Accuracy of single-photon emission computed tomography in differentiating frontotemporal dementia from Alzheimer's disease

Background

Alzheimer''s disease (AD) and frontotemporal dementia (FTD) are the commonest causes of presenile dementia. In the absence of a biological marker, diagnosis is reliant on clinical evaluation. Confirmation is often sought from neuroimaging, including single‐photon emission computed tomography (SPECT). Most previous SPECT studies lack pathological validation.

Aim

To examine the accuracy of SPECT in differentiating FTD from AD in patients with subsequent pathological confirmation.

Methods

Technetium‐99‐labelled hexamethyl propylene amine oxime SPECT images obtained at initial evaluation in 25 pathologically confirmed cases of FTD were examined. These images were visually rated by an experienced blinded nuclear medicine consultant and compared with those of 31 patients with AD, also with pathological validation.

Results

A reduction in frontal cerebral blood flow (CBF) was more common in FTD and was of diagnostic value (sensitivity 0.8, specificity 0.65 and likelihood ratio (LR) 2.25; 95% CI 1.35 to 3.77). A pattern of bilateral frontal CBF reduction without the presence of associated bilateral parietal CBF change is diagnostically more accurate (sensitivity 0.80, specificity 0.81 and +LR 4.13, 95% CI 1.96 to 8.71). Diagnostic categorisation (FTD or AD) on the basis of SPECT alone was less accurate than clinical diagnosis (based on neurology and detailed neuropsychological evaluation). One patient with FTD was initially clinically misdiagnosed as AD, owing to the lack of availability of full neuropsychological assessment. However, SPECT correctly diagnosed this patient, providing a diagnostic gain of 4%.

Conclusion

Technetium‐99‐labelled hexamethyl propylene amine oxime SPECT CBF patterns provide valuable information in the diagnosis of FTD and AD. These data can be better used as an adjunct to clinical diagnosis if pathology is to be correctly predicted in life.Frontotemporal dementia (FTD) is a cortical dementia distinct from other dementing illnesses. It typically presents with personality/behavioural change and decline in social conduct with early loss of insight.^1,2 In the absence of biological markers, the pathological detection of characteristic histological changes remains the gold standard of diagnosis. In life, diagnosis is primarily based on patterns of neurological and neuropsychological findings. However, differentiation from other dementias can be difficult and demands an astute qualitative analysis of various behaviours and neuropsychological test performances.³ With a paucity of experienced neuropsychologists, additional and independent diagnostic information is often sought through imaging, be it structural (CT and MRI) and/or functional (single‐photon emission computed tomography (SPECT) and positron emission tomography).SPECT is used to evaluate patients with dementia and can show purported characteristic changes in FTD and in Alzheimer''s disease (AD).^4,5,6,7,8,9 The technique provides a method of evaluating blood flow in various regions of the brain, which reflects areas of poor function by showing reductions in regional cerebral blood flow (CBF). It has been shown that posterior changes in regional CBF are common in AD,^4,5,6,7 whereas in FTD anterior changes are prevalent^7,8,9 and posterior changes rare.⁷However, CBF changes are neither wholly specific nor invariable in various dementing illnesses. Masterman et al¹⁰ looked at the value of bitemporal hypoperfusion in diagnosing AD, and found that, although a sensitive measure for detecting dementia (0.75), it was poorly specific for AD (0.55). Consequently, bitemporal hypoperfusion on SPECT can be a non‐specific finding in various forms of dementia and is not exclusive to AD. Starkstein et al¹¹ reported deficits in CBF in the frontal (especially orbitofrontal) and anterior temporal cortices in FTD. However, they provided neither the measure of the diagnostic accuracy of SPECT in FTD nor of the diagnostic gain it may provide. Most of these studies are also limited by the fact that the dementia groups are defined clinically. The clinical diagnostic accuracy of FTD in life varies hugely between 14–85%.^12,13,14A few studies have looked at the accuracy of clinical and SPECT findings in relation to the final pathological diagnoses.^{15,16,17,18,19} Although these studies found that SPECT findings do correlate with dementia type, they failed to enquire whether SPECT provides any additional diagnostic gain over clinical judgement. These studies are also severely limited by the small numbers of patients in the FTD groups.The aims of this study include evaluation of the diagnostic accuracy of SPECT in differentiating FTD from AD at initial assessment in a group of patients with final pathological confirmation of diagnosis. We also examined the diagnostic gain SPECT may provide over clinical diagnosis of FTD from among this group of patients with FTD and AD.