Mohammed T. Abou-Saleh

COMPUTED TOMOGRAPHY 353syndrome of depression) have CT scan changes such as increasedlateral ventricular size and decreased tissue density numbers 14 .Inparaphrenia, there is dilatation of the lateral ventricles, preservationof cortical structures and loss of the normal ventricular size/age correlation seen in normal ageing 15 .CT IN ADTwo areas of clinical interest are important in relation to CTscanning in AD. First, what is the diagnostic ability of the CTscan? Second, what CT changes take place in AD and how arethey related to the clinical features of the disorder?DIAGNOSTIC ABILITY OF CTThe second area of clinical interest relates to the ability of CT scanchanges to differentiate patients with dementia from nondementedcontrol subjects. It is well recognized that dementedsubjects can have normal CT scans, whereas normal subjects canhave marked atrophic changes. Discriminant analyses are able todifferentiate the two groups using CT scan appearances in about80% of cases 16 , a rate that has remained virtually constant overtime in spite of advances in CT technology and methods of scananalysis 49 . In a meta-analysis, De Carli et al. 18 estimatedsensitivity and specificity for a variety of CT measures. Specificitywas high for most measures (about 90%, i.e. few normal subjectswere classified as having abnormal CT appearances). Sensitivity(i.e. the number of AD patients regarded as having abnormal CTscans) was lower.Serial CT scans in individual patients have been performed andhave the potential for greater diagnostic accuracy. Increases inventricular size have been shown to outstrip those which takeplace in normal ageing. Luxenberg et al. 19 found that the rate oflateral ventricular enlargement in male AD patients over 12months completely differentiated these patients from controls (i.e.100% sensitivity and 100% specificity). Increase in ventricular sizecorrelates with deterioration in cognitive function 19 and twosubgroups of patients with AD have been described on this basisof one with significantly increasing ventricular size and deterioratingcognitive function and one without these chages 20 .age-matched controls, correlating with degree of cognitiveimpairment 17,24 .In addition to measures of global cerebral atrophy andventricular enlargement, CT scans can provide other informationof clinical interest. Regional cerebral atrophy has been shown tobe related to certain behavioural disturbances 49 . Basal gangliacalcification is found in a significantly greater proportion ofpatients with delusions and demented patients with affectivesymptomatology have less severe CT scan changes, includingrelative preservation of the interhemispheric fissure 49 .Table 63.1 summarizes the relevant studies; 88% of ventricularmeasures show significant correlations with cognitive tests,whereas only 41% of cortical assessments do so (62=11.3,p50.001, d.f.=1).The diagnostic potential of the specific temporal lobe views ofthe brain has attracted some interest in AD 25 . Pathologically, thetemporal lobe discriminates well between AD and normalcontrols 26 and coronal plane CT images can be reformated todisplay the temporal lobes in fine detail.CT has been combined with SPET (single-photon emissiontomography) in order to improve diagnostic accuracy in AD.Jobst and colleagues, in the OPTIMA project in Oxford 27,28 ,reported a series of studies demonstrating that views of thetemporal lobe could be achieved during CT scan, with the planeorientated along the long axis of the medial temporal lobe (20–258anterior to standard CT angle). In this way, 92% of patients withAD were correctly diagnosed compared to a 5% false-positiverate. Simple measurement of the narrowest thickness of the medialtemporal lobe (right or left) was about 50% thinner in patientswith AD compared to controls. Combining these measurementswith SPET in patients with histologically proven AD compared tocontrols, the medial temporal lobe atrophy provides 94%sensitivity and 93% specificity, parietotemporal hypoperfusionon SPET gave 96% sensitivity and 89% specificity, and thecombination of both changes gave a sensitivity of 97%. Theresults of Lavenu et al. 29 who carried out a similar study using CTand SPET, resulting in a diagnostic accuracy rate which was muchless—68%. Stage of disease is an important influencing factor inthis rate and replication of the results of these studies is neededbefore they can be incorporated into clinical practice. O’Brien etal. 7 found reduced temporal lobe width in AD, vascular dementiaand Lewy body dementia compared to controls, suggesting a lackof specificity of the finding with a single cross-sectional measurement.CLINICO–RADIOLOGICAL CORRELATIONSEarly studies demonstrated correlations between the degree ofintellectual impairment and both cortical and subcortical atrophyon CT 21–23 but often included normal controls in the correlations.Normal control subjects tended to be patients referred forinvestigations and found to have normal scans, rather thanpeople screened first and then scanned.With regard to clinico-radiological changes, both corticalatrophy and lateral ventricular enlargement occur with normalageing and tend to accelerate after the age of 60. There is asignificant correlation between cerebral atrophy and age innormal subjects but this has not been as consistently found indementia. Correlations have been described between cognitivefunction and both cortical atrophy and ventricular size. Generally,correlations are higher in the latter relationship (ventricularsize can be measured as a continuous variable, which may partlyexplain the greater association), although some studies havereported no association between degree of dementia (measured byboth specific cognitive tests and global ratings) and either CTmeasure. The third ventricle has been examined in a number ofstudies, and was found to be larger in demented patients than inCONCLUSIONIn summary, the role of the CT scan in old age psychiatry is as arelatively non-invasive and widely available neuroradiologicaltechnique to exclude intracranial mass lesions. Regional changesmay be helpful in the differential diagnosis of the dementiasyndrome. The concordance between the CT changes and aclinical diagnosis of dementia is not absolute and significantoverlap exists between the changes seen in dementia and thoseseen in normal ageing. Some methods of CT scan analysis arebetter than others in this differentiation and serial CT scans onindividual patients may be an even better indicator.REFERENCES1. Bradshaw J, Thomas J, Campbell M. Computed tomography in theinvestigation of dementia. Br Med J 1985; 286: 277–80.2. Larson E, Reifler B, Featherstone J, English D. Dementia in elderlyoutpatients: prospective study. Ann Intern Med 1984; 100: 417–23.