This is a pseudohyponatremia, caused by a measurement artefact. In actual fact the sodium concentration is normal. The college has explored pseudohyponatremia in two previous SAQs:
The best article to reference for a discussion of such pseudohyponatremia would have to be the 2011 article by Fortgens and Pillay, who treat the problem with lucid explanations.
The plasma sample is taken to the biochem lab, and is diluted to 1/10th. Then, this diluted result is run through either a flame emission spectrophotometer (i.e. it gets burned and the emission spectra measured), or run through an indirect ion-sensitive electrode. Either way, the whole plasma is used, not just the water fraction. The amount of sodium found in this way is then “diluted” by calculation, with the assumption that it comes from a sample which originally consisted of 93% water.
Obviously, if you has 20% protein in that sample (eg. in multiple myeloma) then this assumption is false. So, anything that causes there to be less water in your sample than the assumed 93%, will cause the test to show a falsely decreased sodium.
High triglycerides have been observed to increase the-non-water volume of plasma. This was the topic of Question 20.4 from the second paper of 2015. It has also been observed personally by the author. For instant, here is a photograph of a colleague's hand, holding a sampling tube with some blood and supernatant.
The triglyceride level there was about 45g/L, probably the highest level seen in this fat-loving corner of an especially BBQ-obsessed country, and thoroughly beating the Guinness record. We even had to get the pharmacy to make up a batch of special "fatless" parenteral nutrition mixture for this guy, and he narrowly avoided plasma exchange (it was viewed as useless, given that much of the pancreas was already necrotic). With this horrific lipaemia, the serum sodium level was only modestly depressed, down to 130mmol/L (when tested in the ABG machine's ion-sensitive electrode, the level was 135mmol/L).
Historical anecdotes of this abound. Ladenson et al (1981) reported on a similar situation, where serum sodium was measured low as an artifact of flame photometry. The authors made a strong case for the increased use of potentiometric sodium measurement, in an era when blood gas machines were probably absurdly expensive and impractically large.
A more specific example can be found in the 1985 article by Howard et al, where six cases of hyperlipaemic pancreatitis are presented. The authors offer a scenario which explains the importance of recognising this possible measurement error. Of the six patient presented in their study, one was mistakenly resuscitated with hypertonic saline, with intracerebrally disastrous consequences.
What if one has no access to a direct sodium-sensing electrode a'la ABG machine? One may be able to correct the erroneous serum sodium measurement if the triglyceride level is known. For instance, Fortgens and Pillay offer the following equation to correct sodium:
Thus, if one's "formal" sodium is 130 mmol/L and the protein level is 60g/L, the equation gives 133.8 mmol/L as the corrected sodium, which is not far from the actual ABG result in the real-life example offered above. This is startling; for every 10g/L increase in triglyceride levels, the sodium is only depressed by about 0.85mmol/L. Thus, a truly preposterous serum triglyceride level of of 100g/L would be required to depress serum sodium by 8.5mmol/L. For further reference, Fortgens and Pillay produce this table of pre-calculated values, including a scenario where a triglyceride level of 30g/L (with a relatively normal plasma protein) gives a measured sodium of 130 mEq/L and a "true" sodium of 133 mEq/L.
Elevated levels of paraprotein have been observed to increase the non-water volume of plasma. Using the same equation as above (and assuming a reasonably normal lipid concentration) one can estimate that for every 10g/L elevation in protein (from zero), the "formal" serum sodium will be depressed by about 0.53 mmol/L. Thus, a patient with multiple myeloma who has a measured sodium of 130mmol/L and a ridiculous serum protein level of 190g/L would have a corrected sodium level of 139mmol/L. Again, vast elevations of protein would be required to generate modest decreases in sodium. Yu et al (2005) report a case of multiple myeloma where the serum protein was about 99g/L, causing the sodium to drop by about 5mmol/L when compared to ABG values.
The issue is complicated by the fact that multiple myeloma paraproteins are strongly cationic, and can give rise to "true" hyponatremia by participating in the balance of electroneutrality (Bloth et al, 1978) In this manner, the presence of cationic paraproteins can give rise to a falsely depressed anion gap (either by "faking" a low sodium or producing a genuinely low sodium). All the various ways in which paraproteins can affect lab results are discussed with great attention to detail in the excellent 2010 article by King and Florkowski.
