What Is Cavum Septum Pellucidum in the Brain?

A cavum septum pellucidum is a small, fluid-filled space between the two thin membranes that make up the septum pellucidum, the partition sitting between the brain’s left and right lateral ventricles. It forms during normal fetal development and is present in virtually every unborn baby. In most people the two membranes fuse together after birth, but imaging studies consistently find that a residual space persists in a majority of healthy adults, making the CSP one of the most common incidental findings on brain MRI. The structure has drawn attention far beyond its humble anatomy because it keeps turning up in research on head trauma, psychiatric illness, and prenatal brain development, though its clinical significance depends heavily on its size and context.

How It Forms and Why Most People Still Have One

During early fetal life, the septum pellucidum develops as two separate sheets of tissue. A fluid-filled gap between them is a normal part of brain formation and can be seen on prenatal ultrasound from roughly 18 weeks onward. After birth, the two leaves usually begin to fuse from back to front, gradually closing the space. The traditional teaching was that the cavity disappears entirely in most adults, but modern high-resolution MRI tells a different story.

A 2025 study using detailed manual segmentation found a residual CSP in about 96% of young adult brains, compared with only 57% when standard automated software was used on the same subjects.¹ Older MRI studies using lower-resolution scans reported prevalence figures in the range of 68–80%, depending on the age group and imaging technique.² One study of both schizophrenia patients and healthy controls found a CSP in about 75% of each group, with no meaningful difference between them.³ The upshot is that having a CSP is the norm, not the exception. The cavity is nearly universal when imaging resolution is high enough to detect it. What matters clinically is not whether you have one, but how large it is and what other findings accompany it.

The CSP and Its Neighboring Cavities

The CSP is sometimes confused with two related midline fluid spaces. Directly behind the CSP sits the cavum vergae, a continuation of the same gap that extends further back beneath the corpus callosum. Even further back and lower lies the cavum veli interpositi, a space between the layers of the membrane called the tela choroidea. All three are fluid-filled developmental remnants, and all three can show up on brain imaging.

MRI studies have classified several distinct morphologies for each of these cavities based on how the surrounding structures like the fornix and corpus callosum relate to the space.⁴ One study found a cavum vergae in roughly 22–39% of subjects across different age groups.⁵ These cavities occasionally get lumped together in older literature, sometimes under the informal (and anatomically incorrect) label “fifth ventricle.” They are not true ventricles because they do not communicate with the brain’s ventricular system in healthy individuals, but the nickname persists in casual clinical use.

Why Prenatal Ultrasound Focuses on the CSP

The CSP occupies an outsized role in obstetric imaging. During a standard second-trimester anatomy scan, the sonographer is expected to visualize the CSP as one of the required checkpoints for normal fetal brain anatomy.⁶ Seeing a normal-looking CSP on ultrasound provides reassurance that the midline forebrain structures have developed properly.⁷

When the CSP cannot be found on prenatal ultrasound, it raises a flag. Non-visualization of the CSP is associated with a range of brain malformations including agenesis of the corpus callosum, septo-optic dysplasia, holoprosencephaly, schizencephaly, and acquired fetal brain injury.⁸ In plain terms, the CSP is easy to spot and sits in the middle of a group of structures that all form around the same stage of brain development. If the CSP is absent, the odds increase that something went wrong with one of those neighboring structures. That said, isolated absence of the septal leaves without any other anomaly is considered a rare but normal variant.

Interestingly, the CSP can be informative when it is abnormally large, too. Research has found that fetuses with 22q11.2 deletion syndrome, a genetic condition that can cause heart defects and developmental delays, tend to have wider-than-expected CSPs on mid-pregnancy ultrasound compared to controls.⁹ Because the CSP is already part of the standard anatomy scan, measuring its width could theoretically help flag this condition without adding any extra imaging steps. That idea is still being evaluated, but it illustrates how this tiny cavity punches above its weight in prenatal medicine.

