One of the puzzling things about autism has always been the disparity between the sexes. Boys have always been more susceptible than girls. This is not in itself unusual. There are gender differences affecting a whole range of conditions and, if this New York Times article is correct, men frequently come off worse.
But if boys are more susceptible you might have assumed that as the severity of the condition increase this disparity would become more marked. In fact you would have assumed wrong. According to this source:
The greater severity and lower frequency of autism in females has been cited as evidence for a multifactorial polygenic mode of inheritance with differential loading by sex, which predicts greater severity in the less frequently affected sex.
Greater severity is usually taken to include severe cognitive impairment as well and the greater the degree of cognitive impairment the closer the ratio between boys and girls. But there are problems with this model. David Skuse has argued that the association between cognitive impairment and autism is not because they share a common cause but simply because if you have both conditions you are more likely to be seen by a clinician and get a diagnosis. More able people may be just as autistic but have coping strategies that enable them to avoid a diagnosis. And if girls have better coping strategies than boys they will be disproportionately overrepresented amongst autistic people without cognitive impairment who are missed by the system.
Last week Woman’s Hour broadcast a segment on Asperger syndrome took up this argument and suggested that there may be as many girls as boys on the spectrum. Most of them are not getting a diagnosis because they present in ways that are unfamiliar to clinicians who are used to seeing the condition in boys. The programme is no longer available but Sunday’s Observer carried a two page spread on the same story.
Experts like Judy Gould and Tony Attwood cited by the Observer still believe there is a gender difference but they estimate that it is only 2.5:1. Asperger girls may be more passive than boys. They do not assert their difference or draw attention to themselves. Instead they observe and copy other people’s behaviour. Their special interests may be intense but are also likely to be more socially acceptable; reading fiction, following soaps, celebrity culture – the sort of thing that lots of other girls do – and so they do not stand out.
Conformity comes at a cost. The Observer quotes Tony Attwood’s estimate that 20% of anorexic girls are undiagnosed autistics. Then there is self harm and other evidence of psychological stress. There are important differences between men and women. They need to be understood and respected. But it does not help autistic women if autism is described as an extreme male brain syndrome. The Observer ends by quoting professor David Skuse who believes that:
if we can prove the ratio of boys to girls is as high as many of us suspect, it would be as significant a milestone in this field as the discovery that the condition is on a spectrum.”
This post is also appearing on Left Brain Right Brain. You are invited to join the discussion there.
April 14th, 2009
Posted by
Mike |
epidemiology, news |
no comments
Sometimes the world of autism produces an unusual event that challenges existing notions about the condition. These events need investigating in case they turn out to be significant. But too often their novelty value attracts premature speculation in the media. In July 2008 there were press reports that drew attention to a cluster of autism cases in the childen of Somali refugees in Minneapolis. In March 2008 a study was published that drew attention to high rates of autism among Somali children in Sweden.
There was a brief flurry of interest in the online autism world. Kristina Chew’s article provoked much discussion on Autism Vox. David Kirby weighed in of course with Is Autism an “American Disease?” I found his article interesting because he drew attention to the little known phenomenon of higher than average autism rates amongst the children of immigrant families, especially among non-white migrants to Europe and North America. Joseph has written a good introduction to the research literature on this. Despite going back over thirty years the literature is not extensive. But it does suggest that cultural and linguistic differences might be at least as significant as the ethnicity of the migrant populations.
Unfortunately Kirby confuses the question by trying to connect vaccines to immigration. But it is a very muddled connection. According to Kirby foreign migrants come mainly from countries which still use thimerosal in childhood vaccines and have good vaccination records (92% coverage in Mexico for example). He claims that
“many if not most of these children are routinely revaccinated on entry into the United States.”
Even assuming that Kirby is correct, what has this to do with the Somali children? They were all born and vaccinated within the USA. And the most dramatic increase in autism is in Somali children enrolled in early years programmes whose vaccination schedule would have been largely thimerosal free. If the Somali phenomenon is part of a broader trend affecting the children of immigrants then it is clear that vaccination history is not a commonality that they all share.
Kirby mentions vitamin D as another potential commonality. Lots of migrants are dark skinned and come from tropical or sub tropical climate zones. They come to countries in temperate climate zones with much less sunshine. Vitamin D deficiency is a recognized problem. But it does not cause autism.
