Sodium is an important electrolyte in our blood and cells. Concentration of sodium in blood is higher than it is in cells. Sodium and water handling are interlinked in our body and their levels are maintained within quite tight limits. Water tends to foll
Low sodium level in blood is termed hyponatraemia. It is a relatively common electrolyte abnormality. Hyponatraemia may occur in isolation or as part of another medical problem. The normal level of sodium in our blood ranges between 135 to 145 mEq/L and a level below 135 mEq/L qualifies as a diagnosis of hyponatraemia or low sodium. Hyponatraemia is classified as mild moderate or severe depending on sodium level. Mild hyponatraemia refers to serum sodium level between 130 to 134 mEq/L, moderate to a level between 125 and 129 mEq/L and severe hyponatraemia to a level below 125 mEq/L. Severe hyponatraemia, especially when it has developed acutely, poses a threat to life.
Sodium is an important electrolyte in our blood and in cells. Concentration of sodium in blood is higher than it is in cells. Sodium and water handling are interlinked in our body and their levels are maintained within quite tight limits. Water tends to follow sodium. Thus, if sodium concentration rises in blood, water will be drawn from cells to try and bring the level down. Conversely, when sodium level in blood is low water will be pulled out from blood into cells as a result of the osmotic differential, causing them to swell. Clinically, swelling of brain cells is the most important consequence of hyponatraemia. Swelling of brain cells is called cerebral oedema. The problem is that the brain is located in the rigid bony compartment called the cranium formed by the skull bones. The space within this compartment cannot expand. Thus, swelling of brain cells results in rise in intracranial pressure and this pressure is very poorly tolerated by the brain with resultant symptoms, some of which can be severe or even potentially fatal.
Hyponatraemia is usually considered with respect to water content in the body – with deficit of water, normal amount of water and excess of water in the body. Hyponatraemia in association with deficit of water in the body is usually due to excessive loss of fluid from the body which we call dehydration. Causes include vomiting, diarrhea, excessive sweating, etc. One of the responses to these circumstances is for the pituitary to produce anti-diuretic hormone (ADH). This hormone is secreted whenever there is a need to conserve water within the body, its action being to promote reabsorption of water by the kidneys so that a highly concentrated urine is passed to minimize losses of water in urine. Normal volume hyponatraemia refers to low serum sodium level along with normal amount of water in the body. This is usually due to inappropriate secretion of ADH which results in the kidneys reabsorbing water when there is no physiological need to do so. The syndrome of inappropriate ADH secretion (SIADH) may be due to aberrant secretion of this hormone from small cell lung cancer cells or in some cases of pneumonia. SIADH may also arise because of brain tumour or bleeding into the brain. Excess volume hyponatraemia is characterized by low serum sodium along with retention of excess water in the body. The retention of fluid is due to major problems in critical organs. Failure of the heart, kidneys or liver interferes with clearance of fluid from the body. Accumulation of the fluid is often in tissues rather than in blood in such cases and is seen as swelling over the feet or face or as retention within the abdomen causing it to swell up, sometimes massively.
The causes of hyponatremia thus include conditions leading to dehydration which may arise from excessive loss of fluids from the body or inability to maintain adequate intake. Another cause is SIADH for which underlying conditions as discussed above need to be considered. When hyponatraemia due to kidney, liver or heart failure occurs the cause is usually evident. However, additional complications may aggravate the problem in such patients and such cases need to be approached with an open mind. The hormone besides ADH that influences serum sodium level to a substantial degree is aldosterone which is one of the steroid hormones produced by the adrenal glands. Aldosterone is also referred to as the min era loc or ti co id hormone. The other important steroid hormone produced by the adrenal glands is cortisone. Aldosterone promotes retention of sodium by the kidneys. Deficiency of aldosterone may occur in Addison’s disease which may cause sodium loss from the kidneys and hyponatraemia. Deficiency of thyroid hormone also sometimes causes low sodium levels. Drinking unusually large amounts of water may cause dilution of blood with low sodium levels resulting. People who are exercising may drink large quantities of water to prevent dehydration and may develop hyponatraemia. Marathon runners have been known to die of hyponatraemia from drinking too much water without replenishment of sodium and other electrolytes. Drinking really large quantities of water without specific indication may also denote an underlying psychiatric problem.
Symptoms of hyponatraemia depend on how severe is the drop in serum sodium and its rate of development. Mild and even moderate hyponatraemia may not cause any symptoms, the condition being detected incidentally. This is more likely to be the case when hyponatraemia is chronic rather than acute. Most of the symptoms of hyponatraemia are due to cerebral oedema and include headache, mental confusion and lethargy. Nausea and vomiting may also be present. As the condition progresses seizures and drowsiness advancing to loss of consciousness may be seen. Severe hyponatraemia, if allowed to persist, is fatal.
Diagnosis is established by estimation of serum sodium along with other electrolytes. The diagnosis may easily be overlooked clinically but since serum electrolytes is generally ordered as part of laboratory screening in all significantly ill patients, the diagnosis is soon evident. A faulty electrolyte report which may be due to inadvertent dilution of blood or due to a laboratory error may create confusion. A low sodium report in a person without symptoms may also create confusion. However, since quite a few individuals with hyponatraemia may have no symptoms the report needs to be given due weightage. An idea of whether hyponatraemia is with increased, low or elevated water content in the body and what is the probable mechanism of its occurrence can be derived from further tests such as osmolality of serum and urine and from sodium concentration in urine. In many cases of hyponatraemia the cause is obvious. Should this not be clear, further testing is indicated. Problems like lung cancer and brain tumours may need to be ruled out by appropriate imaging tests such as CT or MRI scan.
Treatment of hyponatremia depends on the clinical condition and the cause. Acute hyponatraemia associated with symptoms such as altered consciousness and seizures is a medical emergency since delay in treatment can easily be fatal. The first priority is to ensure that vital parameters such as respiration, circulation and blood pressure are maintained. Severe acute hyponatraemia needs to be corrected by appropriate intravenous fluids. A major problem is that too rapid correction is also dangerous. It can cause permanent damage to the brain leaving the patient with paralysis. Concentrated solutions of saline to elevate the level of serum sodium to a limited extent are administered intravenously to such patients under close monitoring. Should the initial serum sodium level be too low, the intravenous infusion of concentrated saline may need to be repeated on subsequent days. The cause and type of hyponatraemia also needs consideration. If it is due to dehydration, fluid replacement is also a priority and the underlying problem such as diarrhea, vomiting, etc also needs attention. In SIADH fluid restriction alone may be sufficient to reverse the electrolyte changes. Diuretics are drugs given to promote urine flow and reduce swelling in the body. They are also used to lower blood pressure. Strong diuretics that promote urine flow and sodium loss in urine may be indicated in cases not responding to fluid restriction alone. For people with hyponatraemia along with excess water retention the focus is on correction or control of underlying health problems like heart, kidney or liver failure. In general, in chronic hyponatraemia correction of serum sodium level is by tackling the underlying cause while in acute hyponatraemia the line of treatment depends on the clinical condition, severity of hyponatraemia and the rapidity of development. Provided the acute crisis is tided over, the prognosis depends on the underlying cause. When caused by problems like lung cancer or brain cancer or by major organ failure the prognosis is obviously guarded. With other causes chances of a good recovery are quite bright.