Detective Nephron: An Electrolyte Mystery

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Detective Nephron, world-renowned for expert analytical skills, trains budding physician-detectives on the diagnosis and treatment of kidney diseases. L.O. Henle, a budding nephrologist, presents a new case to the master consultant.

Nice Glom (the new medical student) enters the room along with L.O. Henle to present a case.
NephronWhat do you have for me today Henle?
Henle looks at Glom
GlomI have a 65-year-old man with a serum sodium concentration of 112 mEq/L.
NephronHyponatremia! My favorite electrolyte disorder. What is the first question you need to ask?
HenleWhether the patient has symptoms?
NephronExactly. Given the severity of this hyponatremia, we need to know if we need to treat immediately with hypertonic saline to avoid life-threatening cerebral edema. Severe symptoms such as seizures and coma indicate significant cerebral edema and require the use of NaCl 3% 100 mL IV bolus, which you could repeat twice if symptoms persist. Moderate symptoms such as confusion indicate a lesser degree of cerebral edema but still significant enough to be dangerous and also require the use of NaCl 3% but in slow infusion. Remember, severely symptomatic or moderately symptomatic hyponatremia are medical emergencies and need to be treated with hypertonic saline.
HenleI interviewed the patient and did a full neurological exam. The patient is asymptomatic.
Nephron(upset) That is not entirely true, is it? Evidence has emerged over the last several years suggesting that all hyponatremias are symptomatic to a degree. Even mild chronic hyponatremia in the range of 125 to 135 mEq/L is not only associated with increased mortality but also increased morbidity in the form of subtle attention deficits, gait disturbances, falls, fractures, and osteoporosis.
GlomI did not know that.
Nephron(smiling) That is not entirely true, is it? Evidence has emerged over the last several years suggesting that all hyponatremias are symptomatic to a degree. Even mild chronic hyponatremia in the range of 125 to 135 mEq/L is not only associated with increased mortality but also increased morbidity in the form of subtle attention deficits, gait disturbances, falls, fractures, and osteoporosis.
Henle & Glom(looking at each other) No.
NephronRegulatory volume decrease is the process by which cells adapt to swelling. In the brain, astrocytes do swell under hypotonic conditions, and neurons do not because they lack aquaporin 4, the water channel responsible for cerebral edema. Astrocytes achieve regulatory volume decrease by extruding osmoles to the extracellular compartment, therefore reducing intracellular tonicity to avoid further water entry. Initially, during the first 3 hours of this process, K+ and Cl- are the main osmoles extruded, but after that, organic osmoles, such as glutamate and myoinositol, take a primordial role. Glutamate is the main excitatory neurotransmitter in the brain. It is hypothesized that glutamate released in large amounts during this process can cause excitotoxicity and neuronal cell damage. This damage may be manifested as subtle neurological symptoms such as gait disturbances that can only be detected by special neuropsychometric testing.
GlomAmazing!
NephronWell, getting back to our case, since our patient is apparently asymptomatic, there is no need to use hypertonic saline. Mildly symptomatic and the so-called “asymptomatic” (looking at Henle) hyponatremia reflect almost complete adaptation to hypotonicity with mild degrees of cerebral edema. Full adaptation takes 48–72 h and that is where the difference lies between acute and chronic hyponatremia. Hypertonic saline is not needed in this case and we have more time to focus on the underlying pathophysiology causing hyponatremia.
NephronGlom, why don’t you tell me more about this patient?
GlomHe is a 65-year-old homeless man with a significant history of alcohol abuse. He was found down in the street by the police who brought him to our emergency department. It seemed the patient was inebriated initially but was alert and oriented for me. On exam, his BP = 100/60 mm Hg, heart rate = 79 bpm, RR = 20. He looked disheveled and malnourished. Cardiopulmonary exam was unremarkable. Abdominal exam was benign. Neurological exam was normal, as Henle said. Overall, he seemed euvolemic. His initial laboratory examination revealed a Na = 112 mEq/L, K = 2.5 mEq/L, Cl = 95 mEq/L, TCO2 = 25 mEq/L, Glucose = 75 mg/dL, BUN = 3 mg/dL, Cr = 0.3 mg/dL, Alb = 3.2 g/dL. POsm = 280 mOsm/kg. So, it seems he has pseudohyponatremia, but I thought we don’t see this anymore since the way the laboratory measures sodium concentration has changed?
