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  • Kevin M. Rice, MD

Pseudo-Subarachnoid Hemorrhage due to Diffuse Cerebral Edema

Updated: Jul 26, 2021

"Found Down" • Xray of the Week 2016 • Week #37

This 53 year old male known abuser of oxycodone was found down. Urine was positive for opiates and benzodiazepines.

Arterial blood gas results:

pH 6.89 (N = 7.38 - 7.42),

pCO2 126 mmHg (N = 38 - 42),

pO2 19 mmHg (N = 75 - 100),

O2 sat 24% (N = 94 - 100)

What are the arrows pointing to? Why this appearance?

Pseudo-Subarachnoid Hemorrhage due to Diffuse Cerebral Edema

Figure 1: CT Scan of the brain. A & B. high attenuation in the basal cisterns and subarachnoid spaces. C. Diffuse edema with loss of grey and white matter differentiation.

The history compatible with anoxic brain injury and the diffuse cerebral edema on the CT scan should indicate that the findings are not due to subarachnoid hemorrhage (SAH). The increased attenuation in the basal cisterns and subarachnoid spaces is due to diffuse cerebral edema related to anoxic encephalopathy. The high attenuation appearance may be due to a combination of diffuse decreased attenuation of the brain as a result of edema, and the subarachnoid space with less low attenuation CSF and a larger proportion of meninges and blood vessels than usual.

Differential diagnosis of pseudo-subarachnoid hemorrhage on CT:


•Artifacts on CT: anoxic encephalopathy, Spontaneous intracranial hypotension

•Iatrogenic causes: recently administered intrathecal or IV contrast material; and following endovascular procedures such as aneurysm coiling and stroke intervention.

Highly proteinaceous material in the subarachnoid space causes increased attenuation which may be due to purulent fluid seen with meningitis or leptomeningeal carcinomatosis, mimicking SAH.

Anoxic encephalopathy and spontaneous intracranial hypotension both cause a perceptual error due to relatively higher attenuation of the basal cisterns related to the low attenuation seen with these conditions.

Iodinated contrast in the subarachnoid space either due to IV or intrathecal administration causes high attenuation in the subarachnoid space which should be easily differentiated from SAH if the history is known. Likewise, endovascular procedures may result in leakage of iodinated contrast which could be confused with SAH, especially given that the procedures are often done for patients with SAH or aneurysms. Follow up CT scan within 3-4 hours after the procedure will show clearing of the contrast, whereas SAH will persist.


1. Carrie P. Marder, et al. Subarachnoid Hemorrhage: Beyond Aneurysms. American Journal of Roentgenology. 2014;202: 25-37. 10.2214/AJR.12.9749

2. Mendelsohn DB, Moss ML, Chason DP, Muphree S, Casey S. Acute purulent leptomeningitis mimicking subarachnoid hemorrhage on CT. J Comput Assist Tomogr 1994; 18:126–128

3. Tsuchiya K, Katase S, Yoshino A, Hachiya J. FLAIR MR imaging for diagnosing intracranial meningeal carcinomatosis. AJR 2001; 176:1585–1588

4. al-Yamany M, Deck J, Bernstein M. Pseudo-subarachnoid hemorrhage: a rare neuroimaging pitfall. Can J Neurol Sci 1999; 26:57–59

5. Given CA, Burdette JH, Elster AD et-al. Pseudo-subarachnoid hemorrhage: a potential imaging pitfall associated with diffuse cerebral edema. AJNR Am J Neuroradiol. 2003;24 (2): 254-6. AJNR Am J Neuroradiol (full text)

6. Yuzawa H, Higano S, Mugikura S et-al. Pseudo-subarachnoid hemorrhage found in patients with postresuscitation encephalopathy: characteristics of CT findings and clinical importance. AJNR Am J Neuroradiol. 2008;29 (8): 1544-9.

7. Zhang J1, Li Q1, Zhang Z2, Sun X1. Pseudo-subarachnoid hemorrhage in a patient with hypoxic encephalopathy. Neurochirurgie. 2015 Feb;61(1):35-7.

Kevin M. Rice, MD

Kevin Rice, MD serves as the Medical Director of the Radiology Department of Valley Presbyterian Hospital in Los Angeles, California and is a radiologist with Renaissance Imaging Medical Associates. Dr. Rice has made several media appearances as part of his ongoing commitment to public education. Dr. Rice's passion for state of the art radiology and teaching includes acting as a guest lecturer at UCLA. In 2015 Dr. Rice launched Global Radiology CME to provide innovative radiology education at exciting international destinations, with the world's foremost authorities in their field.

Follow Dr. Rice on Twitter @KevinRiceMD

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