Brain Volume: Does Size Matter?

As I've mentioned previously, I've been criss-crossing the country recently giving an update on Alzheimer's disease (AD) to fellow family physicians.  One of the bigger questions that often comes up is with regards to neuroimaging.  In other words, why should we obtain a CT or MRI of the brain?

The practical matter is that while we cannot prove a diagnosis of AD based upon currently available radiologic studies (although cutting edge contrast agents & ligands may be available at research facilities), we can look for masqueraders, such as vascular dementia (from one or more strokes), infection, subdural hematoma, normal pressure hydrocephalus, and even cancer, all of which can alter cognitive function.

With that said, researchers have already started linking brain volume (specifically, hippocampal) & atrophy to cognitive decline.  Back in January 2008, those with greater cognitive reserve were shown to have larger brain volume.  That same month, white matter hypertensities, presumably from small vessel disease due to vascular risk factors, was associated with a progression from normal cognition to mild cognitive impairment (MCI), while decrease brain volume predicted the conversion of MCI into dementia.

So it should come as no surprise then that in a study published in Neurology this month, brain volume & atrophy, specifically cortical thinning, was able to predict AD in asymptomatic, cognitively intact individuals 10 years prior to onset of cognitive decline & dementia.  Granted, this study was rather small, composed of two cohorts of 25 cognitively intact patients each, followed for 11 and 7 years, respectively, of whom 8 patients developed AD in one set and 7 developed AD in the other.  

No we're not quite ready to predict AD based upon neuroimaging for brain volume & atrophy.  However, we do have data that diabetes, conjugated equine estrogen +/- medroxyprogesterone acetate, reduction in lean mass, and an increase in visceral adiposity have been associated with less brain volume while B vitamins, endogenous sex steroid hormones, and physical activity have been associated with greater brain volume.  So it appears that we do have some control over our destiny.  And perhaps size does matter.  But I'd rather not wait to find out how little I can get away with.