Correlation of rCBF (SPECT), CSF tau, and cognitive function in patients with dementia of the Alzheimer's type, other types of dementia, and control subjects.

Background: The diagnosis of Alzheimer's disease (AD) during life remains difficult and a definite diagnosis of AD relies on histopathological confirmation at post-mortem or by cerebral biopsy. It is well known that levels of tau proteins are consistently and significantly increased in the cerebrospinal fluid (CSF) of Alzheimer's patients versus levels in normal controls. However, the sole use of this biochemical marker as a test for AD is hampered by mediocre specificity, since tau concentrations may also be elevated in certain other neurological disorders (OND). Studies of the regional cerebral blood flow (rCBF) are widely performed because of their convenience and usefulness in a variety of neurological disorders. Most studies have reported high diagnostic accuracy for brain perfusion single-photon emission tomography (SPECT) in Alzheimer's disease.

Methods: In order to improve specificity, in this study, correlation of 99mTc-HMPAO SPECT scanning and CSF tau protein levels was made in 117 patients with AD, 67 patients with OND (26 of which had other dementias), and 23 age-matched controls. Means and standard deviations of tau protein levels were 297, 42 +/- 221, 12 in AD patients and 78, 07 +/- 98, 51 in patients with OND (p = 0.0006). No correlation was noted between CSF tau protein levels and age, duration of the disease, and neuropsychological scores of mini-mental state examination (MMSE), Cambridge Cognitive Examination (CAMCOG), and Functional Rating Scale for Symptoms of Dementia (FRSSD).

Findings: There was a bilateral parietal and temporal hypoperfusion in patients with AD in SPECT in comparison to normal subjects (p < 0.05) and there was a statistical correlation between this hypoperfusion and neuropsychological tests, such as MMSE and CAMCOG (p < 0.01). There was no correlation between tau protein levels and hypoperfusion in SPECT.

Interpretation: Conclusively, the correlation between elevated levels of tau proteins and hypoperfusion in SPECT in AD patients therefore cannot improve the specificity of tests in AD and this means that the determination of CSF tau proteins levels is not a specific diagnostic test for AD.