Abnormal retinal hemodynamics found in patients with Alzheimer's disease

"In many cases, visual symptoms are often the initial complaint reported by patients with Alzheimer's disease, and there is evidence from electrophysiologic tests suggesting that morphologic manifestations of Alzheimer's disease may extend to the retina," said Dr. Feke, senior scientist and director of the Ocular Circulation and Imaging Laboratory at Schepens Retina Associates Foundation/Harvard Medical School, Boston. "Impaired cerebral blood flow has been documented in patients with Alzheimer's disease, and, in a 2005 study, impaired cerebral blood flow was evident even in the pre-clinical stage of the disease.

"Among the various mechanisms responsible for reduced cerebral blood flow are beta-amyloid deposition on the arterial side and collagen deposition leading to increased vessel wall thickness on the venous side of the circulation," Dr. Feke said.

Retinal hemodynamics

The investigators measured the blood column diameter, centerline blood speed, and retinal blood flow rate in the major superior temporal retinal vein in one eye of each subject using the Canon CLBF 100 retinal laser Doppler blood flow instrument. The results in the two groups were compared using unpaired t-tests and Mann-Whitney U tests.

There was no significant difference in IOP between the groups, but Snellen visual acuity was decreased (p = 0.031) in the patients with Alzheimer's disease, according to Dr. Feke.

The investigators reported that mean venous blood column diameter was 133 ± 10 µm in the patients with Alzheimer's disease and 147 ± 9 µm in the control group, a statistically significant (p = 0.006) difference. However, the mean venous centerline blood speed was similar in both groups (26.6 ± 9.7 mm/sec and 30.7 ± 5.4 mm/sec, respectively). The resultant mean blood flow rates were 11.2 ± 4.5 µl/min in the patients and 15.6 ± 2.9 µl/min in the controls. The difference in blood flow rate was also statistically significant (p = 0.026).

"We found a marked narrowing of the retinal venous blood column diameter and a reduction in the retinal blood flow rate in the patients compared with the age-matched control subjects using our Canon retinal laser Doppler instrument," he said. "Our ability to 'lock on' to the target vessels provided reliable measurements.

"To the best of our knowledge, this is the first study to demonstrate abnormalities in the retinal circulation of patients with Alzheimer's disease," Dr. Feke said. "It is likely that the mechanisms producing the reduced blood flow in the retina are related to those that produce the cerebral blood flow abnormalities in patients with Alzheimer's disease."

It is tempting to speculate that the narrowing of the venous blood column diameter is related to an increase in the venous wall thickness resulting from collagen deposition as found in cerebral veins, according to Dr. Feke.