The usefulness of CT scanning in diagnosing dementia of the Alzheimer type

Assessment of suspected dementia

At the Second Canadian Consensus Conference on Dementia (CCCD) (February, 1998), a group of neurologists, geriatricians, and psychiatrists met to consider guidelines for evaluation of dementia in Canada. This review paper formed a background paper for their discussion of dementia diagnosis. These experts from across the country concluded that diagnosis of suspected dementia cases continued to rest on skilled clinical assessment. Mental status exam, preferably in some quantifiable form, has become an essential part of the assessment. Selected laboratory tests are advisable in all cases (CBC, TSH, electrolytes, calcium, and glucose), but the CCCD continued to advise that CT scanning was mandatory only in selected cases where clinical findings pointed to another possibility besides Alzheimer's disease. The growing list of other diagnostic measures with potential usefulness in diagnosis of Alzheimer's disease or dementia in general was reviewed, but the evidence was judged as insufficient to support routine use of these tests by physicians. As new treatments for Alzheimer's disease become available, neurologists face new diagnostic challenges--differentiating Mild Cognitive Impairment, Frontotemporal dementias and Mixed dementias, and Lewy Body Dementia. Guidelines to aid in differential diagnosis are presented.

Medial temporal lobe atrophy in stroke patients: relation to pre-existing dementia

OBJECTIVE

The links between stroke and Alzheimer's disease seem to be closer than expected by chance. In a previous study it was shown that up to 16% of patients admitted for stroke had pre-existing dementia. Medial temporal lobe atrophy (MTLA) is strongly associated with Alzheimer's disease. The aim of this study was to determine the prevalence of MTLA and its relation with pre-existing dementia.

METHOD

The study was conducted on 170 consecutive stroke patients (87 women; median age 75 years; 152 infarcts), who underwent non-contrast CT with temporal lobe oriented 2 mm contiguous slices at admission. A cut off point of 11.5 mm was used to differentiate patients with and without MTLA. Pre-existing dementia was assessed using the informant questionnaire on cognitive decline in the elderly (IQCODE) with a cut off score of 104.

RESULTS

Ninety four patients (55.3%) had MTLA, of whom 23 (24.5%) had pre-existing dementia; of 76 patients without MTLA, only four (5.3%) had pre-existing dementia (p=0.0007). The logistic regression analysis with MTLA as dependent variable found the following independent variables: increasing age (p<0.05), and global cerebral atrophy scores (p<0.01). The IQCODE scores just reached significance (p=0.05).

CONCLUSION

Stroke patients with MTLA are more likely to have pre-existing dementia; this suggests that Alzheimer's disease might contribute to the dementia syndrome. A longitudinal follow up is now necessary to determine whether stroke patients with MTLA and without pre-existing dementia are at increased risk of Alzheimer's disease over subsequent years.

Evaluation of selection criteria used in Alzheimer's disease clinical trials

BACKGROUND

In the absence of a biological marker for Alzheimer's disease (AD), diagnosis has to be achieved using clinical criteria sets such as those outlined in DSM-IV, NINCDS-ADRDA, or ICD-10. As these criteria are quite broadly defined, there may be inter-rater variability in interpretation.

METHODS

Using a previously published CT scan measuring technique which correlates well with diagnoses achieved using the NINCDS-ADRDA criteria as interpreted at our clinic, we chose to independently examine and reach a diagnosis in patients selected for participation in clinical trials of therapeutic agents for the treatment of AD. Forty-four CT scans from six investigators across Canada were examined using this model. All patients had been diagnosed as having AD by NINCDS-ADRDA criteria and were deemed acceptable to participate in a clinical trial.

RESULTS

The diagnostic concordance achieved in the original published model was 91.5%. The diagnostic concordance in the population currently being studied was 77.3%. However when examined by site, results ranged from 57.1% to 100%. Using the model, an index of atrophy and a probability of diagnosis of AD can be determined. Across sites, there were statistically significant differences in these measures (p < or = 0.035). The mean probability of diagnosis of AD across sites ranged from 0.56 to 0.94. Although the sites with lower probabilities had slightly lower mean ages and slightly less atrophy, there was no overall correlation of the atrophy measures with age.

