Water: when too much is not good
A healthy person can consume a large quantity of water without any deleterious effect. But in those people who have impaired water clearance, it could result in hyponatremia.
SEVENTY-THREE-year-old Mr. K was admitted to hospital for drowsiness and abnormal behaviour that had been evident for over a week. There was no past history of diabetes, blood pressure or any other illness. His CT scan was normal. A test revealed the diagnosis the plasma sodium returning to normal. Investigation revealed that he had a few litres of water every day.
Mr. V, 83-year-old patient, was brought to hospital with a history of weakness, difficulty in urinating and walking. He had high blood pressure. A few weeks earlier he had complained of feeling weak. He was diagnosed as being depressed and was prescribed anti-depressants. His condition deteriorated and he was unable to move or urinate. Examination was normal except for a distended bladder. Again all investigations were normal except plasma sodium, which was 110meq/l. Anti-depressant drugs were withdrawn. His water intake was restricted and he was given intravenous normal saline. With that he improved remarkably. His plasma sodium also returned to 130meq/l.
Both these cases demonstrate the lurking danger of hyponatremia or low plasma sodium, as a result of indiscriminate water intake in persons whose "free water clearance" is impaired.
What is Hyponatremia?
Our diet varies tremendously both in salt and water content. Sodium, mainly derived from common salt (sodium chloride), is distributed in the body in the extra cellular compartment. This is the exchangeable form of sodium which remains in the blood and interstitial fluid.
The non-exchangeable form is mainly in the bones. Our dietary intake of salt varies from 10-15 grams of sodium choloride a day.
The kidneys can vary the excretion of sodium from almost 0-20 gm/day. Because of this the total body sodium is maintained in balance inspite of a variable diet. But conditions like heart or liver failure or kidney diseases can impair this sodium loss resulting in an accumulation of salt in the body. This mainly manifests itself as swelling of the legs because the excess sodium retains water due to its osmotic activity.
Similarly, our water intake is highly variable, dependent on the sensation of thirst and cultural practices. Two-thirds of the body weight is composed of water. It is freely permeable from the extra cellular to intracellular compartment depending on osmotic activity. There is a osmo centre or osmostat in the brain which senses the tonicity of the blood and which produces the Anti Diuretic Hormone (ADH) which stimulates the thirst centre in the brain as well as reduces water loss in urine. Because of this osmo centre and the response of the kidneys to the ADH, a normal balance is maintained inspite of a variable water intake. The kidney can concentrate urine to an osmolity of 1,200 mosmole/l so that urine output for the day is as low as 300 ml. Similarly, it can dilute urine up to 60 mosmole/l and produce a urine output of 10 litres per day. This ability of the kidneys to get rid of excess water from the body is called "free water clearance." Because of this, plasma sodium is normally maintained between 135-145 meq/l. It is only a ratio and does not show the total body content of sodium or water. A plasma sodium less than 135 meq/l is called "Hyponatremia."
Incidence of Hyponatremia
Hyponatremia is common in hospitals, seen in 10-15 per cent of all patients. In critical care units, its incidence can be as high as 30 per cent. It is a very important factor determining the mortality or outcome in critically ill patients.
Any condition that leads to impairment of free water clearance by the kidneys results in hyponatremia. The causes are expressed along with the volume status (water status) since it has therapeutic implication.
(a) Associated with low volume status, for example vomiting, diarrhoea, burns, use of diuretic drugs (increase urinary output) etc., a patient looses both water and sodium. In these conditions, water is often replaced resulting in hyponatremia.
(b) Associated with increased blood volume, example, heart, liver or kidney failure, where both water and sodium are retained with water in excess of sodium.
(c) Associated with normal blood volume: In this condition, the anti-diuretic hormone is secreted inappropriately because of the disturbance in the osmo stat. This happens in lung disease, brain tumours, head injuries.
Two important groups of drugs are worth mentioning (a) Diuretics which increase water and sodium loss in urine. However the intake of water is high because of thirst and this results in hyponatremia. (b) Anti-depressants which produce dryness of mouth resulting in excess water consumption.
The manifestations of hyponatremia depend on whether the condition has occurred acutely (a short period of time) or chronically (a long period of time). In acute hyponatremia, the brain cells swell since water moves from the blood into the brain cells due to osmotic activity. In chronic hyponatremia, the brain cells adapt by removing certain osmotic substances from the cells. The manifestations with hyponatremia are predominantly neurological, which include weakness, nausea, vomiting, drowsiness, abnormal behaviour, convulsions or fits and, ultimately, coma.
As the manifestations are neurological, patients are taken to either a neurologist or psychiatrist. A estimation of plasma sodium will reveal the diagnosis. Unfortunately, this simple investigation cannot be estimated by small laboratories predominant in the country.
The rapidity with which plasma sodium should be corrected depends on the condition of the patient ad the acuteness of onset. This is important because if asymptomatic chronic hyponatremia is not corrected, the brain system can be damaged irreversibly (centre pontine demyelination). Treatment consists of assessing the volume correctly, replacement with intravenous normal saline and restricting water intake. Underlying diseases like heart or liver failure should be treated and drugs responsible for hyponatremia stopped. There are certain drugs, which are available to correct hyponatremia. These are used only when water restriction fails.
How much water should we take?
A healthy person can consume a large quantity of water without any deleterious effect. However, the body has an excellent thirst centre which helps guide in consuming the right quantity of water. But in those people who have impaired water clearance (examples are heart, liver or kidney failure patients on diuretics, anti-depressants etc.) hyponatremia would result if they drink a lot of water.
Dr. R. RAVICHANDRAN
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