Head Trauma, Contact Sports, and an Enlarged CSP

The link between the CSP and head injury has been recognized since the 1960s, when researchers first described the combination of an enlarged CSP and cognitive decline in retired boxers with what was then called dementia pugilistica. The proposed explanation was that repeated blows to the head tore or stretched the thin septal membranes, allowing the cavity to expand. Over the decades, that connection has been extended from boxing to American football and other contact sports, and it remains one of the most studied aspects of CSP pathology.

A study of retired American pro-football players found that 94% had a CSP graded at 2 or higher on a severity scale, compared with just 18% of age-matched controls. The average length of the CSP in the players was roughly 10.6 mm versus 1.1 mm in the control group.¹⁰ Findings from the larger DIAGNOSE CTE Research Project confirmed that former football players had greater CSP presence and size than unexposed controls, with professional players showing a bigger effect than those who played only at the college level. Rotational forces from cumulative head impacts correlated with increased CSP size.¹¹

A 2024 study in older adults broadened the picture beyond athletes. Participants with a history of both repetitive head impacts and traumatic brain injury had roughly three times the odds of having a CSP compared with those who had neither. Those meeting criteria for traumatic encephalopathy syndrome, a clinical diagnosis linked to chronic brain damage from repeated head trauma, had over eleven times the odds. The strongest effects appeared in people with a history of playing American football, and a CSP length cutoff of about 6 mm showed modest ability to distinguish people with long exposure to repetitive head impacts from those without.¹²

The practical value of an enlarged CSP as a stand-alone marker for chronic traumatic encephalopathy remains limited, however. Because some degree of CSP is present in most people, and because CTE can currently only be definitively diagnosed at autopsy, an enlarged CSP on a brain scan is better understood as one piece of a puzzle than as a diagnosis in itself. Its presence in a former contact-sport athlete adds weight to a clinical picture, but it does not prove that CTE is occurring.

Psychiatric Conditions and Large CSP

Researchers have long wondered whether an unusually large CSP could serve as a visible marker of abnormal brain development that predisposes someone to psychiatric illness. The CSP has been studied in schizophrenia, bipolar disorder, major depression, PTSD, and substance use disorders. The results are more nuanced than early reports suggested.

In schizophrenia, a meta-analysis pooling data from multiple MRI studies found that a large CSP (typically defined as 6 mm or longer) was significantly more common in people with schizophrenia spectrum disorders than in healthy controls, with an odds ratio of about 1.59. However, the presence of any CSP, regardless of size, was not more frequent in schizophrenia than in the general population.¹³ A separate study confirmed this pattern, finding a CSP in roughly equal proportions of schizophrenia patients and healthy controls, with an abnormally large CSP being the more meaningful finding when it appeared.¹⁴ In that study, patients who did have a large CSP performed worse on tests of verbal learning and memory, hinting at a possible link between the structural variant and specific cognitive difficulties.

For bipolar disorder, a meta-analysis found that having a CSP of any size was about twice as common in people with bipolar disorder compared with healthy controls.¹⁵ An older MRI study also reported higher CSP prevalence in bipolar patients compared to controls, while no CSP was observed in patients with major depression.¹⁶ When large CSP was compared across conditions, schizophrenia showed a stronger association than mood disorders.¹⁷

The emerging picture is that a small CSP means very little psychiatrically. It is the large version that has a modest statistical association with certain conditions. Even then, the overlap with healthy populations is enormous, and plenty of people with large CSPs have no psychiatric diagnosis at all. Clinicians do not use the CSP to diagnose mental illness, but its presence in a psychiatric patient can sometimes be noted as consistent with a neurodevelopmental origin for their condition.¹⁸

What Influences Whether a CSP Persists or Enlarges

Both genetic and environmental factors seem to play a role in determining whether the septal leaves fuse or remain open. A twin study of Vietnam combat veterans and their identical twin brothers found that an abnormal CSP appeared in both the combat-exposed twin with PTSD and the unexposed twin, at higher rates than in twin pairs without PTSD. This pattern points toward a familial, possibly heritable vulnerability factor rather than something caused by the traumatic experience itself.¹⁹

On the environmental side, prenatal exposures including alcohol, tobacco smoke, and maternal infections have been proposed as contributors to incomplete fusion of the septal leaves.²⁰ In short, the CSP’s persistence or size is not determined by a single cause. It likely reflects a combination of genetic predisposition and prenatal environment, with postnatal head trauma capable of widening it further in some people. This multifactorial origin is part of what makes the CSP such a frustrating biomarker: the same finding can arise from very different pathways.

