Benign enlargement of the subarachnoid space, often abbreviated BESS, is a condition in which extra cerebrospinal fluid accumulates in the space surrounding an infant’s brain, causing the head to grow faster or larger than expected. It is the most common cause of a big head (macrocephaly) in babies and is generally considered harmless and self-limiting, though the word “benign” in the name has sparked debate among specialists because the condition can come with temporary developmental delays and, in rare cases, fluid collections that complicate the clinical picture.
How BESS Typically Looks
The classic scenario goes like this: a baby’s head circumference crosses upward on the growth chart during the first several months of life, sometimes jumping from an average percentile to well above the 95th in just a few weeks. A parent or pediatrician notices the rapid growth, and imaging reveals widened fluid spaces over the surface of the brain, with the brain’s internal fluid chambers (ventricles) looking normal or only slightly enlarged.1PubMed Central. Benign Enlargement of Subarachnoid Space in Infancy: “A Review with Emphasis on Diagnostic Work-Up” One published case report documented a jump from the 50th percentile to above the 95th in a single month.2PubMed Central. Benign enlargement of sub-arachnoid spaces in infancy
Aside from the large head, most babies with BESS look and act fairly normal. The fontanelle (soft spot) may feel full but not tense or bulging, and the infant is usually alert, feeding well, and gaining weight. Some babies show mild delays in sitting up, crawling, or early language, but obvious signs of increased pressure inside the skull, such as persistent vomiting, bulging eyes, or irritability that nothing relieves, are not part of the typical picture.
Why It Runs in Families
One of the most consistent findings across studies is that BESS clusters in families. In one cohort, about 72% of infants with the condition had a family member, usually a parent, with a large head.3PubMed Central. Neurodevelopmental Outcomes of Infants with Benign Enlargement of the Subarachnoid Space An earlier study put that figure even higher, at 88%, and noted that some families had members with frank external hydrocephalus alongside relatives with simple large-headedness spanning multiple generations.4Pediatrics. Idiopathic External Hydrocephalus: Natural History and Relationship to Benign Familial Macrocephaly
This family pattern is one reason many pediatricians ask to measure a parent’s head at the first visit after BESS is suspected. If mom or dad also has a head above the 98th percentile and developed normally, that substantially raises the probability that the baby’s large head is a benign inherited trait rather than something more worrisome. That same study concluded that idiopathic external hydrocephalus is closely related to benign familial macrocephaly and generally resolves without treatment.4Pediatrics. Idiopathic External Hydrocephalus: Natural History and Relationship to Benign Familial Macrocephaly Infants who had both the imaging findings and a parent with macrocephaly showed a characteristic pattern of neuromotor development in the first year and carried a good developmental prognosis overall.5PubMed. Developmental prognosis for infants with benign enlargement of the subarachnoid spaces
How It Is Diagnosed
Ultrasound through the open fontanelle is usually the first imaging step because it is quick, does not involve radiation, and can show the widened fluid spaces clearly. If more detail is needed, MRI is preferred over CT because it avoids exposing a baby’s brain to ionizing radiation. On either type of imaging, the hallmark is prominent fluid over the frontal portions of the brain, with normal or only mildly enlarged ventricles.
There is no single, universally agreed-upon cutoff for “too wide.” A systematic review of subarachnoid space measurements found that in infants without BESS or macrocephaly, the upper range of the cranio-cortical width on ultrasound ran from about 5 mm to roughly 11 mm, depending on the study.6PubMed Central. Subarachnoid space in infancy and early childhood: a systematic review of measurements The lack of standardized diagnostic criteria is a genuine limitation; clinicians often combine the imaging appearance with clinical features like family history, normal neurological exam, and a reassuring growth trajectory to arrive at the diagnosis. Researchers have been working on dedicated BESS growth charts that model head-circumference trajectories over time, with the goal of distinguishing benign macrocephaly from pathologic causes more reliably and reducing unnecessary imaging.7PubMed Central. Development of Benign Enlargement of Subarachnoid Spaces Growth Charts
Developmental Delays and How They Tend to Resolve
This is where parents often have the most anxiety. Roughly a third of infants diagnosed with BESS show some form of developmental delay, and the most common type by far is gross motor delay, such as being late to sit, crawl, or walk. In one large study, about 32% of patients were diagnosed with developmental delay, and among those, 53% had gross motor delay specifically. The reassuring part: gross motor delay resolved in 86% of those children, on average by about 22 months of age.8Journal of Neurosurgery: Pediatrics. Subdural hematoma prevalence and long-term developmental outcomes in patients with benign expansion of the subarachnoid spaces
A smaller, earlier study found similar numbers: developmental delay in about 21% of patients, mostly in gross motor skills, with resolution in 15 out of 20 affected children during follow-up. Fine motor delay, present in a small percentage, resolved in all four children who had it. Verbal delay had a more mixed trajectory: it resolved in two of the four children who had it initially, but was newly detected in six others during the follow-up period.9PubMed. Long-term health status in benign external hydrocephalus That language finding is a reminder that “benign” does not necessarily mean “nothing to watch.” Regular developmental check-ins are worthwhile even when the overall outlook is good.
