By Michael Rubin, MD
Professor of Clinical Neurology, Weill Cornell Medical College

Dr. Rubin reports no financial relationships relevant to this field of study.

Synopsis: Although many specific neuromuscular disorders are rare, in aggregate, these all add up to a large number, roughly twice as common as multiple sclerosis, and about the same prevalence as Parkinson’s disease.

Source: Deenen JCW, et al. The epidemiology of neuromuscular disorders: A comprehensive overview of the literature. J Neuromusc Dis 2015; DOI 10.3233/JND-140045.

Studies on population frequencies of inherited neuromuscular diseases around the world were first published in 1991,1,2 with additional reports surfacing in 2010 from the Muscular Dystrophy Campaign, and in 2013 from the Orphanet Report Series, the latter of which included rare neuromuscular disorders. Given improved diagnostic capabilities with the advent of molecular genetics, and consensus on diagnostic criteria, an up-to-date review of the scientific literature since 1990 was undertaken to determine occurrence rates, and gender and age distributions of various neuromuscular disorders.

Using keywords “epidemiology,” “incidence,” and “prevalence,” and searching for original English language articles whose titles included the terms neuromuscular or neurological disorder, and neuromuscular or muscle disease, PubMed was searched for peer-reviewed articles appearing between January 1990 and July 2014, covering 30 relatively frequent or phenotypically distinguishable neuromuscular diseases. Arranged anatomically, these conditions included anterior horn cell disorders, such as progressive spinal muscular atrophy and amyotrophic lateral sclerosis, and peripheral nerve disorders, such as Charcot-Marie-Tooth disease, chronic inflammatory demyelinating polyneuropathy, Guillain-Barre’ syndrome, and chronic idiopathic axonal polyneuropathy. Myasthenia gravis and Lambert-Eaton myasthenic syndrome were the neuromuscular junctionopathies included, while muscle diseases studied included the entire gamut of muscular dystrophies, inflammatory myositides, chronic progressive external ophthalmoplegia, and Pompe’s and McArdle’s disease. Secondary disorders, such as diabetic or HIV neuropathies, were excluded, and where multiple publications incorporated the same data, only the most recent data were included.

Among 169 relevant articles identified, incidence rates were identified for 11 disorders, ranging from 0.05/100,000 for Lambert-Eaton myasthenic syndrome to 9/100,000 for spinal muscular atrophy. Prevalence rates were identified for 24 disorders, ranging from 0.1/100,000 for oculopharyngeal muscular dystrophy to 60/100,000 for post-polio syndrome. Prevalence rates lower than 50/100,000 were found in 23 diseases, making them rare by definition, but summating the prevalence rates for the 24 disorders so obtained brought the total to 160/100,000. Age distribution was ambiguous in five disorders, but occurred early in life in Friedreich ataxia and in Duchenne and congenital muscular dystrophies, and later in life in amyotrophic lateral sclerosis, post-polio syndrome, and Lambert-Eaton myasthenic syndrome. Overall, disorders were equally distributed between the sexes for the 18 disorders in which it was available, but men featured more prominently in amyotrophic lateral sclerosis, chronic inflammatory demyelinating polyneuropathy, Guillain-Barre syndrome, Lambert-Eaton myasthenic syndrome, and Duchenne, Becker, and facioscapulohumeral dystrophy, whereas women were twice as affected in myasthenia gravis, non-dystrophic myotonia, polymyositis, and dermatomyositis. No information was available for Pompe’s or McArdle’s disease, chronic idiopathic axonal polyneuropathy, idiopathic neuralgic amyotrophy, or progressive spinal muscular atrophy. Although individually rare, as a group neuromuscular diseases are not, being twice as high as multiple sclerosis (80/100,000 in Europe) and similar to that seen with Parkinson’s disease (100-300/100,000 worldwide). Since 1990, considerably increased prevalence rates seem to have occurred in Charcot-Marie-Tooth disease, and in Becker, facioscapulohumeral, and myotonic muscular dystrophy, with the latter prevalence appearing twice as high compared to 1991. Genetic testing may be a significant contributing factor.

Commentary

To determine the incidence and prevalence of inflammatory myopathies, original articles in English or French were reviewed, of which 46 were found, published between 1966-2013, wherein prevalence or incidence were discussed.3 Prevalence ranged from 2.4-33.8 per 100 000, and incidence from 1.16-19 per million/year, with no clear geographical disparities found. In the northern hemisphere, possibly due to ultraviolet radiation and its immunomodulatory effects, the incidence of dermatomyositis followed a latitudinal gradient, whereas sporadic inclusion body myositis prevalence correlated with HLA-DR3 frequency. Inflammatory myopathies are rare and may be influenced by genetic and environmental factors.

References

  1. Emery AE. Population frequencies of inherited neuromuscular diseases—a world survey. Neuromuscul Disord 1991;1:19-29.
  2. Emery AE. Population frequencies of neuromuscular diseases—II. Amyotrophic lateral sclerosis (motor neuron disease). Neuromuscul Disord 1991;1:323-325.
  3. Meyer A, et al. Incidence and prevalence of inflammatory myopathies: A systematic review. Rheumatology 2015;54:50-63.