Subdural Empyema

Subdural empyema is a collection of pus between the dura and the arachnoid. Because the subdural space has no barriers, infection spreads quickly over the convexity and along the falx. Spread is usually from the frontal or ethmoid sinuses or from the middle ear and mastoid, through valveless emissary veins, septic thrombophlebitis or direct erosion of the sinus wall^[1]; other causes include cranial surgery, trauma, meningitis (particularly in infants) and haematogenous spread. In the largest modern series of 699 patients, sinusitis and otitis were the main sources, and most patients were young men^[2]. Streptococci (particularly the Streptococcus anginosus group), anaerobes and staphylococci predominate, and infections are often polymicrobial.

Patients present with fever, headache, signs of meningism and rapidly progressive neurological signs: altered consciousness, hemiparesis, aphasia and seizures. Signs of the source, such as frontal swelling from osteomyelitis (Pott's puffy tumour), sinus tenderness or ear discharge, may be present. Deterioration can be very rapid, and the level of consciousness at presentation is the strongest predictor of outcome^[2].

Contrast-enhanced MRI is the most sensitive test, showing a crescentic or lentiform extra-axial collection with marked restricted diffusion and enhancement of the surrounding membrane. Small collections, particularly along the falx, are easily missed on CT. Imaging also shows the source (sinus or mastoid opacification), cerebritis or abscess, and venous sinus or cortical vein thrombosis. Lumbar puncture is dangerous and usually contraindicated.

Treatment is urgent surgical evacuation, together with high-dose intravenous antibiotics and treatment of the source (often by ENT surgeons at the same anaesthetic). In Nathoo's series, patients treated by craniotomy had better outcomes and fewer repeat operations than those treated by burr holes, which they attributed to more complete evacuation of loculated and thick pus^[2]. Burr holes or limited craniotomy can still be appropriate in critically ill patients or for early, liquid collections.

• A wide craniotomy exposing the full extent of the collection, including the interhemispheric component.
• Gentle irrigation and evacuation without stripping adherent membranes off the cortex.
• Send pus for Gram stain, aerobic and anaerobic culture before antibiotics if possible.
• Do not replace the bone flap if it is infected (or treat the bone as indicated).
• Anticonvulsants are often given; seizures are common.

Mortality has fallen in the CT and MRI era but remains significant, and survivors may have epilepsy and focal deficits. Repeat imaging is needed if the patient fails to improve, because re-accumulation and new collections are common. Antibiotics are usually given for four to six weeks or longer.