Neurological disorders such as multiple sclerosis and Alzheimer's disease are becoming more common in the US, a large new analysis suggests.
Deborah Hirtz at the National Institute of Neurological Disorders and Stroke in Bethesda, Maryland, US, and colleagues reviewed about 500 research articles describing the prevalence of 12 diseases commonly identified and treated by neurologists.
The team focused on studies published between 1990 and 2005 and ranked them according to various criteria, including scope and diagnostic precision.
Nearly 1 out of every 1000 people in the US - around 266,000 people - suffers from multiple sclerosis (MS), Hirtz's team calculates. This figure is 50% higher than that estimated by a similarly large literature review published in 1982 (Neurology, vol 32, p 1207).
Higher or lower?
Some scientists speculate that increased amounts of certain pollutants or a reduction in dietary vitamin D have caused a rise in MS cases. These hypotheses remain unproven, according to Douglas Goodin, director of the Multiple Sclerosis Center at University of California San Francisco Medical Center, US. But Goodin adds that "if the prevalence of MS is increasing we need to find out why".
Hirtz notes that the higher MS estimate might be due to better diagnosis rather than an actual increase in prevalence.
Conversely, some experts argue that the MS prevalence determined by Hirtz's team is too low. "The National MS Society alone has over 300,000 people in its data base who have self-identified themselves as having multiple sclerosis," says Nicholas La Rocca of the society, based in New York.
Hirtz's analysis also estimates that 67 out of every 1000 people aged 65 or older in the US suffers from Alzheimer's disease. She says that this represents a "substantial increase" in the prevalence of this disease and attributes this partly to the growing proportion of very elderly people in the US.
According to the new analysis:
- Nearly six out of every 1000 children have some form of autism.
- About two out of every 1000 youngsters have Tourette's syndrome, which is characterised by uncontrolled movements and speech. Hirtz says the collection of reliable data on these disorders is relatively recent.
- The number of traumatic brain injuries is down by 50% over the past few decades, perhaps partly due to better car safety. An estimated 300,000 people in the US suffer such injury each year, researchers report.
- About 13,000 people each year suffer an injury to the spinal cord.
- The prevalence of certain disorders, such as cerebral palsy, migraine and epilepsy, has remained steady in the US over the past 25 years. Rates of Motor neuron/Lou Gehrig's disease (ALS) and Parkinson's disease also appear stable. However, a recent study projected that the number of people living with Parkinson's disease will double from about 4.3 million to 9 million people worldwide over the next 25 years (Neurology, vol 68, p 384).
Journal reference: Neurology (vol 68, p 326)