Benign Enlargement of the Subarachnoid Space (BESS) is a condition observed in infants characterized by excess cerebrospinal fluid (CSF) accumulating around the brain. The term “benign” indicates the condition is generally self-limiting and does not represent a severe underlying disease. BESS is the most frequent cause of an unusually large head circumference, or macrocephaly, in otherwise healthy babies. This temporary finding is often viewed as a variation of normal development in early infancy.
Defining the Subarachnoid Space Enlargement
The subarachnoid space lies between the pia mater and the arachnoid mater, two of the brain’s protective meningeal layers. This space is filled with cerebrospinal fluid (CSF). In BESS, the subarachnoid space, particularly over the frontal lobes, becomes widened due to CSF accumulation. The enlargement is identified via medical imaging, such as ultrasound or MRI, when the fluid-filled space appears larger than expected for the infant’s age.
Radiologically, the condition is defined by specific measurements. A craniocortical width—the distance between the inner skull and the brain’s surface—greater than 5 millimeters is generally considered widened in infants younger than one year old. This excess fluid collects externally and is not contained within the brain’s internal fluid chambers (ventricles), distinguishing it from internal hydrocephalus.
The mechanism involves the CSF system, which constantly produces and reabsorbs fluid. The most accepted theory suggests a transient imbalance where CSF production temporarily outpaces reabsorption. Reabsorption occurs primarily through arachnoid villi, whose function is thought to be delayed in infants with BESS, leading to a temporary fluid backlog. Crucially, the expansion does not lead to a build-up of pressure inside the skull, meaning BESS is not associated with the severe symptoms of true hydrocephalus.
Identifying Features and Theories of Cause
The primary sign leading to a BESS diagnosis is a rapid increase in head circumference (macrocephaly). This rapid growth typically begins between three and twelve months of age, often causing the head circumference to cross upward over multiple growth percentile lines. The mean age of presentation is around seven months, with head size frequently reaching above the 90th percentile. Importantly, most infants appear healthy and do not exhibit signs of increased intracranial pressure, such as irritability or a bulging soft spot.
Genetic factors often play a role, as a positive family history of macrocephaly is frequently noted. It is common for a close relative to have a larger-than-average head size, suggesting an inherited predisposition. Another theory suggests a temporary mismatch where the skull expands more quickly than the underlying brain tissue. This creates a larger subarachnoid space that is then filled by the normal volume of circulating CSF.
The temporary dysfunction of the arachnoid villi eventually resolves as the child matures. Maturation of these absorption pathways is often complete around eighteen months of age, corresponding to when the fluid collections typically begin to decrease. This physiological delay explains why the condition is limited to the first year or two of life and carries a favorable outcome.
Distinguishing BESS from Serious Conditions
The main goal of imaging is to differentiate BESS from serious causes of head enlargement, such as obstructive hydrocephalus or chronic subdural hematomas. This distinction relies on evaluating imaging characteristics, including the fluid distribution, surrounding brain structures, and presence of mass effect. In BESS, the excess fluid is uniformly distributed CSF that follows the brain’s contours without causing compression. The brain tissue appears normal, and the ventricles are either normal in size or only mildly enlarged.
Obstructive hydrocephalus involves a CSF flow blockage, causing pressure to build up and the ventricles to swell dramatically. This internal pressure pushes on the surrounding brain tissue, a finding absent in BESS. Chronic subdural hematomas, collections of blood products, can sometimes mimic BESS but are fundamentally different.
A key diagnostic feature is the appearance of the small bridging veins that connect the brain surface to the inner skull lining. In BESS, these veins are often visible extending across the widened subarachnoid space, confirming the fluid is in the correct anatomical layer. Subdural collections displace or obscure these veins and may contain septations or have a higher density than pure CSF, indicating blood products. The lack of cortical flattening or midline shift in BESS confirms that intracranial pressure remains normal, separating it from pathological fluid collections.
Monitoring and Expected Outcome
Management of BESS is conservative, focusing on careful monitoring of head growth and neurodevelopmental milestones. The condition is self-limiting and rarely requires medical or surgical intervention. Pediatricians regularly measure head circumference to ensure the growth curve stabilizes, even if it remains above the 95th percentile. Developmental assessments check for temporary delays, particularly in gross motor skills, which are sometimes observed in the first year of life.
The prognosis for infants diagnosed with BESS is excellent, with the vast majority achieving normal neurodevelopmental outcomes. The enlarged subarachnoid spaces generally resolve spontaneously by two years of age as the CSF reabsorption mechanism matures. Surgical procedures, such as shunt placement, are almost never necessary for BESS alone. Follow-up is maintained until resolution, typically confirmed by imaging around eighteen to twenty-four months of age.