Intracranial Epidural Abscess Workup

Updated: Nov 12, 2014
  • Author: Tarakad S Ramachandran, MBBS, MBA, MPH, FAAN, FACP, FAHA, FRCP, FRCPC, FRS, LRCP, MRCP, MRCS; Chief Editor: Niranjan N Singh, MBBS, MD, DM, FAHS, FAANEM  more...
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Workup

Laboratory Studies

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  • Findings from routine laboratory tests are not diagnostic but are essential in the preparation of the patient for operation. These tests may reveal polymorphonuclear (PMN) leukocytosis and an elevated erythrocyte sedimentation rate (ESR).
  • Results of blood cultures may be positive.
  • Hyponatremia has been reported in approximately 30% of cases.
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Imaging Studies

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  • Neuroimaging narrows the potential diagnoses and enables prompt empirical therapy until a specific microbiological diagnosis is made.
  • Radiography of the skull may demonstrate the responsible sinusitis, mastoiditis, or osteomyelitis.
  • Before the advent of CT scanning, cerebral angiography was often required. Cerebral angiography demonstrated an avascular mass that displaced the dural sinuses away from the inner table of the skull.
  • CT scanning of the brain without enhancement is often used as a screening tool in the assessment. Abscess appears as a poorly defined lentiform area of low or intermediate density (see the image below). CT scanning can also show bony destruction and fragmentation in patients with underlying mastoiditis. When contrast is administered, the convex inner side of the low-density lesion becomes enhanced and appears as rim enhancement caused by the inflamed dura.
    CT scan showing lenticular-shaped intracranial epi CT scan showing lenticular-shaped intracranial epidural abscess.
  • Because MRI is free from bony artifacts and easily demonstrates fluid collections outside the brain, it is the diagnostic procedure of choice to delineate a cranial epidural abscess.
  • Epidural fluid is observed as higher signal intensity than the ventricular cerebral spinal fluid (CSF) on both T1- and T2-weighted MRI. Use of gadolinium can significantly enhance the dura on T1-weighted MRI. MRI is also useful for visualizing small fluid collections that can be missed by CT scanning and in differentiating postoperative abscesses from hematomas or sterile effusions. MRI is particularly useful in differentiating subdural empyema from cranial epidural abscess. The characteristic MRI abnormality includes a crescentic or lentiform fluid collection overlying the hemisphere or in the interhemispheric fissure, which is mildly hyperintense relative to the CSF on T1-weighted images and isointense to CSF on T2-weighted images. A hypointense medial rim, representing the displaced dura is very characteristic of cranial epidural abscess. See the images below.
    Intracranial epidural abscess. Enhanced MRI of the Intracranial epidural abscess. Enhanced MRI of the brain, axial section, revealing a left temporal epidural abscess with an abscess cavity and a thickened enhancing capsule. Adjacent thickened dura enhances as well. In addition, mass effect is evident.
    Intracranial epidural abscess. A coronal section o Intracranial epidural abscess. A coronal section of the MRI revealing a left temporal epidural abscess with an abscess cavity and a thickened enhancing capsule. Adjacent thickened dura enhances as well. In addition, mass effect is evident.
    Intracranial epidural abscess. MRI of the brain, u Intracranial epidural abscess. MRI of the brain, unenhanced. A T1-weighted image (axial view) showing a left temporal epidural abscess with an abscess cavity, surrounding capsule, and the thickened dura underneath. Mass effect is evident.
  • The vein of Labbe may masquerade as an epidural abscess. Recognition of the vein of Labbe on CT scan is therefore essential for the appropriate management of otological and neurotological disease.
  • Kraus et al present a 12-month-old male with acute coalescent mastoiditis and a subperiosteal abscess. [12] An epidural abscess was suspected on preoperative CT scan. No abscess was found on surgery. Based on the surgical finding, they determined that this misdiagnosis was due to a vascular variant, the occipitotemporal vein (OTV, vein of Labbe) that masqueraded as an abscess on the CT scan. The OTV runs in an anterior-to-posterior direction along the lateral surface of the left temporal lobe and drains into the transverse sinus near its junction with the sigmoid sinus. It can be recognized on unenhanced MRIs as a prominent flow void apposed to the lateral aspect of the temporal lobe, and is readily demonstrated on MR and computed tomographic (CT) venographic images and on cerebral arteriograms obtained during the venous phase of enhancement.
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Other Tests

Lumbar puncture carries the risk of precipitating herniation in the setting of increased ICP. Risks and benefits should be carefully weighed before a decision is made to proceed with a spinal tap. Findings on CSF studies can often be unremarkable, with reference range glucose and protein levels. CSF pressure may be increased. Spinal fluid may contain excess cells that are usually polymorphonuclear cells. The cell count is usually less than 200 cells, but it can be as high as 7000/mm3. Protein may be elevated as much as 100 mg/dL and the glucose level is often within the reference range unless associated meningitis is present, in which case it may be decreased.

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