CONCLUSIONS

Current results raise the possibility that the selection of patients for AD clinical trials using current diagnostic criteria sets may not be adequate and conclusions with respect to agent efficacy could be flawed.

Regional HmPAO SPECT and CT measurements in the diagnosis of Alzheimer's disease

BACKGROUND

This study investigated the hypothesis that the combination of regional CT brain atrophy measurements and semiquantitative SPECT regional blood flow ratios could produce a diagnostic test for Alzheimer's disease (AD) with an accuracy comparable to that achieved with the present clinical gold standard of the NINCDS-ADRDA criteria.

METHODS

Single proton emission computed tomography (SPECT) and CT head scans were performed on 122 subjects referred an UBC Alzheimer clinic and diagnosed as either 'not demented' (ND-37) or 'possible/probable AD' (AD-85) by the NINCDS-ADRDA criteria. Stepwise discriminant analysis (SDA) was performed on the bilateral SPECT regions of interest and compared to bilateral CT qualitative/quantitative assessment in the frontal, parietal and temporal lobes to determine which were most accurate at ND/AD distinction. Receiver operating curves (ROC) were then constructed for these variables individually and for their combined discriminant function.

RESULTS

The left temporal qualitative cortical atrophy score (CT) and left temporal perfusion ratio (SPECT) were selected in the SDA. The combined discriminant function was more specific at AD/ND distinction than either of CT or SPECT alone. The accuracy of AD/ND distinction with the combined discriminant function was below that achieved by clinical diagnosis according to the NINCDS-ADRDA criteria and was not significantly different from that achieved with SPECT or CT alone as defined by ROC curve analysis.

CONCLUSION

The measurements of left temporal cortical atrophy and regional cerebral blood flow were most indicative of AD; however they lacked the sensitivity and specificity to recommend their use as a diagnostic test for AD.

Brain atrophy in normal ageing and Alzheimer's disease. Volumetric discrimination and clinical correlations

BACKGROUND

We examined the differences in volume of the ventricular and extracerebral cerebrospinal fluid spaces in normal ageing and in probable Alzheimer's disease (AD) and we tried to investigate the effects of the severity of illness on the morphometric differentiation of AD and ageing, the principal components underlying brain atrophy in both conditions, and the correlations of these measurements with clinical findings.

METHOD

Forty patients with probable AD were matched with 40 non-demented elderly controls. Both groups underwent standardised clinical tests and unenhanced cranial computed tomography for post hoc volumetric analysis.

RESULTS

The lateral and third ventricles and the anterior and lateral fissures were significantly larger in AD than in normal ageing. The volumes of the lateral ventricle and lateral fissure permitted a highly efficient differentiation between normal ageing and AD even at the mild stage of dementia, and this differentiation was improved further in the more severe stages of illness. We identified one principal component underlying brain atrophy in normal ageing and two components in AD: a 'grey matter' component accounting for sulcal and third ventricular enlargement, and a 'white matter' component for lateral ventricular enlargement. In AD, most of the volumetric measurements were significantly correlated with cognitive impairment, but in the group of non-demented elderly controls they were correlated with age. CONCLUSION. Volumetric indices of brain atrophy permit a highly efficient differentiation between normal ageing and AD even in the mild stages of illness and this demonstrates that substantial structural brain changes have developed in the preclinical phase of illness. We suggest that there is an uncoupling between lateral ventricular enlargement and cortical brain atrophy in AD.

Methodological problems and clinical relevance of structural neuroimaging in dementia research

Structural neuroimaging and dementia are conceptually different being only loosely correlated. Computed tomography or magnetic resonance imaging can never "prove" a clinical syndrome such as dementia, but yield clues as to its cause and the site and extent of pathological changes. Conversely, the type and degree of intellectual deterioration can hardly predict neuroradiological findings. The value of structural neuroimaging lies in detecting or excluding possible causes of dementia and quantifying linear or volumetric parameters of tissue and fluid volume. If based on a presumed or established etiology, however, specific neuropsychological and dementia syndromes may correspond to focal pathological changes seen in well-defined cerebral areas as recent investigations have shown with hippocampal atrophy in Alzheimer's dementia.