A small but real proportion of children with BESS are later diagnosed with autism spectrum disorder. In the larger study mentioned above, about 5% received an autism diagnosis during follow-up.8Journal of Neurosurgery: Pediatrics. Subdural hematoma prevalence and long-term developmental outcomes in patients with benign expansion of the subarachnoid spaces Whether the enlarged fluid spaces play a causal role or simply share underlying risk factors with autism is not settled. For parents, the practical takeaway is that ongoing developmental monitoring matters regardless of how reassuring the imaging looks.
The Subdural Collection Problem
One of the more complicated aspects of BESS is that some infants develop thin layers of blood or fluid (subdural collections) between the brain and the skull. The leading theory is mechanical: when the subarachnoid space is wider than normal, the tiny veins that bridge from the brain surface to the skull’s inner lining get stretched. Stretched veins are more fragile, so even very minor bumps or jostling could theoretically cause them to leak.10American Journal of Neuroradiology. Subdural Hematomas in Infants with Benign Enlargement of the Subarachnoid Spaces Are Not Pathognomonic for Child Abuse
Not every researcher agrees with this explanation, though. A finite-element modeling study that simulated the physics of an infant’s head concluded that wider subarachnoid spaces actually had a damping effect, reducing the relative motion between the brain and skull during shaking. That group suggested BESS may not be a risk factor for subdural bleeding at all.11PubMed. Influence of the benign enlargement of the subarachnoid space on the bridging veins strain during a shaking event: a finite element study The disagreement is unresolved, and it makes clinical decision-making harder, especially in scenarios where child abuse is a concern.
Child Abuse Investigations
Subdural blood in an infant is one of the findings that can trigger a child-protection investigation, because it is associated with inflicted head trauma. When a baby with known BESS turns up with a subdural collection on imaging, clinicians face a genuine dilemma: did the fluid arise spontaneously or from trivial trauma because of the underlying condition, or is it evidence of abuse?
Several research groups have weighed in on this. One neuroradiology study concluded that subdural hematomas in children with BESS are “not pathognomonic for child abuse,” meaning they can occur spontaneously or after accidental trauma and should not be treated as automatic proof of abuse.12PubMed Central. Subdural hematomas in infants with benign enlargement of the subarachnoid spaces are not pathognomonic for child abuse Another study reached a similar conclusion, stating that incidental subdural collections in the setting of BESS are not necessarily indicative of abusive head injury.13Journal of Neurosurgery: Pediatrics. Macrocephaly in infancy: benign enlargement of the subarachnoid spaces and subdural collections
At the same time, the presence of BESS does not rule abuse out either. A 2023 review in a pediatric radiology journal cautioned that abuse cannot be safely excluded in infants with BESS and subdural collections without thorough investigation, and recommended that even when initial evaluations look reassuring, full skeletal surveys should be considered.14PubMed Central. Benign enlargement of the subarachnoid spaces and subdural collections-when to evaluate for abuse The medical community has not reached consensus on how aggressively to investigate in every case, and families should be aware that a workup for abuse, if it happens, is not an accusation but a standard part of the clinical evaluation for any infant with unexplained subdural blood.
When Surgery Enters the Picture
The vast majority of infants with BESS never need any surgical intervention. The condition typically resolves on its own as the child grows, usually by age two or three, though some children carry a larger-than-average head permanently without any functional consequence. Treatment, when it happens, is reserved for a minority of cases.
A Norwegian population-based study found that about 28% of children diagnosed with the condition underwent surgery. The most common procedure was placement of a shunt to drain excess fluid, used in about 90% of the surgical cases. Other options included endoscopic fenestration of the fluid spaces and, rarely, subdural-to-peritoneal shunts.15Pediatric Neurology. Epidemiology of Benign External Hydrocephalus in Norway—A Population-Based Study An interesting detail from that study: only about 29% of the surgically treated children had clear signs of raised intracranial pressure. The remaining 71% were treated for other reasons, often persistent rapid head growth or parental and physician concern. This highlights how much clinical judgment plays into the decision.
