Hello -
First off, a brief introduction to subdurals:
Subdural hematomas are the result of blood accumulation beneath the
dura (the outermost layer of the covering of the brain, the meninges,
and the arachnoid, the middle layer of the covering of the brain).
This bleeding is usually from veins which course in the subdural space
and is most often associated with trauma, in particular shear stress
which pulls the veins away (they are anchored to the dura which is
attatched to the skull) from the brain (which floats in the
cerebrospinal fluid). This shear stress is often seen in
motor-vehicle accidents, falls, boxing injuries, and shaken-baby
syndrome. Anticoagulation (which does not start the bleeding, but
presumeably keeps small bleeds from stopping on their own) whether
from drugs (coumadin, plavix, asprin, ethanol) or from medical
conditions (cirrhosis, etc) is associated with subdural hematomas.
Increased age, and other factors causing atrophy of the brain,
increase the risk of subdural bleeds by increasing the tension on the
subdural veins. Other shifts of the brain (such as by rapidly
expanding/contracting masses, hydrocephalus, etc) may also produce
subdural bleeds by stressing the bridging veins.
A subdural is classified as acute within 1-4 days of the bleed,
subacute between 4-21 days, and chronic after 21 days. This
terminology has more to do with the appearance of the bleed on CT/MRI
scans than with the cause of the bleed.
Chronic subdurals are known for their tendency to rebleed, even when
treated, with rebleed chances between 20 and 40%
(http://uscneurosurgery.com/classroom/science/pathophysiology/topics/trauma.htm).
Chronic subdurals are associated with hypertension (along with
cardiovascular diseases, likely due to the anticoagulants given to
treat these disorders, alcoholism, and diabetes):
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9173674&dopt=Abstract
Despite this association, there is no known link between hypertension
directly causing chronic subdural hematomas - teleologically it seems
less likely due to the site of bleeding (low pressure venous system)
versus the site of most of the pressure in hypertension (the arterial
system). Hypertension is associated with other types of arterial
hemmorhages in the brain (most commonly intraparenchymal bleeds in the
basal ganglia, but also can be associated with acute subdural
hematomas from arterial venous malformations).
A distinct possibility in the case of chronic subdural hematomas is
that a chronic subdural, which may have been quite small to begin
with, expands through a series of rebleeds until it is compressing a
significant amount of the brain. This type of compression can lead to
clinical signs of a "midline shift" and "herniation" (shifts of the
normal brain across the midline of the skull and down towards the
spinal cord respectively) - which in combination reduce cerebral
vascular flow. This reduction in blood flow to the brain triggers
compensatory reflexes in the body which increase the blood pressure -
thus, a chronic subdural might also be the cause of hypertensive
episodes, particularly in the ED. A case like this is summarized
here:
http://www.learningfile.com/learning_file/viewcase.php?section=nu&case_num=810
Additionally, the compression of blood vessels by the shifting brain
may lead to strokes throughout various brain regions (encompassing
most of the brain).
An excellent article from eMedicine.com on subdural hematomas can be
found here, although it may not be terribly approachable without some
medical knowledge:
http://www.emedicine.com/neuro/topic575.htm
A more approachable series of articles from healthcentral.com,
subdurals : http://www.healthcentral.com/mhc/top/000781.cfm
herniation: http://www.healthcentral.com/mhc/top/001421.cfm
Please let me know if you would like further explanation.
synarchy |
