What are the symptoms of polio?
The incubation period for poliomyelitis is commonly 6 to 20 days with a range from 3 to 35
days. The response to poliovirus infection is highly variable and has been
categorized based on the severity of clinical presentation. Up to 95% of all polio
infections are inapparent or subclinical without symptoms. Estimates of the ratio of
inapparent to paralytic illness vary from 50:1 to 1,000:1 (usually 200:1). Infected
persons without symptoms shed virus in the stool, and are able to transmit the virus to
others.
Approximately 4%-8% of polio infections consist of a minor, nonspecific illness without
clinical or laboratory evidence of central nervous system invasion. This syndrome is known
as abortive poliomyelitis, and is characterized by complete recovery in less than a week.
Three syndromes, which are indistinguishable from other viral illnesses, are observed with
this form of poliovirus infection:
- upper respiratory tract infection (sore throat and fever)
- gastrointestinal disturbances (nausea, vomiting, abdominal pain, constipation or,
rarely, diarrhea), and
- influenza-like illness
Nonparalytic aseptic meningitis (symptoms of stiffness of the neck, back, and/or legs),
usually following several days after a prodrome similar to that of minor illness, occurs
in 1%-2% of polio infections. Increased or abnormal sensations can also occur. Typically
these symptoms will last from 2 to 10 days, followed by complete recovery.
Less than 2% of all polio infections result in flaccid paralysis. Paralytic symptoms
generally begin 1 to 10 days after prodromal symptoms and progress for 2 to 3 days.
Generally, no further paralysis occurs after the temperature returns to normal. The
prodrome may be biphasic, especially in children, with initial minor symptoms separated by
a 1- to 7-day period from more major symptoms. Additional prodromal signs and symptoms can
include a loss of superficial reflexes, initially increased deep tendon reflexes and
severe muscle aches and spasms in the limbs or back. The illness progresses to flaccid
paralysis with diminished deep tendon reflexes, reaches a plateau without change for days
to weeks, and is usually asymmetrical. Strength then begins to return. Patients do not
experience sensory losses or changes in cognition.
Many persons with paralytic poliomyelitis recover completely and, in most, muscle function
returns to some degree. Patients with weakness or paralysis 12 months after onset will
usually be left with permanent residua.
Paralytic polio is classified into three types, depending on the level of involvement:
- Spinal polio is most common, and accounted for 79% of paralytic cases
from 1969-1979. It is characterized by asymmetric paralysis that most often involves the
legs.
- Bulbar polio accounted for 2% of cases and led to weakness of muscles
innervated by cranial nerves.
- Bulbospinal polio accounted for 19% of cases and was a combination of
bulbar and spinal paralysis. The death-to-case ratio for paralytic polio is generally
2%-5% in children and up to 15%-30% in adults (depending on age). It increases to 25%-75%
with bulbar involvement.
Post-Polio Syndrome
After an interval of 30-40 years, 25%-40% of people who contracted paralytic poliomyelitis
in childhood experience new muscle pain and exacerbation of existing weakness, or develop
new weakness or paralysis. This disease entity is referred to as post-polio syndrome.
Factors that enhance the risk of post-polio syndrome include increasing length of time
since acute poliovirus infection, presence of permanent residual impairment after recovery
from the acute illness, and female gender. The pathogenesis of post-polio syndrome is
thought to involve the failure of oversized motor units created during the recovery
process of paralytic poliomyelitis. Post-polio syndrome is not an infectious
process, and persons experiencing the syndrome do not shed poliovirus. |
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