Size and growth are some pretty important concepts related to lots of different features of the human condition. Although universal connections related to health rarely (never?) exist, there are some interesting data linking small birth weight and later childhood intellectual development for example, as well as brain size and the intelligence quotient (at least to a degree). Size and growth tend to be determined by a variable combination of both genes and environment. This meta-analysis for example, showed how the physical environment affects various growth measures, height and weight, but perhaps not as much parameters such as head circumference. This leads me to the subject of this post - head size and autism - and some of the published research and its potential implications.
Macrocephaly is the technical term where head circumference is larger than about 2 standard deviations from the average (mean); the opposite condition being microcephaly, where head circumference is smaller. There are various reasons why a larger head may occur, such as an enlargement of the brain and cases of hydrocephalus (water on the brain). In most cases in infants and children, larger head size is thought to be governed by more genetic factors such as heredity or the presence of one or more various genetic disorders.
Macrocephaly has, down the years, been associated with some cases of autism spectrum conditions. Kanner first noted that some of his original cohort had larger heads (among other things). An observation which has subsequently been reported again and again and again with prevalence estimates of macrocephaly in autism ranging from approximately 10-30%. The data does suggest some degree of heritability linked to macrocephaly, although the relationship is not entirely straight forward and the heritability implications not immediately clear. In more recent years there has been some debate about macrocephaly and autism and how factors such as ethnicity might also show an effect. This study for example first published as an abstract at IMFAR 2009, based on an Israeli cohort found little evidence of elevated rates of macrocephaly in autism in comparison to asymptomatic controls.
Why might macrocephaly be important to autism? Well I mentioned that outside of heritability, generally macrocephaly is tied into one or more genetic conditions. The theory in autism is that such findings might indicate some genetic component and hence help point towards candidate areas of interest. To see this theory in action, readers may wish to have a look at these studies (here and here) on how macrocephaly from both a case study and group study point of view might help untangle a few research strands in such a heterogeneous condition. The 'endophenotypes' way of study is something which really is the future of autism research.
There are perhaps more immediate effects potentially tied into macrocephaly presenting in autism also. This study suggested specific autistic behaviours tend to be more severe in those with larger heads, including delays in acquiring language; the language side of things however seems to be the source of some speculation. Uta Frith joins a number of other researchers in saying that macrocephaly may imply abnormal neural connectivity. One might also expect that a larger head may indicate a larger brain and hence the maturational issue of brain growth could be implicated. Certainly studies of total brain volume in autism seem to suggest some discrepancy with control populations, driven by things like an increased cortical surface area. Such studies also seem to point to a 'brain overgrowth' as potentially being present, at least during early infancy with lots of different areas of the brain involved. Thinking about this, I was reminded of some work I have heard discussed quite a few years ago about synaptic pruning in relation to autism and the various instruments of such pruning including the MHC and endogenous opioid peptides. Indeed the work of Zagon and McLaughlin is I think absolutely fascinating, particularly their characterisation of the zeta opioid receptor and their various studies on the effects of opioid antagonists such as naltrexone on rat postnatal development. I don't know if and how it might tie into these latest findings on synaptic pruning.
I think it is important also to note that larger head size is not exclusive to autism spectrum conditions. This paper reported similar trends in head size and various other growth parameters from non-autism controls with a psychiatric disorder. Importantly however they indicated that there may be subtle differences in the timing of measurements between autism and controls which might provide some clues for the underlying processes involved.
What we can ascertain from this collected body of work is that larger heads show more than a passing association to autism spectrum conditions, and perhaps tie into some elements of the presented condition including some interesting findings on serotonin. Having said that, the recent meta-analysis of neonatal factors in connection to autism included as part of this post suggesting that the connection to head circumference might not be all it has been cracked up to be, perhaps needs to be taken on board. What I would like to see is more information on is any link to important autism co-morbidities such as epilepsy and if such 'overgrowth' is confined to early infancy, and whether it is controlled by genes or environment (or both)?
Regarding size matters...