Benign enlargement of the subarachnoid spaces (BESS)

Case contributed by Salvo Fedele


Neuroimaging in the workup of macrocephaly. The anterior fontanelle was enlarged and pulsatile. Two symmetric protrusions were evident laterally in the frontoparietal region. A prominent venous pattern was seen widely in the head. Neurologically normal and have no history of trauma.

Patient Data

Age: 6 months
Gender: Female

Growth chart


Macrocephaly was evident at the first visit (age 1 month). Two cranial ultrasounds were inconclusive at ages 2 and 4 months. The anterior fontanelle was pulsatile at age 5 months.

Patient was scanned for RMI at 6 m old


On T2-Weighted sequences water, including CSF, is bright. Here you can appreciate the normal sulcus posteriorly and the normal subarachnoid space posteriorly by contrast to the enlargement of the subarachnoid space in the frontoparietal regions and to the wide sulcus anteriorly

T1 Inversion Recovery produces more pronounced grey-white matter differentiation than regular T1-weighted images. Here you can appreciate the widening of the bi-frontal and anterior interhemispheric CSF spaces without flattening of adjacent gyri.

X - widening of bifrontal subarachnoid space
S - widening of the anterior sulcus (note the normal posterior sulcus)

Follow-up at 24 months old


Follow-up: The child is now 24 months old. Macrocephaly is very evident. She is neurologically normal, The anterior fontanelle is open but not pulsatile. The ocular fundus is normal. A new MRI would be scheduled in close temporal proximity to a substantial reduction of the anterior fontanelle.

Case Discussion

Although usually in the BESS the macrocephaly persists beyond 2 years of age, the subarachnoid space fluid collection becomes minimal as the child grows older 1.

In the case of a child with enlargement of the subarachnoid spaces who does not fit the expected normal pattern of development, or in whom the finding does not resolve after 18-24 months of age, further investigations may be warranted. Several genetic conditions, e.g. glutaric aciduria type I, can feature enlargement of the subarachnoid spaces as either an early or an associated finding on neuroimaging. 

In the case of this girl with a normal pattern of development and normal neurological examination, depending on the future clinical data the neurosurgeon will decide whether redo an MRI.

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