eMedicine Specialties > Physical Medicine and Rehabilitation > Disorders of the Motor Unit

Acute Poliomyelitis

Author: Christine Weiler, MD, Staff Physician, Department of Physical Medicine and Rehabilitation, University of Colorado Health Sciences Center
Coauthor(s): Selina Yingqi Xing, MD, MS, Staff Physician, Department of Physical Medicine and Rehabilitation, Temple University; Harry Schwartz, MD, Director of Medical Rehabilitation Program & Spinal Cord Injury Program, Moss Rehabilitation Hospital; Clinical Assistant Professor, Department of Physical Medicine and Rehabilitation, Temple University School of Medicine
Contributor Information and Disclosures

Updated: Apr 27, 2009

Introduction

Background

Acute poliomyelitis is a disease of the anterior horn motor neurons of the spinal cord and brain stem caused by poliovirus. Flaccid asymmetric weakness and muscle atrophy are the hallmarks of its clinical manifestations, due to loss of motor neurons and denervation of their associated skeletal muscles. Because of the success of poliovirus vaccine, poliomyelitis, once one of the most feared human infectious diseases, is now almost entirely preventable by proper immunization. (See image below and Image 1.)

Pathophysiology

Acute poliomyelitis is caused by small ribonucleic acid (RNA) viruses of the enterovirus group of the picornavirus family. The single-stranded RNA core is surrounded by a protein capsid without a lipid envelope, which makes poliovirus resistant to lipid solvents and stable at low pH. Three antigenically distinct strains are known, with type I accounting for 85% of cases of paralytic illnesses. Infection with one type does not protect from the other types; however, immunity to each of the 3 strains is lifelong.

The enteroviruses of poliomyelitis infect the human intestinal tract mainly through the fecal-oral route (hand to mouth). The viruses multiply in oropharyngeal and lower gastrointestinal tract mucosa during the first 1-3 weeks of the incubation period. Virus may be secreted in saliva and feces during this period, causing most host-to-host transmission. After the initial alimentary phase, the virus drains into the cervical and mesenteric lymph nodes and then into the blood stream. Only 5% of infected patients have selective nervous system involvement after viremia. It is believed that replication in extraneural sites maintains the viremia and increases the likelihood that the virus will enter the nervous system.

The poliovirus enters the nervous system by either crossing the blood-brain barrier or by axonal transportation from a peripheral nerve. It can cause nervous system infection by involving the precentral gyrus, thalamus, hypothalamus, motor nuclei of the brainstem and surrounding reticular formation, vestibular and cerebellar nuclei, and neurons of the anterior and intermediate columns of the spinal cord. The nerve cells undergo central chromatolysis along with an inflammatory reaction while multiplication of the virus precedes onset of paralysis. As the chromatolysis process goes on further, muscle paralysis or even atrophy appears when fewer than 10% of neurons survive in the corresponding cord segments. Gliosis develops when the inflammatory infiltrate has subsided, but most surviving neurons show full recovery.^1,2

Frequency

United States

Because of widespread use of the poliovirus vaccine, the incidence rate has been less than 0.01 cases per 100,000 population since 1965. The last case of wild-type polio in the United States was in 1979. Only a few cases of paralytic poliomyelitis are reported each year in the United States. Vaccine-derived poliovirus infections are seen primarily in low-vaccination communities. Rare cases of poliomyelitis are reported due to live attenuated poliovirus vaccine. Small pockets of poliomyelitis epidemics still exist among isolated religious sects, such as the Amish. These groups usually choose not to participate in government-sponsored public health services, such as immunizations for infectious diseases.

International

Acute poliomyelitis has a worldwide distribution, with the peak season being from July to September and the concentration being in tropical areas of the Northern Hemisphere. As of 2006, 6 countries were endemic to polio: Afghanistan, Egypt, India, Niger, Nigeria, and Pakistan. The condition continues to occur epidemically in nonimmunized populations in developing regions. Poor sanitation and crowded circumstances are 2 additional factors associated with dissemination. Internationally, importation of polio continues to occur into polio-free countries. From 2002-2005, 21 previously polio-free countries experienced a resurgence of wild-type polio.³ In 8 of those countries, the cases were limited and no further spread was observed. In the remaining 13 countries, multiple cases were observed, with the outbreak lasting less than 6 months.^4,5

Mortality/Morbidity

Of acute poliovirus infections, 4-8% show only nonspecific illness, and 1-2% of infections finally result in neurologic symptoms. The incidence of paralytic diseases increases with young age, advanced age, recent hard exercise, tonsillectomy, pregnancy, and impairment of B-lymphocyte defenses. The mortality from acute paralytic poliomyelitis is 5-10%, but it can reach 20-60% in cases of bulbar involvement.

Race

Acute poliomyelitis has no racial predilection.

