Subdural empyema represents loculated infection between the outermost layer of 
the meninges, the dura, and the arachnoid. The empyema may develop 
intracranially or in the spinal canal. Intracranial subdural empyema is most 
frequently a complication of sinusitis or, less frequently, otitis or 
neurosurgical procedures. Spinal subdural empyema is rare and may result from 
hematogenous infection or spread of infection from osteomyelitis. The most 
common organisms in intracranial subdural empyema are anaerobic and 
microaerophilic streptococci, in particular those of the Streptococcus milleri 
group (S. milleri and Streptococcus anginosus). Staphylococcus aureus is present 
in a minority of cases, and multiple additional organisms, including 
Gram-negative organisms, such as Escherichia coli, and anaerobic organisms, such 
as Bacteroides, may be present. Pseudomonas aeruginosa or Staphylococcus 
epidermidis may be present in cases related to neurosurgical procedures, and 
Salmonella species have been detected in patients with advanced AIDS; multiple 
organisms may be present simultaneously. Spinal subdural empyemas are almost 
invariably caused by streptococci or by S. aureus. Subdural empyema--whether it 
occurs in the skull or the spinal canal--may cause rapid compression of the 
brain or spinal cord, and represents an extreme medical and neurosurgical 
emergency. The diagnostic procedure of choice for intracranial and spinal 
subdural empyema is MRI with gadolinium enhancement. Computed tomography scan 
may miss intracranial subdural empyemas detectable by MRI. Conversely, occasion 
spinal subdural empyemas may be detected by CT myelography where MRI is 
negative. Treatment in virtually all cases of intracranial or spinal subdural 
empyema requires prompt surgical drainage and antibiotic therapy. Pus from the 
empyema should always be sent for anaerobic, as well as aerobic, culture. 
Because intracranial subdural empyemas may contain multiple organisms, 
provisional antibiotic therapy of intracranial subdural empyema, where the 
organism is unknown, should be directed against S. aureus, microaerophilic and 
anaerobic streptococci, and Gram-negative organisms. Antibiotics should include 
1) nafcillin, oxacillin, or vancomycin; plus 2) a third generation 
cephalosporin; plus 3) metronidazole. Provisional antibiotic therapy of spinal 
subdural empyemas should be directed against S. aureus and streptococci, and 
should include nafcillin, oxacillin, or vancomycin. Morbidity and mortality in 
intracranial and spinal subdural empyema relate directly to the delay in 
institution of therapy. Both conditions should, thus, be treated with great 
urgency.