Imaging of the septum pellucidum: normal, variants and pathology

Abstract

The septum pellucidum is a largely neglected anatomical midline structure during post-natal neuroimaging interpretation. Conversely, it is one of the anatomical landmarks used on pre-natal ultrasound to access normal midline formation. Because of its importance during the pre-natal period, the awareness of its primary malformative abnormalities is much higher than its disruptive acquired pathologies, often leading the misinterpretation. In this article, we will review the normal septum pellucidum formation, anatomy, and anatomical variants and will describe the imaging findings in primary malformative and secondary disruptive abnormalities affecting the septum pellucidum.

Introduction

The septum pellucidum is a midline structure composed of two thin membranes (leaflets) located between the anterior horns of the lateral ventricles. During fetal life, the leaflets of the septum pellucidum are separated by a cerebrospinal fluid (CSF) space called the “cavum septum pellucidum”. Lack of visualization of the cavum septum pellucidum during an anatomical prenatal screening ultrasound (between 18 and 22 weeks of gestation) raises the concern for additional midline abnormalities. An absent septum pellucidum can either represent a primary malformative abnormality, with or without associated commissural abnormalities, or a secondary disruptive etiology. Callosal agenesis and septo-optic dysplasia are well-known abnormalities associated with an absent septum pellucidum. However, these two associations are only a small portion of the potential abnormalities associated with an abnormal septum pellucidum. Our aim of this review article is to describe the normal formation, anatomy, anatomical variants, and the spectrum of abnormalities that are associated with an abnormal septum pellucidum. Our goal is to improve the understanding of this anatomical structure and decrease the rate of misinterpretations related to septum pellucidum abnormalities.

Normal formation

The septum pellucidum formation starts at 10–12 weeks of gestation and continues until approximately 17 weeks of gestation. It originates embryologically from the “lamina reuniens”, which is a primitive commissural plate at the rostral wall of the telencephalon, formed as a thickening of the superior lamina terminalis. The lamina reuniens is also responsible for the formation of the forebrain commissures (anterior commissure, corpus callosum, hippocampal commissure) and the forniceal columns. The septum pellucidum develops as the inter-hemispheric fissure deepens and divides the lamina reuniens at the midline, with its lateral walls forming the septum pellucidum leaflets, and the CSF gap in between forming the cavum septum pellucidum et vergae. ^1–3

Normal anatomy

The septum pellucidum is a thin translucent double membrane midline structure extending craniocaudally from the inferior surface of the corpus callosum superiorly, to the body and columns of the fornix inferiorly Figure 1 A–C). Its width varies from 1.5 to 3.0 mm. Both leaflets of the septum pellucidum are lined laterally by ependymal cells, forming the medial wall of the anterior horn of the lateral ventricles. Each leaflet is made of a thin layer of gray matter, a thin layer of white matter and an inner pial layer facing the contralateral leaf. ⁴ Its vascular supply is provide from medial lenticulostriate branches of the anterior cerebral arteries, and its surface is coursed by two to three veins that connect with the choroid plexus veins. ^1,5

Figure 1.

A–C. Normal SP anatomy on post-contrast T ₁ weighted sequences of a 10-year-old patient. (A) Axial view showing the normal juxtaposed leaflets of the SP (arrow). (B) Coronal view showing the normal juxtaposed leaflets of the SP (arrow) and inferiorly the forniceal columns (arrowheads). (C) Sagittal midline view showing the septum pellucidum (yellow) as a triangular-shaped structure located below the corpus callosum and above the forniceal columns (arrowheads) and forniceal body (arrows). SP, septum pellucidum.

During fetal life, the two leaflets of the septum pellucidum are separated by CSF, forming a space known as “the cavum septum pellucidum et vergae” (Figure 2A–C). This space constitutes an extra pial space that does not communicate with the ventricular system or the subarachnoid space. The two leaflets of the septum pellucidum start fusing at approximately 6 months of gestation in a caudal to rostral direction such that the posterior portion (the cavum vergae) is normally closed at birth in term neonates and the anterior portion (the cavum septum pellucidum) is closed by 3–6 months of age ¹ (Figure 3A). However, this closure may occur within a wide range of time, as further discussed under anatomical variants.

