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42         SA JOURNAL OF RADIOLOGY • September 2006

Often one is confronted with an extracerebral fluid col-
lection/effusion when interpreting computed tomogra-
phy (CT) of the brain in children. To determine whether 
the effusion is subarachnoid or subdural, a few basic 
principles should be kept in mind.

Subdural effusions are crescent-shaped,1 especially 
over the frontal and parietal lobes, with the vessels 
‘pasted’ against the cerebral surface (Fig. 1).2 The inter-
hemispheric fissure anteriorly can be V-shaped or 
asymmetrical, as the hemisphere is displaced away from 
the falx at an angle (Figs 2a and b).2

Subarachnoid effusions emphasise the gyral outlines 
as the subarachnoid spaces enlarge, especially over the 

Subarachnoid or subdural effusion?

M Pienaar, BSc, MB ChB
A Brandt, MB ChB 

G Dekker, MB ChB, MMed (Fam)  
D van der Merwe, MB ChB 

S Hlongwane, MB ChB
S I Dajee, MB ChB, DA (SA), MMed (Rad D)

Department of Radiology, Tygerberg Hospital and  

Stellenbosch University

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Fig. 1. Contrasted CT brain showing enhancing dura and meninges, with 
the contrast-filled vessels (arrows) compressed against an effaced cerebral 
surface in the presence of a subdural effusion.

Figs 2a and b. CT studies illustrating the brain surfaces at the interhemi-
spheric fissure pushed away by subdural fluid from the flax, either bilaterally 
(a) or unilaterally (b). Note marked effacement of the gyri and sulci. 

a

b

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43         SA JOURNAL OF RADIOLOGY • September 2006

frontal lobes and interhemispheric fissure when the 
patient lies supine (Fig. 3). It is important to note sym-
metry of the subarachnoid effusion.2  

Clinical relevance of this determination is in the evalu-
ation of collections in non-accidental injury (which are 
often hypodense – indicating chronic effusions). This is 
also important in pyogenic meningitides with subdural 
collections,3 where these must be distinguished from 
the normal subarachnoid spaces in infants less than 1 
year of age.2

1.   Kirkwood JR. Essentials of Neuroimaging, 2nd ed. New York: Churchill Livingstone, 1995: 378.
2.    Barkovich AJ. Pediatric Neuroimaging, 3rd ed. Philadelphia: Lippincott Williams & Wilkins, 2000: 606-

609.
3.   Barkovich AJ. Pediatric Neuroimaging, 3rd ed. Philadelphia: Lippincott Williams & Wilkins, 2000: 371.

 

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Fig. 3. Uncontrasted CT study illustrating normal frontal enlarged subarach-
noid spaces in an infant. The frontal lobes stay parallel to the interhemi-
spheric fissure with subarachnoid effusions, in contrast to subdural effusions. 
The surface gyri and sulci are prominent, whereas in a subdural collection 
these would be effaced.   

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