Subdural empyema (ie, abscess) is an intracranial focal collection of purulent material located between the dura mater and the arachnoid mater. About 95% of subdural empyemas are located within the cranium; most involve the frontal lobe, and 5% involve the spinal neuraxis. This article focuses on the intracranial type, which causes clinical problems through extrinsic compression of the brain by an inflammatory mass and inflammation of the brain and meninges.
Subdural empyema is a life-threatening infection that was first reported in the literature about 100 years ago. It accounts for about 15-22% of focal intracranial infections. The mortality rate approached 100% before the introduction of penicillin in 1944 and has declined since that time. Because the symptoms might be very mild initially, rapid recognition and treatment are important; the early institution of appropriate treatment gives the patient a good chance of recovery with little or no neurological deficit.
Subdural empyema is a primarily intracranial infection located between the dura mater and the arachnoid mater. It has a tendency to spread rapidly through the subdural space until limited by specific boundaries (eg, falx cerebri, tentorium cerebelli, base of the brain, foramen magnum). The subdural space has no septations except in areas where arachnoid granulations are attached to the dura mater. Subdural empyema is usually unilateral.
With progression, subdural empyema has a tendency to behave like an expanding mass lesion with associated increased intracranial pressure and cerebral intraparenchymal penetration. Cerebral edema and hydrocephalus also may be present secondary to disruption of blood flow or cerebrospinal fluid (CSF) flow caused by the increased intracranial pressure. Cerebral infarction may be present from thrombosis of the cortical veins or cavernous sinuses or from septic venous thrombosis of contiguous veins in the area of the subdural empyema.
In infants and young children, subdural empyema most often occurs as a complication of meningitis. In such cases, subdural empyema should be differentiated from reactive subdural effusion (ie, sterile collection of fluid due to increased efflux of intravascular fluids from increased capillary wall fenestrations into the subdural space). In older children and adults, it occurs as a complication of paranasal sinusitis, otitis media, or mastoiditis.
Infection usually enters through the frontal or ethmoid sinuses; less frequently, it enters through the middle ear, mastoid cells, or sphenoid sinus. This often occurs within 2 weeks of a sinusitis episode, with the infection spreading intracranially through thrombophlebitis in the venous sinuses. Infection also may extend directly through the cranium and dura from an erosion of the posterior wall of the mastoid bone or frontal sinus. Direct extension also could be from an intracerebral abscess. Rarely, infection spreads hematogenously from distant foci, most commonly from a pulmonary source or as a complication of trauma, surgery, or septicemia. The sphenoid sinus also could be a source of infection.
Subdural empyema accounts for 15-22% of focal intracranial infections. Sinusitis is the most common predisposing factor in the developed world.
In the pre-antibiotic era, the mortality rate approached 100%; this still may be the case in developing countries.
In the developed world, the mortality rate has improved tremendously: it is about 6-35% (variance depending on areas and hospitals); however, about 55% of patients have neurological deficits at the time of hospital discharge.
The mortality rate has continued to decline because of early diagnosis and treatment, more accurate localization with head CT scan, early sinus drainage, and recognition of the prominent role of anaerobes in the disease.
The high incidence of morbidity (ie, neurological deficits) is attributed to the short follow-up period and low mortality rate. Very ill patients who would have died in the past now survive with deficits.
Sex- and age-related demographics
Subdural empyema is more common in males, who can account for up to 80% of cases. The reason for this predominance is unknown. One theory is that the normal development of the paranasal sinuses in males results in anatomic differences that predispose them to recurrent sinusitis.
Subdural empyema can occur at any age, but about two thirds of patients are aged 10-40 years.
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