Mechanisms of formation (pathogenesis)
In a healthy body, the filtration of fluid from capillaries into tissues and its return to the bloodstream are in balance. This process is regulated by several forces:
- Filtration factors (push fluid into tissues): blood hydrostatic pressure (normally higher in the legs in a vertical position) and the colloid-osmotic pressure of the interstitial fluid itself.
- Retention factors (return fluid to vessels): the main one is plasma oncotic pressure (created by blood proteins), as well as tissue fluid pressure.
- Vascular wall: the volume of fluid passing through the capillaries depends on their permeability.
When this equilibrium shifts towards filtration, water begins to retain in the tissues.
Main causes of edema
Depending on the disrupted mechanism, several types of edema are distinguished:
- Venous hypertension (increase in hydrostatic pressure): due to local stasis (varicose veins, thrombosis, valve insufficiency) or systemic stasis (in heart failure).
- Hypoproteinemia (drop in oncotic pressure): shortage of plasma proteins due to starvation, malabsorption, liver pathologies (reduced synthesis), or protein loss through the kidneys (nephrotic syndrome) and intestines.
- Lymphostasis: obstruction of lymphatic pathways. Water leaves, but proteins remain in the tissue, drawing in new fluid. Occurs in erysipelas, post-mastectomy, or filariasis.
- Membranogenic: increased vascular permeability during inflammation (with mediators like histamine) or allergies (e.g., life-threatening angioedema of the face and neck).
The body responds to the loss of circulating blood volume (CBV) compensatorily: it activates renal mechanisms (RAAS) and releases ADH to retain sodium and water, which paradoxically only exacerbates the edema.
Diagnostics and severity grades
The first sign of fluid retention is latent edema, detected only by weighing (weight gain against the background of decreased diuresis). Visible changes and a pitting indentation upon palpation appear when body weight increases by at least 10–15%.
Physical examination for edema is performed by pressing fingers onto an area with a bony backing (inner lower leg, lower back). If the pit persists for 2–3 minutes, it is overt edema. The skin often appears glossy and peeling, and bluish (cyanosis) in venous stasis.
Severity grades:
- Pastosity: minimal swelling of the legs, the pit hardly remains.
- Anasarca: total edema of all subcutaneous tissue, pits remain even from a stethoscope.
Besides subcutaneous tissue, fluid can accumulate in cavities: abdominal (ascites), pleural (hydrothorax), and the pericardial sac (hydropericardium).
Differential diagnosis: heart vs kidneys
The clinical presentation heavily depends on the cause:
| Sign | Cardiac edema | Renal edema |
|---|---|---|
| Where do they start? | On the legs (lowest points) | On the face (especially eyelids) |
| When are they worse? | In the evening (after a vertical position) | In the morning (after sleep) |
| Symptoms | Tight shoes by the evening, ring won't come off | Close ones notice facial puffiness |
Generalized (symmetrical) edema indicates a systemic disease, while local or asymmetrical edema points to an issue with specific veins, lymphatic vessels, or an allergy.
Briefly: answers to topical questions
Which inflammatory mediators cause increased vascular wall permeability in membranogenic edema?
Inflammatory mediators/factors that are directly documented in sources as increasing vascular wall permeability include:
- Histamine — in inflammatory edema, it is associated with vasodilation and a sharp increase in vascular permeability.
- Bradykinin — in inflammatory edema, it is associated with vasodilation and a sharp increase in vascular permeability.
- Hageman factor — acts as an inflammatory mediator, activates plasma mediators, and increases vascular permeability.
- Basophil mediators — the source indicates their effects include increased vascular wall permeability; basophils also release platelet-activating factor.
What clinical tests and methods are used for the precise diagnosis of latent edema?
The following methods are used to diagnose latent edema:
- Body weight monitoring — daily weigh-ins to detect weight gain before visible edema appears.
- Comparison of diuresis with fluid intake volume.
- McClure-Aldrich test (blister test) — intradermal injection of 0.2 ml of saline; in latent edema, the papule resolves in less than 40 minutes.
What trophic and visual skin changes, besides cyanosis, are characteristic of prolonged venous stasis?
With prolonged venous stasis and long-standing edema, the following changes are confirmed:
- Skin thinning.
- Hyperpigmentation.
- Coldness of the extremities.
- In chronic venous insufficiency — lower extremity trophic ulcers.
Class and exam questions
Why does edema appear on the legs in the evening during heart failure?
Due to increased hydrostatic pressure in the veins. Prolonged vertical positioning during the day promotes blood pooling in the lowest points of the body (legs).
How to distinguish pastosity from pronounced edema?
Pastosity is mild puffiness where pressing produces practically no pit or it disappears quickly. In true edema, the pit persists for 2–3 minutes.
Why is angioedema dangerous?
This is an allergic edema that develops very rapidly. It is dangerous due to its localization on the neck, lips, and tongue, which can lead to airway obstruction and asphyxiation.
What else to study on this topic
- Pathogenesis of edema in heart failure (hemodynamics, hormones, kidneys)
- Mechanisms of formation of nephrotic and acute nephritic syndromes
- Causes and Types of Hypoproteinemia
- Classification and Diagnosis of Lymphostasis (Elephantiasis, Post-Mastectomy Syndrome)
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