Following a severe brain injury the goal of the clinician is to prevent secondary brain injury. This entails increasing oxygen delivery to the brain by preventing hypoxia and increasing cerebral perfusion. 

• Cerebral Perfusion Pressure (CPP) = Mean Arterial Pressure - Intracranial Pressure 
• To increase CPP, increase MAP (>85 mmHg) and/or decrease ICP (<20 mmHg)

Hyperosmolar therapy, including mannitol and hypertonic saline, is often used to decrease ICP. The goal of these agents is to, in part, decrease brain volume in order to increase blood flow into the closed space of the cranium. A study conducted by Boone et al. reviewed 7 articles comparing mannitol vs. hypertonic saline in severe TBI. The study concluded that both agents are effective at reducing ICP with mixed results regarding superiority.

Another meta analysis conducted by Rickard et al. found similar results with a trend towards favoring hypertonic saline. A limiting factor in both studies is varying methodology between different studies. While there is no clear winner between mannitol and hypertonic saline it is clear that both decrease ICP, thereby increasing CPP and reducing the risk of secondary brain injury.

References:
1. Boone, M.D., Oren-Grinberg, A., Robinson, T.M. et al, Mannitol or hypertonic saline in the setting of traumatic brain injury: what have we learned?. Surg Neurol Int. 2015;6:177
2. Rickard, A.C.; Smith, J.E.; Newell, P.; Bailey, A.; Kehoe, A.; Mann, C. Salt or sugar for your injured brain? A meta-analysis of randomised controlled trials of mannitol versus hypertonic sodium solutions to manage raised intracranial pressure in traumatic brain injury. Emerg. Med. J 2013.