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

The Gulf Today - Time Out - - TRAVEL - Dr Asheesh Me­hta In­ter­nal Medicine Spe­cial­ist

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.

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