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A 28-year-old woman with a three-year history of epilepsy was evaluated with intracranial monitoring, which consisted of two subdural strip electrodes and eight depth electrodes. On the third day of monitoring, she developed a headache but no focal signs or symptoms. What is your diagnosis? Elucidation on page 346.


SURGICAL DIAGNOSIS: Subdural hematoma not affecting brain parenchyma


Figure 1. There are two 5 x 2 subdural strip electrodes present.

Figure 2. There is a subdural strip electrode present. There is metallic artifact present, as well as a small extra-axial hyperdensity most consistent with a fluid collection. The ventricles are symmetric, and there is no midline shift.

Figure 3. There is an extra-axial ovoid hyperdensity present adjacent to the subdural strip electrode. This represents fluid below the subdural strip electrode. The ventricles are asymmetric, and there is a 2 mm midline shift. Due to there being a change in the imaging findings, this fluid most likely represents a subdural hematoma.

Figure 4. There is a relative flattening of the electroencephalogram, which confirms the presence of fluid below the subdural strip electrode.


Brain imaging is typically ordered in the immediate postoperative period to help determine the location of the subdural grid electrodes, which helps guide future resection. (1) When additional imaging is acquired, it is usually to investigate neurological compromise. The differential diagnosis for extra-axial masses on CT is the following: acute subdural hematoma; epidural hematoma; foreign body; meningioma; meningeal metastases; dural sinus thrombosis; cortical venous thrombosis; neurosarcoidosis; lymphoma; tuberculosis; dural A-V fistula; and rarely, extramedullary hematopoiesis; leukemia; venous varix; hemangiopericytoma; and malignant tumor. (2) Of this differential diagnosis, since we know the grid is in the subdural space, the diagnosis is almost certainly subdural hematoma.

Radiologically, all patients develop fluid collections underneath the subdural electrodes. (3) Clinically significant hematomas are a dreaded complication of intracranial monitoring with subdural strip and grid electrodes. Large published series of patients with intracranial monitoring have shown a risk of 0-3% (3% in three series, 0% in one). (4-7) The most common signs and symptoms of hematomas include headache, aphasia, and decreased level of consciousness. (3,7,8) Headache is a very common occurrence in the immediate postoperative state and can persist. Headaches that appear days after implantation are the ones that are concerning for hematoma formation. One study of patients with intracranial monitoring looked at risk factors for complications, (6) but after Bonferonni correction for multiple comparisons, none of the risk factors for complications reached statistical significance.

The presence of a fluid collection does not seem to be related to risk of clinically significant hematoma formation. (3,8) The presence of ventricular asymmetry was correlated with clinically significant hematoma formation in one study. (3) The presence of midline shift but not the amount of shift was correlated in one study (3) but not in another. (8) Since size of the normally-occurring fluid collection and associated midline shift may not be good predictors of clinically significant hematoma, we would like to suggest that a change in the shape of the fluid collection may be important. This is the first case of which we are aware that a documented change in the shape of the fluid collection associated with clinical signs can be found in a subdural hematoma at surgery.


(1.) Morris K., O'Brien T.J., Cook M.J., et al. A Computer-Generated Stereotactic "Virtual Subdural Grid" to Guide Resective Epilepsy Surgery. Am J Neuroradiol 2004;25:77-83.

(2.) No author given. Extra-axial Spaces and Subarachnoid Cisterns. In: A.G. Osborne, K.R. Moore, K.L. Saltzman(eds.) Expert Differential Diagnosis: Brain and Spine. Salt Lake City, Utah: Amirsys, Inc, 2009: I-4-1 to I-4-79.

(3.) Albert G.W., Dahdaleh N.S., Reddy C. et al. Postoperative radiographic findings in patients undergoing intracranial electrode monitoring for medically refractory epilepsy. J Neurosurg 2010;112:449-454.

(4.) Wyler A.R., Walker G., Somes G. The morbidity of long-term seizure monitoring using subdural strip electrodes. J Neurosurg 1991;74:734-737.

(5.) Rydenhag B., son Silander H.C. Complications of Epilepsy Surgery after 654 Procedures in Sweden, September 1990-1995: A Multicenter Study based on the Swedish National Epilepsy Surgery Register. Neurosurgery 2001;49:51-57.

(6.) Hamer H.M., Morris H.H., Mascha E.J., et al. Complications of invasive video-EEG monitoring with subdural grid electrodes. Neurology 2002;58:97-103.

(7.) Fountas K.N. and Smith J.R. Subdural Electrode-Associated Complications: A 20-Year Experience. Sterotact Funct Neurosurg 2007;85:264-272.

(8.) Mocco J., Komotar R.J., Ladouceur A.K., et al. Radiographic Characteristics Fail to Predict Clinical Course After Subdural Electrode Placement. Neurosurgery 2006;58:120-125.

David S. Gloss, MD; Jay Varma, MD; Laura Lehnhoff, MD; Harold Neitzschman, MD, FACR, FACNM, FAAP

Dr. Gloss is the Director of Intraoperative Monitoring at Barrow Neurological Institute in Phoenix, Arizona. Drs. Varma and Lehnhoff are Epileptologists at Barrow Neurological Institute in Phoenix. Dr. Neitzschman is a Professor of Radiology at Tulane University Health Sciences Center in New Orleans.
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Title Annotation:Radiology of the Month
Author:Gloss, David S.; Varma, Jay; Lehnhoff, Laura; Neitzschman, Harold
Publication:The Journal of the Louisiana State Medical Society
Article Type:Case study
Date:Nov 1, 2013
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