It seems that photoplethysmography is some kind of measuring blood flow in the vein through light. They have found an important correlation.
Transcranial brain photoplethysmography to study the venules of cerebral cortex in patients with multiple sclerosis.http://www.ncbi.nlm.nih.gov/pubmed/24335101
To evaluate the utility of a transcranial brain photoplethysmography parameter as a potential marker for patients with multiple sclerosis.
We investigated 38 patients affected by multiple sclerosis, according to the revised McDonald criteria (12 males and 26 females, mean age 41.1 ± 8.5 years, Expanded Disability Status Scale mean value 2.6 ± 2.1) and compared them with 20 age- and sex-matched healthy controls. By means of transcranial brain photoplethysmography, a safe and non-invasive technology, we measured the increase in cerebral blood volume during compression for 10 s of the internal jugular veins of both sides in sitting position.
The cerebral blood volume increase was significantly smaller in the multiple sclerosis patients (left frontal cortex: -58%, p < 0.0001; right frontal cortex: -59%, p < 0.0001) compared with the controls.
Our study reveals that a minor increase in cerebral blood volume on the frontal cortex of both sides in sitting position is associated with a diagnosis of multiple sclerosis and might be a new marker.