Subdural Empyema Workup

Updated: Oct 28, 2022
  • Author: Segun Toyin Dawodu, JD, MD, MS, MBA, LLM, FAAPMR, FAANEM; Chief Editor: Niranjan N Singh, MBBS, MD, DM, FAHS, FAANEM  more...
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Laboratory Studies

See the list below:

  • CBC count may show a toxic leukocytosis.

  • Erythrocyte sedimentation rate (ESR) may be elevated.

  • Blood should be cultured for aerobic and anaerobic organisms.

  • Preoperative tests should include electrolytes, BUN, liver function tests, and CBC count if surgical intervention is being considered.


Imaging Studies

Cranial MRI is now the imaging study of choice, being superior to cranial CT scan in outlining the extent of subdural empyema and demonstrating the convexity and interhemispheric collections.

MRI also shows greater morphological detail than CT scan.

The sensitivity of MRI is improved by using gadolinium contrast medium. See the image below.

MRI scan of a subdural empyema in the left parieta MRI scan of a subdural empyema in the left parietal area.

Cranial CT [8] scan was the standard technique for quick diagnosis before the advent of MRI. The use of high-resolution, contrast-enhanced CT scan increases diagnostic yield, although it sometimes gives equivocal or normal results.

On CT scan, subdural empyema shows as a hypodense area over the hemisphere or along the falx; the margins are better delineated with the infusion of contrast material. Cerebral involvement also is visible.

Cranial osteomyelitis may be seen.

CT scan is the modality of choice if the patient is comatose or critically ill and MRI is not possible or is contraindicated. See the image below.

CT scan of a subdural empyema in the left temporal CT scan of a subdural empyema in the left temporal/parietal area.

Cranial ultrasound [9] has been helpful in differentiating subdural empyema from anechoic reactive subdural effusion in infants with meningitis accompanied by complex features (eg, increased echogenicity in the convexity collections, presence of hyperechoic fibrinous strands or thick hyperechoic inner membrane, and increases in echogenicity of the pia-arachnoid).


Other Tests

See the list below:

  • Preoperative - ECG, chest radiograph

  • Studies to define causes - Chest radiograph for pulmonary source, CT scan of paranasal sinuses and mastoid cells, sputum culture, nasal drip culture



Lumbar puncture is currently contraindicated because of possible cerebral herniation from increased intracranial pressure.

Lumbar puncture may be performed in the course of a workup to rule out meningeal infection when increased intracranial pressure has been excluded.

CSF examination [10] is an adjunctive test in the diagnosis of subdural empyema and may be obtained in addition to the other diagnostic tests previously outlined. CSF findings include the following:

  • WBC count (predominantly polymorphonuclear neutrophils) is increased. A significant increase (> 50/µL) may be seen, although a slightly elevated cell count of 5–20/µL (reference range, 0–5/µL) does not rule out the possibility of subdural empyema.

  • Increased protein level greater than 100 mg/dL may be seen (reference range, 20–40 mg/dL), although less substantial elevations (50–90 mg/dL) do not rule out the possibility of subdural empyema.

  • Decreased glucose levels of 40 mg/dL or less usually are seen (reference range, 50-80 mg/dL). CSF glucose levels should be normalized with a blood glucose level obtained concurrently.

  • Occasionally, the CSF is normal and sterile in these cases.

  • The specific CSF findings should be compared with the accepted normal values of the treating physician's laboratory.