Category:Arterial Pathology

Functional Anatomy

Arteries, capillaries and veins form a continuous system lined by endothelium. The endothelial layer is low friction to allow smooth blood flow. The endothelium forms the innermost layer of arteries;

• Tunica intima.

The tunica intima is enclosed externally by the internal elastic membrane. The outer tunics are:

• Tunica media.

Composed of elastic tissue and smooth muscle, this layer is the most variable, depending on location but is often the thickest layer.

• Tunica adventitia.

Fibrous tissue limiting expansion and so guarding against rupture.

Classification of arteries:

• Large elastic arteries are required to expand considerably E.g. the aorta as it receives the ventricular outflow.
• Large muscular arteries consist mainly of smooth muscle, the arterial lumen diameter is controlled by the autonomic nervous system.
• Arterioles regulate resistance to the flow of blood and therefore regulate peripheral blood pressure. The muscle layer is smaller than that of the larger muscular arteries. Opening or closing of the arterioles directs flow to specific capillary beds.

Hypertrophy

Occurs as a response to increased arterial blood pressure and increased load on the vessels. The elastic layers of the intima and the media increase in size.

Typically seen in the pulmonary vessels of cats with Aelurostrongylus abstrusus infection. The pneumonia regresses and leaves behind the hypertrophied pulmonary arteries which are clinically insignificant.

Degeneration

Arteriosclerosis

Encompasses a group of diseases characterised by degeneration of the arterial walls with fibrous thickening. The disease is chronic and non-inflammatory. As the vessels lose their elasticity the lumen becomes narrowed and the end result is ischaemia if the tissue bed supplied.

Incidence:

• Horse: mainly in the aorta and branch points of arteries.
• Dog: commonly seen in th myocardium. Contributes to myocardial scarring and damage preceding myocardial failure.
• Also occurs in cattle.

Thought to be aetiologically related to haemodynamic damage to the endothelial layer. Perhaps initiated by:

• Platelet endothelial interaction.
• Plasma constituents entering the sub-endothelial layer.

Changes may be:

Hyperplastic

Chronically raised intraluminal pressure stimulates hyperplasia of the smooth muscle layer, reducing the size of the vessel lumen. The elastic lamina may become duplicated or split allowing penetration of hyperplastic smooth muscle cells into the intima. Fibrous plaques form.

Hyaline

May occur alongside hyperplastic lesions or alone as a primary lesion. The intima becomes more permeable, allowing penetration of plasma contents into the vessel wall which becomes degenerate.

A combination of hyerplastic and hyaline change is often seen in the coronary arteries of the dog. Associated with ischaemic necrosis of the myocardium and therefore heart failure.

Also seen in uterine and ovarian arteries of pregnant animals. Thought to be clinically insignificant but indicated raised intraluminal pressure.

Atherosclerosis

Deposition of fatty substance within the vessel wall seen in man. Very rare in domestic species. May be seen in elderly pigs, turkeys, birds of prey and dogs with hypothyroidism.

Medial calcification

Often associated with arterioscleosis.

• Dystrophic: associated with vessel damage.
• Metastatic: elevated plasma calcium levels E.g. Vitamin D poisoning.

Calcification often seen in the horse:

• Small vessels of the bowel with unknown aetiology. Not thought to be clinically significant.
• Aorta due to healed strongyle parasite lesions.

Amyloidosis

Amyloid is an eosinophilic, homogenous, hyaline material. Due to its beta-pleated-sheet structure it is almost insoluble. Amyloid may be:

• AA: Serum amyloid A, alpha-2 globulin.
• AL: Derived from immunoglobulin light chains.

Disease may be truly idiopathic (dogs and cats) or may be secondary to another disease process, often chronic inflammation or neoplasia. Chronic antigenic stimulation induces the overproduction of AA protein which may become deposited throughout the body.

Deposits are found:

• Renal vessels and glomeruli.
• Splenic white pulp.
• Space if Disse.
• Coronary arteries.
• Meningeal arteries.

Affected organs are non-functional and appear waxy and pale.

Inflammatory-Arteritis

Presence of inflammatory cells within and around the vessel wall. Vasculitis often increases vessel permeability, presenting as oedema and haemorrhage. Petichial and ecchymotic haemorrhages within mucosae are characteristic.

Multiple aetiologies:

Infective

• Bacterial: Often toxin damage E.g. Salmonellosis, Erysipelas.
• Viral: Epitheliotropic viruses E.g. Equine arteritis, Canine distemper.
• Mycotic: Mucor Mycosis.

Parasitic

The main parasitic lesion of the arteries in th UK is Strongylus vulgaris of horses. Larvae and mmatures migrate along arterial walls, particularly cranial mesenteric and ilio-caecal arteries with occasional aberrent migration to the ascending aorta.

Non-infective

Polyarteritis nodosa

Fibrinoid necrosis of the media of arteries and arterioles. An intense neutrophil infiltration may be identified which, when the lesions become chronic, is accompanied by cells such as eosinophils, plasma cells and lymphocytes.

Aetiology unknown but thought to be an antibody-antigen reaction.

Fibrinoid necrosis is also a feature of vessel damage. Seen with uraemia in dogs where the ammonia within the bloodstream irritates the endothelial layer. The affected arteries include the gastric arteries which, when damaged, result in ischaemia and gastric ulceration.

Aneurysm and Rupture

Aneurysm is a local abnormal dilation of a vessel due to weakening of loss of elasticity of the vessel wall.

Seen proximal to a stenotic lesion. If the wall partially ruptues blood can track within the wall producing a dissecting aneurysm. Verminous anurysm occurs in horses with strongylus vulgaris infestation.

Rupture of the vessels is rare. aortic rupture may occur in some situations:

• Traumatic.
• Dog: Spirocerca lupi (NOT IN UK)
• Horse: Verminous aneurysms which rupture. Rupture within the pericardium leads to haemopericardium and Cardiac tamponade.

Vascular Pathology

Thrombosis may be due to:

• Endothelial injury.
• Altered blood flow E.g. abnormal stasis.
• Hypercoagulability.

Often associated with other disease processes for example Disseminated intravascular coagulation.

Commonly see posterior paralysis of cats with cardiomyopathy.

Nasal thrombosis.