Figure 2.

Fetal cavum septum pellucidum at vergae. (A) Coronal MRI of a 22 weeks gestation fetus demonstrating the two leaflets of the septum pellucidum (arrows) and the CSP (*). (B) Axial MRI of a different 22 weeks fetus showing the two leaflets of the septum pellucidum (arrows), the CSP and the CV. (C) Sagittal MRI of a 35 weeks fetus demonstrating the CSP (red) and CV (green). Note the forniceal column (arrow) and the body of the fornix (arrowhead). The forniceal column is the landmark separating the CSP (anterior) and the CV (posterior). CSP, cavum septum pellucidum; CV, cavum vergae.

Figure 3.

A. Axial steady-state T ₂ weighted sequence of a 3-day-old patient demonstrating a persistent CSP without a CV. Note the forniceal columns (arrows) which make up the posterior margins of the CSP. (B) Axial T ₂ weighted sequence of a 5-day-old patient demonstrating a persistent CSP (red) and a persistent CV (green). (C) Coronal view of a head ultrasound showing the SP leaflets (arrows) and the CSP (*). CSP, cavum septum pellucidum; CV, cavum vergae; SP, septum pellucidum.

Whether in the adult or pediatric population, when the two leaves are fused, the septum pellucidum appears as a single thin midline structure that is vertically oriented and forms the internal walls of the frontal horns of the lateral ventricles (Figure 1A–B). Seen in the lateral projection, it appears as a triangular structure (Figure 1C).

Function

The function of the septum pellucidum is poorly understood. It appears to be more than just an anatomical bridge between the corpus callosum and the fornix. It has important connections with the hippocampus and hypothalamus, and therefore, is part of the limbic system. The septal nuclei, a group of neurons that form the septum verum just below the septum pellucidum proper, have a role in memory, consciousness, sleep and emotional responses to the environment. ¹

Absence of septum pellucidum has variable prognosis depending on the presence of associated anomalies. Symptoms may include learning difficulties, behavioral changes, seizures, and psychiatric disorders. Literature suggests that isolated agenesis of septum pellucidum tends to have a better neurodevelopmental outcomes than complex agenesis of septum pellucidum, where additional abnormalities are present. ⁶

Anatomical variants

When the leaflets of the septum pellucidum fail to completely fuse after birth, the cavum of the septum pellucidum et vergae can persist into adult life. The space anterior to the forniceal columns is the cavum septum pellucidum (Figures 2C and 3B) and the space posterior to forniceal columns is the cavum vergae (Figure 2C). These two cavities communicate through the columns of the fornix. When the forniceal columns are very close, the communication is narrowed and this narrow communication has been termed the aqueduct septi. ⁷ As the cavum vergae normally closes before the cavum septum pellucidum, when persistent it is usually seen in association with a cavum septum pellucidum, although rarely it can be seen in isolation. ⁸

Persistence of the cavum septum pellucidum in both the pediatric and adult population is generally considered a normal anatomical variant although some studies have reported association with developmental delay and psychiatric disorders, such as schizophrenia. ^9–11 Also, an enlarged cavum septum pellucidum has been described in association with chromosomal abnormalities such as the trisomies. ¹²

In the pediatric population, the cavum septum pellucidum is normally seen in all premature infants, in 85% of full-term neonates, and 12% of children between 6 months and 16 years of age. ¹³

The reported prevalence of persistent cavum septum pellucidum and cavum vergae in adults varies based on the definition used and the modality of diagnosis.

Cavum septum pellucidum

This space is anatomically defined anteriorly by the genu of the corpus callosum, superiorly by the anterior body of the corpus callosum, posteriorly by the columns of the fornix and inferiorly by the rostrum of the corpus callosum and the anterior commissure (Figure 2C). The lateral walls, consisting of the two leaflets of the septum pellucidum, are parallel and less than 10 mm apart (Figure 2A–B).

