Poliomyelitis — Comprehensive Review

1. Etiology

2. Epidemiology

3. Pathogenesis

Stages of Viral Spread

1. Entry & primary replication: Virus is ingested, survives gastric acid, and infects epithelial cells of the oropharyngeal mucosa, M cells, lymphocytes in Peyer's patches, and enterocytes of the ileum. Tonsils and intestinal lymphoid tissue are key replication sites.
2. Primary viremia: Virus drains into regional lymph nodes (mesenteric, cervical), then enters the bloodstream. This first viremia seeds the reticuloendothelial system (lymph nodes, liver, spleen), where secondary replication occurs.
3. Secondary viremia: A second, larger viremia (detectable 3–5 days post-infection) occurs coinciding with the "minor illness." This phase is associated with fever and systemic symptoms. Neutralizing antibodies begin to rise at this point, and in ~95% of infected individuals, immunity halts further spread.
4. CNS invasion (minority of cases): Two pathways are proposed:
   • Hematogenous route: Virus crosses the blood-brain barrier during viremia.
   • Neural (retrograde axonal) route: Poliovirus binds to its receptor (CD155, an immunoglobulin superfamily member) at the neuromuscular junction end-plate, then travels retrograde up motor axons to anterior horn cells. Studies in transgenic mice show that cutting the sciatic nerve before IM injection prevents spinal cord infection, supporting this neural route.
5. Cytolysis of motor neurons: The virus is directly cytolytic to motor neurons of the anterior horns of the spinal cord and the motor nuclei of the brainstem (especially nucleus ambiguus, facial nucleus). Viral RNA has been detected within anterior horn motor neurons. The number and location of neurons destroyed determines the extent and distribution of paralysis.

Immunity

• Humoral (primary): Specific IgM (transient, <6 months) and IgG (lifelong) provide protection. VP1 is the dominant neutralization target. Humoral immunity must precede neurological invasion — antibodies cannot clear virus already within CNS tissue.
• Secretory IgA: Prevents intestinal replication and shedding; particularly important for mucosal immunity (better induced by OPV than IPV).
• Cellular immunity: Of uncertain significance; defects in cellular immunity do not worsen enterovirus disease.
• Agammaglobulinemia dramatically worsens outcome — emphasizing humoral immunity's dominance.
• Maternal antibody provides passive protection to neonates for the first ~6 months.
• Immunity is type-specific; infection with one serotype does not protect against the others.

4. Morphogenesis (Histopathology)

Acute Phase

• Spinal cord: Perivascular mononuclear cell cuffing and neuronophagia of anterior horn motor neurons. Central chromatolysis of affected neurons is among the earliest changes — preceding paralysis by 1–2 days in experimental models.
• The inflammatory reaction is typically confined to the anterior horns but can extend into the posterior horns; in severe cases, cavitation may occur.
• A brief local leukocytic (neutrophilic) reaction is seen in the first few days, replaced by persistent mononuclear perivascular infiltrates lasting months.
• Cranial motor nuclei are sometimes involved.
• No inclusion bodies are formed (distinguishing polio from herpesviruses, rabies, etc.).
• In fatal cases, lesions are also found in: precentral (motor) cortex, hypothalamus, thalamus, vestibular nuclei, roof nuclei of cerebellum, reticular formation of brainstem, and lateral horn cells of spinal cord.

Chronic/Late Phase

• Loss of neurons and gliosis in affected anterior horn regions.
• Atrophy of anterior (motor) spinal nerve roots.
• Neurogenic atrophy of denervated skeletal muscles.
• Collateral reinnervation by surviving motor neurons enlarges their motor units, which later fail in post-polio syndrome.

5. Clinical and Morphological Characteristics — Periods of the Disease

Period 1: Incubation Period (1–3 weeks)

Period 2: Minor Illness (Abortive Poliomyelitis) — ~5% of infected

• Fever (38–40°C), malaise, headache, sore throat, anorexia, nausea/vomiting, occasionally mild diarrhea or constipation.
• Corresponds to the period of viremia and viral dissemination.
• In the majority, an effective immune response terminates infection here.
• No CNS involvement.

