Brain Scans & Pupil Dilation
Some possible -- though not definitive -- tests for Alzheimer's disease include brain scans and pupil dilation.
Brain scans
One test that can help diagnose early-stage Alzheimer's disease involves new brain-imaging technologies: positron emission tomography (PET) and single photon emission computed tomography (SPECT).
PET scans reveal regional differences around the brain in the metabolism of the body's main fuel: blood sugar, or glucose. People with early-stage Alzheimer's disease show unusually decreased glucose metabolism in one region of the brain.
Unfortunately, people with atherosclerosis of the brain -- the narrowing of arteries that causes most strokes and mini-strokes -- also show similar impairment of glucose metabolism. So it's not clear exactly how valuable PET scans will ultimately be in diagnosis of Alzheimer's disease.
In addition, PET scans are experimental and very expensive. This technology is not widely available, and where it is available, health insurance usually does not cover it. Nonetheless, PET scan research continues. Several researchers have paired PET scans with APOE genetic evaluations in hopes that the combination can help diagnose Alzheimer's more effectively than either test by itself.
SPECT scans help evaluate blood flow through the brain, and brain metabolism. Compared with PET scans, the information provided by SPECT scans is less precise, but SPECT scans are considerably less expensive, so some clinicians use them to support a diagnosis of probable Alzheimer's disease.
PET and SPECT scans can help diagnose early-stage Alzheimer's disease, but by themselves, they are not definitively diagnostic.
Pupil dilation
This possible test for Alzheimer's disease is based on dilation (opening) of the pupil of the eye. In routine ophthalmologic exams using standard chemicals to dilate the pupil, the pupils of people with Alzheimer's disease seem to dilate unusually quickly.
In one study, a chemical called tropicamide had no effect on the pupils of 30 out of 32 people who were free of Alzheimer's, but it dilated the pupils of 18 out of 19 people with the disease. The effect was equally clear in people with mild and severe Alzheimer's disease. ]
However, results were disappointing when Mayo Clinic researchers attempted to duplicate this study. They used tropicamide on 20 people with Alzheimer's disease and 20 age-matched cognitively normal controls, and found no differences in pupil dilation between the two groups.
It's possible that the pupil dilation test has merit, but the Mayo study casts significant doubt on it.