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Infantile spinal muscular atrophy

Spinal Muscular Atrophy (SMA) is a term applied to a number of different disorders, all having in common a genetic cause and the manifestation of weakness due to loss of the motor neurons of the spinal cord and brainstem. more...

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Caused by mutation of the SMN gene

The most common form of SMA is caused by mutation of the SMN gene, and manifests over a wide range of severity affecting infants through adults. This spectrum has been divided arbitrarily into three groups by the level of weakness.

  • Infantile SMA - Type 1 or Werdnig-Hoffmann disease (generally 0-6 months). SMA type 1, also known as severe infantile SMA or Werdnig Hoffmann disease, is the most severe, and manifests in the first year of life with the inability to ever maintain an independent sitting position.
  • Intermediate SMA - Type 2 (generally 7-18 months). Type 2 SMA, or intermediate SMA, describes those children who are never able to stand and walk, but who are able to maintain a sitting position at least some time in their life. The onset of weakness is usually recognized some time between 6 and 18 months.
  • Juvenile SMA - Type 3 Kugelberg-Welander disease (generally >18 months). SMA type 3 describes those who are able to walk at some time. It is also known as Kugelberg Welander disease.

Other forms of SMA

Other forms of spinal muscular atrophy are caused by mutation of other genes, some known and others not yet defined. All forms of SMA have in common weakness caused by denervation, i.e. the muscle atrophies because it has lost the signal to contract due to loss of the innervating nerve. Spinal muscular atrophy only affects motor nerves. Heritable disorders that cause both weakness due to motor denervation along with sensory impairment due to sensory denervation are known by the inclusive label Charcot-Marie-Tooth or Hereditary Motor Sensory Neuropathy. The term spinal muscular atrophy thus refers to atrophy of muscles due to loss of motor neurons within the spinal cord.

  • Hereditary Bulbo-Spinal SMA Kennedy's disease (X linked, Androgen receptor)
  • Spinal Muscular Atrophy with Respiratory Distress (SMARD 1) (chromsome 11, IGHMBP2 gene)
  • Distal SMA with upper limb predominance (chromosome 7, glycyl tRNA synthase)


The course of SMA is directly related to the severity of weakness. Infants with the severe form of SMA frequently succumb to respiratory disease due to weakness of the muscles that support breathing. Children with milder forms of SMA naturally live much longer although they may need extensive medical support, especially those at the more severe end of the spectrum.

Although gene replacement strategies are being tested in animals, current treatment for SMA consists of prevention and management of the secondary effect of chronic motor unit loss. It is likely that gene replacement for SMA will require many more years of investigation before it can be applied to humans. Due to molecular biology, there is a better understanding of SMA. The disease is caused by deficiency of SMN (survival motor neuron) protein, and therefore approaches to developing treatment include searching for drugs that increase SMN levels, enhance residual SMN function, or compensate for its loss. The first effective specific treatment for SMA may be only a few years away, as of 2005.


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A fearful reminder stalks polio's survivors - includes related article defining poliomyelitis
From FDA Consumer, 12/1/85 by Richard C. Thompson

In the first half of this century it was known as infantile paralysis, then poliomyelitis, appearing in dreaded summer epidemics that frightened parents as it struck down their children, killing many and leaving those who survived in wheelchairs and iron lungs and facing a lifetime of limited activity.

All that changed beginning in the 1950s. Mass inoculations of infants and children throughout the United States with Salk and Sabin vaccines made polio a sad memory. It was a classci public health triumph.

But now polio is back, not as a crippling and wasting scourge of children but as new symptoms for adults who once had the disease and--though handicapped--thought themselves partially or completely recovered.

For the 300,000 living survivors of polio, the threat of post-poliomyelitis muscular atrophy (PPMA) is a fearful reminder of the paralysis and suffering they experienced decade ago. An estimated 40,000 of these survivors may have or be developing late PMMA, which seems to occur about 30 years after the original illness.

The condition appears in its victims as extreme fatigue, often severe muscle pain, and a muscular weakness that may be slowly progressive over many years. Except for its association with childhood polio, the cause of late PPMA is not known and there is as yet no prevention, cure or treatment.

Dr. Marinos Dalakas, a neurologist at the National Institute of Health who is studying PPMA, says it is a condition that affects perhaps 15 percent of the polio survivors. Why 85 percent are spared no one can say.

Although it is not life-threatening nor severely disabling, late PPMA can be very alarming to people who have already lost some of their muscle strength because of childhood polio. Some researchers believe it may even be affecting muscles that were not affected by the original polio.

Even if their recovery from polio left them without severe handicaps, the muscle weakness and fatigue brought on by PPMA can make it difficult for these patients to perform such simple tasks as opening jars or climbing stairs. Muscles may be sore or tender or actually painful. Sufferers also fear they are losing muscle mass, saying a particular set of muscles in their arms or legs appears thinner to them than it used to.

Dalakas stresses that late PPMA is not a new phenomenon; both patients and researchers have known about it in a general way for several years. "But now," he adds, "we have defined and classified the symptoms and are doing studies to see what the causes and therapies might be."

There are several theories as to what may be producing these late post-polio symptoms. One is based on the fact that as people grow older they can be expected to lose nerve cells in the spinal cord. Ordinarily, the loss is not noticeable because other cells will compensate. But people who had polio have already lost some of the cells that control muscle movement; those are the cells the polio virus attacked. Now the remaining healty cells, according to this theory, have become overworked and are failing.

