Edema (also known as oedema also) is the abnormal accumulation of the tissue fluids due to excessive filtration by increased capillary pressure or poor reabsorption at the venous end of the capillaries.
Edema refers to the excessive accumulation of fluid in tissue spaces and body cavities, involving a disturbance in the mechanism of fluid exchange between capillaries, tissue spaces, and lymphatic vessels. These are usually accompanied by endocrine, circulatory, hepatic, and renal changes. It is considered to be a clinical sign rather than a disease.
Etiology
From the etiological view, edema is of two types:
- Inflammatory edema
- Non-inflammatory edema
Inflammatory edema
Inflammatory edema is caused by local inflammations, live boils, cellulitis, trauma, infection, or chemical irritation that result in increased capillary permeability that may cause ema. The main biochemical observations in inflammatory edema are low pH, accumulation of inflammatory cells, and blockage of lymphatics.
Non-inflammatory edema
Non-inflammatory edema occurs as a result of a rise in hydrostatic pressure or a fall in colloidal osmotic pressure.
Depending on the etiological factor or involvement of tissue, it may be of various types:
- Cardiac edema
- Nephrogenic edema
- Allergic edema
- Pulmonary edema
- Hepatic edema
- Edema of nutritional origin
- Obstructive edema
Cardiac edema
The venous congestion that occurs in CHF increases the permeability of the vein, leading to the escape of fluid into tissue spaces and the development of edema. The CVP is elevated, capillary pressure becomes higher, and fluid is forced into tissue spaces.
As a result of low cardiac output, blood supply to the organs becomes inadequate, and metabolites are accumulated in the tissue spaces, leading to a higher solute concentration. To overcome this, more and more fluid flows into such spaces from the capilleries. The excretion of sodium ions is greatly reduced, and their concentration in extracellular fluid is elevated, resulting in water retention.
Retention of salts is the chief cause of edema in CHF, which ultimately enhances renal venous pressure. Excessive aldosterone secretion increases tubular resorption, while anoxia damages the capillary endothelium, resulting in increased capillary permeability.
Nephrogenic edema
In acute glomerular nephritis, there is capillary damage, which results in the retention of sodium ions and excessive loss of plasma proteins. The development of edema depends on kidney damage, protein loss, capillery damage, retention of sodium ions, or a combination of these effects.
As a result of renal damage, erythropoietin production is reduced, which may also help in edema development.
Allergic edema
As a result of contact with allergens, histamine-like substances induce angioedema, urticaria, and wheals develop on the body. Capillary permeability and hydrostatic pressure increase. These forms facilitate the passage of fluids and proteins into the interstitial space, and edema develops.
Pulmonary edema
Circulatory volume is affected by hemodynamic factors, and there is an overload of pulmonary circulation. Physiochemical factors that regulate fluid exchange in tissues, along with the effects of the nervous system, combine together to induce acute pulmonary edema.
Hepatic edema
In hepatic fibrosis, fluid is accumulated in the dependent parts, which is the chief outcome. Due to fibrosis of cells, portal circulation is obstructed, which helps in the acclimation of fluid in tissue spaces and the peritoneal cavity. Hepatic damage also causes lowered protein synthesis and hypoproteinemia, which helps in further fluid accumulation.
Edema of nutritional origin
Plasma proteins help maintain osmotic pressure. A fall in colloidal osmotic pressure induces edema as a result of the diffusion of fluid from the vascular system into tissue spaces. Since plasma albumin exerts a greater osmotic pressure, edema is directly correlated with its concentration.
A deficiency of protein in the diet, its malabsorption, or excessive excretion may lead to hypoproteinemia. It can also occur in long-standing diarrheal disorders and the infestation of blood sucking parasites or liver flukes.
Deficiency of vitamin A in diet also causes generalised edema.
Obstructive edema
In the first calving of bovines or the first foaling of mares, edema of the udder and ventral abdomen is usually noticed. It is usually physiological or may be caused by the compression of venous drainage by the developing foetus. Such edema is relieved in a few days.
Tumours, fibrosis, vascular disease, lymphangitis, thrombo-phlebitis, or periphlebitis impede blood or lymphatic circulation, and there is a rise in hydrostatic pressure resulting in edema.
In congenital genetic obstruction in calves and pigs and sporadic ulcerative lymphangitis in horses, obstructive edema develops.
Pathogenesis
When hydrostatic pressure increases or osmotic pressure is reduced, the fluid is leaked into the tissue spaces since it cannot return to the capillaries. Accumulation of fluid in tissue spaces or escape of fluid into serous cavities lead to the development of edema.
Clinical Findings
The fluid may accumulate in the following spaces, as could be seen in clinical findings:
- Subcutaneous tissue: Anasarca
- Peritoneal cavity: Ascietes
- Pleural cavity: Hydrothorax
- Pericardial space: Hydropericardium
In bovines, fluid may accumulate in the intermandibular space. In equines, it may be seen in limbs if venous return is obstructed or there is a lack of muscular movement. In African horse sickness, local edema of the head is also noticed.
Edematous swellings are soft, painless, and pits on pressure.
If fluid is accumulated in the abdominal cavity, there is abdominal distention, and on tactile percussion, fluid thrills can be heard.
In the hydrothorax and hydroperitoneum, respiratory and cardiac movements are restricted, and the ventral part of the lungs collapses. Heart and respiratory sounds become dull. In cerebral edema, severe nervous signs are noticed.
Diagnosis
- Based on history and clinical signs, a history of involvement of a specific organ and swelling at the site may reflect edema.
- Palpation and percussion: Palpation and percussion of the edematous area may give an idea about the disease.
- Exploratory puncture: Examination of fluid collected from the edematous area reveals transudate or exudate.
- Radiological examination: The edematous swelling present inside the body, like the hydrothorax or hydropericardium, may be diagnosed by the radiological examination.
- Blood examination: edema suspected to be caused by a lack of protein may be detected by blood analysis, as in such cases, total protein and albumin levels are reduced. Liver function tests are valuable in those cases where hepatic edema is suspected.
Differential Diagnosis
Uroperitoneum
In the case of the uroperitoneum, edema should be differentiated from the accumulation of urine in the peritoneal cavity. In such cases, a rectal examination reveals an empty bladder due to its rupture.
Pericarditis
In pericarditis, besides pleurisy and peritonitis, swelling is noticed, but such swelling is usually inflammatory and accompanied by toxaemia, pain, and other signs of inflammation. Examination of fluids helps in confirmation.
Treatment
It is essential to treat the primary cause of the disease. Animals should be given rest, and sodium and water intake should be regulated.
- Usually, animals are provided with a sodium-free diet.
- The use of diuretics like frusemide is valuable in relieving edema.
- Since most of the affected animals have hypoproteinemia, they should be given a high-protein diet.
- Cardiac edema: Digitalis therapy is recommended.
- Renal edema: Corticosteroids and amino acids are beneficial.
- Hepatic edema due to parasites: specific anthelmenthic therapy in addition to liver tonics. Such cases should be given an excessively carbohydrate rich diet and aminoacids, along with calcium and oral antibiotics. Fat should be restricted.
- Nutritional origin: Whole blood or plasma expanders may be infused in addition to a diet rich in minerals, vitamins, and amino acids.
- Allergic edema: antihistamines (chlorpheniramine maleate) and corticosteroids (Dexamethasone). If there is excessive fluid accumulation, it may be removed slowly after an exploratory puncture at the site. If fluid is not removed slowly, it may cause acute dilatation of the splanchnic vessels and peripheral circulatory failure.