treatments for polio include hydrotherapy, electrotherapy, massage and passive motion
exercises, and surgical treatments, such as tendon lengthening and nerve
grafting.[13]
Prognosis
Patients with abortive polio infections recover
completely. In those who develop only aseptic meningitis, the symptoms can be
expected to persist for two to ten days, followed by complete recovery.[58] In cases of
spinal polio, if the affected nerve cells are completely destroyed, paralysis
will be permanent; cells that are not destroyed, but lose function temporarily,
may recover within four to six weeks after onset.[58] Half the
patients with spinal polio recover fully; one-quarter recover with mild
disability, and the remaining quarter are left with severe disability.[59] The degree
of both acute paralysis and residual paralysis is likely to be proportional to
the degree of viremia, and inversely proportional to the
degree of immunity.[33] Spinal
polio is rarely fatal.[34]
A child with a deformity of her
right leg due to polio
Without respiratory support, consequences of
poliomyelitis with respiratory involvement
include suffocation or pneumonia from
aspiration of secretions.[57] Overall,
5–10% of patients with paralytic polio die due to the paralysis of muscles used
for breathing. The mortality ratevaries by age: 2–5% of children
and up to 15–30% of adults die.[4] Bulbar
polio often causes death if respiratory support is not provided;[40] with support, its mortality rate ranges from 25 to 75%, depending on the
age of the patient.[4][60] When
intermittent positive pressure ventilation is available, the mortality can be
reduced to 15%.[61]
Many cases of poliomyelitis result in only
temporary paralysis.[13] Nerve
impulses return to the formerly paralyzed muscle within a month, and recovery
is usually complete in six to eight months.[58] The neurophysiological processes
involved in recovery following acute paralytic poliomyelitis are quite
effective; muscles are able to retain normal strength even if half the original
motor neurons have been lost.[62] Paralysis
remaining after one year is likely to be permanent, although modest recoveries
of muscle strength are possible 12 to 18 months after infection.[58]
One mechanism involved in recovery is nerve
terminal sprouting, in which remaining brainstem and spinal cord motor neurons
develop new branches, or axonal sprouts.[63] These
sprouts can reinnervate orphaned
muscle fibers that have been denervated by acute polio infection,[64] restoring the fibers' capacity to contract and improving strength.[65] Terminal
sprouting may generate a few significantly enlarged motor neurons doing work
previously performed by as many as four or five units:[35] a single motor neuron that once controlled 200 muscle cells might
control 800 to 1000 cells. Other mechanisms that occur during the
rehabilitation phase, and contribute to muscle strength restoration, include myofiber hypertrophy—enlargement of muscle fibers
through exercise and activity—and transformation oftype II muscle fibers to type I muscle fibers.[64][66]
In addition to these physiological processes, the
body possesses a number of compensatory mechanisms to maintain function in the
presence of residual paralysis. These include the use of weaker muscles at a
higher than usual intensity relative to the muscle's maximal
capacity, enhancing athletic development of previously little-used
muscles, and using ligaments for
stability, which enables greater mobility.[66]
Complications
Residual
complications of paralytic polio often occur following the initial recovery
process.[12] Muscle paresis and paralysis can sometimes result in skeletal deformities, tightening of the joints and movement
disability. Once the muscles in the limb become flaccid, they may interfere
with the function of other muscles. A typical manifestation of this problem isequinus foot (similar to club foot).
This deformity develops when the muscles that pull the toes downward are
working, but those that pull it upward are not, and the foot naturally tends to
drop toward the ground. If the problem is left untreated, the Achilles tendons at the back of the foot retract and the foot cannot
take on a normal position. Polio victims that develop equinus foot cannot walk
properly because they cannot put their heel on the ground. A similar situation
can develop if the arms become paralyzed.[67] In some cases the growth of an affected leg is slowed
by polio, while the other leg continues to grow normally. The result is that
one leg is shorter than the other and the person limps and leans to one side,
in turn leading to deformities of the spine (such as scoliosis).[67] Osteoporosis and increased likelihood of bone fractures may occur. An intervention to prevent or lessen length
disparity can be to perform an epiphysiodesis on the distal femoral and proximal tibial/fibular
condyles, so that limb's growth is artificially stunted, and by the time ofepiphyseal (growth)
plate closure, the legs are more equal in
length. Other surgery to re-balance muscular agonist/antagonist imbalances may
also be helpful. Extended use of braces or wheelchairs may cause compression neuropathy, as
well as a loss of proper function of the veins in the legs, due to pooling of blood in paralyzed
lower limbs.[40][68]Complications
from prolonged immobility involving the lungs, kidneys and heart include pulmonary edema, aspiration pneumonia, urinary tract infections, kidney stones, paralytic ileus, myocarditis and cor pulmonale
- Friskila Damaris Silitonga, SKEP, NS, MPH
fris, you are truly inspirational blogger, good job, certainly I will promote about this blog on my FB, success always 4 u :)
BalasHapusAde Wahyuni - friend in SMUNSA Binjai
excellent well-researched article on polio, once such a horrible disease which still persists in some parts of the world
BalasHapus