Some researchers have trouble with this reasoning. Although it helps explain the weakness that develops in muscles affected by the earlier polio, it does not explain such symptoms in muscles that were not originally affected. And a few researchers discount the aging factor altogether, saying that many late PPMA patients are still in their 30s and 40s, too young to be affected by the natural loss of nerve cells.

There is other speculation that late PPMA may be caused by a dormant polio virus that has reactivated, in much the same way that a dormant chicken pox virus can "reawaken" after many years to cause shingles. But this theory is not acceptable to many researchers. They point out that although the polio virus can persist in animals, there is no evidence that it persists in human who have normal immune systems. Critics of this theory say emphatically that late PPMA is not recurrent polio attacking new muscle groups; it is instead, they believe, an increased weakness over time in the same muscles that were damaged years ago.

A variation of the reactivated virus theory holds that late PPMA may be related to a deficiency in the patients' immune systems. Researchers are looking at this possibility, but immune studies require long work before answers become available.

Whatever theories they espouse, researchers agree that late PPMA is not a form of amyotrophic lateral sclerosis, or ALS. (See "Lou Gehrig's Disease Still Needs a Cure" in the December 1983-January 1984 FDA Consumer.) Although the same spinal cord cells are affected, late PPMA does not at all follow the course of ALS, and laboratory tests confirm that the two conditions are completely different. In particular, late PPMA is not life-threatening and its victims do not encounter the difficulty in breathing or swallowing that is symptomatic of ALS.

Late PPMA is enigmatic. Not only is it virtually impossible to predict which polio survivors will be afflicted with PPMA, it is also difficult to forecast the course of the symptoms once they occur.

The severity of fatigue, muscle weakness, and aching pain may level off and remain stable for several years. Or the symptoms may worsen every two or three years. But whatever the course, patients probably face some loss of muscle strength because PPMA seems to affect muscles that have already been weakened.

Dr. Lauro Halstead of the Institute of Rehabilitation and Research in Houston, himself a polio survivor, has studied over 200 post-polio cases. He found that almost all had experienced generalized, increased physical fatigue as well as renewed weakness in muscles that had been affected by their polio years earlier. He seems to see such fatigue and weakness as almost the expected and delayed consequences of polio. The symptoms appeared about 30 years after the original illness.

Most of his patients have also experienced severe joint and muscle pain as well as new weakness in muscles not affected by the earlier polio. Some who had been getting around on crutches reported they had to go back to wheelchairs, some even to motorized chairs.

Medication such as aspirin to control joint and muscle pain has worked for many of Halstead's patients. Beyond that, he suggests that good nutrition, good health practices, and especially no overexertion may be the best therapy.

The NIH team headed by Dr. Dalakas reported last year on an initial study of 17 persons, aged 31 to 65, who had had polio and had recovered to various degrees. All 17 were experiencing recent neuromuscular symptoms.

The team found that the patients fit into two distinct groups. The first, a group of seven patients, had had generally severe paralytic and other symptoms at the time of their original illness. In their immediate recovery they tended to have little endurance, experienced joint pain, and were prone to falling and injuries. They had substantial disability and required corrective devices and braces. Over the years, their condition would remain stable for long periods, then deteriorate, requiring that adjustments be made to their braces and that their work demands be reduced.

This group was not experiencing muscular weakness or wasting in the late PPMA sense. In effect, their basic condition had been defined as they recovered from their original illness, and that condition could be stabilized with minor adjustments to their corrective devices and in their life styles.

The second group of 10 patients had had less severe symptoms during their original illnesses. They were now experiencing late PMMA symptoms of a slowly progressive muscle weakness and wasting. One woman in this group, for example, had been functioning well with only mild, left-side weakness from her earlier (1950) polio. Some 30 years later she began developing focused pain and weakness in her right arm, which had not been affected in her original illness.

Laboratory tests and biopsies with this second group suggest to the NIH researchers that "late PMMA is a benign form of motor-neuron (nerve cell) disease" that may be slowly progressive. The researchers also see the patients' immune systems as perhaps playing a role in late PPMA. They accept the possibility that aging and a diminished reserve of strength in older polio patients may be a factor. Again, they strongly rule out Lou Gehrig's disease as well as myasthenia gravis, a chronic, progressive muscular paralysis, as explanations for PPMA.

The NIH researchers are now going beyond the initial group of 17 to do a study of 2,500 patients who survived polio. They are investigating how many now have late PPMA; to what degree; how it is affecting them; what seems to be the cause; and how it might be treated.

Whatever the cause and course of late PPMA, persons who have recovered from polio and who now--years later--discover new muscle weakness and pain should get immediate medical attention. It must first be determined whether the affliction is due to another cause and not related to their earlier polio. If the condition is diagnosed as late PPMA, medication may be helpful in relieving painful symptoms, and the changes in life style suggested by Dr. Halstead, such as maintaining good health and avoiding overexertion, may be useful.

Once past their initial fears and misgivings, post-polio patients should understand that--whatever the theories being offered--the experts agree that this new complication is not life-threatening. Accepting that it is an added burden to an already burdened body, post-polio patients should still be able to adjust to these changed conditions and to continue to live productive lives.

COPYRIGHT 1985 U.S. Government Printing Office
COPYRIGHT 2004 Gale Group

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