Cavum septum pellucidum cyst

A cyst of the septum pellucidum should be suspected when the cavum septum pellucidum is larger than 10 mm in transverse diameter with lateral bowing of the septal leaflets ¹ (Figure 4). This may be considered an anatomical variant if there are no symptoms related to this finding. However, this has been reported as a potential cause of obstruction to the foramen of Monro. The incidence of cysts of the cavum septum pellucidum is of 0.04% with only a few symptomatic cases described in the literature. Symptoms have been described as the result of a ball-valve phenomenon where there is obstruction of the foramen of Monro leading to increase intracranial pressure; compression of the hypothalamic-septal triangle resulting in neuropsychiatric symptoms or optic pathway compression; and chronic venous impairment from stretching of the internal cerebral and ependymal veins. ^14,15

Figure 4.

Coronal steady-state sequence of the brain in a 14-year-old patient demonstrating a prominent size of the cavum septum pellucidum, measuring more than 10 mm in transverse dimension, and showing outward bowing of the septal leaflets (arrows). Findings are compatible with a cavum of the septum pellucidum cyst. This patient was clinically asymptomatic.

Cavum vergae

The cavum verge is defined anatomically with the anterior margins composed of the forniceal columns, superior margin composed by the posterior body of the corpus callosum, and inferior margin composed by the forniceal body and hippocampal commissure (Figure 2C). The cavum vergae should be distinguished from the cavum velum interpositum, described below.

Cavum velum interpositum

The cavum velum interpositum is another potential CSF midline space not connected with the ventricular system. It’s also considered a normal variant. However, it is a distinct structure compared to the septum pellucidum. It is described here due to its close location relative to the cavum vergae and due to the often confusing differentiation. The cavum velum interpositum is located in front of the quadrigeminal cistern, above the roof of the third ventricle, and below the fornix ¹⁶ (Figure 5). A cyst of the cavum velum interposition can also be present, only causing symptoms if there is mass effect upon adjacent structures, such as the tectal plate.

Figure 5.

Axial and sagittal T ₂ weighted sequences of a 20-month-old patient with history of developmental delay. (A) Axial plane showing the body of the fornix splayed outward (arrows) by a cyst of the CVI (blue). (B) On the sagittal plane, the CVI can be seen posterior to the third ventricle (third) and bellow the forniceal body (arrows). The location bellow the fornix helps differentiate from a cavum vergae (which should be located above it). CVI, cavum velum interpositum.

Primary (malformative) abnormalities of the septum pellucidum

A congenital absent septum pellucidum can be either due to a primary failure of formation or a secondary disruption. Primary malformative absence can be due to isolated septal deficiency, associated with other commissural abnormalities such as with corpus callosum agenesis and holoprosencephaly; or be part of septo-optic dysplasia syndrome.

The distinction between a primary malformative cause and secondary disruption can be challenging. Careful analysis of the remaining brain structures (especially the midline structures), associated hemorrhage, and ventriculomegaly are key to determining the differential diagnosis. ¹⁷

Isolated absence of the septum pellucidum

Absence of the septum pellucidum is commonly associated with additional brain malformations, and isolated septal deficiency is rare. Its diagnosis depends on the absence of any additional brain malformation and a negative post-natal evaluation for septo-optic dysplasia syndrome, including ophthalmic evaluation, endocrine screening, and post-natal MRI. ¹⁶

Although controversial and formerly believe to be rare, cases of isolated absence of the septum pellucidum with no clinical, endocrine or ophthalmologic abnormalities have been reported, suggesting that true isolated septal deficiency is more common than previously thought. ^1,16

Aside from the lack of visualization of the septum pellucidum leaflets (Figure 6), squaring of the frontal horns of the lateral ventricles can be seen on coronal views. This has been described as box-like frontal horns. There is also a low position of the fornix that can be seen on both coronal and sagittal views. ¹⁸

Figure 6.

