Benign enlargement of the subarachnoid space in infancy

Benign enlargement of the subarachnoid spaces in infancy (BESS or BESSI) also known as benign external hydrocephalus (BEH), is as the name implies, a benign enlargement of subarachnoid spaces seen in infancy. It usually involves the frontal lobe subarachnoid spaces, and it is characterised clinically by macrocephaly or frontal bossing.


  • most common cause of macrocephaly (macrocrania) 9
  • more common in males than in females
  • in the majority of cases, there is a family history of macrocephaly

Clinical presentation

Often discovered incidentally or when patients undergo neuroimaging in the workup of macrocephaly. These patients are usually neurologically normal and have no history of trauma.


The aetiology is not clear. It has been described as a variation of normal development of the brain, whereby there is a transient accumulation of cerebrospinal fluid in the frontal region. It is also thought to be due to the delayed development or delayed function of the arachnoid villi at the sagittal sinus.


Radiographic features

Ultrasound, CT and MRI imaging of the brain may all demonstrate the characteristic findings seen in BESS:

  • widening of the bifrontal and anterior interhemispheric CSF spaces
    • no consensus on cut-off values exists 5
    • findings should be correlated with patient age 5,7
      • an estimated equation for both interhemispheric width (IHW), craniocortical width (CCW) and sinocortical width (SCW) has been suggested 7
      • IHW >5 mm in neonates
      • IHW >8.5 mm in 1 year olds
  • no flattening of adjacent gyri
  • CSF space follows the gyral contour
  • usually normal sulci posteriorly
  • the anterior fontanelle is frequently enlarged with the enlargement of the subarachnoid space in the frontoparietal regions
  • normal ventricular size, no pressure effects on the surrounding brain tissue, no cerebral atrophy
  • no blood products on MRI study
  • another key distinction between benign enlargement of the subarachnoid spaces and a subdural fluid collection, is that in the former the cortical veins will be adjacent to the inner table of the calvarium on MR and ultrasound; whereas in the latter the veins are displaced away from the inner table, as the arachnoid membrane and subarachnoid space are displaced


While these findings are benign in many cases, there is an increased risk of subdural haemorrhage, either spontaneously or following minor trauma. Subdural hematoma in a patient with BESS should not be interpreted as suggestive of non-accidental injury without other stigmata 4

A low percentage of patients may develop communicating hydrocephalus, which may warrant treatment.

Treatment and prognosis

The condition resolves spontaneously by the age of 2 years. Although the macrocephaly may persist, the subarachnoid space fluid collection will resolve or become minimal as the child grows older.

Studies show that infants with macrocephaly or rapid head-growth, CT findings of enlarged subarachnoid spaces, normal-to-minimally increased ventricular size and who have a parent with macrocephaly, have a good developmental prognosis and a characteristic pattern of neuromotor development in the first year.

A review of the literature has described that although most children with external hydrocephalus do well, a substantial number show temporary or permanent psychomotor delay. 

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Article Information

rID: 13118
Section: Pathology
Synonyms or Alternate Spellings:
  • BESS
  • Benign subdural effusions of infancy
  • Benign macrocephaly of infancy
  • Benign subarachmoid space enlargement
  • Benign external hydrocephalus of infancy
  • Benign external hydrocephalus
  • BEH
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