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 per the name, a benign enlargement of the subarachnoid spaces in infants. 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 majority of cases, there is a family history of macrocephaly

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.

Ultrasound, CT, and MRI 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
  • anterior fontanelle is frequently enlarged, with enlargement of the subarachnoid space in 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 calvaria on MRI and ultrasound; whereas in the latter the veins are displaced away from the inner table, as the arachnoid membrane and subarachnoid space are displaced (see cortical vein sign)

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. 

While these findings are benign in many cases, there is an increased risk of subdural haemorrhage, either spontaneously or following minor trauma. Subdural haematoma 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.

  • chronic subdural haematoma / non-accidental trauma
    • MRI may be useful to confidently exclude blood products, which would manifest as collections which do not follow CSF signal on all sequences
    • collection may exert mass effect (flattening) of the cerebral contour
  • cerebral atrophy
    • associated with small head circumference
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Article information

rID: 13118
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
  • BESSI
  • Benign macrocrania of infancy

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