When a CSP Becomes a Problem on Its Own

In the vast majority of people, the CSP is clinically silent. It sits between the ventricles, does nothing noticeable, and is found only incidentally on imaging done for another reason. Rarely, though, the CSP can become a symptomatic cyst. This happens when fluid accumulates under pressure within the cavity, causing the septal walls to bow outward and compress the surrounding structures, including the lateral ventricles and the veins that drain nearby brain tissue.

Symptomatic CSP cysts are defined by lateral bowing of the walls with the membranes separated by 10 mm or more.²¹ Symptoms can include headaches that do not respond to standard medication, visual disturbances, and sometimes hydrocephalus caused by the cyst blocking the normal flow of cerebrospinal fluid. The condition is rare enough that case reports still get published when one is encountered.

When a symptomatic CSP cyst requires treatment, endoscopic fenestration is the standard approach. A neurosurgeon uses a small camera-guided instrument to make an opening in the cyst wall, allowing the trapped fluid to drain into the adjacent ventricle. Studies have found this to be a safe procedure with generally good results.²² In children, even cases without frank hydrocephalus have been successfully treated with neuronavigation-assisted fenestration through a single-sided approach.²³ Neurocognitive testing before and after surgery is considered important for evaluating outcomes, since the symptoms of a CSP cyst can be subtle and overlap with other conditions.

Does the CSP Affect Children’s Development?

Because the CSP has been linked, at least statistically, to various psychiatric and neurological conditions in adults, parents whose child is found to have a persistent CSP sometimes worry about developmental consequences. A large population-based study in school-aged children addressed this directly. The researchers found no relationship between the presence of a CSP and cognitive function, emotional problems, or behavioral difficulties.²⁴ Their conclusion was straightforward: in the general pediatric population, the CSP is a normal structural brain variation without clinical implications.

This finding is worth emphasizing because it directly addresses a common source of parental anxiety. If a child’s brain MRI, obtained for headaches or another routine reason, mentions a CSP, the report is describing something present in the majority of children. By itself, it does not predict learning difficulties, mental illness, or behavioral problems.

Incidental Findings and What to Do With Them

Brain MRI is increasingly common, not just in clinical care but in research settings where healthy volunteers undergo scanning. One study of young volunteers found that a CSP was among the incidental findings noted, appearing more often in males and showing a statistical association with psychosis-related symptoms in that particular sample.²⁵ Findings like these can create a feedback loop: a person gets a brain scan for a research study, the report mentions a CSP, and the person spends weeks worrying about a structure that was almost certainly present since before they were born.

The challenge for radiologists is knowing when to mention the CSP and when to let it go unremarked. A small or average-sized CSP in an adult or child is typically not worth flagging. A large one, especially in the context of other midline abnormalities, may warrant a note and possibly follow-up imaging. The 6 mm threshold that keeps appearing in the psychiatric literature has become an informal dividing line between “unremarkable variant” and “worth mentioning,” though it was never established as a formal clinical cutoff. In prenatal imaging, the calculus is different: the CSP’s presence or absence, and its width, are both considered meaningful parts of the fetal anatomy evaluation, for all the reasons covered above.

If you or someone you know has been told they have a cavum septum pellucidum on a brain scan, the overwhelming likelihood is that it is a normal finding. Mention it to your doctor if you have questions, but treat it with the same level of concern you would give to any other common anatomical variant, which is to say, very little unless there are symptoms that need explaining.