This article includes items about: myotonic dystrophy type 1, myotonic dystrophy type 2, spinal muscular atrophy, spinal-bulbar muscular atrophy
A significantly higher-than-average risk of sudden death exists for adults with type 1 myotonic dystrophy (MMD1, also known as DM1) and either a severely abnormal electrocardiogram (ECG) or a diagnosis of atrial tachyarrhythmia (an abnormally fast, irregular heart rate originating in the upper chamber, or atrium, of the heart), according to a study in the June 19 issue of the New England Journal of Medicine. The study was supported by MDA, the National Institutes of Health and the medical electronics manufacturer Medtronic.
A severely abnormal ECG correlated with a risk of sudden death that was 3.3 times above average. A clinical diagnosis of atrial tachyarrhythmia resulted in a fivefold increase in the risk of sudden death compared to the average risk.
The investigators defined a severe ECG abnormality as either a heart rhythm not originating from the normal place in the heart; a significant lengthening of the normal time it takes for signals to travel through the heart; or a partial blockage of signals from the upper to the lower part of the heart.
All these abnormalities reflect malfunctions of the heart’s conduction system, which regulates heart rate and rhythm via electricity-like signals. Conduction-system abnormalities, which are common in MMD1, can cause sudden death if they progress to complete blockage of cardiac signals and stoppage of the heartbeat; or if they lead to a fast, nonfunctional rhythm pattern in the lower heart chambers (ventricular tachyarrhythmia) that prevents adequate pumping action.
Cardiologist William Groh, who received MDA support at Indiana University’s Krannert Institute of Cardiology in Indianapolis, and colleagues studied 406 adults with MMD1 seen at 23 MDA clinics beginning in April 1997.
During an average follow-up period of 5.7 years, there were 81 deaths among study participants, 27 of which were classified as sudden. In addition, there were 32 deaths from respiratory failure due to respiratory muscle weakness, five nonsudden cardiac deaths and 17 deaths from other causes.
The investigators note that 10 percent of study participants had cardiac pacemakers by the last follow-up visit but that these did not appear to decrease the rate of sudden death or death from any cause.
A small number of participants had implantable cardioverter-defibrillators, which can bring an abnormally slow heartbeat up to a normal rate and also slow a dangerously fast heart rhythm, by delivering an electric shock. Sudden death occurred in two patients with these devices.
The investigators say their findings suggest cardioverter-defibrillators may be helpful in preventing sudden death in MMD1 but that their data didn’t allow firm conclusions to be drawn.
“In conclusion,” the authors write, “our study shows that adult patients with myotonic dystrophy type 1 are at high risk for sudden death.” They noted that the presence of a severe ECG abnormality or a clinical diagnosis of atrial tachyarrhythmia increased the risk of sudden death, whether or not the other condition existed.
A type of cardiac abnormality known as “left ventricular noncompaction,” a condition in which the muscular wall of the heart’s main (lower left) pumping chamber is spongy and meshlike, rather than tightly compacted, might be specifically associated with type 2 myotonic dystrophy (MMD2, also known as DM2), says a group of researchers in France who published their findings in the April issue of Neuromuscular Disorders.
Karim Wahbi, at René Descartes University in Paris, and colleagues, describe a 61-year-old man with MMD2 with this abnormality, which can (but doesn’t always) lead to blood clots and interference with the heart’s pumping capacity. It was detected with an echocardiogram, which uses sound waves to produce images of the heart.
The researchers note that most cases of left ventricular noncompaction have been seen in association with a neuromuscular disorder, including MMD1. They recommend careful followup of MMD2 patients for cardiac complications.
A new study says brain MRI abnormalities known as “white matter lesions,” known to be common in type 1 myotonic dystophy (MMD1, or DM1), are more similar among family members affected by the disease than they are among people who share other disease characteristics, such as the size of the DNA abnormality that causes their disease.
Alfonso Di Costanzo at the University of Molise in Campobasso, Italy, and colleagues, who published their findings in the April issue of Neuromuscular Disorders, studied 60 people (30 of each gender) with the noncongenital form of MMD1. Each of the study participants, who came from 22 families, had at least two close relatives who also were affected.
When they analyzed the extent and location of the brain abnormalities in the study participants, the researchers found these were not correlated with the person’s gender or age at MMD1 onset, or with the gender of the parent from whom the disease was inherited; nor did they correlate with the size of the MMD1-causing DNA expansion, measured in blood cells, even though larger expansions are generally correlated with more severe disease symptoms and earlier onset.
The authors suggest that additional, as yet undefined, genetic factors, and/or environmental factors shared by families could be influencing the development of the MRI abnormalities. They also note, however, that MMD1-associated DNA expansions measured in blood cells don’t always match those measured in brain or other cells.
