computed tomography ( ct ) remains the preferred diagnostic method for the chronic subdural hematoma ( csdh)3 ) . chronic sdh has a variety of imaging characteristics in ct ; low , intermediate , or high density relative to brain parenchyma3,6 ) . chronic sdh frequently appears to be mixed density7,9 ) . with current high - resolution ct scanners , acute trauma on the patients with csdh may develop acute bleeding over the csdh , which would produce mixed density10 ) . repeated episodes of acute bleeding may result mixed densities of sdh . in other words , we retrospectively examined the medical records and ct scans of 259 consecutive patients who diagnosed as csdh from january 2006 to december 2011 . we excluded 17 patients who diagnosed by magnetic resonance imaging ( mri ) only before surgery . the csdhs were classified into four groups ; hypodensity ( < 25 hu ) , homogeneous isodensity ( 25 - 35 hu ) , layered type , and mixed type on the basis of ct scans ( fig . although we examined the history of head trauma minutely , the etiology was identifiable in only 122 patients ( 50.4% ) . statistical analysis was performed using the chi - square test or fisher 's exact test . for the statistical significance , we divided the etiology into either known or unknown groups . the density of csdh was isodense in 115 patients , hypodense in 31 patients , mixed in 79 cases , and layered in 17 cases ( table 1 ) . mixed or layered types were more common in the oldest age , while isodense or hypodense sdhs were more common the age of less than 70 years ( p=0.0002 by fisher ) . the csdh was on the left side in 115 patients , on the right side in 70 patients , and bilateral 40 patients ( table 2 ) . bilateral hematomas tended to be hypodense , while isodense one tended to locate on the left side ( p=0.015 by fisher ) . the etiology could be identified in 67.7% of the hypodense hematomas , while the cause was obscure in 59.5% of the mixed hematomas ( p=0.047 by chi - square ) . the preferred surgical technique was single burr - hole drainage under local anesthesia ( table 4 ) . we performed craniotomy for 2 patients with mixed density hematomas and one patient with isodense hematomas . , we used an endoscopy to suck out the semisolid clot around the corner of the hematoma cavity . the density of csdh was isodense in 115 patients , hypodense in 31 patients , mixed in 79 cases , and layered in 17 cases ( table 1 ) . mixed or layered types were more common in the oldest age , while isodense or hypodense sdhs were more common the age of less than 70 years ( p=0.0002 by fisher ) . the csdh was on the left side in 115 patients , on the right side in 70 patients , and bilateral 40 patients ( table 2 ) . bilateral hematomas tended to be hypodense , while isodense one tended to locate on the left side ( p=0.015 by fisher ) . the etiology could be identified in 67.7% of the hypodense hematomas , while the cause was obscure in 59.5% of the mixed hematomas the preferred surgical technique was single burr - hole drainage under local anesthesia ( table 4 ) . we performed craniotomy for 2 patients with mixed density hematomas and one patient with isodense hematomas . , we used an endoscopy to suck out the semisolid clot around the corner of the hematoma cavity . there are some reports that the hypodensity was the most common type in ct scanning3,6 ) . however , isodense csdhs were often reported as more common than the hypodense lesions1,5,7,9 ) . it may depend on the patient population , resolution of the ct scanner , and methods of density classification . mixed density was relatively common , being roughly one third of cases in this study . a layered type of the hematoma density may result from prolonged recumbency , which separates the blood components and fluid8,20 ) . if the patient maintains lying position for a long time , more heavier components sink by the gravity . elderly people are more likely to stay lying down for a long period of declining physical activity . the blood components are separated by gravity and layered density is appeared in the ct scan . theoretically , the mixed density results from three hypotheses . the first situation is in hyper - acute bleeding ; the difference between solid clot and liquid blood may produce the mixed density15 ) . the second situation is in subacute sdhs ; resolving hematoma may appear peripheral hypodensity with central hyperdensity during transitional period9 ) . fibrinolysis in csf is activated from the outside to the inside in order after trauma . the change into the hypodensity is faster in the peripheral region than in the center , where the density of hematoma remains hyperdense for a long time . however , these two situations are relatively short to get the mixed density images in the ct scans . it is well known that the repeated microhemorrhages were