A team of US scientists has established that millions of Americans, across all age groups, have some degree of recognized neurological illness.

The study is published in the journal Neurology.

The study team comprises six scientists representing the National Institutes of Neurological Disorders and Strokes (NINDS) and the National Institutes of Health (NIH) in Bethesda, Maryland; and the National Center for Chronic Disease Prevention and Health and Promotion, and the Centers for Disease Control and Prevention in Atlanta, Georgia. The study was led by Dr. Deborah Hirtz, of the NIH/NINDS in Bethesda.

Using data from the US and other developed countries when the US data wasn't good enough, the researchers estimated the prevalence and incidence of 12 different neurological disorders among Americans.

The 12 disorders they included were:

- Autistic spectrum disorders, including Asperger's Syndrome
- Cerebral palsy
- Tourette syndrome
- Migraine
- Epilepsy
- Multiple Sclerosis
- Alzheimer's Disease
- Parkinson's Disease
- Stroke
- Major traumatic brain injury
- Spinal cord injury
- Amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease - a type of motor neurone disease)

In some cases they used prevalence, or the proportion of people with the disorder, which is a more suitable way of estimating impact for long lasting diseases, such as epilepsy, and in other cases they used incidence or rate, which is more suitable for measuring impact of diseases of shorter duration, or that can strike more than once in the same person, such as injuries.

In the case of children, Dr Hirtz and colleagues estimated that for every 1,000 children, the prevalence is 5.8 for autistic spectrum disorders and 2.4 for cerebral palsy. There was insufficient data to give a good estimated prevalence for Tourettes, but they said if they were asked to guess it they would say it was around 3.5 per 1,000 children.

In the general population, they estimated that the 1-year prevalence for migraine was 121, while for epilepsy it was 7.1 and for multiple sclerosis it was 0.9.

And in the elderly population the prevalence of Alzheimer's disease was 67 per 1,000, while that of Parkinson's was 9.5.

In terms of those illnesses measured by rate or incidence, the measure used is incidence per 100,000. Among these were stroke at 183, major traumatic brain injury at 101, spinal cord injury at 4.5 and ALS at 1.6.

When these figures are translated into proportions of the overall population, they add up to millions of Americans living with one or more of these 12 neurological illnesses. For example, half a million American children have an autistic disorder, over 2 million Americans have epilepsy, nearly 3 million have had a stroke, more than 35 million suffer migraines.

In the same edition of the journal, Dr Steven Alpert of the University of Pittsburgh points out that the studies that contributed to these findings did not all use the same methods for measuring rates and prevalence, and neither did they use the same methods to diagnose illnesses. So in his view while the figures are interesting, they are probably not accurate.

Dr Hirtz and colleagues left out many other conditions that could have counted as being equally burdensome. For instance mental retardation, sleep disorders, chronic pain. The reason was that these conditions are often treated by non-neurologists and do not have as well defined criteria for diagnosis.

They also acknowledged that these figures do not capture the full extent of the burden of these disorders. For a more realistic assessment of the burden of these illnesses, the figures would have to include estimates of the duration, intensity, degree of disability, frequency, impact on life expectancy, and some measure of the suffering they cause, as well as the wider impact on family and society at large.