The definitive diagnosis of Alzheimer’s disease requires microscopic examination of brain tissue, something that cannot be performed safely in a living patient. Instead, physicians use a constellation of symptoms and patterns of progression, combined with exclusion of all other causes of dementia, to diagnosis patients with the disease. But there are many alternative causes of memory loss in the elderly that must be excluded in order to make the diagnosis of Alzheimer’s. Many of these have signs and symptoms that overlap, making it difficult to determine the correct diagnosis.

Because each of these disorders is treated differently, with some responding well to specific therapy, it is imperative to determine the true underlying cause of a patient’s symptoms.

Near-infrared optical spectroscopy may provide some help in clarifying this problem. This type of light is nonionizing and travels safely through the brain tissue with no adverse effects. The light is scattered by the physical components of the brain, and the brains of Alzheimer’s patients scatters this light differently, allowing for a noninvasive and simple diagnosis.

Currently, the diagnostic finding in Alzheimer’s disease is the presence of amyloid plaques and neurofibrillary tangles, which are only visible under the microscope. The amyloid plaques accumulate as the disease progresses, and this plaque accumulation affects the way near-infrared light scatters when it hits the brain. Optical spectroscopy detects the pattern of light scatter in living patients without the need for biopsy or invasive procedure.

Researchers used this technique recently to accurately predict the presence or absence of Alzheimer’s associated amyloid plaques in brain tissue. The technique is currently being studied in living patients to determine its viability as a diagnostic and prognostic tool.

In addition to allowing for accurate differentiation from other causes of dementia, near-infrared optical spectroscopy may also be used to detect the disease at an earlier state, before symptoms are apparent, and allow for early initiation of therapy. It could also prove useful as a measurement of response to treatment and as a method of following Alzheimer’s patients over time.

Reference

Hanlon, E.B., Perelman, L.T., Vitkin, E.I., Greco, F.A., McKee, A.C., Kowall, N.W. (2008). Scattering differentiates Alzheimer disease in vitro. Optics Letters, 33(6), 624. DOI: 10.1364/OL.33.000624