Mohammed T. Abou-Saleh

Principles and Practice of Geriatric Psychiatry.Editors: Professor John R. M. Copeland, Dr Mohammed T. Abou-Saleh and Professor Dan G. BlazerCopyright & 2002 John Wiley & Sons LtdPrint ISBN 0-471-98197-4 Online ISBN 0-470-84641-063Computed Tomography (CT)Alistair Burns 1 and Godfrey Pearlson 21 University of Manchester, UK, and 2 Johns Hopkins Hospital, Baltimore, MD, USAComputed tomography (CT) was introduced in the early 1970sand has become one of the standard investigations in the clinicalneurosciences. In cranial CT, X-radiation is passed through thehead in the form of a tightly collimated beam and is measured bya series of detectors. Radiation is absorbed by the interveningstructures, which results in ‘‘attenuation’’ of the beam. Attenuationis maximal in high-density regions such as bone and minimalin low-density regions such as cerebrospinal fluid (CSF). Theinformation from the detectors is processed by computer and theproduct is a numerical output. The brain is divided into threedimensionalvolume elements (or voxels) and each is given anattenuation number, which represents the average attenuation inthat area. The numerical output is transferred to a grey scale andeach voxel is represented by a two-dimensional pixel. The familiarCT scan images are the result of the pictorial representation of thepixels on the grey scale. Areas of high attenuation are representedby white and areas of low attenuation by black. Areas withintermediate attenuation (such as brain substance) appear grey.The amount of radiation exposure in an average CT scan isslightly less than that in a set of conventional skull X-rays.USE OF CT IN CLINICAL PRACTICECT has several advantages over magnetic resonance imaging(MRI) in the investigation of dementia. Acute haematomas can beeasily distinguished from areas of infarction, the increased densityof the former contrasting significantly with the hypodensity of thelatter. In practice, CT is more widely available than MRI and theexamination is less arduous and can be completed much faster.The presence of a cardiac pacemaker or the presence of surgicalclips from previous brain surgery are not contra-indications to CTscanning in the same way that they are for MRI.Guidelines have been published that suggest the circumstancesunder which a CT scan should be performed for the work-up ofdementia 1,2 . Essentially, where the duration of the illness is short(56 months and certainly 53 months), and where the features ofthe illness indicate that there may be cerebral pathology, then thechances of a CT scan detecting a clinically significant lesion isincreased. Such features include: focal neurological signs; epilepticfits; variations in the course of the illness; and indicators of thepresence of normal pressure hydrocephalus (gait disturbance, andincontinence in the presence of dementia). One study specificallyexamined a population sample aged 65+ and found thatpotentially treatable lesions (subdural haematoma, hydrocephalus,non-metastatic intracranial tumour) were present in 145 outof a possible 137 000 patient years at risk 33 . Specific featurespredicting the detection of such a lesion were: cognitiveimpairment for 1 month or less; head trauma in the week beforemental state change; rapid onset of change over 48 h; history ofCVA; seizures or incontinence; focal neurological signs; papilloedema;visual field defects; gait abnormalities; postural instability;or headaches. Paris et al. 3 estimate that the yield of potentiallytreatable conditions is about 3%. Factors on CT scan that predictwho will respond positively to a CSF shunt have beendocumented by Vanneste et al. 4 —cerebral atrophy and thepresence of white matter disease were poor predictors of responseto the insertion of a shunt.DIFFERENTIAL DIAGNOSISCT scanning is helpful in confirming certain diagnoses, e.g.Alzheimer’s disease (AD), but is not the definitive investigation,whereas in other disorders, e.g. ‘subdural haematoma’ the CT isthe definitive investigation.The CT scan has been used to differentiate vascular dementiafrom primary degenerative dementia. Generally speaking, goodconcordance between the presence of vascular lesions on CT andthe presence of vascular dementia (defined purely clinically orusing the Hachinski score) has been achieved 5–7 .White matter lesions on CT scan have been widely reported 8.9and are associated with impaired cognitive function (in both ADpatients and non-demented subjects) and neurological signs (gaitdisturbance and extensor plantar response). Scheltens et al. 10described a number of rating scales used to detect white matterchanges on CT and MRI brain imaging, concluding that the idealrating scale is not yet in existence but that different rating scalesserve individual purposes.Excessive ingestion of alcohol can result in cerebral atrophy andventricular dilatation on CT scan, particularly affecting thefrontal lobe and cerebellar vermis. It is apparent that the changesoccur relatively early (but do not antedate alcohol excess), areapparent before any clinical evidence of declining cognitivefunction and may be partially reversible with abstinence 11 .There is also evidence that third ventricular size is correlatedwith memory impairment in alcoholics without Korsakoff’sdeterioration. Patients with Korsakoff’s psychosis have morecortical atrophy and lateral ventricular enlargement, but the sizeof the third ventricle is particularly increased.Depression has been shown to be accompanied by both cerebralatrophy and ventricular enlargement 12,13 . CT scan appearances indepressed patients appear to be midway between those of normalcontrols and demented subjects, tending to be nearer the latter.More recently, it has been shown that patients with reversibledementia secondary to depression (pseudodementia, or dementiaPrinciples and Practice of Geriatric Psychiatry, 2nd edn. Edited by J. R. M. Copeland, M. T. Abou-Saleh and D. G. Blazer&2002 John Wiley & Sons, Ltd