Atherosclerosis is the most common vascular disease. It may begin in childhood with the formation of fatty streaks and is progressive. It is characterised by thickening of the intima with focal deposition of lipids, proliferation of smooth muscle, influx of monocytes from the blood and fibrosis. It particularly affects the upper end of the internal carotid arteries, the intracranial part of the vertebral arteries, and the basilar artery (Figure 3 and 4).
| Figure 3.|
Ventral surface of a brain showing severe atheroma of the blood vessels of the Circle of Willis.
| Figure 4.|
Section of basilar artery showing marked narrowing of the lumen by atheroma and haemorrhage into the plaque.
The main risk factors include age, genetical factors, diabetes, abnormalities of lipid metabolism, cigarette smoking, hypertension, and obesity. It affects both extracranial and intracranial blood vessels. Patients may present with transient ischaemic attacks and these patients have a high risk of stroke. Platelet-fibrin emboli or cholesterol crystals can sometimes be seen in the retina on fundoscopy. Strokes are frequently embolic in origin and the emboli often arise from extracranial arteries and from the heart. Atherosclerosis may involve the vertebral arteries and lead to thrombosis and cerebral infarction (Figure 5). Dissection of the intraosseous part of the vertebral arteries and dissection of the latter will require removal of the relevant vertebral bodies at post mortem.
| Figure 5.|
Coronal section of a brain containing two infarcts, each in a temporal lobe, one recent and haemorrhagic, and the other old and partly cystic.
Raised pressure exposes smaller blood vessels to excessive pressure and blood flow which, following breakdown of auto regulation, results in an increase in blood flow proportional to the rise in blood pressure. High pressure results in disruption of the blood-brain barrier with transendothelial transport of plasma constituents across the vessel wall (Figure 6). The deposition of plasma proteins within the wall of small blood vessels results in progressive disruption of smooth muscle cells, fibrosis (Figure 7), and eventually fibrinoid necrosis. Weakening of the wall may result in the formation of small aneurysms (Charcot-Bouchard microaneurysms) (Figure 8) which are liable to rupture. The latter are particularly likely to appear at sites showing fibrinoid change, and in small arteries and arterioles in the region of the basal ganglia. Weakening of the vessel wall may also result in small haemorrhages (Figure 9), often multiple, and occurring over a long period of time; these haemorrhages may resolve, leaving perivascular spaces of variable size (Figure 10). This will result in progressive deterioration of cerebral function, depending upon the number and site.
| Figure 6.|
Blood vessel in hypertension showing thickening due to seepage of plasma constituents into the wall with progressive destruction of the muscle coat.
| Figure 7.|
Section of kidney showing the effects of chronic hypertension on a small artery - note the marked fibrosis and replacement of the muscle layer (elastic stain).
| Figure 8.|
The effect of weakening of the wall of a small intracerebral blood vessel, with the formation of a small aneurysm. This section has been stained for fibrin, which appears red. Note seepage of fibrin into the wall.
| Figure 9.|
Small perivascular haemorrhage in hypertension.
| Figure 10.|
Small blood vessel surrounded by a space in chronic hypertension. This will result in poor perfusion of adjacent cells.
Chronic hypertension also increases the degree of atheroma, leading to progressive involvement of smaller blood vessels. Small vessel disease may be associated with small haemorrhages or infarcts (lacunar infarcts), less than 1 cm. In diameter, and these may be associated with focal neurological signs, depending on the site. This process may continue over a long period of time and result in a stepwise clinical deterioration with variable degrees of dementia associated with focal neurological signs, including a pseudobulbar syndrome This condition is sometimes referred to as Binswanger's subcortical leukoencephalopathy.
Sudden elevation of blood pressure, such as may occur in malignant hypertension and in toxaemia of pregnancy, can result in severe cerebral oedema, seepage of fibrin into vessel walls, resulting in fibrinoid necrosis of small vessels, and rupture of small vessels with the formation of haemorrhages.
Acute hypertension may cause "hypertensive encephalopathy" with diffuse cerebral dysfunction, headache, nausea, vomiting, and deterioration in conscious level including deep coma.
Chronic hypertension results in a shift of the auto regulation curve to the right (i.e. towards higher pressure levels). This process has a protective reaction against vessel rupture but it also raises the lower limit at which adequate cerebral blood flow can be maintained. Therefore, focal or diffuse cerebral ischaemia may result at a level of mean cerebral perfusion pressure that would be tolerated by a normal person.
