Victims of trauma with severe head injury who have bled into their brains are at high risk of developing blood clots in their legs. These blood clots can break off and travel through the bloodstream to the lungs causing death. Blood thinners can be given to patients to prevent blood clots from developing but this can leave patients at risk for additional bleeding in the brain causing further damage or death. The earlier blood thinners are started the more effective they are at preventing blood clots but some patients with severe head injury who have bled into their brains will develop further bleeding even if they do not receive blood thinners. Even though a growing body of research has shown that the majority of bleeding in the brain stops within the first 24 hours after injury and that it is safe to start blood thinners as early as 24 hours after injury, doctors are still waiting longer than 4 days to start blood thinners in these patients over concerns of worsening bleeding. In Canada, almost half of the patients with severe head injury do not receive blood thinners until at least five days after injury. Delays in starting blood thinners appear to put patients at increased risk of developing blood clots, unnecessarily. This study will compare the benefits of starting low-molecular weight heparin (LMWH), a type of blood thinner, early (less than 48 hours) versus the current practice (waiting until the 5th day after being injured) in preventing blood clots in patients who have bled into their brains after severe head injury. The investigators believe that starting LMWH earlier will be more effective in preventing blood clots without worsening any bleeding when compared to waiting to start blood thinners. This study is called OPTTTICH (Optimal Timing of Thromboprophylaxis in Traumatic IntraCranial Haemorrhage) and will be the largest Canadian investigator-initiated randomized control trial on blood clot prevention in trauma patients with severe head injury who have bled into their brains.

Inclusion Criteria:

1. Multi-system trauma patients referred to the trauma service with a non-progressing tICH documented on 24-hour repeat head CT scan

Exclusion Criteria:

1. Less than 16 years of age
2. Unexpected to survive or to remain in hospital >72 hours
3. Known malignancy under active care at time of admission
4. Known DVT, PE, or other condition requiring anticoagulation at time of admission
5. Coagulopathy (defined as international normalized ratio (INR) values >1.5 times the upper limit of normal, or partial thromboplastin time (PTT) values >1.5 times the upper limit of normal) at 24 hours after admission
6. Platelet count <75 x 109/L at 24 hours after admission
7. Bilateral lower limb amputation
8. History of allergy to heparin or suspected or proven HIT
9. Limitation of life support or palliative care
10. Prior enrolment in this trial or currently in a confounding randomized trial
11. Pregnancy
12. Study drug (LMWH or placebo) not administered within 36-48 hours post-injury
13. Grade V liver or splenic injuries that have not received definitive care (e.g. embolization, surgical intervention) within 36-48 hours after injury
14. Persistent intracranial pressure >20 mm Hg
15. Spinal subdural haematoma or spinal epidural haematoma
16. Intracranial haemorrhage progression on 24-hour repeat CT head scan