What is a subdural empyema?
A subdural empyema is a collection of pus in the subdural space, the naturally occurring space between the dura and arachnoid mater (outer and middle meningeal layer). This is a consequence of an infection and often spreads from neighboring sites like the paranasal sinuses. Most cases occur within the cranium but can extend to the subarachnoid space of the spinal column. A collection of pus anywhere in the body is known as an abscess and an empyema, which is an accumulation of pus in a body cavity/space, also falls under the definition of an abscess. A subdural empyema may be therefore be categorized as a brain abscess but should be differentiated from a cerebral or cerebellar abscess where the pus collects within the brain parenchyma.
How does pus collect around the brain?
Since the subdural space is a free space, the infection and accumulation of pus can spread rapidly. It is usually unilateral, affecting only one hemisphere, and limited by the dural folds like the falx cerebri and tentorium cerebelli. A subdural empyema should also be differentiated from a subdural effusion, which is an accumulation of sterile fluid around the brain in the subdural space. As the pus collects, it may compress brain tissue and give rise to features of raised intracranial pressure just as with a subdural effusion.
Cerebral edema (swelling of the brain) as a result of inflammation and the possibility of an obstruction in the flow of cerebrospinal fluid (hydrocephalus) may further contribute to pressure on the brain. If the bridging veins that cross the subdural space become occluded (thrombophlebitis), cerebral infarction (death of brain tissue) may then occur further contributing to the disease process. Although a subdural empyema is a serious condition, it is not common and proper treatment can lead to complete recovery. Resolution of the empyema may ultimately contribute to thickening of the dura mater.
Causes of Pus Around the Brain
The causes and risks are similar to those of a brain abscess. Bacteria are by far the most common cause, followed by fungi and protozoa. A subdural empyema may be a complication of meningitis but this is more likely to occur in infants and children. Pathogenic microorganisms may reach the site via various routes including :
- Trauma with a penetrating head injury.
- Post-operatively – after surgery with spread of bacteria from the skin.
- Infiltration from neighboring sites – paranasal sinuses (sinusitis), middle ear (otitis media), mastoiditis or osteomyelitis of the skull.
- Distant sites – dissemination from lungs, heart valves.
- Associated with other infections – malaria, tuberculosis.
Signs and Symptoms of a Subdural Empyema
Symptoms of a subdural empyema are usually a consequence of the raised intracranial pressure, meningitis and the source of infection. It can be more distressing if a cerebral infarction occurs.
Fever, a persistent headache, nausea, vomiting and malaise are often reported at the outset, along with symptoms relating to the source of infection. This is not conclusive for a subdural empyema. Neck stiffness, papilledema, lethargy and confusion or drowsiness should highlight the involvement of the intracranial structures.
Seizures, one-sided weakness, numbness or abnormal sensations may be an indication of the progression of the condition. Blurred vision, difficulty speaking or coma are severe signs and could be an indication of cerebral infarction as well.