REVIEW ARTICLE The Lateral Ventricles: A Detailed Review of Anatomy, Development, and Anatomic Variations C.L. Scelsi, T.A. Rahim, J.A. Morris, G.J. Kramer, B.C. Gilbert, and S.E. Forseen ABSTRACT SUMMARY: The cerebral ventricles have been studied since the fourth century BC and were originally thought to harbor the soul and higher executive functions. During the infancy of neuroradiology, alterations to the ventricular shape and position on pneumo- encephalography and ventriculography were signs of mass effect or volume loss. However, in the current era of high-resolution cross-sectional imaging, variation in ventricular anatomy is more easily detectable and its clinical significance is still being investi- gated. Interpreting radiologists must be aware of anatomic variations of the ventricular system to prevent mistaking normal variants for pathology. We will review of the anatomy and development of the lateral ventricles and discuss several ventricular variations. he cerebral ventricles were the center of attention among phi- ventricular size and morphology but have been studied exten- Tlosophers, priests, anatomists, and physicians as far back as sively and will be left out of this review. Aristotle in the fourth century BC.1 They were originally thought to harbor the soul and “vital” spirits responsible for higher func- ANATOMY tions. After the influence of Christianity and the Renaissance, the Lateral Ventricles ventricles were conceptualized as 3 cavities where common sense, The lateral ventricles are paired C-shaped structures comprising creative imagination, and memory were individually allocated. It a body and atrium along with 3 projections into the frontal, tem- was not until the 16th century that anatomists Andreas Vesalius poral, and occipital lobes, termed “horns.” The lateral ventricles and Constanzo Varolio identified ventricles as being filled with communicate with the third ventricle through the interventricu- 2 CSF. lar foramina of Monro.5 Each lateral ventricle has an estimated Before the era of cross-sectional imaging, when ventriculog- capacity of 7–10 mL.6 raphy and pneumoencephalography were the only modalities The frontal horn (Fig 1) extends anteriorly from the foramina available, variations in ventricular morphology were used as of Monro and communicates with the body of the lateral ven- 3,4 signs of parenchymal volume loss or mass effect. Currently, tricles posteriorly. The anterior wall and roof are formed by the asymmetries and anatomic variations are frequently identified genu of the corpus callosum, and the floor is formed by the ros- and accepted as normal variations. Although they are not fre- trum. The head of the caudate nucleus forms the lateral wall. The quently a diagnostic dilemma, imaging interpreters must be columns of the fornix form the inferior portion of the medial aware of these anatomic variants to prevent mistaking them for wall.6-8 pathology. In this review, we discuss normal anatomy, ventricu- The body of the lateral ventricle communicates with the lar development, and anatomic variations of the lateral ven- atrium posteriorly from the foramina of Monro to the corpus cal- tricles along with a deep look into the investigation of these losum and psalterium of the fornix, also called the hippocampal entities. Pathologic states (ie, hydrocephalus, degenerative dis- commissure. The roof is formed by the body of the corpus cal- ease, traumatic encephalopathy, and so forth) can affect losum, and the floor is formed by the thalamus. The septum pellucidum and body of the fornix form the superior and infero- medial walls, respectively. The lateral wall is formed by the cau- Received November 12, 2019; accepted after revision January 6, 2020. date nucleus and thalamus. In the groove between the caudate From the Departments of Radiology (C.L.S., T.A.R., J.A.M., G.J.K.) and nucleus and thalamus along the lateral wall sits the stria termina- Neuroradiology (B.C.G., S.E.F.), Medical College of Georgia, Augusta, Georgia. Please address correspondence to Chris L. Scelsi, DO, 1120 15th St, Augusta, GA, lis, a main outlet pathway of the amygdala and the location of the 30912; e-mail: [email protected] thalamostriate vein.9 Indicates open access to non-subscribers at www.ajnr.org The atrium (Fig 2) is a triangular cavity that communicates http://dx.doi.org/10.3174/ajnr.A6456 with the body, temporal horn, and occipital horn. The body and 566 Scelsi Apr 2020 www.ajnr.org the thalamus, tail of the caudate nu- cleus, and tapetum. The striothalamic sulcus separates the caudate tail and thalamus. The tapetum also runs infe- riorly to comprise the lateral wall and separates the lateral wall from the optic radiations. The choroidal fis- sure runs along the medial wall.12 The occipital horn curves posteri- orly and medially from the atrium and varies in size. The tapetum forms the roof and lateral wall and separates it FIG 1. Anatomy of the frontal horns of the lateral ventricles. Coronal T1-weighted MR image 7,9,11 from a healthy 22-year-old man. CN indicates caudate nucleus; CC, body of the corpus callosum; from