A search of PubMed for autism and vitamin D deficiency found 3 articles: two were case studies of rickets in autistic children with restricted diets. One was about possible vitamin D deficiency in autistic Somali children in Sweden. But it is only a brief note (less than two pages) in Swedish and there is no abstract available in English. I did turn up another article, this time in Medical Hypotheses, that suggested a link between vitamin D and autism. It argues that lack of sunlight, particularly UVB penetration in northern climates makes dark skinned people more prone to vitamin D deficiency and hence more prone to autism. This link to autism has been described as no more than “an interesting speculation,” by Dr. Wendy Roberts, a professor of pediatrics at the University of Toronto. It is also interesting that the author of the speculation is founder of the Vitamin D Council whose mission is
to put an end to the worldwide epidemic of Vitamin D Deficiency.
Perhaps it is not about commonalities. What if the Somalis are a special case? Over on Age of Autism Kirby speculates that it could be more “evidence” of mitochondrial influence on autism.
I wonder if the fact that Somalians reflect a different mitochondrial haplogroup than most of the rest of Africa (M1 rather than L) might provide a clue here?
I had no idea what haplogroups were before now, but I do know that the distribution of mitochondrial haplogroups as depicted in this mitochondrial map of the world is far too complicated to support Kirby’s “wonderings” without some very hard data. What if we compare the data from Sweden and Minneapolis?
The first thing to note is that there is no relation between the two datasets. In Sweden an epidemiological study found a rate of 0.19% among children with a degree of learning difficulty(Europe)/mental retardation(USA). This is consistent with the consensus figure of 20 in 10000 for ASD amongst children with additional learning difficulties. The comparable figure for Somali children in Sweden is 70 in 10000. In Minnesota there is no reliable epidemiological data for autism. There is the administrative data for children in receipt of autism services. But this is based on teacher assessment and does not require diagnosis by a trained clinician. A clinical diagnosis requires
A total of six (or more) items from (1), (2), and (3), with at least two from (1), and one each from (2) and (3):
The three categories comprise the famous Triad of Impairments. In Minnesota, providing you satisfy their criteria for the first part of the Triad, meeting their criteria for either of the other parts is sufficient to access autism services. This is much less demanding than the DSM-IV criteria and probably explains why Minnesota has more than twice the national average of autism cases on its books. Even though the figures for Somali children in Minneapolis, at 1 in 28 or 3.6 per cent. are at least 4 times the national average they are only twice the average for Minnesota. Compared with the Swedish figures of 20 in 10000 for natives and 70 in 10000 for Somali immigrants, Minneapolis has a baseline figure of 180 in 10000 for natives and 360 in 10000 fo Somali immigrants.
All we can say with certainty is that
- no matter what criteria you use, the children of foreign migrants are more likely to be identified as autistic than native children.
- the higher the “going rate” for autism in a specific community, the higher the corresponding rate will be in the migrant community.
- clinical criteria are more parsimonious than administrative criteria.
Finally, the Age of Ignorance makes the important points that
- Somalia is a war zone and the refugees who have made it to America have suffered unspeakable hardship and trauma along the way.
- The children of psychologically distressed parents frequently display evidence of psychological distress themselves.
Whatever the actual rate of autism in Somali communities it requires careful investigation. I agree with Steve Novella that
What we don’t need are people with an agenda politicizing this issue.
I concur completely with the Age of Ignorance conclusion
One further comment. What Kirby, et al, are doing with the poor people of Somalia, i.e. using them to further their agenda, disgusts me. The people of Somalia are living in some of the poorest conditions in the world. They are being subjected to war, war where people die, where people are psychologically damaged with PTSD, depression, etc. They are uprooted and forced to leave their homes to protect their families. It is well known that children of the victims of war, e.g. survivors of the Holocaust, develop psychological problems.
Kirby and his acolytes on the blogs Age of Autism, Injecting Sense and Adventures in Autism are trying to exploit the situation in Somalia. The people of Somalia have suffered mass murder, rape, torture, famine, disease - all the usual features of a bloody civil war. Half of all children under five in 1993 died in Somalia. A third of all refugees have been tortured. Maybe having traumatized parents who are strangers in a strange land and growing up between two cultures is a more significant environmental difference for these children than vaccines or sunlight.
November 20th, 2008
Posted by
Mike |
epidemiology, vaccines |
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