For patients with Alzheimer's disease, it helps to have a big head.

That's the conclusion of a new study that examined the head circumferences of 270 participants in the Multi-Institutional Research in Alzheimer's Genetic Epidemiology study (or MIRAGE for short). Apparently, the extra cranial capacity affords patients some cognitive reserve, resulting in better brain function at any given level of cerebral atrophy.

Researchers had previously noted an inverse relationship between cognitive performance and head circumference. But whether one's "maximum attained brain size" affected the relationship between brain pathology and Alzheimer's symptoms remained unknown, according to the new report.

To find out, German researchers working on the MIRAGE study gathered all sorts of data on 270 patients, whose average age was 75. They measured their heads (as a proxy for brain volume). They gave them MRI scans to ascertain their degree of cerebral atrophy (as a proxy for the damage wrought by Alzheimer's). They took blood to see which variant of the APOE gene was in their DNA (having one or two copies of the e4 version of APOE is thought to increase one's risk of Alzheimer's). They looked up the results of each patient's most recent mini-mental state examination (MMSE) to measure cognitive function. They also took into account each patient's age and ethnicity, how long they'd had Alzheimer's and whether they had diabetes, hypertension or major depression.