Coronal HASTE sequence of a 24 weeks fetus with absent septum pellucidum (arrow). No other abnormalities were identified in-utero. Post-natal evaluation of the optic nerves and endocrine system is recommended to differentiate an isolated septal deficiency versus septo-optic dysplasia.

Holoprosencephaly spectrum

Holoprosencephaly (HPE) is a spectrum of brain malformations characterized by failure of differentiation and midline cleavage of the prosencephalon, caused by either teratogens or genetic factors. ¹⁸ The four commonly recognized subgroups are alobar, semi-lobar, lobar, and midline interhemispheric variant. A less well-known subtype has been described as a septo-preoptic variant. ¹⁹ Aside from the septo-preoptic variant, the other HPE subtypes have been described as lacking a septum pellucidum. ²⁰

Alobar HPE is the most severe form with complete absence of midline cleavage. This results in a large monoventricule with complete absence of the interhemispheric fissure, commissural structures, and the septum pellucidum. There is fusion of the basal ganglia, hypothalamus, and thalamic nuclei at the midline resulting in absence of the third ventricle. Most commonly, the cerebrum is described as a pancake-like mass of tissue in the anterior part of the calvarium. A crescent shape monoventricle is continuous with a large dorsal cyst, usually occupying most of the volume of the calvarium ¹⁸ (Figure 7A). This subtype is associated with the most severe facial abnormalities. ²⁰

Figure 7.

A. Axial HASTE sequence of a 30 weeks gestation fetus showing absent septum pellucidum and other midline structures with a single LV, fused cerebral hemispheres (arrows) and a DC. Findings are compatible with alobar holoprosencephaly. (B) Axial HASTE sequence of a 33 weeks gestation fetus showing absent of septum pellucidum with lack of anterior forebrain cleavage (*). Note the preserve cleavage of the posterior brain structures (arrows) compatible with semi-lobar holoprosencephaly. DC, dorsal cyst; LV, lateral ventricle.

Semi-lobar HPE is characterized by posterior separation of the cerebral hemispheres and lateral ventricles, with presence of the posterior interhemispheric fissure. The anterior interhemispheric fissure, commissures, and septum pellucidum are absent. The separation of the deep nuclei varies, but the thalami and hypothalami remain unseparated ^20,21 (Figure 7B). The hippocampal commissure and splenium of the corpus callosum may be present.

The diagnosis of lobar HPE is more subtle than the previous described forms. ¹⁸ The interhemispheric fissure is present along nearly the entire midline, and the thalami are almost completely separated. The frontal lobes are more fully developed. The frontal horns of the lateral ventricles are present. Only the most anterior portion of the frontal lobes are fused. The third ventricle is present and the callosal dysgenesis is limited to the rostrum or genu, but the septum pellucidum is always absent such that the frontal horns of the lateral ventricles appear squared on the coronal images. ^17,21

The middle interhemispheric variant, also termed syntelencephaly, is characterized by a lack of separation of the posterior frontal and parietal regions, with normal interhemispheric separation of the anterior and occipital regions, in contrast to the above-described lobar form.

The second characteristic of this subtype is the pattern of callosal dysgenesis, which is identified within the body of the corpus callosum while the remaining genu and splenium appear normal. ^18,22

The septo-preoptic variant is a very mild form of HPE where the non-separation is restricted to the septal (subcallosal) and/or the preoptic region with no significant fusion of the rostral frontal lobes. The associated callosal abnormalities are usually restricted to a hypoplastic or absent rostrum with nearly normal appearance or slightly hypoplastic corpus callosum. The septum pellucidum is usually present, unlike the other forms of HPE. The septo-preoptic variant is commonly included in the lobar spectrum and represents a milder form of lobar HPE. It is often associated with midline craniofacial malformations including solitary median maxillary central incisor and congenital nasal pyriform aperture stenosis.