A review of the surgical literature noted that signs of increased intracranial pressure, such as a bulging fontanelle, vomiting, or sunsetting eyes, are the most accepted indication for shunting. Developmental delay alone has not been reported as a stand-alone reason for surgery in any published series.16PubMed Central. Benign external hydrocephalus: a review, with emphasis on management In borderline cases where the clinical picture is ambiguous, some centers have used direct intracranial pressure monitoring to guide the decision. A small study found that patients whose baseline and average pressures stayed below 10 mmHg were managed with observation alone.17PubMed. Intracranial pressure measurement in infants presenting with progressive macrocephaly and enlarged subarachnoid spaces
The Prematurity Connection
While BESS is overwhelmingly seen in otherwise healthy, full-term babies, premature infants appear to be at elevated risk. A study of 100 children with BESS found that 19% had been born premature, a rate roughly 2.4 times higher than the general prematurity rate. Most of those were late preterm births, and their average age at diagnosis was about 12 months.18PubMed. Evaluation of Association of Prematurity with Benign Enlargement of Subarachnoid Space in Infants Referred for Macrocephaly
Separately, research on very premature infants (born at or before 28 weeks or weighing under about 1,250 grams) has shown that temporary widening of the subarachnoid spaces is common in this group, particularly in those who had respiratory distress and needed mechanical ventilation.19Journal of Diagnostic Medical Sonography. Temporary Enlargement of the Subarachnoid Space in Very Premature Infants In very preterm infants, the widening may reflect immaturity of the fluid-absorption pathways rather than the familial trait seen in typical BESS. Whether these represent the same condition or merely look alike on imaging remains an open question.
Ruling Out Conditions That Mimic BESS
The word “benign” in the name carries a lot of weight, but it only applies once other diagnoses have been excluded. Several conditions can look strikingly similar on imaging yet require very different management. Communicating hydrocephalus, in which fluid accumulates because of impaired absorption rather than a benign anatomical variant, can present with widened subarachnoid spaces and a big head. The difference often becomes clear over time: in true hydrocephalus, the ventricles tend to enlarge progressively, whereas in BESS they stay normal or only mildly dilated.
Certain metabolic disorders also deserve mention. Glutaric aciduria type 1, a rare inherited metabolic disease, can cause widened fluid spaces over the brain in infancy and is sometimes initially mistaken for BESS. Missing this diagnosis has serious consequences because the condition can lead to irreversible brain damage during a metabolic crisis, and early treatment with dietary management and emergency protocols can prevent that. Some specialists advocate for metabolic screening (a urine organic acids test) in any infant whose enlarged subarachnoid spaces lack the reassuring context of a clear family history of macrocephaly and an otherwise normal exam.
Other things on the differential include chronic subdural collections from birth trauma, meningitis that has resolved but left fluid behind, and, in very rare cases, brain atrophy from an underlying neurological condition. The imaging pattern in atrophy tends to look different (the fluid spaces are widened because the brain itself is smaller, not because extra fluid is accumulating around a normal brain), but in borderline cases, follow-up imaging and clinical monitoring are needed to tell the two apart.
What the Naming Debate Tells You
You may encounter different names for this condition depending on when and where the literature was published. “Benign external hydrocephalus,” “benign extracerebral fluid collections,” “idiopathic external hydrocephalus,” and “benign enlargement of the subarachnoid spaces” all refer to essentially the same entity. The variety of terms reflects a decades-long uncertainty about whether this is truly a form of hydrocephalus (implying a problem with fluid drainage) or simply a normal variant in babies who happen to inherit big heads. The current consensus leans toward BESS as the preferred term, partly because it avoids the word “hydrocephalus,” which can alarm parents unnecessarily and implies a pathological process that may not be present.
This naming issue is not purely academic. Parents who search online for “external hydrocephalus” will encounter far more alarming literature than those who search for “benign enlargement of subarachnoid spaces,” even though the two searches often lead to the same condition. If your child has been diagnosed and you are researching it, be aware that the older terminology may overstate the severity. A conversation with the treating pediatrician or pediatric neurologist, with specific reference to the imaging findings and your family history, is more useful than a keyword-driven internet deep dive.