The relationship between white matter low attenuation on brain CT and vascular risk factors: a memory clinic study - computed tomography

Age and Ageing, Sept, 1995 by Khaled Amar, Tim Lewis, Gordon Wilcock, Margaret Scott, Romola Bucks

Introduction

White matter low attenuation (WMLA), or leucoaraiosis are hypodense areas in the cerebral white matter described on brain CT scans since 1980 [1, 2]. WMLA are observed in about 10% of normal elderly people and their prevalence increases with age [3, 4]. They are commonly seen in demented patients with a prevalence of approximately 30% in Alzheimer's disease (AD), and 80% in vascular dementia (VAD) [5-7]. On pathological examination, WMLA are areas of demyelination, gliosis, perivascular oedema, and lipohyalinosis of the deep penetrating arteries [8, 9]. The exact cause and significance of WMLA is not understood but a relationship to vascular risk factors particularly hypertension has been demonstrated in a number of studies [6, 10-12, 14]. However Raiha et al. have recently reported a relationship between WMLA and low rather than high blood pressure, suggesting that haemodynamic factors probably play a dominant role in the aetiology of white matter lesions [13].

The purpose of this study was to examine the prevalence of WMLA in our Memory Disorders Clinic patients including those who are borderline for dementia, and to investigate its relationship with vascular risk factors, particularly blood pressure.

Methods

We assessed brain CT scans of 202 patients referred to our Memory Disorders Clinic between January 1991 and December 1992. All patients are assessed with full history (from patients and carers), physical examination which includes cardiovascular and neurological examination, and are tested with a neuropsychological test battery. They are investigated with a laboratory dementia screen and brain CT scans. In appropriate cases EEG, MRI and/or SPECT scans are also performed. Diagnosis for each patient is made in a case conference which involves at feast two physicians with considerable experience in dementia, a psychiatrist and at least two psychologists who are also experienced in this field. McKhan's criteria are used for diagnosing Alzheimer's disease, while the diagnosis of vascular dementia is usually considered in the light of the clinical findings, the Hachinski Ischaemic scale and the DSM III R criteria for diagnosing vascular dementia [15-17].

Brain CTs were interpreted by an experienced neuroradiologist (T.L.) who was not aware of the clinical data. He recorded the presence or absence of WMLA, cerebral infarcts and atrophy. Areas of WMLA are normally seen in the periventricular and subcortical white matter, have an ill defined and patchy appearance (unlike infarcts which are well demarcated and usually follow a specific vascular territory), and have a density between normal white matter and CSF.

The extent of WMLA was recorded on a four-point scale (from 0 to 3) where 0 = no visible WMLA, 1 = WMLA localized to the frontal and/or occipital periventricular regions of the lateral ventricles, 2 = as in 1 but WMLA spreading towards the centrum semiovale, and 3 = extensive WMLA coalescing with the centrum semiovale. Intensity of WMLA was also graded on a four-point scale from 0 to 3, where 0 = absent, 1 = mild, 2 = moderate, and 3 = severe. Using a modified formula devised by Blennow et al., a leucency score, reflecting WMLA severity, was calculated by adding the scores of WMLA extent and intensity (minimum score = 0 and maximum = 6) [11].

The following information was gathered from the patients' notes; age, sex, diagnosis, presence of the vascular risk factors; hypertension, diabetes, hypercholestrolaemia, heart disease and peripheral vascular disease. Blood pressure, focal neurological signs on examination, blood cholesterol and random blood glucose were also recorded Patients were divided into six diagnostic groups: (1) probable Alzheimer's disease, (2) probable vascular dementia (VAD), (3) mixed AD and VAD, (4) other dementias, (5) possible dementia, (6) isolated memory loss, and (7) no dementia.

Patients in the 'other dementia' group included patients with dementia other than AD or VAD such as senile dementia of the Lewy body type. Parkinson's disease and alcoholism. Those in the 'possible dementia' group included patients with minimal cognitive impairment who were borderline for the diagnostic criteria for dementia. The majority of patients with isolated memory loss were suffering from age-associated memory impairment and memory loss secondary to cerebrovascular disease.

For statistical analysis, Student's t test was used to detect the difference between means, and Pearson's [x.sup.2] was used to compare groups. All statistical analyses were performed using SPSS/PC (Statistical Package for Social Studies).

Results

As can be seen from Table I, 45% of our patients were men (91 patients), and 55% were women. The mean age was 71 years (SD 10.2), and ranged from 45 to 93 years. One hundred patients (49.5%), with a mean age of 74.5 years (SD 8.4), had visible WMLA on their brain CTs, compared with the 102 patients (55.5%), with a mean age of 67.5 years (SD 10.6) with no visible WMLA. The difference in age between these two groups did not reach statistical significance (p = 0.067).