Vascular Dementia Ingmar Skoog

BACKGROUND

Cerebrovascular disorders are generally believed to be the second most common cause of dementia, after Alzheimer’s disease (AD). Dementia caused by cerebrovascular disease is often labelled vascular dementia (VaD). AD and cerebrovascular disease often co-exist in the same patient. This condition is termed mixed dementia¹. Cerebrovascular disorders are also associated with cognitive decline which does not reach diagnostic thresholds for dementia^2–6. The term vascular cognitive impairment was introduced by Bowler and Hachinski in 1995⁷ to capture the whole range of cognitive dysfunction associated with vascular disease. This term thus includes VaD, mixed dementia and other forms of cognitive decline caused by cerebrovascular and cardiovascular diseases. It is important to recognize that cognitive impairment associated with cerebrovascular or cardiovascular disorders is potentially preventable or treatable.

DIAGNOSTIC CRITERIA AND DEFINITIONS

The most often used criteria for VaD were developed almost two decades ago. ICD-10 was published in 1993⁸, DSM-IV in 1994⁹ and the National Institute of Neurological Disorders and Stroke and the Association Internationale pour la Recherché et l’Enseignement en Neurosciences (NINDS-AIREN) criteria were published 1993¹⁰ These criteria show inconsistencies, and give rise to very different rates of VaD^11,12.

Definition of Dementia and Cognitive Decline

Cognitive function declines with increasing age. The decline may be accelerated by a range of insults to the brain, such as AD, cerebrovascular disease, other brain disorders, and various peripheral disorders such as cardiovascular diseases (CVDs). When the cognitive function reaches a certain threshold, and gives rise to difficulties in everyday life, the term dementia is used.

In criteria such as those of the ICD-10⁸ and DSM-IV⁹, dementia is a global decline in intellectual function involving memory, orientation, visuospatial abilities, executive function, language, thinking and often changes in personality and emotions. One problem with these criteria is that they state that memory impairment is mandatory for a diagnosis of dementia. This concept is based on the symptoms of AD, and misses a large proportion of individuals with severe cognitive dysfunction associated with impairments in activities of daily living (ADLs) and personal function, as cerebrovascular disease may cause significant cognitive dysfunction with relatively preserved memory function⁷.

Definition of Cerebrovascular Disease

Many different cerebrovascular diseases may cause cognitive decline and dementia, including stroke, silent infarcts, ischaemic white matter lesions (WMLs), hereditary cerebral haemorrhage with amyloidosis, granular cortical atrophy, hypertensive encephalopathy, cerebral amyloid angiopathy and cerebral vasculitis (Table 56.1). Most cases of VaD have a mixture of cerebrovascular changes, which could be expected, as different cerebrovascular diseases share similar risk factors. The two most common causes of VaD are stroke and WMLs. However, most criteria highlight clinical strokes in their definitions of cerebrovascular disease. Thus, the definition of cerebrovascular disease is based on the presence or history of focal neurological motor symptom/signs, or brain imaging findings of cerebrovascular ^, disease. DSM-IV lists examples of signs, while the ICD-10 specifies that at least one should be unilateral spastic weakness of the limbs, unilateral increased tendon reflexes, extensor plantar response or pseudobulbar palsy.

The DSM-IV requires that there should be signs andsymptoms orlaboratory evidence of cerebrovascular disease (i.e. that brain imaging reveals ‘multiple infarctions in the cortex and subcortical white matter’), while ICD-10 requires evidence from history, examination ortests of significant cerebrovascular disease (i.e. history of stroke or evidence of cerebral infarction on brain imaging). In the NINDSAIREN criteria¹⁰, a diagnosis of probably VaD requires that focal signs consistent with stroke andrelevant cerebrovascular disease by brain imaging should be present. One of the authors of the NINDSAIREN criteria published a modified version a year later¹³,inwhich this criterion was changed to focal signs consistent with stroke orrelevant cerebrovascular disease by brain imaging. The first version is far too strict and underestimates the occurrence of VaD¹¹, since it will exclude individuals without stroke symptoms (including those with silent infarcts or white matter lesions). In a study on 85-year-olds¹⁴, 13% of the demented had VaD based on the first version (i.e. ‘focal signs consistent with stroke andrelevant cerebrovascular disease by brain imaging’), while 47% had VaD with the second version (i.e. ‘focal signs consistent with stroke orrelevant cerebrovascular disease by brain imaging’). In a study conducted on 100 demented patients, the prevalence of VaD was 14% using the first NINDS-AIREN criteria and 76% using DSM-III-R (which uses ‘focal signs consistent with stroke orrelevant cerebrovascular disease by brain imaging’)¹². The NINDS-AIREN criteria allow for a diagnosis of ‘possible’ VaD in patients with dementia and focal neurological signs in whom brain imaging is missing, and in patients with subtle onset and variable course.

Table 56.1 Causes of vascular dementia

CADASIL, cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy.