These different types of HPE are associated with varying clinical presentations and severity and are important to detect in the early fetal life. With the more severe subtypes, the early ultrasound diagnosis can be straight forward, whereas in more subtle subtypes the diagnosis requires further imaging work-up and is usually made on fetal or post-natal MRI. Overall, a hallmark of HPE is absence of the cavum septum pellucidum. This highlights the vital role of early detection of the septum pellucidum to ensure diagnosis of the milder forms of HPE. ²⁰

Septo-optic dysplasia syndrome

This entity is also known as hypoplastic optic nerve syndrome and de Morsier syndrome. It was first described in 1956 by de Morsier who reported post-mortem diagnosis of patients with optic nerve hypoplasia and agenesis of the septum pellucidum. ²³ The diagnosis is the result of various genetic abnormalities and pre-natal insults, with heterogeneous clinical presentations. Classically characterized by a triad of hypoplasia of the optic nerve and chiasm, hypothalamic-pituitary dysfunction and complete or partial absence of the septum pellucidum. It can be associated with various malformations such that some papers have proposed to categorize it as “septo-optic dysplasia complex”. ²⁴ The main finding on fetal MRI is the absent cavum septum pellucidum with squaring of the frontal horns of the lateral ventricles on coronal views, and low-lying fornix (Figure 8C). Most cases of optic hypoplasia are missed pre-natally with only 50% of the optic nerve hypoplasia cases detectable on post-natal MRI. ^18,25 Associated MRI findings can be one or more of the following: dysgenesis of the corpus callosum, anomalies of the hypothalamic-pituitary axis (with or without imaging findings), and anomalies of cortical development, mostly schizencephaly. ¹⁶ When schizencephaly or other malformations of cortical development are present, the term “septo-optic dysplasia plus” has been proposed ²⁶ (Figure 8A–C).

Figure 8.

Septo-optic dysplasia in a 3-day-old patient. (A) Sagittal T ₁ weighted sequence demonstrating a hypoplastic optic chiasm (arrow) and absent visualization of the posterior pituitary bright spot (arrowhead). (B) Axial T ₂ weighted sequence showing absent septum pellucidum (arrow) and associated close lip schizencephaly (arrowhead). (C) Coronal view showing the absent septum pellucidum (*), low-lying forniceal columns (arrows) and squaring of the frontal horns of the lateral ventricles (arrowheads).

The diagnosis is established by both ophthalmologic examination and neuroimaging, showing hypoplasia of the optic discs and complete or partial absence of the septum pellucidum. ¹⁸

Association with corpus callosum anomalies

Pre-natal documentation of the presence of the cavum septum pellucidum is one of the key elements of the standard morphological fetal brain evaluation, performed between 18 and 22 weeks of gestation. A pre-natal identification of the cavum septum pellucidum excludes a complete agenesis of the corpus callosum ^16,27 and an absent cavum septum pellucidum raises the concern for callosal agenesis/dysgenesis. Dysgenesis is a term use to refer to a present but malformed corpus callosum, which may be partially absent or hypoplastic. ²⁸

When evaluating a fetus with absent cavum septum pellucidum, additional secondary findings of callosal agenesis/dysgenesis are helpful clues to the correct diagnosis of a primary malformative cause. These secondary findings of callosal agenesis include: enlargement of the posterior aspect of the lateral ventricles (giving the classic appearance of colpocephaly), a parallel configuration of the lateral ventricles on axial images, upturned anterior horns of the lateral ventricles in a “bull’s horn” shape, and a high riding third ventricle (Figure 9A–C). The absence of the cingulate sulcus in cases of complete callosal agenesis allows the medial hemispheric sulci to extend all the way to the third ventricle roof, best seen in the midline sagittal view.

Figure 9.

Fetal MRI of a 32 weeks fetus with callosal agenesis. (A) Axial view showing absent corpus callosum and septum pellucidum (*) and parallel configuration of the lateral ventricles (arrows). (B) Axial view at the level of the atrium of the lateral ventricles showing ventricular dilatation compatible with colpocephaly. (C) Coronal view showing the “bull’s horn” configuration of the lateral ventricles (arrows).

