From cavum septi pellucidi to anterior complex: how to improve detection of midline cerebral abnormalities

Abstract

On routine examination, according to the guidelines of the International Society of Ultrasound in Obstetrics and Gynecology, midline anomalies are mainly depicted based on analysis of a cardinal anatomical structure, the cavum septi pellucidi (CSP)1. Indeed, failure to identify the CSP is a key feature in the diagnosis of the large spectrum of both holoprosencephalies and commissural anomalies, such as complete corpus callosal agenesis, septo-optic dysplasia (malformative origin), schizencephaly (ischemic insult) and obstructive ventriculomegaly (barotraumatic insult)2-6. As absence of the CSP is one of the main findings of midline anomalies, it should be considered as a marker of abnormal central nervous system organization and not as a malformation per se, since isolated absence of the CSP can be observed in normal individuals4, 7. In our experience, some patients referred to our institution with isolated ventriculomegaly in fact had undiagnosed corpus callosal agenesis, despite the presence of the CSP being mentioned in their previous ultrasound reports (Figure 1). In fact, the apparent CSP is a false image created by the anterosuperior displacement of the third ventricle mimicking the CSP. To improve detection of midline anomalies, we suggest that it is important to investigate not only the CSP but also what we refer to as the ‘anterior anatomical complex of the midline’ (or anterior complex). The anterior complex is illustrated in a routine axial plane in Figure 2. From posterior to anterior, the anterior complex includes the CSP, which is bordered on both sides by the frontal horns, the genu of the corpus callos, the pericallosal sulcus and the anterior part of the interhemispheric fissure. The shape of the pericallosal sulcus and the anterior part of the interhemispheric fissure, taken together, can be likened to that of an anchor. When one focuses specifically on the anterior complex, a clear difference is observed between Figures 1 and 2. In Figure 1 (complete corpus callosal agenesis), the anterior complex is not identified, owing to the absence of the genu of the corpus callosum and upward displacement of the third ventricle, whereas in Figure 2, the anterior complex is clearly identified as a normal case. Identification of the frontal horns on both sides of the CSP is also important, since free communication between the frontal horns highlights the absence of the CSP. Furthermore, unilateral enlargement of the proximal frontal horn (close to the transducer) can be useful for the diagnosis of proximal unilateral ventriculomegaly since the proximal atrium is often overlooked because of the presence of bony artifacts8. Finally, the presence of an interhemispheric fissure (IHF) has been shown to be another clue indicating midline anomalies, and is an anatomical feature of the neurosonogram1. If incomplete cleavage of the IHF is a common feature of mild forms of holoprosencephalies, distortion of the IHF, associated with impaction of the medial borders of the frontal lobes, may also be an important anatomical feature in the investigation of midline pathologies, but more significantly, it may indicate supratentorial central nervous system disorganization, as well as potential cytogenetic anomalies9. Therefore, we suggest integrating investigation of the anterior complex, which is demonstrated on both the routine supratentorial axial and coronal planes, as part of routine cerebral sonographic analysis in order to improve the detection of midline anomalies. M. Cagneaux and L. Guibaud* Université Claude Bernard Lyon I, Imagerie Pédiatrique et Foetale, Hôpital Femme Mère Enfant, 59, Boulevard Pinel, 69677 Lyon-Bron, France *Correspondence (e-mail : laurent.guibaud@chu-lyon.fr)