Why does kidney disease cause lower limb edema? What can be done to help eliminate edema?

Author: Zuo Li, Chief Physician, Peking University People's Hospital

Chairman of Blood Purification Branch of Chinese Society of Research Hospitals

Reviewer: Li Mingxi, Chief Physician, Peking Union Medical College Hospital

Member of the Medical Science Branch of the Chinese Medical Association

Many common diseases in clinical practice can cause edema, such as kidney disease, liver disease, heart disease, lung disease, and hypothyroidism. These five conditions are the most common causes of edema.

Today we will talk about edema related to kidney disease.

Kidney disease-related edema often manifests as symmetrical edema of both lower limbs. After gently pressing the front of the tibia for a while and then releasing it, a depression that slowly subsides will appear. Doctors call this type of edema "pitting edema."

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Edema is also clinically graded according to the location of the edema. Edema below the ankle joint is mild edema, edema below the knee joint is moderate edema, and edema above the knee joint is severe edema. In severe cases, it may even cause pleural or abdominal effusion.

1. Why does kidney disease cause edema?

Edema caused by kidney disease usually has two causes.

One reason is nephritic edema, which may be due to renal insufficiency or glomerular filtration disorder. The kidney's ability to excrete water is reduced. Compared with the amount of water taken in, the amount of water excreted is too little. The water cannot be excreted in a timely and effective manner, and edema occurs.

Another reason is the decrease in blood osmotic pressure, which leads to renal edema. The reason why water is in the blood vessels is that the protein and crystals in the blood form a high osmotic pressure to maintain the balance of water inside and outside the blood vessels. When there is kidney disease, the protein in the blood vessels decreases, resulting in a decrease in osmotic pressure, and water flows into the tissues outside the blood vessels, causing tissue edema.

2. What are the symptoms of renal edema?

As mentioned earlier, renal edema is divided into nephritic edema and nephrotic edema.

Nephritic edema is caused by excessive water in the blood vessels, which cannot be excreted by the kidneys, resulting in increased hydrostatic pressure in the blood vessels, which can only push the water into the interstitial space. Therefore, nephritic edema usually occurs in areas with loose interstitial spaces, such as the eyelids and vulva. Especially the eyelids, patients can easily find swollen eyelids when they wake up in the morning.

Nephrotic edema is caused by low plasma osmotic pressure, so it often occurs in low-hanging parts. For example, patients who are sitting often have more severe swelling in their lower limbs, while patients who have been bedridden for a long time will have edema in their lower backs. Therefore, during clinical physical examinations, in addition to checking whether the patient's lower limbs are edematous, we also need to check whether the back is edematous.

3. If only one leg has edema, could it be kidney disease?

Generally not, because nephrotic edema usually affects both lower limbs at the same time, and the degree of swelling is the same. If the two legs of a patient with nephrotic edema swell to different degrees, it is usually due to complications. For example, patients with nephrotic syndrome are prone to thrombosis, especially those with less activity or even immobilization. They are more likely to develop thrombosis in the lower limbs, which prevents venous blood from returning. Generally, thrombosis will not suddenly form in both legs at the same time. Usually, one leg will have thrombosis and the other will not, which will cause the two legs to swell to different degrees.

Of course, other diseases besides kidney disease can also cause venous thrombosis and leg edema. Generally speaking, when the degree of swelling of the lower limbs is different, it is often due to venous thrombosis, or it may be due to lymphatic blockage, resulting in lymphatic reflux obstruction, lymph fluid seeping into the interstitial space and causing edema, but this edema is less concave.

For patients, it is usually difficult to tell what causes leg edema. Therefore, it is recommended to seek medical attention promptly after edema is discovered, and let a professional doctor identify whether the edema is caused by cardiac, renal, hepatic, pulmonary, or endocrine factors.

4. If edema is suspected to be caused by kidney disease, what tests need to be done?

If edema is suspected to be caused by kidney disease, a urine routine test is required to see if there is hematuria or obvious proteinuria. Because whether it is nephritic edema or nephrotic edema, urine routine tests usually show hematuria or proteinuria, and if there is no hematuria or proteinuria, kidney disease can often be ruled out.

But there is another situation which is renal insufficiency, in which the filtration function of the kidney is weakened. If the renal insufficiency is not caused by nephritis, hematuria and proteinuria may not be obvious. At this time, it is necessary to check whether the blood creatinine is elevated to help make a judgment.

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So, on the whole, a routine urine test should be conducted to see whether there is hematuria or proteinuria, and then to see whether the blood creatinine is high. Combining the two, we can basically determine whether the edema is caused by kidney disease.

5. In order to control edema and urine volume, do I need to control the amount of water I drink?

Patients are generally advised to maintain water balance through diet and avoid using medication as much as possible.

How to maintain water balance through diet? The first thing is to eat a light diet.

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There is a device in our brain that regulates blood sodium levels, and this device does not allow blood sodium to be too high. If you eat too much salt, the device that regulates blood sodium levels will reduce the amount of water discharged by the kidneys, causing more water in the blood vessels to dilute the blood sodium, which will cause edema.

On the other hand, a light diet can reduce the feeling of thirst, thereby reducing water intake. Therefore, in order to control edema, in order to avoid medication, the diet must be as light as possible to better achieve the purpose of controlling edema.

6. Are there any massage techniques that can help eliminate edema?

Nephrotic syndrome can cause blood stasis, especially in bedridden patients or those who stand for long periods of time. This stasis can easily lead to blood clots. Therefore, for hospitalized patients with nephrotic syndrome, doctors sometimes ask family members to passively massage the upper and lower limbs of the patients, turn them over, etc., which actually passively promotes blood flow and makes it difficult for blood clots to form.

However, manual massage alone usually cannot eliminate edema. A lower limb drive pump can be used to assist blood return through massage to avoid venous thrombosis.

In addition, you can also apply pressure to the lower limbs, which can promote the return of interstitial water to the blood vessels to a certain extent and help reduce swelling.