A study of the survival and functional status of 39 people with spinal-bulbar muscular atrophy (SBMA, or Kennedy’s disease) conducted at the Mayo Clinic in Rochester, Minn., showed patients’ survival was only slightly altered compared to healthy subjects’ survival and that most patients had only mild neurologic impairment many years after diagnosis.
Eric Sorenson and colleagues, who published their findings in the May 20 issue of Neurology, reviewed the medical records of 39 people with a confirmed SBMA diagnosis who were seen at the Mayo Clinic between 1990 and 2005. They compared them to people unaffected by SBMA but otherwise similar.
Of the 39 SBMA-affected study participants, 33 (85 percent) were alive at the study’s start. In the six who had died, the average age of death was 76.
Eight could not be contacted, but the other 25 agreed to take part in the unctional study, which used a standard rating scale to assess their abilities. The average score on this scale was 37, out of a possible 48.
The most common impairment was difficulty climbing stairs, although only five people said they were unable to do so.
Twenty reported some impairment in walking, but only two required a wheelchair.
Minor difficulty with swallowing was reported by 20 subjects, but all maintained normal oral intake and none used a feeding tube. No one required a speech augmentation device, and only one person needed respiratory support, using noninvasive positive pressure ventilation during sleep.
The results of the study suggest that “in addition to a good long-term survival, patients with SBMA continue to be independent in most of their activities of daily living,” the authors write. They say they don’t want to “diminish the significant impairment” SBMA causes in some patients; but that their findings suggest that “in the majority, SBMA follows a relatively benign course with comparatively good functional status years after the diagnosis.”
Researchers at several German and U.S. institutions recently announced they’ve identified a protein that can almost completely compensate for the genetic defect that causes chromosome-5-linked spinal muscular atrophy (SMA), which is by far the most common type of the disease. They say their findings may open new treatment pathways, as well as provide better understanding of the biology of SMA and possibly other, related diseases.
A research team coordinated by Brunhilde Wirth at the University of Cologne in Germany identified the activity level of the gene for the plastin 3 protein as a modifying factor in SMA in females.
In the process, they found that defects in the growth and maintenance of nerve fibers, or “axons,” which carry chemicals from cell to cell, are the primary problem in chromosome-5 SMA and are a potential point of intervention in its treatment. They published their findings in the April 25 issue of Science.
Chromosome-5 SMA, a genetic disease in which spinal nerve cells that control voluntary muscles (motor neurons) are lost, is caused by mutations in the SMN1 gene that result in varying degrees of deficiency of the nerve-cell protein SMN.
People with moderate SMN loss have weakness and limited mobility but generally live into adulthood, while babies born with almost no SMN have severely compromised breathing and swallowing functions and often don’t survive more than a few years.
Wirth’s team identified six families in which some of the females with specific SMN gene mutations and protein levels had the expected SMA symptoms, while others with the same SMN mutations and protein levels had very mild disease. Suspecting the presence of modifying genetic factors, the researchers measured activity levels of the siblings’ other genes.
Only one gene, carrying the code for the protein plastin 3, showed significantly different levels of protein-producing activity in cells from mildly affected, compared to more severely affected, siblings.
In boys, the level of plastin 3 protein didn’t make a difference in the degree of severity of SMA; but in the girls, high plastin 3 protein levels resulted in far less severe SMA symptoms or no symptoms at all with the same degree of SMN deficiency. The investigators don’t know the reason for this gender difference.
Further experiments in human cells and in mice and zebrafish showed that plastin 3 is important for bundling filaments that make up nerve axons and that extra plastin 3 can almost completely compensate for the detrimental effects of reduced SMN protein levels on these cellular structures.
The researchers note that their findings “support the view that axon biology is crucial for SMA ... and that proteins stabilizing the axon can modify the disease.”
Eugenio Mercuri from Catholic University in Rome, with researchers from several Italian institutions and the Dubowitz Neuromuscular Centre in London, found daily treatment with oral salbutamol (called albuterol in the United States) yielded some encouraging results in a small trial involving young children with type 2 spinal muscular atrophy (SMA2).
The investigators, who published their findings online June 23 in Neuromuscular Disorders, gave albuterol at a dosage of 2 milligrams three times a day for a year to 23 children who were between 30 months and 6 years old at study entry.
They assessed motor function using a standardized rating scale six months before the treatment started, when the treatment started, six months after starting treatment and a year after starting treatment.
There were no significant changes in function when the children were not on treatment, but the functional scores after six months and a year of treatment with albuterol were significantly higher than those recorded when the children started albuterol. There were no major side effects.
The researchers say their results suggest albuterol may be beneficial in SMA2 but that a larger trial, in which albuterol is compared to a placebo (inactive substance), is needed to confirm these early findings.
They also note that the drug’s actions in SMA are not well understood. It may reduce muscle wasting and/or increase production of the SMN protein, which is insufficient in this disease.