Nephron(shocked) That is a common misconception. Two-thirds of all clinical laboratories in the US still use indirect ion selective electrode technology to measure sodium concentration on a routine basis. This technology is prone to error in the presence of high protein or lipid levels in the blood. So we still need to be alert for those, however, pseudohyponatremia does not seem to be the problem here. Do you have a serum ethanol level from his initial laboratory examination in the emergency department?
GlomSure, it was 192 mg/dL.
NephronMmmm …. It seems we need to go over the concepts of osmolality and tonicity. Henle?
HenleTonicity is effective osmolality. Tonicity does not take into account the contributions of solutes that cross cell membranes and therefore do not exert an osmotic effect. Urea is one of these solutes. But the BUN is low in this patient?
NephronCan you think of another solute that crosses cell membranes and does not contribute to tonicity but does contribute to osmolality?
Henle(jumps in) Ethanol!
NephronPrecisely, and this is what happened to this patient. The ethanol level was high enough that it made the plasma osmolality normal, but if you discard its effects, the tonicity will still be low, roughly 228 mOsm/kg. This is really a hypotonic hyponatremia. Classically, this is seen in hospitalized ESRD patients with hyponatremia associated with normal osmolality. Let’s stop expensive workups looking for hypertriglyceridemia and multiple myeloma without realizing that what makes osmolality normal in these patients is the contribution of high urea levels.
Glom(blushing) Thanks for telling me. I will cancel my orders for a SPEP and a lipid profile. So following the algorithm, then, this patient has hypotonic euvolemic hyponatremia.
NephronYou like algorithms, don’t you Glom? Clinicians use algorithms as a way of chunking separate pieces of information and to increase space in their working memory. That is a valid and effective cognitive technique, but when algorithms are abused and used as cookbook medicine without understanding the underlying pathophysiology they fail miserably. There are several issues with the classic hyponatremia algorithm: first, clinicians’ assessment of volume status in hyponatremia has been studied and found to have a low sensitivity and specificity. Second, the effectiveness of the classic algorithm only enables 10% of clinicians to correctly diagnose hyponatremia. Finally, the classic algorithm suggests you could arrive to a single diagnosis; however, hyponatremia is usually multifactorial.
HenleUnderstood. What is your approach then?
NephronIn a series of pivotal studies done by Dr. Edelman and reported in a landmark paper published in the Journal of Clinical Investigation in 1958, he described what is known now as the Edelman equation by which the serum sodium concentration could be viewed as a function of total body contents of sodium, potassium, and water. The equation goes like this: [Na+] = (NaE + KE)/TBW. Where [Na+] is the plasma sodium concentration, NaE and KE are the total body exchangeable sodium and potassium respectively, and TBW is total body water.
HenleExchangeable sodium and potassium?
NephronExchangeable sodium and potassium represent the total body sodium and potassium that exert an osmotic effect. For instance, bone tissue can store almost a third of total body sodium, but it is non-exchangeable, meaning it does not exert an osmotic effect and therefore does not contribute to plasma sodium concentration. It is hypothesized that movement of sodium from bone to plasma during chronic hyponatremia is responsible for osteoclast activation and subsequent osteoporosis.
HenleVery interesting.
NephronFollowing the Edelman equation, then, hypotonic hyponatremia is produced when total body water is increased relative to total body exchangeable sodium and potassium, or simply put, when there is excess solute-free water.
GlomIs that why you always said hyponatremia is mainly a water disorder, and not a sodium disorder?
NephronYes. By the way, what are the sodium disorders?
GlomMmm ….
HenleHypovolemia and hypervolemia.
NephronExcellent! Then an excess of solute-free water can develop as a consequence of either increased water intake or decreased water excretion.
GlomThis is finally making more sense for me.
NephronPatients can ingest large amounts of water without developing hyponatremia because kidneys can handle huge water loads, but there is a limit as to how much the kidneys can excrete…
Pause…and the limit is about 18 liters per day.
GlomI see.
NephronWe said most hyponatremias are due to an increase in water compared to total exchangeable cations. We talked about increase in water intake. What about decrease in renal water excretion? What pathophysiological mechanisms do contribute to a decrease in renal water excretion?
HenleHigh antidiuretic hormone (ADH).
NephronYes! High ADH activity is the most common mechanism of hypotonic hyponatremia.
NephronAnd what stimulates ADH to be released in large amounts?