Haemorrhage and hypertension occurs most commonly in the region of the basal ganglia (Figure 11), cerebellum, and pons.
| Figure 11.|
Horizontal section of brain showing a major and fatal haemorrhage into the region of the basal ganglia. This is a common site for haemorrhage in Hypertension.
An aneurysm is a focal arterial dilatation. These may be saccular (Figure 12), fusiform, miliary, or infectious.
| Figure 12.|
Berry aneurysm on a vertebral artery causing sudden fatal subarachnoid haemorrhage. This is an uncommon site for a berry aneurysm - they usually occur towards the front end of the Circle of Willis.
The prevalence of intracranial saccular aneurysms appears to be about 3% and the incidence increases with age up to the sixth decade. Occasionally they are familial. Approximately two thirds rupture before death and one third of these will die within 72 hours of the initial haemorrhage. A further third die or become disabled due to complications of rupture. Massive haemorrhage can lead to coma and death within minutes. They almost always occur at points of bifurcation of the main intracranial arteries where there are frequently gaps in the muscle coat and where there is a tendency for the formation of fibrous pads within the intima. Haemodynamic stress is also greatest at points of bifurcation. There is an increased tendency for saccular aneurysms to occur in association with arteriovenous malformations, where there is increased regional blood flow. Almost 90% occur on the terminal part of the internal carotid artery or on the major branches of the anterior portion of the circle of Willis. They are more common in women than men. Smoking and alcohol intake increase the incidence of rupture.
These usually occur in the intracranial part of the internal carotid artery and the basilar artery and are usually seen in elderly patients with advanced cerebral atherosclerosis. They may result in transient ischaemic attacks, infarction due to thrombosis, or haemorrhage.
There are a variety of different types of vascular malformation, classified on the basis of the type of blood vessels present and on the configuration of the vascular channels. These include arteriovenous malformations, cavernous angioma, venous angioma, and mixed types. They are regarded as congenital in origin.
Arteriovenous malformation is the most significant of the group and is the most likely to bleed. They may present at any age but most commonly present in the second to the fourth decade. They may be asymptomatic. They frequently extend from the brain parenchyma into the subarachnoid space and they contain arteries and veins and abnormal vessels of variable thickness (Figure 13).
| Figure 13.|
Arteriovenous malformation within the cerebral cortical grey and white matter.
Cavernous haemangioma often occurs as a familial disorder and may be multiple. They may occur anywhere in the brain or within the leptomeninges that are most common in the subcortical white matter, pons and internal capsule. They usually present in young or middle aged adults with seizures, headache, and focal neurological signs. Occasionally they cause haemorrhage.
Venous angiomas are the most common incidental vascular malformation encountered at autopsy and they are usually asymptomatic. Rarely, they present with epileptic fits or haemorrhage.
Capillary teleangectasia are composed of dilated capillaries. They are usually found incidentally at autopsy and rarely cause haemorrhage.
This is characterised by the deposition of amyloid fibrils within the walls of blood vessels of the brain and meninges. It may cause circulatory disturbances, including intracranial haemorrhage. It is the second most frequent cause of non-traumatic cerebral haemorrhage, accounting for as many as 15% of all cases. It is commonly associated with Alzheimer's disease and with ageing, but is also seen in Down's syndrome and sometimes following head-injuries(dementia pugilistica). Some degree of congophilic angiopathy is found in approximately one third of autopsied patients over the age of 60 years and this percentage increases with advancing age. It occurs in both sporadic and hereditary forms. The sporadic form is usually most severe in the parietal and occipital lobes and within the meninges and it is the most common cause of lobar intracerebral haemorrhage. It may also result in patchy loss of myelin within the white matter with progressive dementia. A variety of hereditary forms have been described in various parts of the world, with numerous genetical abnormalities and some variation in clinical presentation and behaviour.
Arterial dissection usually occurs in young or middle-aged people. It is usually spontaneous but may follow trauma, particularly to the neck and side of the head. It may involve the external or intracranial parts of the vertebral arteries or the carotid artery. If haemorrhage tracks through the adventitia it may rupture into the subarachnoid space. Alternatively, haemorrhage within the wall may occlude the lumen and result in occlusion and infarction.
First described in the renal arteries. The pathogenesis is unknown and it involves medium and small-sized arteries. Some cases are familial. It results in narrowing of the lumen, alternating with dilatation. It affects any layer of the arterial wall, but usually mainly the media, with hyperplasia of both muscle and fibrous tissue.