The late effects of Polio Information for Health Care Providers
Some of the recent concepts of the causes of the late effects of polio (LEOP) are summarized below:
(I) PROGRESSIVE DETERIORATION OF RESIDUAL MUSCULOSKELETAL ABNORMALITES
It is well established that residual polio is a cause of functional deterioration in many people who have previously had polio. Some clinicians are therefore skeptical about the presence of a new distinct disease entity, rather considering the "new" symptoms as a logical progression of musculoskeletal abnormalities. It is important to keep in mind that symptoms of LEOP can occur also in people who were mildly affected by polio and who have no, or virtually no, obvious residual symptoms and signs of the disease (1).
(II) NEURONAL ATTRITION DUE TO AGEING
Paralytic polio results in a diminished number of motor neurones. It is possible that post-polio people are more vulnerable than others, when further motor neurone depletion occurs with normal ageing. Consequently, some attribute the symptoms of LEOP to ageing.
This hypothesis has been contested, since it has been noted that the onset of "new" symptoms is associated with the length of the interval from the onset of polio rather than with age of the person. New symptoms may, in fact, commence at any age and commonly well before the age of 60, when significant age-related loss of motor neurones ordinarily occurs (2).
(III) MOTOR NEURONE DISEASE
Amyotrophic lateral sclerosis (ALS) is a progressive disease, which usually begins between the ages of 40 and 60. The initial symptoms resemble some of those of LEOP (weakness and wasting of extremities and bulbar involvement). Similarities of onset and symptoms have prompted the theory that poliomyelitis may evolve into ALS in later years.
Martyn et al (3) noted a striking similarity in the geographical relation between past notification rates of poliomyelitis and current mortality from motor neurone disease in England and Wales. However, people with ALS were not more likely to have had polio than controls according to two retrospective studies (4,5).
Corticospinal tract degeneration, a prominent aspect of ALS, is rare in poliomyelitis (6,7) and muscle weakness in LEOP is not due to a loss of whole motor neurones, as is the case in ALS (8). Furthermore, there are marked clinical differences. ALS has a rapid, relentless progression whereas the muscle weakness and atrophy of LEOP is benign and much more gradual. ALS is characterized by hyperreflexia and extensor plantar reflexes. Such findings are uncommon following paralytic polio, which usually causes flaccid paralysis and hyporeflexia (9). The present consensus appears to be that the neuro-muscular aspect of LEOP is a separate entity from other motor neurone diseases.
(IV) RECURRING POLIO INFECTION, IMMUNOLOGICAL AND INFLAMMATORY PROCESSES
Since some of the new symptoms of LEOP resemble those of acute paralytic poliomyelitis (muscle weakness, respiratory distress, pain), they were thought to be caused by a reactivation of a dormant polio virus. Serological tests failed to identify the virus and there is no evidence that it can remain active in the central nervous system for so long (10).
One investigation of long term polio survivors noted alterations in their immune status, which differed both from normal controls and from people with other neurological diseases (11). Other studies have demonstrated immunoglobulin abnormalities in the cerebro-spinal fluid and peripheral lymphocytes (8,12).
Pezeshkpour and Dalakas (7) examined spinal cords from deceased people with a history of poliomyelitis. They found signs of a mild to moderate perivascular and interparenchymal inflammation and concluded that slow disease activity continues silently for many years in the spinal cord. This is an interesting finding, because polio is conventionally thought to be a self-limiting infection.
Perivascular and interstitial inflammatory cells were detected in fully recovered and originally spared muscles in post-polio people (13). Others have failed to confirm findings of immunological and inflammatory processes (10).
(V) OVERUSE DEGENERATION OF THE NEUROMUSCULAR COMPLEX
The hypothesis of relative overuse of deficient neuromuscular complexes is at present the most commonly held theory.
Each motor unit is made up of a motor neurone, its axon and all muscle fibres innervated by that axon. Following a polio infection, surviving motor neurones enlarge ("sprout") and become responsible for up to 5-10 times as many individual muscle fibres (14).