Stroke

Silent infarcts

Ischaemic white matter lesions (WMLs)

CADASIL

Hereditary cerebral haemorrhage with amyloidosis

Granular cortical atrophy

Hypertensive encephalopathy

Cerebral amyloid angiopathy

Cerebral vasculitis

Definition of an Association between Cerebrovascular Disease and Dementia

The influence of a stroke or an infarct on the cognitive symptoms is not easy to decide in the individual case, even if epidemiological – studies suggest a strong statistical relationship^{6,15 19}. The temporal relationship between symptoms of stroke and onset of dementia is often regarded as strengthening the possibility that the two disorders are related. The recent reports that non-symptomatic infarcts are common in the elderly and related to an increased risk for dementia and stroke^20,21 may question this statement. The NINDS-AIREN criteria suggest an arbitrary limit of three months for onset of dementia after stroke. However, a stroke which occurred years ago indicates the presence of cerebrovascular disease, including silent infarcts. The NINDS-AIREN criteria allow for a diagnosis of ‘possible’ VaD in the absence of a clear temporal relationship between dementia and clinical symptoms of stroke.

Both DSM-IV and ICD-10 criteria leave the clinician to decide on whether cerebrovascular disease and dementia are related, as both state that ‘CVD should be (reasonably) judged to be etiologically related to the disturbance’. There are no specifications as to what this judgement should be based on.

EPIDEMIOLOGY

Almost all epidemiological studies reporting on the frequency of VaD are concerned with the subtype related to clinically manifest stroke or transitory ischaemic attacks (TIA). The proportion of vascular dementia varies widely between studies: between 15 and 45%. The variation may reflect disparities in diagnostic criteria, or differences in the rate of cerebrovascular disorders in different geographical areas²². It may also reflect differences in the efforts made to detect and diagnose cerebrovascular disease, and if brain imaging has been used. Although the prevalence of dementia is similar in most parts of the world, there are differences regarding the type of dementia. Stroke-related dementia is traditionally reported to be more common in Finland, the former Soviet Union and Asian countries, including Japan and China, than in Western Europe and the USA, where AD is generally reported to be the most common type of dementia. However, more recent studies report proportions of VaD in China and Japan similar to western countries²³. Whether this is due to changes in diagnostic procedures or in risk factors for stroke is not clear.

Stroke

As mentioned, all criteria for VaD encompass history of stroke. Stroke patients typically have history of stroke or TIA including acute focal neurological symptoms and signs, such as hemiparesis or acute aphasia. Other cardiovascular manifestations, including myocardial infarction and hypertension, are common in the patients. Stroke is most often caused by cortical infarcts due to thromboembolism from extracranial arteries and the heart, and is often related to large vessel disease.

The typical clinical course of stroke-related dementia includes sudden onset, stepwise deterioration and a fluctuating course. In the early stages, the expression of cognitive impairment is variable and depends on the site of the lesions. A large proportion of patients with cerebrovascular disease have a gradual onset of dementia with a slowly progressive course²⁴, with or without focal signs or infarcts on brain imaging, which makes it difficult to differentiate from AD or other types of dementia. It has even been suggested that individuals with cortical strokes show less decline in cognitive function than those with subcortical cerebrovascular disease²⁵.

Stroke is an essential part of most criteria for VaD or vascular cognitive impairment. In line with this, most epidemiological studies report an increased frequency of dementia in individuals with stroke (Table 56.2). In the studies by Tatemichi etal.^15–17, relatively young individuals with ischaemic stroke had at least nine-times greater risk for dementia than stroke-free controls, and those with dementia had a higher mortality rate and worse prognosis which was independent of stroke severity. Also Pohjasvaara etal.¹⁸ reported an increased prevalence of dementia in stroke victims, as well as a decrease in independent living for those with dementia. Lindén etal.⁶ reported that stroke victims aged above 70 had an odds ratio (OR) of 4.7 for dementia. The odds for dementia were higher in those aged 70–80 (OR 6.7) than in those aged above 80 (OR 4.8), but the frequency of dementia after stroke was higher after age 80 (34% versus 18%). In addition, 60% of non-demented stroke victims had some cognitive dysfunction, showing that very few elderly stroke victims are free from cognitive disturbances. In a population study on 85-year-olds, Liebetrau etal.¹⁹ reported that the odds ratio for dementia in stroke was 4.3, but the prevalence of dementia in those with stroke was 57%. It may be that the increased risk for dementia with stroke decreases with age although the prevalence increases.

Table 56.2 Prevalence of dementia in stroke patients and in the general population

Age group	Stroke (%)	Population (%)
60–64	10	1
65–69	20	1
70–74	25	2
75–79	30	5
80+	50	20–30

The most important risk factors for stroke are hypertension, diabetes mellitus, atherosclerosis, atrial fibrillation, smoking, being overweight, and hypercholesterolaemia, especially high levels of low-density lipoprotein (LDL) cholesterol²⁶

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