what is siadh? how does your body know when to retain fluidsand when to get rid of them? it’s not like you just think to yourself “i’ve had toomuch water, better get rid of some.†(if you do, and it works, call us). your bodyuses chemicals called hormones to send widespread messages, kind of like how the p.a. systemat school tells everyone to ignore the smoke
billowing out of the science wing. the antidiuretic hormone, abbreviated as adh,is the hormone that controls water retention in the body. it also constrict blood vessels,and incidentally the vasoconstrictor drug called vasopressin is just adh. cool! butthat’s not what we’re talking about right
now. anyways, the more adh floating aroundin your blood, the more fluid you retain. the less adh in your blood, the more fluidyou excrete. the nephrons in the kidneys are the structuresthat physically control how much water is excreted from your body. nephrons are mostlya series of tubes attached end-to-end that type fluids and wastes towards the bladder.these tubes though also allow fluids and electrolytes to move through the tube walls and back intothe blood if needed. adh affects the last two-thirds of these tubes, called the distalconvoluted tubule and the collecting ducts. these tubes focus almost exclusively on reabsorbingwater back into the blood. the wall of these tubes are unsurprisingly made up of cells,a common trait of living things, but these
cells have proteins called aquaporins. aquaporinsallow water to move quickly in and out of the cells. the more adh floating around inthe blood, the more aquaporins are available to... ahem...facilitate water movement throughthe cell (yo, wata, come over here for a sec). so when adh is low, most of the water flowsthrough the the distal convoluted tubule and the collecting duct, giving us diluted urine.when adh is high, aquaporins grab much of the water passing through the these tubesand throws them back into the blood. when i drink a glass of water and that wateris absorbed into my blood, my plasma osmolality drops, which means i’m diluting my bloodwith the water. that means there’s more fluid for all those blood cells to bouncearound in (wooo parrttayy). the part of my
brain called the hypothalamus sees this dropin plasma osmolality and tells the pituitary gland to slow down the release of adh (that’senough!). low adh leads to lots of diluted urine (urine with low osmolality), which bringsour plasma osmolality back to normal. what a nifty feedback loop. now suppose adh continues to be released eventhough my plasma osmolality has dropped (stop it! stop it! pituitary gland whistling). we’regoing to continue retaining water, and as we drink more and more water, we might expectour plasma osmolality to continue dropping. however this isn’t exactly the case. asmore water is retained, it dilutes the other solutes floating around in our blood, likesodium. the extra fluid also takes up more
space in our blood vessels. this taking upmore space issue triggers another mechanism in our body that causes the hormone aldosteroneto stop being released. less aldosterone floating around in the blood causes the body to startdumping sodium from the blood into the urine. concentration gradients cause water to followsodium, so we end up with the excess water being excreted in the urine with the sodium,normalizing the fluid volume in the blood. so now our body is removing sodium from bloodthat already has a lower concentration of sodium. uh oh! this means the plasma sodiumosmolarity is dropping significantly. this whole fiasco we’ve just talked about iscalled syndrome of inappropriate antidiuretic hormone, often abbreviated as siadh.
there are four patterns of adh release inpeople with siadh. type a is completely erratic and is independentof the plasma osmolality. adh levels tend to be very high so the maximum amount of fluidis retained, causing urine osmolality to be very high.type b is a constant release of a moderate amount of adh.type c is the “baseline†plasma sodium concentration level is set lower than normal.it’s like you turned down the thermostat in your house to a new lower level. this typeis particularly unique because the plasma sodium concentration is stable, unlike othersiadh’s where it would continue to fall. type d is the least common type of siadh whereadh secretion is completely normal, yet urine
osmolality is still high. the symptoms a person with siadh experiencesis caused by the dilution and loss of sodium in the blood. when your body has a lower sodiumconcentration than normal, you experience symptoms similar to dehydration or any othercondition where sodium is low. symptoms like headaches, nausea, and vomiting are commoninitially, along with muscle cramps and tremors. as the sodium concentration continues to getlower in your blood, the neurons in your brain begin to swell leading to cerebral edema.this causes symptoms like confusion, mood swings, and hallucinations. if left untreatedit will lead to the common downwards trend in most illnesses of seizure, coma, death.
it’s not too hard to suspect someone mighthave siadh. all we have to do is consider your symptoms in addition to your lab results.low blood sodium levels and low plasma osmolarity combined with high urine osmolality and highurine sodium is a giant red flag for siadh. so what causes siadh? pretty much anythingthat messes with the secretion of adh. conditions like strokes, hemorrhages, or traumato the brain can mess up the brain’s ability to release adh.similarly, some drugs that act on the brain like mood stabilizers or anti-epileptics canchange the way adh is released. surgery in general often causes an increasesecretion of adh, although we’re not entirely sure why. obviously brain surgery, specificallyto the pituitary gland also might cause extra
adh to be released.adh can also be produced ectopically by tumors, which means the tumors themselves produceadh outside of the pituitary gland and release adh into the bloodstream. small cell carcinomain the lungs is the type of cancer most likely to release adh this way.infections in the lungs and brain are also linked to increase the risk of adh secretion.if some of your family members have had siadh before, there’s also a possibility you maydevelop it. the best treatment for siadh is to figureout what the underlying cause of the excessive adh is, and treat that problem. otherwisewe can manage siadh by restricting your daily
Diabetes Insipidus Labs,intake of fluid. you can also start a high-saltand high-protein diet to help replace the
excess loss of sodium. drugs that inhibitadh secretion can also be used in chronic siadh situations. for people who have reallysevere acute hyponatremia symptoms, hypertonic iv fluids are usually administered.
Since you now understand how to manage your condition, you will be better willing to create the proper alternatives and modifications in your life. Keep in mind that these tips will only be related if you use as most of them as you possibly can, and thus you need to start off today to see quick results.
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