Although the cavum septum pellucidum is always absent in cases of callosal agenesis, the leaflets of the septum pellucidum are variably present and can be folded within the everted cingulate gyri. In some cases of partial agenesis of the corpus callosum, a rudimentary cavum septum pellucidum may be present. ¹⁷

Secondary (disruptive) abnormalities of the septum pellucidum

Partial or complete absence of the septum pellucidum may be an acquired phenomenon, such as in cases of severe hydrocephalus, post-surgical related causes, hemorrhage, or ischemic insult.

Hydrocephalus

A partial or complete absence of the septum pellucidum can be seen in cases of long-standing hydrocephalus. This is particularly true in cases of severe congenital hydrocephalus. It is presumed to be the result of septal necrosis, secondary to increased intraventricular pressure, with fraying and fenestration of the leaflets of the septum pellucidum. ⁵ This disruptive process has been described in patients with Chiari II malformation and aqueductal stenosis, ²⁹ both malformative and acquired causes of aqueductal stenosis (Figure 10A–B). The literature supports this hypothesis by the observation of development of increasing large areas of septal fenestration as the hydrocephalus progresses over time, and the complete septum pellucidum absence in most severe hydrocephalus cases. ²⁹ When the absence is partial, it is easier to confirm the acquired nature of the process. A complete absence of the septal leaflets is, however, not uncommon in acquired causes, such as severe prolong hydrocephalus. It does not occur as the result of mild to moderate ventriculomegaly without increased ventricular pressure. This highlights the fact that not all cases of absent septum pellucidum are developmental in nature, and that acquired causes should be considered in the differential.

Figure 10.

Septal deficiency secondary to hydrocephalus. (A) Axial T ₂ weighted sequence of a 2-year-old patient with history of Chiari II that showed progressive hydrocephalus. Note the large gap in the septum pellucidum leaflets (*) and its small anterior remnants (arrows). (B) Coronal T ₂ weighted sequence of a 0-day-old patient with congenital aqueductal stenosis showing the markedly enlarged LVs with loss of cerebral parenchyma and a ventricular diverticulum along the left vertex (arrowheads). Note the septum pellucidum defects (arrows) that can be seen in cases of hydrocephalus. (C) Axial T ₂ weighted sequence in a 2-month-old patient with post-hemorrhagic hydrocephalus showing a gap in the septum pellucidum leaflets (*) and normal posterior juxtaposed leaflets (arrow). LV, lateral ventricle.

Insult

Injuries from hemorrage, infarction, infection and trauma can also cause absence of the septum pellucidum, most commonly partial absence. The presence of blood products, ischemia, porencephaly or even hydranencephaly are additional clues to the diagnosis. ¹⁶

Spontaneous hemorrhage into the septum pellucidum is extremely rare in older children and adults but has been repeatedly reported in early infancy due to immature cerebrovascular reactivity and autoregulatory mechanisms. ³⁰ Most common causes of adult septum pellucidum hemorrhage are trauma (Figure 11), neoplasm and underlying vascular anomalies (Figure 12). ^30,31

Figure 11.

Post-traumatic septum pellucidum hemorrhage. Non-contrast CT of a 72-year-old female with history of head trauma. Hemorrhage is seen along the posterior septum pellucidum (arrow).

Figure 12.

Non-contrast head CT of a 41-year-old female with ruptured anterior cerebral artery aneurysm (non-showed here). Note bilateral subarachnoid hemorrhage (arrowheads) and hemorrhage within the bilateral lateral ventricles and cavum septum pellucidum (arrow).

Septum pellucidum infarction is a very rare condition due to occlusion of the subcallosal artery, one of the branches of the anterior communicating artery. Cases of septum pellucidum infarction associated with fornix or cingulate gyrus infarctions have been reported. ^32,33 To our knowledge, isolated infarction of the septum pellucidum has not been reported.