GlomHypertonicity and hypovolemia.
NephronHypertonicity is a well-known stimulus for ADH release, but remember true hyponatremia is hypotonic so this does not really apply here.
GlomWhat about hypovolemia?
NephronTo be more precise, you should say “decreased effective arterial blood volume.” Hypovolemia refers to decreased extracellular fluid (ECF) volume. Remember, extracellular fluid volume can be divided into 2 compartments, interstitial and intravascular. The intravascular compartment can also be divided into a venous sub-compartment that contains 85% of the blood volume, and an arterial sub-compartment that contains only 15% of the blood volume. The baroreceptors are located in the arterial portion of the intravascular space and sense changes in this compartment. When the volume of this compartment goes down, the baroreceptors are activated and the end result is the release of ADH from the posterior pituitary. Baroreceptors do not sense changes in ECF volume. ECF volume changes can parallel changes in the arterial compartment such as in individuals who develop hemorrhage, vomiting, or diarrhea, the so-called hypovolemia, but sometimes not. That is why you could have patients with low effective arterial blood volume despite an expanded ECF volume. Can you think of clinical scenarios where this happens?
GlomLiver cirrhosis and heart failure.
NephronVery good! What about other non-physiological causes for ADH release?
Henle & GlomMmm….
NephronWhat do you call the condition where ADH is secreted autonomously in the absence of a physiologic stimulus?
HenleSIADH (syndrome of inappropriate antidiuretic hormone secretion)!
NephronExactly! Any other mechanism by which ADH could be high?
HenleAdrenal insufficiency?
NephronPrimary, secondary, or tertiary?
HenleI don’t know.
Nephron(surprised look) All! But by different mechanisms. Cortisol exerts a negative feedback loop on ADH release. So, in the absence of cortisol, ADH is uninhibited. This occurs in any adrenal insufficiency. In the specific case of primary adrenal insufficiency where the problem is the adrenal gland, aldosterone secretion is also compromised, and aldosterone regulates renal sodium excretion. In the absence of aldosterone, you have renal salt wasting and decreased effective blood volume causing ADH release as well.
GlomFascinating!
NephronWhat other mechanisms of decreased renal water excretion do you know?
HenleDecreased GFR.
NephronVery well. This is a very common problem in our AKI, CKD, and ESRD patients. They don’t need to drink 18 L of water to develop hyponatremia. With much less water intake they will be in trouble because their kidneys cannot excrete the extra water load. When you see an anuric ESRD patient with hypotonic hyponatremia, it is almost always due to this. Don’t do a million dollar workup looking for another cause.
NephronAny other mechanisms of decreased renal water excretion?
GlomI don’t know any other.
NephronLow solute intake. The amount of solutes excreted in the urine, also known as urine solute load, determines the volume of urine being produced. Under steady state conditions, the amount of solutes you eat is equal to the urine solute load. So, if you eat a low solute diet, then you will excrete a small urine solute load and therefore your urine volume will be low, and with low urine volumes the ability to excrete water will be limited.
Glom(confused) What is considered a low solute intake?
NephronThe normal diet contains 600 to 900 mOsm per day of solutes.
GlomI must be eating a good amount of solute because I love spaghetti.
NephronThat is a common misconception; carbohydrates do not produce any meaningful solutes. Most solutes are derived from either proteins because they metabolize to urea, or salt.
HenleI guess this occurs in the so-called tea and toast diet?
NephronExactly. These patients eat toast, which is mainly carbohydrates, with very little solute intake, and drink tea all day, which is mainly water, exceeding the capacity of their kidneys to excrete water. Any other clinical scenarios?
HenleNot sure.
NephronHave you heard of beer potomania?
HenleOh yes!
NephronThis condition occurs in alcoholics. They usually do not eat enough solute and therefore they have a limited capacity to excrete water. On top of that, they drink beer all day and beer is 90% water. They usually end up retaining water and developing hyponatremia. I am afraid this is what is happening to this patient. How much does he drink? Does he eat enough solutes?
GlomYes, I asked him. He said he drinks 2 six packs of beer every day and eats very little food. He looks very malnourished.
HenleYes, his BUN is only 3!
NephronDo you have a urine osmolality in this patient?
GlomIt is 79 mOsm/kg.