Protein and lipid are the two main naturally occurring substances that can cause a pseudohyponatremia. Theoretically, however, this could happen in the presence of any large molecules. For example, Yip et al (2006) were able to demonstrate this in samples adulterated with heparin.
Following a TURP using glycine as an non-conducting irrigation fluid, one can fall prey to the so-called "TURP syndrome". This bizarre complication can occur in as many as 5-10% of TURP cases. It is due to absorption of irrigant solution through the distended urethra.
In the course of a trans-urethral prostatectomy, small prostatic veins are cut. To keep the view clear, the irrigant solution needs to pump at pressure higher than venous pressure. This solution is iso-osmolar, but it can't be conductive, or the monopolar diathermy won't work. The solution is made iso-osmolar by addition of glycine or sorbitol. As much as 6 litres of this crap gets infused into the periprostatic veins as the TURP is conducted. The bloodstream is thus inundated with glycine or sorbitol; these act in the same way as high lipids and high paraprotein, confusing the indirect ion-sensing electrode. Additionally, the sorbitol or glycine can act as osmotic agents, expanding the ECF volume.
This is a thing of the past: nowadays, progressive urologists use normal saline to irrigate, and a bipolar diathermy probe which doesn't care how conductive your irrigant is. However, the topic is still dear to the College of Intensive Care Medicine. Question 21 from the second paper of 2009 asked a horrendous amount of detail about TURP syndrome, extending all the way to the metabolytes of glycine and its influence on the metabolism of ammonia. In addition, in Oh's Manual Delaney and Finfer spend about as much ink on TURP syndrome as they did on SIADH and CSW combined, so it must seem important to them in some way. In proportion to this interest from figures of authority, some extra space will be devoted here to TURP syndrome.
Glycine is the smallest possible amino acid, consisting only of two carbon atoms, some hydrogen and the NH2 / COOH groups. According to Wikipedia, it is sweet-tasting. A 1.5% glycine solution (220 mmol/L) is usually the culprit behind TURP syndrome. Risk factors for TURP syndrome include prolonged operation time (over 1 hour), resection of over 60g of prostate tissue, a perforation of the prostate or bladder, and hight irrigation fluid bag pressure (i.e. hanging higher than 70cm above the patient). TURPs are not the only situations where this can happen; irrigation fluid of the same type has also been used in hysteroscopy.
Glycine itself has a well-known toxicity syndrome. The specific constellation of features includes haemodynamic instability, a decreased level of consciousness (with or without seizures) and hyponatremia which is usually iso-osmolar. On top of that, the metabolism of glycine by oxidative deamination can result in a massive excess of ammonia, with its own delirium-generating effects. Traditionally, glycine toxicity causes blindness, but stupour and coma are also common. A good article on this topic has a table (Table 1) which lists other unpleasant CNS manifestations, including dilated unreactive pupils, seziures and paralysis. The table is so good that it is reproduced below with minimal alteration (specifically "coma" and "death" were removed from the bottom of each category).
The abnormalities can develop immediately, or be delayed by up to 24 hours. Other irrigant solutions (eg. sorbitol and mannitol) can also do this, but only glycine causes blindness and hyperammonaemia. It is an inhibitory neurotransmitter in the spinal cord (where its famous antagonist is strychnine) and the retina, and it passes freely into the intracellular compartment. Likely, at least some of the problems from TURP syndrome are due to this molecule.
Glycine toxicity is associated with the following biochemical changes:
"Some candidates appeared to misunderstand the pathogeneses of TURP syndrome and discussed glycine toxicity and hyperammonaemia interchangeably within the answer", remarked the examiners in their comments to Question 6 from the second paper of 2022. From this, the implication is that the college wanted to set aside all the contributions made by glycine, and to focus on the iso-osmolar hyponatremia as the main subject of their question. That is slightly unfair, as "TURP syndrome" usually implies "all the things that happen together as a bouquet of abnormalities", not "just the hyponatremia associated with the TURP procedure". To be clear, the literature is also divided on what contributes most, or in whom. Jensen (1991) and Vijayan (2011) give glycine and ammonia a position of prominence, whereas Gravenstein (1997) and Moorthy et al (2001) deemphasise its role. On balance of things, the CICM trainee should side with the examiners, and mainly consider TURP syndrome in terms of iso-osmolar hyponatremia.