Some studies have described association of septum pellucidum fenestration and chronic brain injuries in the adult population and reported higher prevalence of septal fenestration in a population of boxers. The authors stated that repeated bouts and rotational injuries may result in stretching and tearing of the superior part of the septum pellucidum. ^1,34

In addition, studies have investigated the relationship between head trauma and the presence of a cavum septum pellucidum with controversial results regarding its association. Some studies reported higher incidence of cavum septum pellucidum in patients after traumatic brain injury as well as larger cavum septum pellucidum than healthy controls. ^35–37

Post-surgical

Another possible origin of septum pellucidum disruption is iatrogenic. In fact, the septum can be targeted in endoscopic septostomy, as a treatment for lateral ventricle hydrocephalus caused by obstruction of the foramen of Monroe and in cases of septum pellucidum cysts felt to be symptomatic (Figure 13A–B). Endoscopic septostomy is a safe and successful treatment for restoring normal CSF circulation in these obstructive cases, and results in post-surgical disruption of the septum pellucidum. ^38,39

Figure 13.

A, B. Surgical fenestration of a cavum septum pellucidum cyst. (A) Coronal T ₂ weighted sequence demonstrating a markedly enlarged cavum septum pellucidum cyst (arrowheads) with outward bowing of the septum pellucidum leaflets (arrows). (B) Coronal T ₂ weighted sequence showing decrease size of the cavum septum pellucidum cyst after surgical fenestration with resolved outward leaflet bowing (white arrows). Note also the left septal defect (black arrow) and left frontal lobe surgical tract (arrowhead).

Endoscopic third ventriculostomy is an increasingly common technique in the treatment of hydrocephalus with successful results in the obstructive type, in selected patients. It creates an internal diversion of the CSF flow via the floor of the third ventricle, allowing the CSF to flow from the third ventricle directly to the interpeduncular cistern and the subarachnoid space. ⁴⁰ Endoscopic third ventriculostomies can be associated with septostomy, therefore demonstrating surgical disruption of the septum pellucidum (Figure 14A–B).

Figure 14.

(A) Axial T ₂ weighted sequence in a patient with hydrocephalus prior to surgical procedure. Note the intact septum pellucidum (arrows). (B) Axial T ₂ weighted sequence of the same patient after endoscopic third ventriculostomy (not shown) and a septostomy (arrow). Note the associated CSF flow artifact and the overall mild decrease size of the lateral ventricles.

Ventricular shunt is the most commonly used procedure for the treatment of hydrocephalus, especially in children. It creates an external diversion of the CSF flow using a tube that begins in the ventricular system and carries CSF into the peritoneum (most commonly), into the right atrium or into the pleural space. The ventriculostomy tube is inserted either in the frontal or occipital horn of the lateral ventricles through a small hole in the calvarium. ^18,41 The tube may at times perforate the septum pellucidum, creating a small focal septal defect.

While endoscopic septostomy technique directly targets and fenestrates the septum pellucidum, external and internal ventricular shunting can result in a secondary septum pellucidum disruption, which may appear as partial defect in the septum pellucidum. This defect is usually larger in the setting of endoscopic septostomy compared to ventricular shunts.

Tumors involving the septum pellucidum

Neoplasms that primarily originate from the septum pellucidum are rare. Most commonly, the septum pellucidum is involved from direct extension of tumors arising from nearby structures, especially the corpus callosum ¹ such as glioblastoma and primary central nervous system lymphoma. ⁵ Septal invasion can be seen with other tumors and usually presents as thickened septum pellucidum. ⁴²

De novo tumors that arise from the septum pellucidum can be of various histological origins. The most commonly reported tumors in a neurosurgical series of 19 patients are central neurocytoma, subependymal giant cell astrocytoma and subependymoma. ⁴³

Central neurocytomas are intraventricular tumors most commonly located in the lateral ventricles, usually near the foramen of Monroe, and arise from the ventricular walls including the septum pellucidum. They most commonly affect young adults. Central neurocytomas typically present as a well-circumscribed lobulated heterogeneously enhancing mass, with a broad attachment to the lateral ventricle wall or the septum pellucidum. Intratumoral cystic areas and punctate calcifications are present in two-thirds and half of cases respectively. Prominent flow voids may be seen as well as areas of hemorrhage ^44,45 (Figure 15A).

Figure 15.

