There are numerous reported ways to treat chronic subdural hematomas (CSDH) and practice is still differing considerably between departments. Except for a recent randomized controlled trial (RCT) that found that postoperative subdural drainage was better than no drain, there is no higher level evidence. Another recent RCT did not replicate these findings, but the study was severely underpowered.

Aim of this population based study is to compare clinical results (reoperation rates, complications, perioperative death, and survival) between neurosurgical departments treating CSDH with different treatment policies.

Inclusion Criteria:

• All patients treated with evacuation of primary chronic subdural hematoma(CSDH) from 2005 through 2010 at St.Olav University Hospital
• All patients treated with evacuation of primary chronic subdural hematoma(CSDH) from 2005 through 2010 at University Hospital North Norway
• All patients treated with evacuation of primary CSDH from 2006 through 2010 at Karolinska University Hospital (not 2005 due to practical reasons since major changes occurred in electronic surgery protocols 2005).

Exclusion Criteria:

• Chronic subdural hematoma in arachnoid cyst(s)
• Previous CSDH surgery
• External hydrocephalus (hydrocephalus with cerebrospinal fluid (CSF) in the subdural space rather than in the ventricles)
• CSDH due to previous intracranial surgery (within 6 months).

The patients have been selected to the given therapy solely on the basis of geography. All patients treated with evacuation of primary chronic subdural hematoma(CSDH) from 2005 through 2010 at St.Olav University Hospital and UNN, and primary CSDH from 2006 through 2010 at Karolinska University Hospital (not 2005 due to practical reasons since major changes occurred in electronic surgery protocols 2005).