• When treating patients with severe brain injury, a major goal 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 (MAP) - Intracranial Pressure (ICP). To increase CPP, one must either increase MAP and/or decrease ICP. When ICP is elevated, hyperosmolar therapy (i.e., mannitol and hypertonic saline), is used to decrease ICP by decreasing the brain volume and hence the ICP. 
• 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

• 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
• 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 randomized controlled trials of mannitol versus hypertonic sodium solutions to manage raised intracranial pressure in traumatic brain injury. Emerg. Med. J 2013.