Neuroradiology Case of the Week

Case 22

Ramon de Guzman, MD, and P-L Westesson, MD, PhD, DDS

Clinical Presentation: The patient is a 60-year-old male with bilateral frontal burr holes for bilateral subdural hematoma. There is evidence of infection of the right burr hole.

Radiographic Findings: There is a large right-sided extra axial fluid collection, overlying the right posterior frontal and parietal lobes, which contains an air-fluid level. The fluid appears hypointense on T1WI, hyperintense on T2WI, FLAIR, and DWI, and presents with ring enhancement on post-gadolinium administration. It produces mass effect over the posterior right frontal and parietal cortex, effaces the adjacent sulci and displaces the cortex medially.

Figure 1: Axial T1WI shows hypointense subdural fluid collection along the right fronto-parietal convexity (small arrows) with minimal air-fluid level (arrowhead).	Figure 2: The right subdural fluid collection is hyperintense in axial T2WI (arrows).
Figure 3: Axial post-contrast T1WI shows ring enhancement of the right subdural fluid collection (arrow) with associated meningeal enhancement.	Figure 4: Axial diffusion weighted images shows increased signal of the right subdural fluid collection (arrow) with low apparent diffusion coefficient.

Diagnosis: Subdural Hematoma.

Discussion:
Clinical Discussion
   Subdural and epidural empyemas are uncommon, accounting for approximately 20% to 33% of all intracranial infections [1]).Nearly half of all cases are caused by sinusitis ; the frontal sinus is the most common source [2]. Postcraniotomy infection accounts for another 30%. Between 10% to 15% of empyemas are complications of meningitis. Remaining cases are related to previous head trauma. Empyema carries an 8% to 12% mortality rate [2]).
   Subdural empyema (SDE) should be considered a neurosurgical emergency, and neuroimaging is essential to early diagnosis. When empyema results from sinusitis or mastoiditis it is often associated with seizures, focal deficits and rapid neurological deterioration, progressing from obtundation to coma [3]. Empyemas that occur secondary to prior trauma or surgery are usually more indolent clinically. Mortality associated with SDE now approaches 10% [3], compared with approximately 40% in the pre-CT era [4].

Neuro Imaging Discussion
   Currently MRI is the study of choice because the presence and extent of extra-axial collections are better defined by MR [1]. The increased sensitivity and specificity of MR are due to direct multiplanar imaging, increased contrast resolution, and the absence of artifact from bone. SDE is lentiform or crescentic in shape while epidural empyema (EDE) is typically lentiform. It is possible to differentiate SDE from EDE when the latter is continuous across the midline and/or when there is a hypointense rim on both T1 and T2-weighted images, representing medially displaced dura, seen at the interface between a collection and the brain [1].
   SDE is most commonly located over the cerebral convexity and is frequently bilateral. A paratentorial location is the least common [5]. On T1-weighted images convexity and interhemispheric SDE appear hypointense relative to brain and hyperintense to CSF. On proton density, T2- and diffusion-weighted images, the collections are hyperintense to brain and isointense relative to CSF. These signal change are typical of proteinaceous fluid with T1 and T2 values intermediate between gray matter and CSF. A surrounding membrane that enhances intensely and uniformly following contrast administration is typically identified [6]. Cortical vein thrombosis and venous infarction and cerebritis or abscess may ensue.

References:

1. Weingarten K, Simmerman RD, Becker RD et al.: Subdural and epidural empyemas: MR imaging, AJNR 10:81-87, 1989
2. Chanalet S, Gense de Beaufort D, Greselle JF et al: Clinical and radiological aspects of extracerebral empyemas: 39 cases, Neuroradiol 33(suppl):225-228, 1991
3. Sze G, Zimmerman RD. The magnetic resonance imaging of infections and inflammatory diseases. Radiol Clin North Am 1988;26:839-859
4. Nelson JD, Watts CC, Calcified subdural effusion following bacterial meningitis. Am J Dis Child 1969;117:730-733
5. Lee SH. Infectious diseases. In: Lee SH, Rao KCVG, eds. Cranial Computed Tomography. New York: McGraw-Hill Book Co;1983:505-546
6. Tsuchiya K, Makita K, Furui S et al: Contrast-enhanced magnetic resonance imaging of sub- and epidural empyemas, Neuroradiol 34;494-496, 1992