It is thought that such hypertrophied post-paralytic motor neurones are vulnerable to overuse fatigue. EMG studies have revealed signs of active focal degeneration (15). In fact, Cashman et al (16) established that extensive re-innervation of previously denervated muscles may be followed by a new phase of denervation.
Since muscle fibres which lack neurological command become inactive, others compensate by excessive overuse.
Such compensating muscles have been noted to adapt by developing a predominance of type 1 muscle fibres with marked hypertrophy (17).
Excessive use of post-polio muscles appears to precede their deterioration. It is important to know that muscles which were thought to be spared from the acute disease or believed to have recovered fully on the basis of manual muscle testing may, in fact, have lost up to 50% of their function (18). Since poliomyelitis is a systemic infection, subclinical involvement is likely to have occurred in many cases. Infected individuals may therefore not have been aware of the full extent of muscle involvement.
Excessive muscle use through walking was detected by Ferry et al (19) in subjects with gait abnormalities caused by residual paralysis of some leg muscles. Post-polio muscles have also been shown to need longer time to recover from activity than normal muscles (20). Reduced capacity of the capillary bed may have a negative effect on endurance activity in post-polio muscles (2).
Weakness of the muscles of the extremities and trunk is but one aspect of LEOP. Similar fatigue may occur in the diaphragm and in muscles supplying the throat and mouth.
Although there is evidence of a mild, slow, focal degenerative process in muscles of subjects who once contracted paralytic polio, no definite clinical, electromyographic (EMG), immunological or muscle biopsy findings have, as yet, been able to distinguish clearly between symptomatic and asymptomatic subjects (7,10,11,17,21). Notably age of onset has not been found to correlate with the presence of symptoms (17,21,22) nor has the psychological profile (21).
Some workers have reported differences between stable and deteriorating groups depending on the severity of initial paralysis (22,23) and the relative subsequent degree of functional recovery (22).
Waring et al. (24) noted almost twice as high serum creatine kinase levels in symptomatic as in asymptomatic post-polio people. This is likely to have occurred due to muscle damage and necrosis. A case of post-polio weakness, fatigue and muscle pain with markedly elevated levels of creatine kinase was documented by Peach (25). Following clinical intervention and resolution of symptoms these levels declined. Lange (10) recorded lower amplitude macro-EMG signals in weakened versus non-weakened post-polio muscles. Further studies are needed to confirm or refute these findings.
Polio appears to have a previously unrecognized, second degenerative phase.
It is reasonable to assume that people who have had polio may fatigue their neuro-muscular system even with seemingly modest levels of activity. The overuse fatigue theory provides not only a possible aetiology of muscle fatigue but also explains how muscles and joints may become strained and painful due to insufficient muscle strength. It also provides a rationale for the prevention and management of LEOP - the concept of energy management. Fortunately, it is possible that the symptoms are reversible.
The presence of immunological and/or inflammatory changes may also explain the complaints of unpredictable, often influenza-like muscle pain, exhaustion and excessive need for rest as well as the slow deterioration of muscle strength.
There are signs of slow deterioration during the late post-convalescent period in most post-polio muscles but for unknown reasons only some people experience symptoms. More research is needed to identify the long-term effects of previous polio infection and, in particular, predictors of symptomatic deterioration. It will also be necessary to compare the development of symptoms with that of an ageing polio-free population.
The variation of symptoms relating to LEOP suggest that individual susceptibility plays a role in the development of the disease and it may have a multifactorial cause.
If the senescence theory is at least partially correct, it is to be expected that, as the post-polio population ages, the incidence of new complaints and/or the severity of dysfunction will increase.
© Copyright The Lincolnshire Post-Polio Network 1997 - 2010.
This document comprises an index, foreword, introduction and seventeen other sections or subdocuments. Permission for printing copies is granted only on the basis that ALL sections are printed in their entirety and kept together as a single document.
Document preparation: Chris
Think-tank, Cornwall, United Kingdom.
Created: 7th July 1997
Last modification: 20th January 2010.
Last information content change: 6th June 2000