Sex

The male-to-female ratio for acute poliomyelitis is 1:1.

Age

Most cases of acute poliomyelitis occur in the pediatric population. Infection or immunization against poliovirus provides lifelong protection.

Clinical

History

Most patients (95%) with poliomyelitis virus infections are asymptomatic or have only mild systemic symptoms, such as pharyngitis or gastroenteritis. These cases are referred to as minor illness or abortive poliomyelitis. The mild symptoms are related to viremia and immune response against dissemination of the virus. Only 5% of patients exhibit different severities of nervous system involvement, from nonparalytic poliomyelitis to the most severe form of paralytic poliomyelitis.¹

• Nonparalytic poliomyelitis or preparalytic poliomyelitis
  • The prodromal symptoms include generalized, nonthrobbing headache; fever of 38-40 º C; sore throat; anorexia; nausea; vomiting; and muscle aches. These symptoms may or may not subside in 1-2 weeks.
  • Headache and fever, as well as signs and symptoms of nervous system involvement (eg, irritability, restlessness, apprehensiveness, emotional instability, stiffness of the neck and back) and Kernig and Brudzinski signs because of meningitis, then may follow.
  • Children generally exhibit milder systemic symptoms than do adults.
  • Preparalytic symptoms also may develop into paralytic ones.
• Paralytic poliomyelitis
  • The incubation period from virus exposure to the neurologic phase can last 4-10 days but may extend to 4-5 weeks.
  • Severe muscle pain and spasms, followed by weakness, develop. Muscle weakness tends to become maximal within 48 hours but may develop for longer than a week. No progression of weakness should be noted after the temperature drops to normal for 48 hours. Weakness is asymmetric, with the lower limbs affected more than upper limbs.
  • Muscle tone is flaccid, and the reflexes initially are brisk but then become absent. The transient or occasionally persistent coarse fasciculations also are observed frequently in patients with paralytic poliomyelitis.
  • Patients also complain of paresthesias in the affected limbs without real sensation loss.
  • Paralysis remains for days or weeks before slow recovery occurs over months or years. Which factors favor development of paralytic disease remains unclear, but some evidence exists that physical activity and intramuscular injections during the prodrome may be important exacerbating factors.¹
• Paralytic poliomyelitis with bulbar involvement
  • The purely bulbar form of poliomyelitis without limb weakness may occur in children, particularly in those whose tonsils and adenoids have been removed.
  • Bulbar paralysis with spinal involvement is more common in adults, most frequently involving the medulla and leading to dysphagia, dysphonia, respiratory failure, and vasomotor disturbance.
  • Patients may have symptoms and signs, such as hiccough, shallowness and slowing of respiration, cyanosis, restlessness, and anxiety.
  • When paralysis of diaphragmatic and intercostal musculature also occurs, patients need immediate respiratory assistance and intensive care because of life-threatening respiratory failure. Cranial nerve and bulbar involvement can cause obstruction, due to decreased respiratory drive and associated problems with mucus plugging or actual pharyngeal weakness-induced direct airway obstruction. The loss of vasomotor control with circulatory collapse also contributes to high mortality.
  • The encephalitic form of poliomyelitis
    • This form is very rare and manifests as agitation, confusion, stupor, and coma.
    • Autonomic dysfunction is common, and it has a high mortality.

Physical

Vital signs are the key to monitoring patients with poliovirus infection.

• Muscle weakness can be assessed by muscle strength testing.
  • Usually asymmetric proximal weakness is present with more involvement of lumbar than cervical segments and more spinal cord than brainstem segments.
  • The trunk muscles are affected least.
  • Sensation should be within normal limits objectively.
  • Deep tendon reflexes are diminished or absent.
  • Atrophy of muscle may be detected 3 weeks after onset of paralysis, which becomes maximal at 12-15 weeks and remains permanent.
• Fifty percent of adult patients with poliomyelitis experience transient acute urinary retention.
• Stiffness and pain in the neck and back because of meningeal irritation, as well as abnormalities of autonomic function, also can be seen in some patients.
• Cranial nerve involvement
  • Approximately 10-15% of cases affect the lower brainstem motor nuclei.
  • When the ninth and tenth cranial nerve nuclei are involved, patients develop paralysis of pharyngeal and laryngeal musculature. Unilateral or bilateral facial muscles, as well as the tongue and mastication muscles, may become paralyzed.
  • External oculomotor weakness with pupil sparing may occur in rare cases.
  • Direct infection of the brainstem reticular formation can cause breathing and swallowing disruption, as well as loss of control of the cardiovascular system.

Causes

The carrier with poliomyelitis virus infection is one major source of virus spread from person to person. The major route is oral-fecal transmission. The greatest dissemination of virus occurs within families with poor sanitation and hygiene or crowded circumstances.

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