Osmotherapy is a crude but effective means of desiccating the brain parenchyma and decreasing its contribution to intracranial pressure. This issue is interesting to the time-poor exam candidate because hypertonic saline has already been a subject of an SAQ (Question 4 from the first paper of 2007). In the more distant past (Question 8 from the first paper of 2001) the college demanded that you compare and contrast hypertonic saline with mannitol. It is only a matter of time before they do it again.
An excellent overview of this is afforded by Diringer et al (2013). In brief, osmotherapy works by the following mechanism:
The history of osmotherapy stretches back to the early 20th century, when early cat-haters injected cats with hypertonic saline and noted that their thecal sacks had become flaccid, making it difficult to acquire CSF from them. Mannitol became popular later- if urea causes such splendid osmotic effects, then why not mannitol, Shenkin argued. Indeed, his patients demonstrated a rise in serum osmolality by 20-30 mOsm/L, and a fall in ICP by 30-60%. The physiological consequences of infusing 20% mannitol are discussed elsewhere, and the ICU fellowship candidate should be at least dimly aware of them.
The BTF guidelines (4th edition) are suprisingly ambivalent towards this practice. Their previous Level II and III recommendations were not carried forward from the previous edition because they were based on studies which do not meet their new (stricter) criteria. The BTF committee acknowledged the utility of this strategy (they were "universal in their belief" that hyperosmolar agents are useful) but did not make an actual recommendation in support of their use.
As far as published literature goes, the BTF statement is good enough as an overview of the reasons behind their non-recommendation. For an even larger overview of the evidence, one can look to the excellent 2017 review by Witherspoon et al.
Good question. Let us face it, with the silliness of glycerol and urea behind us, these two are the main contenders. We might appeal to the authority of higher beings for an answer. Indeed, the old 3rd edition of the Brain Trauma Foundation guidelines will impress the casual reader with the steel of their firm conviction.
" Mannitol is effective in reducing ICP", they say, but as for hypertonic saline, "Current evidence is not strong enough". That was in 2007. What about now?
Well. The (2016) publication of the BTF Guidelines was again unable to make a firm recommendation in favour of one agent over another. Sure, many low-quality case control and observational studies support hypertonic saline. But to make recommendations on such weak foundations would undermine the quality of the BTF package as a whole, and discolour all the other recommendations with a blush of shame.
A 2008 trial ran the two substances head to head in an equiosmolar contest. Both substances "equally and durably reduced the ICP" in a case series of 20 stable TBI patients. The conclusion of the investigators was still in favour of mannitol, given the proposed "rheological effects" on the quality of blood flow. The belief in these effects is reasonably well founded in serial measurements of blood viscosity,which revealed that red cell "deformability" increases with mannitol therapy, thereby improving the passage of these soft flexible red cells through the microcirculation.
A 2011 review of all available literature on this matter, however, had scraped together some support for hypertonic saline, finding a small statistically significant benefit for its use among a slush of poorly designed heterogenous trials. Given that little extra evidence became available in the subsequent 2 years, a 2013 review reached the same conclusion.
Overall, it seems the trend these days is away from the mannitol, and towards the saline. Specifically, today's public seem to favour super-high osmolarity saline solutions, such as 20% saline. So, what are the advantages of saline over mannitol?
The use of hypertonic saline has been much maligned by allegations of platelet function impairment and increased risk of bleeding. Fortunately, one is unlikely to ever see these effects in a realistic clinical setting. Studies have demonstrated that one would need to replace 10% of their blood volume with hypertonic saline before one experiences any of these effects.