NephronThat is very diluted urine with suppressed ADH, which can only be explained by drinking in excess of the kidney’s ability to excrete water. You can see low urine osmolality in patients with primary polydipsia who drink huge loads of water, or patients who drink a normal amount of water but their kidneys have a limited ability to excrete water due to non-ADH mechanisms such as low GFR or low solute intake. Given his normal kidney function, the history of alcohol abuse, and his nutritional status, I say this is likely low-solute-intake hyponatremia caused by beer potomania.
GlomWhat is the treatment?
NephronIncrease his solute intake, give him a hamburger! I mean, give him regular hospital food.
HenleAny other recommendations?
NephronI am glad you asked. Patients with low-solute-intake hyponatremia will overcorrect once dietary solute is increased. These patients usually start having a brisk urine output due to massive water diuresis, so the primary team needs to be aware of this and ready to act.
GlomAct?
Nephron(with confidence) Overcorrection of hyponatremia is a medical emergency. Overcorrection is the main risk factor for osmotic demyelination syndrome.
GlomOK, I will tell them the correction rate should be 0.5 mEq/L/h.
NephronThat is the classic teaching in medical school but there is actually not much evidence to support the notion of correction rates. There are, however, correction limits that when you cross put your patient at risk for osmotic demyelination syndrome. The limits have traditionally been 10–12 mEq/L in 24 h. The 0.5 mEq/L/h is an extrapolation of this, i.e., 12 mEq/L divided by 24 h. The absolute magnitude of correction is more important than the rate. You could theoretically correct the sodium by 10 in the first hour as long as you keep the sodium the same in the next 23 hours. There are animal studies that support this concept.
HenleSo, should we tell the team our goal is no more than 10 mEq/L in 24 h?
NephronNo. 10 mEq/L in 24 h is a limit, not a goal. When you use digoxin or phenytoin, you do not aim for one minus the toxic dose, right? You aim for a smaller level that is effective but also safe. The same happens in hyponatremia. We do not aim for 10 mEq/L; 10 mEq/L is our limit. Case series have demonstrated that correcting the sodium concentration by 6 mEq/L in any 24-h period is a safe and effective goal.
GlomOK, so aim for 6 mEq/L but definitely no more than 10 mEq/L?
Nephron(in teaching mode) Also incorrect. 10 mEq/L is the limit for patients with average risk of osmotic demyelination syndrome. Patients with liver cirrhosis, malnutrition, alcoholism, and hypokalemia are at high risk of osmotic demyelination syndrome and the limit should be no more than 8 mEq/L.
GlomWhat happens if the sodium corrects too fast? What can we do?
NephronThey need to carefully monitor his urine output and once they see an increase in urine output or a rapid elevation of serum sodium concentration they need to relower the serum sodium. Relowering serum sodium concentrations has been proven to be an effective prevention strategy to avoid osmotic demyelination syndrome in case series and animal studies. I recommend using D5W at 3 mL/kg IV over 1 hour and repeat sodium right after. We can also add desmopressin 2–4 µg subcutaneously if it seems that D5W is not enough. Some clinicians follow a protocol where they start with NaCl 3% along with desmopression Q8h from the get-go, creating a state of artificial SIADH and avoiding water diuresis, which is the main state responsible for overcorrection. In that way, they are able to correct the hyponatremia very nicely without complications.
GlomI will also tell them to replace the potassium of 2.5 mEq/L.
NephronHold on! You have to be very careful about replacing potassium in patients with hyponatremia. Following the Edelman equation, any addition to the total body potassium will increase serum sodium concentration. In other words, correcting hypokalemia will also correct hyponatremia. So, I will try to minimize the potassium correction for now unless the patient has arrhythmias or muscle weakness until the sodium concentration is at a safer level.
Henle(yawning) It looks like I might need to be awake all night to monitor the sodium concentration in this patient.
NephronThat is a good idea Henle but also it is part of being a good nephrologist. We always do the best for our patients. Make sure you bring your coffee with you Henle, as you will need it!
Special thanks to Dr. Helbert Rondon, Assistant Professor of Medicine, Renal-Electrolyte Division at the University Of Pittsburgh School Of Medicine (writer and submitter for this case).
The concept of Detective Nephron was developed by Kenar D. Jhaveri, MD, Associate Professor of Medicine at Hofstra North Shore LIJ School of Medicine and an Attending nephrologist at North Shore University and Long Island Jewish Medical Center in Great Neck, NY. Send correspondence regarding this section to kjhaveri@nshs.edu or kdj200@gmail.com.