Period 3: Preparalytic (Meningeal/Nonparalytic) Phase — 1–2% of infected

• Recrudescence of fever and headache after apparent improvement.
• Meningeal signs: stiff neck, positive Kernig/Brudzinski signs, tight hamstrings.
• Muscle pain and tenderness, especially in the neck and back.
• Neurological prodrome: irritability, restlessness, emotional instability — often a prelude to paralysis.
• CSF: Aseptic meningitis pattern — elevated lymphocytes (10–500 cells/mm³), mildly elevated protein, normal glucose.
• Some patients recover fully at this stage without progressing to paralysis.

Period 4: Paralytic Poliomyelitis — <1% of infected

• Flaccid (lower motor neuron) paralysis — hypotonia, hyporeflexia/areflexia, later muscle wasting.
• Asymmetric distribution is characteristic; any skeletal muscle may be affected, but leg muscles are most commonly involved (often one limb — monomelic).
• Progression is rapid: maximal weakness within 48 hours to one week; no further weakness after temperature has been normal for 48 hours.
• Coarse fasciculations during the phase of weakness.
• Subjective paresthesias in affected limbs, but objective sensory loss is rare (motor neuron disease, not sensory).
• Urinary retention common in adults (transient).
• Muscle atrophy detectable within 3 weeks; maximal at 12–15 weeks; permanent.

• Involvement of motor nuclei of the brainstem (nucleus ambiguus → dysphagia, dysarthria; facial nucleus → facial palsy).
• More common in young adults; usually accompanied by spinal disease.
• Critical hazards: disturbances of respiration (shallow breathing, hypoventilation, cyanosis) and vasomotor control (hypertension → hypotension and shock).
• Respiratory failure from diaphragmatic/intercostal muscle paralysis or from brainstem respiratory center depression.

• Seizures, altered consciousness — occurs with diffuse CNS involvement.

Period 5: Recovery Phase

• Begins when fever subsides and progression of paralysis ceases.
• Maximal recovery within 6 months (returning function due to collateral reinnervation by surviving motor neurons).
• Residual paralysis persists beyond this.
• Physiotherapy is critical to prevent contractures, deformities (foot drop, scoliosis).

Period 6: Residual Effects / Outcome

• Permanent flaccid paralysis with muscle atrophy in proportion to neuronal loss.
• Deformities: scoliosis, foot drop, limb length discrepancy.
• Respiratory insufficiency if diaphragmatic neurons destroyed.

6. Complications

7. Post-Polio Syndrome (PPS)

• New onset progressive weakness, decreased muscle mass, fatigue, and pain in previously affected (and sometimes unaffected) muscle groups.
• Mechanism: The enlarged motor units created by compensatory reinnervation cannot be maintained by aging motor neurons; gradual degeneration of these over-stressed neurons leads to renewed denervation.
• NOT due to viral reactivation — no convincing evidence of recurrent poliovirus infection.
• Estimated to affect 25–50% of polio survivors.

8. Outcomes and Causes of Death

9. Diagnosis (Laboratory)

• Viral culture: Throat swabs (early) and stool/rectal swabs (weeks to months); cytopathic effect in 3–6 days in human/monkey cell culture.
• RT-PCR: Most rapid; detects and types poliovirus RNA in stool, throat, or CSF.
• Serology: Paired sera showing 4-fold rise in neutralizing antibody titer.
• CSF: Aseptic meningitis pattern (lymphocytic pleocytosis, mildly elevated protein, normal glucose); virus rarely isolated from CSF.
• Note: Poliovirus is uncommonly recovered from CSF — unlike coxsackieviruses and echoviruses.

10. Prevention

Active Immunization (Vaccines)

Non-Vaccine Prevention

• Improved sanitation and hygiene (clean water, sewage disposal) — reduces fecal-oral transmission.
• Enteroviruses are inactivated by chlorine-containing compounds; standard disinfectants (alcohol) are ineffective.
• Surveillance and rapid response to identify imported cases and VDPV outbreaks.
• Contraindications to OPV: Immunodeficient individuals and their household contacts should receive IPV only.

Global Eradication Strategy

• Type 2 wild poliovirus declared eradicated in 2015; trivalent OPV replaced by bivalent (types 1+3).
• All countries are now recommended to include at least one dose of IPV in their schedule.
• The final step will be cessation of OPV once wild-type transmission is confirmed interrupted worldwide.

Summary Table: Disease Periods at a Glance