the optic radiations. The col- F, columns of the fornices; SP, septum pellucidum. Illustration adapted with permission from lateral trigone forms the floor. Similar Gray.64 to the atrium, the bulb of the corpus callosum and calcar avis form the medial wall.9 Choroid Plexus and Choroidal Fissure The choroid plexus is present within the lateral, third, and fourth ventricles and is the primary source of CSF, pro- ducing approximately 500 mL per day.13,14 Within the lateral ventricles, the choroid plexus runs along a cleft between the fornix and thalamus called the choroidal fissure. The choroidal fis- sure forms a C-shape extending from the foramina of Monro to its inferior terminal point, which is termed the “inferior choroidal point.”9,15 Within the atria, there is a prominent triangu- lar tuft called the glomus.15 The tela FIG 2. Anatomy of the atrium of the left lateral ventricle. Coronal T1-weighted MR image from a choroidea is an invagination of the pia healthy 21-year-old woman. OB indicates occipital bulb containing fibers of forceps major; CA, mater and ependyma, which gives rise calcar avis; T, collateral trigone; CC, tapetum of the corpus callosum; solid arrow, calcarine sulcus; to the choroid plexus within the cho- open arrow 65 , collateral sulcus. Illustration adapted with permission from Gray. roidal fissure and along the roof of the third ventricle.9,16 splenium of the corpus callosum form the roof. The tapetum, The choroidal fissure is an important neurosurgical structure which is a sheetlike bundle of decussating fibers in the sple- because it opens up access to the basal cisterns and to structures nium of the corpus callosum, arches over the atrium and forms that would be otherwise inaccessible through an extracerebral the roof. The tapetum continues laterally and comprises the route.9 Recently, excess CSF has been shown to pass through the lateral wall and the caudate nucleus. The floor is created by the inferior choroidal point in patients with idiopathic normal pres- collateral trigone, a continuation of the collateral eminence sure hydrocephalus. Its anatomic relationship may explain dispro- formed by the collateral sulcus. The contour of the medial wall portionate enlargement of the basal cisterns and Sylvian fissures 17 is formed by the calcar avis and bulb of the corpus callosum, in patients with idiopathic normal pressure hydrocephalus. which is a bulging created by the forceps major.5,9 The calcar avis was previously known as the hippocampus minor and rep- DEVELOPMENT resents an indentation formed by the calcarine fissure.10 When After closure of the neural tube during the fourth week of gesta- prominent, the calcar avis can mimic hemorrhage on cranial tion, the lateral ventricles begin forming from the 2 outpouchings sonography.11 of the telencephalon, a derivative of the prosencephalon.18,19 The The temporal horn is the longest and largest horn, extending lateral walls of the telencephalon evaginate, and there is rapid anteriorly from the atrium below the thalamus and terminating growth of the medial basal portion of the cerebral hemispheres at the amygdala. The collateral eminence and hippocampus form called the corpus striatum, which ultimately gives rise to the basal the floor, which is separated from the hippocampus by a thin ganglia. This rapid growth shapes the floor and narrows the fo- layer of white matter called the alveus.11,12 The roof is created by ramina of Monro (Fig 3).18,20-22 AJNR Am J Neuroradiol 41:566–72 Apr 2020 www.ajnr.org 567 FIG 3. Histologic sections of the telencephalon in the developing embryo from weeks 5 to 8. The lateral ventricles (L) arise as outpouchings from the telencephalon. Rapid growth of the corpus striatum (CS) shapes the floor of the lateral ventricles and narrows the foramina of Monro (F). The primitive choroid plexus (CP) comprises a large portion of the ventricular cavity by the eighth week. Also note that the lateral ventricles occupy a large majority of the cerebral hemisphere at 8 weeks. Images courtesy of Dr John Cork, PhD, Department of Cell Biology and Anatomy at Louisiana State University Health Sciences Center and the Virtual Human Embryo Project. Images used were the following: Carnegie Stage 17, section 410; Carnegie stage 18, section 302; Carnegie stage 20, section 131; Carnegie stage 23, section 50. https://www.ehd.org/virtual-human- embryo. lumen.27 When focal, coarctation can isolate a portion of the ventricle, creating multiple small rosettes or an ependymal- lined cyst, commonly referred to as a connatal cyst (Fig 4).28,29 The incidence of unilateral or bilateral frontal horn coarctation has been reported to be between 0.38% and 6.0%, with a major- ity of studies reporting incidences below 1%.30-32 Coarctation of the occipital horns is much more common, with a reported inci- dence of 21.3%.33 Coarctation is thought to occur at sites where opposing walls are in close proximity as is in the case with the ventricu- lar horns, but it also has been described as occurring in the body.28,31,33 The exact mechanism of ventricular coarctation is currently unknown.