Vessel Pathology

Vessel Pathology

·        Arteries

o  3 layers:

§  Intima: thin, includes endothelium, underlying thin layer of connective tissue containing smooth muscle, and elastic interna – elastic fibre layer

§  Media: thick, smooth muscle and collagen.  Large arteries have elastic fibres as well

§  Adventitia: thin layer containing elastic fibres in loose connective tissue

o  3 sizes: large (elastic), medium (muscular), small

·        Veins: thin wall, large lumen, IEL intact only in large vessels, scant media, contain valves

Atherosclerosis

Epidemiology

·        Causes 50% of all deaths in US – including coronary, cerebral and peripheral vascular disease

·        A disease of Western civilisation.  Absent in certain 3^rd world ethnic groups

Aetiology

·        Multifactorial

·        Endothelial damage ® ­permeability, monocyte adhesion and endothelial proliferation ® intimal hyperlipidaemia with invasion of foamy macrophages ® cytokines (IL1 and TNF) and growth factors (PDGF and FGF) cause inflammatory response and proliferation of smooth muscle with sclerosis; plaque thickened by organisation of superimposed thrombi

Gross Morphology

·        Lesions appear in childhood as fatty streaks

·        Adult plaques: discrete, yellow white random elevations, more prominent around ostia of large branches

·        Plaques may have sclerotic firm surfaces or ulcerate exposing soft cholesterol laden material

·        Severity increases with age

Microscopic Morphology

·        Plaque: intimal lesion – deposition of cholesterol esters, necrotic debris, smooth muscle and foam cells. Chronic fibrotic inflammatory response forming a superficial fibrous cap

·        Complications are ulcers with thrombi, bleeding into plaque, embolisation, calcifications, atrophy of media ® aneurysms

·        Reduplication of internal elastic lamina: shows with Elastin Van Geesen (EVG) stain

·        Adventitial fibrosis and chronic inflammation

Complications

·        Calcification ® rigid pipe ® ­pulse pressure ® distal atherosclerosis. (NB Calcium laid down in two ways: dystrophic calcification – Ca laid down in necrotic tissue – and metastatic calcification - ­serum Ca ® Ca laid down abnormally

·        Ulceration: fibrous cap cracks – debris discharged into lumen ® embolisation/thrombis

·        Thrombis: can embolise, or occlude artery. Cause of majority of myocardial infarcts and cases of unstable angina pectoris. If collateral circulation, can recannalise thrombis

·        Haemorrhage: a weak little new artery in the plaque bursts ® pushes plaque against opposite wall

Aortic Aneurysm

Aetiology

·        Severe arteriosclerosis

·        20% familial incidence ® defect in connective tissue component (?type III procollagen)

·        Syphilis and other bacterial infections

·        Cystic medial necrosis ® denudation of elastic layer

·        Trauma

·        Common in Marfan's syndrome (eg Abraham Lincoln). Long ulnar, femur, weak aorta, and high arched pallet

Clinical

·        75% occur in abdominal aorta.  Easy to repair cf thoracic and thoraco-abdominal cases

·        Often asymptomatic ® incidental finding

·        Can cause back pain (due to retroperitoneal blood).  Differential ® pancreatitis

·        44% of symptomatic aneurysms rupture.  ­Distension ® inevitable rupture (Law of La Place)

Pathogenesis

·        Arteriosclerosis ® gradual destruction of media ® focal weakening of wall ® ­distensibility ® ¯w + ­r ®­tension + ¯blood velocity (T µ Pr/W)

·        ­Pressure ® ­radius ® ­tension ® ­radius, etc

Gross Description

·        Fusiform dilatation of severely atherosclerotic aorta with sharp superior and inferior margins

·        Typically abdominal aorta, from just below ostia of renal arteries to bifurcation of aorta

·        Larger aneurysms contain thick old laminated thrombus reducing patent luminal size

·        Aneurysmal thrombus does not organise due to the paucity of functioning vasa vasorum in fibrotic wall

Microscopic Description

·        Aneurysm wall: barely identifiable media, fibrotic lesions with focal aggregates of mononuclear cells

·        Adventitia is fibrotic with chronic inflammation

Complications

·        Thrombus ® distal gangrene, calcification, bacterial infection (salmonella, shigella), rupture, dissection, fistula (eg aorta-vena cava)

Dissecting Aneurysms

·        Usually involves the aorta

·        Fatal in 75 – 90% of cases

·        Causes: atherosclerosis, also hypertension, Marfan‟s syndrome, trauma, inflammation of media

·        Pathogenesis:

o   Cystic medial necrosis: mucoid cysts in the media, elastic fragmentation and fibrosis

o   Commences as a transverse intimal tear, 90% in ascending aorta

o   Splits the media between the mid and outer 1/3

o   Proceeds down occluding branches

·        Outcomes:

o   Acute perforation ® sudden death

o   Subacute progression ® perforation in several days

o   Chronic ® rupture back into the lumen ® double barrel aorta

Arteritis

·        Diverse group of diseases classified by aetiology, vessel size or histologic changes

Infectious arteritis

·        Wide range of organisms, pyogenic, TB, parasites, viruses, fungi, syphilis

·        Vessel infected by septic emboli (® lodges and forms mycotic aneurysm) or direct extension from adjacent abscesses

·        Histology: oedema, fibrin, dense neutrophilic infiltrate

·        Outcome: scarring, obliteration of lumen ® distal infarction

·        Syphilis: occludes vasa vasorum ® ischaemic damage to artery, small vessel occlusion ® obliterative end arteritis, perivascular lymphocyte and plasma cell cuffing. Famous for causing proximal aortic aneurysms

Physical/Chemical Agents

·        Irradiation, trauma, vascular toxins, sulphonamides, penicillin

·       Arteritis syndromes

·        See Vasculitis

Other Vessel Abnormalities

·        Arteriosclerosis:

o  = Thickening and loss of elasticity of arterial walls. Seen in chronic hypertension, and to a lesser degree with ageing

o  Hyaline arteriosclerosis: blood vessel takes on glassy „hyaline‟ appearance. Reflects mild or „benign‟ hypertension. Particularly seen in kidneys

o  Hyperplastic arteriosclerosis: concentric rings of increased connective tissue and smooth muscle give arteries an onion skin appearance. Signifies acceleration/malignancy of the hypertension

·        Fibromuscular dysplasia: non-inflammatory thickening of large and medium sized muscular arteries causing stenosis. Most significant in renal arteries ® secondary hypertension

·        Thrombophlebitis: inflammation and secondary thrombosis of veins, usually small veins as part of a local reaction to bacterial infection

·        Varicose veins: enlarged, dilated, tortuous blood veins and incompetent venous valves – mainly in legs. Predisposing factors include older age, female, heredity, posture and obesity. Varicose veins at other sites include haemorrhoids (rectal), oesophageal varices and varicocoele (scrotum)

·        Vasculitis: inflammation and necrosis of blood vessels – including arteries, veins and capillaries. May be due to infection, trauma, radiation, toxins or immune (eg disposition of immune complexes)

·        Leukocytoclastic vasculitis: a form of hypersensitivity angiitis in the skin presenting as purpura