responsible for the enlargement of csdh2,11 ) . the bleeding from the sinusoidal vessels of the outer neomembrane makes the hematoma grow without coagulation13 ) . even though the csdh continue to enlarge , it may remain asymptomatic , when the reserving capacity was remained or well balanced . the hematoma pressure was 15 cmh2o or below in a half of unilateral csdhs , especially in the elderly18 ) . although some patients with csdh are still asymptomatic , they are prone to fall or slip down . if they slip , even though the injury itself is trivial , it may tear the cortical bridge veins or fragile vessels in the neomembrane . repeated trauma may cause acute bleeding , which would make a lump or a layer of hyperdensity within hypo- or isodense hematoma . like the repeated microhemorrhages from the outer membrane , repeated trauma may cause acute bleeding over the csdh as a mechanism of hematoma enlargement10 ) . sometimes repeated trivial trauma may cause a subdural hygroma , which became a csdh . although the ages of the sdhs were different , such a chronic - on - chronic sdh may produce the mixed density . membranes were frequently observed within the sdhs , especially in hematomas with the mixed density2,16 ) . the acute - on - chronic sdhs tend to produce so - called multi - lobule type membrane , while chronic - on - chronic sdhs would produce multi - layer type membrane . mixed or layered types were more common in the oldest age , while isodense or hypodense sdhs were more common the age of less than 70 years . this uneven distribution result from not only the fact that the reserving capacity is maximal in the oldest age , but also the oldest patients are vulnerable to repeated trauma . also , elderly patients can not remember their trivial trauma events because of cognitive impairment . the oldest age implies brain atrophy and too high reserving capacity , which may cause overlapping hematomas or recurrence . bilateral hematomas tended to be hypodense , while isodense one tended to locate on the left side , in this study . a definite history of head injury was often obtained only in a half of cases , and more than 90% of previous head injuries were mild12 ) . considering the facts that mixed types were more common in elderly patients and the cause of trauma was obscure in the mixed hematomas , we could deduce mixed types occurred by multiple trivial trauma in elderly people . burr - hole drainage is sufficient for most patients and this became the procedure of choice7,11,14 ) . we could remove the hematoma by single or two burr - holes in most cases . we used double burr - holes with saline irrigation in only a few patients . in patients with mixed density csdh , even in acute - on - chronic sdhs the clot was not so hard due to preexisting hemolytic activity of the csdh . we placed a soft silicon drain in all cases , which was usually removed within 2 days . semisolid clot was usually drained out or resolved within a few days with or without urokinase or tissue plasminogen activator . septation within the hematoma was usually not complete in either multi - lobule or multi - layer hematomas . craniotomy may be necessary for those instances in which the subdural collection reaccumulates , the brain fails to expand , or there is solid hematoma11 ) . mixed density of csdh results from multiple episodes of trauma , especially in the oldest age . the etiology was frequently obscure in the mixed density csdhs , since it was hard to remember all the trivial traumas . although there were membranes within the hematoma , burr - hole was usually enough to drain the hematomas .
objectivedensity of the chronic subdural hematoma ( csdh ) is variable . 
 it often appears to be mixed density . 
 multiple densities of csdh may result from multiple episodes of trauma . 
 we investigated the frequency of mixed density and the causes of head injuries representing each density.methodswe could collect 242 cases of chronic sdh . 
 the csdhs were classified into four groups ; hypodensity , homogeneous isodensity , layered type , and mixed type on the basis of ct scans.resultsthe density of csdh was isodense in 115 patients , hypodense in 31 patients , mixed in 79 cases , and layered in 17 cases . 
 the csdh was on the left side in 115 patients , on the right side in 70 patients , and bilateral in 40 patients . 
 the history of trauma was identifiable in 122 patients . 
 the etiology could be identified in 67.7% of the hypodense hematomas , while it was obscure in 59.5% of the mixed hematomas.conclusionmixed density of csdh results from multiple episodes of trauma , usually in the aged . 
 it is hard to remember all the trivial traumas for the patients with the mixed density csdhs . 
 although there were membranes within the mixed density hematomas , burr - holes were usually enough to drain the hematomas .