(A) Axial T ₂ weighted sequence of a 19-year-old patient showing a heterogenous mass within the mid portion of the left lateral ventricle (arrow) with involvement of the septum pellucidum (arrowhead) compatible with a central neurocytoma. (B) Axial post-contrast T ₁ weighted sequence of a 9-year-old patient with tuberous sclerosis and a subependymal giant cell astrocytoma which involves the septum pellucidum (arrows). Note also the multiple subependymal nodules seen in cases of tuberous sclerosis (arrowheads). (C) Axial FLAIR sequence of a 75-year-old patient demonstrating a hyperintense mass within the anterior horn on the right lateral ventricle (arrow) with involvement of the septum pellucidum (arrowhead). This lesion was a subependymoma.

Subependymal giant cell astrocytomas are benign slow growing tumors, occurring almost exclusively in the setting of tuberous sclerosis, but can happen without other manifestations of tuberous sclerosis. They typically arise in the wall of the lateral ventricles near the foramen of Monro and can arise from the septum pellucidum. Subependymal giant cell astrocytomas commonly display partial calcifications and markedly enhance after contrast administration. They can cause hydrocephalus given their preferred location near the foramen of Monro. ^45,46 Despite the benign nature of this tumor, the outcome can be poor due to obstructive hydrocephalus and possible intra tumoral hemorrhage ⁴⁷ (Figure 15B).

Subependymomas are benign neoplasms most often found within the fourth and lateral ventricles. Lateral ventricle subependymomas can arise from the septum pellucidum or be attached to it. Subependymomas are usually hypo-to isointense on T ₁ weighted images and hyperintense on T₂ weighted images. They typically present lobulated margins, display cystic changes and typically show little or no enhancement. ^45,48 Peritumoral edema and intratumoral hemorrhage are rare ⁴⁹ (Figure 15C)

Mixed glioneuronal tumors are a recently described entity that involves the anterior septum pellucidum (although not exclusively), previously known as septal dysembryoplastic neuroepithelial tumors. It has recently been considered as a distinct type of lesion based on the DNA-methylation profile. The lesion appears classically as a well-defined solid mass, hypointense on T1, hyperintense on T2 in a cystic-like fashion, with incomplete suppression on FLAIR (Figure 16A–C). None of the published cases showed contrast enhancement. ^50,51

Figure 16.

(A–C) MRI of the brain in a 26-year-old patient showing a mass centered at the septum pellucidum compatible with a myxoid glioneural tumor of the septum pellucidum.(A) Axial FLAIR sequence showing the low signal tumor (arrow) originating/involving the septum pellucidum (arrowhead). (B) Axial T ₁ weighted sequence showing the tumor (arrow) with signal similar to CSF. (C) Sagittal steady-state sequence showing high T2 signal (arrow) classically seen in these tumors. CSF, cerebrospinal fluid; FLAIR, fluid attenuated inversion recovery.

Different glial neoplasms can involve the septum pellucidum including astrocytomas and oligodendrogliomas. ⁵ Cases of germinomas and embryonal tumors arising at the septum pellucidum have also been reported, appearing as heterogeneously enhancing masses on MRI (Figure 17A-B). ^52,53

Figure 17.

(A) Axial T ₂ weighted sequence of the brain in a 5-year-old patient showing a pilocytic astrocytoma (arrow) with involvement of the septum pellucidum (arrowhead). (B) Axial steady-state sequence of the brain in a 2-year-old patient with a large ETMR (arrow) causing mass effect and involving the septum pellucidum (arrowheads). ETMR, embryonal tumor with multilayered rosettes.

Cavernomas can arise in the septum pellucidum, although are extremely rare. MR imaging features are similar to that of parenchymal lesions, but the rarity of this lesion in this location precludes a definitive diagnosis. ^5,54

Conclusion

The septum pellucidum is an anatomical structure that can be abnormal in a host of different pathologies. Its visualization is an import clue to a normal embryonic midline formation. Knowledge of its normal formation, anatomy and pathological spectrum of abnormalities is important when interpreting fetal and post-natal brain imaging.