A subdural hematoma or subdural haematoma, also known as a subdural haemorrhage, is a
type of hematoma, usually associated with traumatic brain injury.
Blood gathers between the dura mater, and the brain.
Usually resulting from tears in bridging veins which cross the subdural space, subdural hemorrhages
may cause an increase in intracranial pressure, which can cause compression of and damage
to delicate brain tissue.
Subdural hematomas are often life-threatening when acute.
Chronic subdural hematomas, however, have better prognosis if properly managed.
In contrast, epidural hematomas are usually caused by tears in arteries, resulting in
a build-up of blood between the dura mater and skull.
Classification Subdural hematomas are divided into acute,
subacute, and chronic, depending on the speed of their onset.
Acute subdural hematomas that are due to trauma are the most lethal of all head injuries and
have a high mortality rate if they are not rapidly treated with surgical decompression.
Acute bleeds often develop after high speed acceleration or deceleration injuries and
are increasingly severe with larger hematomas.
They are most severe if associated with cerebral contusions.
Though much faster than chronic subdural bleeds, acute subdural bleeding is usually venous
and therefore slower than the typically arterial bleeding of an epidural hemorrhage.
Acute subdural bleeds have a high mortality rate, higher even than epidural hematomas
and diffuse brain injuries, because the force required to cause them causes other severe
injuries as well.
The mortality rate associated with acute subdural hematoma is around 60 to 80%.
Chronic subdural bleeds develop over a period of days to weeks, often after minor head trauma,
though such a cause is not identifiable in 50% of patients.
They may not be discovered until they present clinically months or years after a head injury.
The bleeding from a chronic bleed is slow, probably from repeated minor bleeds, and usually
stops by itself.
Since these bleeds progress slowly, they present the chance of being stopped before they cause
significant damage.
Small chronic subdural hematomas, those less than a centimeter wide, have much better outcomes
than acute subdural bleeds: in one study, only 22% of patients with chronic subdural
bleeds had outcomes worse than "good" or "complete recovery".
Chronic subdural hematomas are common in the elderly.
Signs and symptoms
Symptoms of subdural hemorrhage have a slower onset than those of epidural hemorrhages because
the lower pressure veins bleed more slowly than arteries.
Therefore, signs and symptoms may show up in minutes, if not immediately but can be
delayed as much as 2 weeks.
If the bleeds are large enough to put pressure on the brain, signs of increased ICP or damage
to part of the brain will be present.
Other signs and symptoms of subdural hematoma can include any combination of the following:
Causes Subdural hematomas are most often caused by
head injury, when rapidly changing velocities within the skull may stretch and tear small
bridging veins.
Subdural hematomas due to head injury are described as traumatic.
Much more common than epidural hemorrhages, subdural hemorrhages generally result from
shearing injuries due to various rotational or linear forces.
Subdural hemorrhage is a classic finding in shaken baby syndrome, in which similar shearing
forces classically cause intra- and pre-retinal hemorrhages.
Subdural hematoma is also commonly seen in the elderly and in alcoholics, who have evidence
of cerebral atrophy.
Cerebral atrophy increases the length the bridging veins have to traverse between the
two meningeal layers, hence increasing the likelihood of shearing forces causing a tear.
It is also more common in patients on anticoagulants, especially aspirin and warfarin.
Patients on these medications can have a subdural hematoma with a minor injury.
A further cause can be a reduction in cerebral spinal fluid pressure which can create a low
pressure in the subarachnoid space, pulling the arachnoid away from the dura mater and
leading to a rupture of the blood vessels.
Risk factors Factors increasing the risk of a subdural
hematoma include very young or very old age.
As the brain shrinks with age, the subdural space enlarges and the veins that traverse
the space must travel over a wider distance, making them more vulnerable to tears.
This and the fact that the elderly have more brittle veins make chronic subdural bleeds
more common in older patients.
Infants, too, have larger subdural spaces and are more predisposed to subdural bleeds
than are young adults.
For this reason, subdural hematoma is a common finding in shaken baby syndrome.
In juveniles, an arachnoid cyst is a risk factor for a subdural hematoma.
Other risk factors for subdural bleeds include taking blood thinners, long-term alcohol abuse,
and dementia.
Pathophysiology
Collected blood from the subdural bleed may draw in water due to osmosis, causing it to
expand, which may compress brain tissue and cause new bleeds by tearing other blood vessels.
The collected blood may even develop its own membrane.
In some subdural bleeds, the arachnoid layer of the meninges is torn, and cerebrospinal
fluid and blood both expand in the intracranial space, increasing pressure.
Substances that cause vasoconstriction may be released from the collected material in
a subdural hematoma, causing further ischemia under the site by restricting blood flow to
the brain.
When the brain is denied adequate blood flow, a biochemical cascade known as the ischemic
cascade is unleashed, and may ultimately lead to brain cell death.
The body gradually reabsorbs the clot and replaces it with granulation tissue.
Diagnosis
It is important that a person receive medical assessment, including a complete neurological
examination, after any head trauma.
A CT scan or MRI scan will usually detect significant subdural hematomas.
Subdural hematomas occur most often around the tops and sides of the frontal and parietal
lobes.
They also occur in the posterior cranial fossa, and near the falx cerebri and tentorium cerebelli.
Unlike epidural hematomas, which cannot expand past the sutures of the skull, subdural hematomas
can expand along the inside of the skull, creating a concave shape that follows the
curve of the brain, stopping only at the dural reflections like the tentorium cerebelli and
falx cerebri.
On a CT scan, subdural hematomas are classically crescent-shaped, with a concave surface away
from the skull.
However, they can have a convex appearance, especially in the early stage of bleeding.
This may cause difficulty in distinguishing between subdural and epidural hemorrhages.
A more reliable indicator of subdural hemorrhage is its involvement of a larger portion of
the cerebral hemisphere since it can cross suture lines, unlike an epidural hemorrhage.
Subdural blood can also be seen as a layering density along the tentorium cerebelli.
This can be a chronic, stable process, since the feeding system is low-pressure.
In such cases, subtle signs of bleeding such as effacement of sulci or medial displacement
of the junction between gray matter and white matter may be apparent.
A chronic bleed can be the same density as brain tissue, meaning that it will show up
on CT scan as the same shade as brain tissue, potentially obscuring the finding.
Treatment Treatment of a subdural hematoma depends on
its size and rate of growth.
Some small subdural hematomas can be managed by careful monitoring until the body heals
itself.
Other small subdural hematomas can be managed by inserting a temporary small catheter through
a hole drilled through the skull and sucking out the hematoma; this procedure can be done
at the bedside.
Large or symptomatic hematomas require a craniotomy, the surgical opening of the skull.
A surgeon then opens the dura, removes the blood clot with suction or irrigation, and
identifies and controls sites of bleeding.
Postoperative complications include increased intracranial pressure, brain edema, new or
recurrent bleeding, infection, and seizure.
The injured vessels must be repaired.
Depending on the size and deterioration, age of the patient, and anaesthetic risk posed,
subdural hematomas occasionally require craniotomy for evacuation; most frequently, simple burr
holes for drainage; often conservative treatment; and rarely, palliative treatment in patients
of extreme age or with no chance of recovery.
In those with a chronic subdural hematoma, but without a history of seizures, the evidence
is unclear if using anticonvulsants is harmful or beneficial.
See also
References
External links Subdural Hematoma article - Medscape
Imaging and Mechanism of Subdural Hematoma - MedPix