Intended for healthcare professionals

CCBYNC Open access

Rapid response to:


Incidental findings on brain magnetic resonance imaging: systematic review and meta-analysis

BMJ 2009; 339 doi: (Published 17 August 2009) Cite this as: BMJ 2009;339:b3016

Rapid Response:

MRI and blood viscosity.

Although the paper dealt with incidental findings, the authors noted,
"Reassuringly, the age related trends in silent brain infarcts and white
matter hyperintensities that we observed were consistent with existing
population based studies, etc." But there was no discussion about why age
related changes occurred.

Because MRI identifies regions with higher than usual water content
as areas of brightness, it is very likely that such changes in the elderly
reflect the effects of increased blood viscosity on intra-capillary
pressure, as increased blood viscosity is a part of the aging process.

In 1934, Landis reported that the extent to which water passed out
through the capillary wall was determined by the intra-capillary pressure,
and the higher the pressure the greater the level of transudation. In
accordance with the Poiseuille formula, intracapillary pressure would be
determined by viscosity but mainly by the capllary radius.
This means that in the chance event of a cluster of smaller than usual
capllaries, the increased pressure needed to sustain flow of viscous blood
would increase the amount of water passing out into the tissues. That
water would be identified by MRI as an area of brightness. But any event
(such as an emotional upset) or of some factor (such as smoking) which
increased blood viscosity, could be expected to involve larger capillaries
and thus increase the frequency of areas of brightness. This is the
likely mechanism of the MRI hyperintensities seen in the elderly.

The occurrence of such changes in the elderly draw attention to the
need to reduce blood viscosity, and several approaches have been shown to
be effective. The lowering of the level of fibrinogen could be the most
effective move. If the haematocrit is raised then it should be
normalised. In the long term, 6 grams daily of fish oil, which increases
the fluidity of the red cell membrane, should be considered. It seems
inappropriate to recognise age-related changes and not treat them.

Competing interests:
None declared

Competing interests: No competing interests

19 August 2009
Les O. Simpson
retired experimental pathologist
Dunedin, New Zealand, 9077