Approach through the intraparietal sulcus to the ventricular atrium: A cadaveric microanatomical and morphometric study

Abstract

Background:

The intraparietal sulcus (IPS) provides a natural transsulcal corridor to the lateral ventricular atrium that minimizes disruption of white matter tracts. However, no previous study has quantified the dissection angles from multiple topographic points along the sulcus.

Methods:

Descriptive study conducted in the 9 adult human cerebral hemispheres available for dissection. IPS macroscopic morphology was classified and its length was measured. From the anterior, midpoint, and posterior points of the sulcus, straight pins were directed toward the ventricular atrium, and their angles relative to the anterior-posterior commissure line were measured. Subsequently, the depth was measured from each point to the atrium. Means, standard deviations (SD), ranges, and coefficients of variation (CV%) were calculated.

Results:

The IPS exhibited continuous morphology in all hemispheres, with a mean length of 3.96 ± 0.71 cm and a mean of 3.22 ± 0.97 accessory branches. Angular data were obtained in 6 of 9 hemispheres. Mean dissection angles were 4.5 ± 15.9° from the anterior point, 19.33 ± 11.99° from the midpoint, and 29.33 ± 12.14° from the posterior point. Approach depth, determined in 8 of 9 hemispheres, was 4.86 ± 0.74 cm, 5.24 ± 0.79 cm, and 5.4 ± 0.82 cm from the anterior, midpoint, and posterior points, respectively.

Conclusion:

IPS surface morphology and approach depth showed moderate interindividual variability, whereas dissection angles showed high variability, supporting the need for individualized presurgical trajectory planning. Based on the limited specimens available, these preliminary findings warrant validation in larger cohorts.

INTRODUCTION

The intraparietal sulcus (IPS) was first described in 1866 by William Turner, who noted that although the fissure had been illustrated in anatomical drawings previously, it lacked a description until he designated it as “intraparietal” due to its location within the parietal lobe.[18] Modern measurements detail that this sulcus has an average length of 8.05 cm and an average depth of 1.78 cm.[7] Surgical access to the atrium of the lateral ventricles has historically presented a challenge and was classically performed using transcortical approaches, which forced the surgeon to make incisions through the cerebral cortex and underlying white matter to reach the cavity.[6] To offer a less invasive alternative, Ribas et al. proposed the approach to the ventricular atrium through the IPS, taking advantage of this natural corridor to minimize the disruption of white matter tracts.[14] In addition to its excellent utility for treating lesions in the atrium, this approach can be used to reach deeper and more complex areas, such as specific sectors of the cerebral central core (CCC), including the posterior lateral thalamus and the posterior portion of the caudate nucleus. This corridor stands out fundamentally because it offers a direct view of the structures and allows the preservation of neurologically critical fascicles, such as the optic radiations and the superior longitudinal fasciculus, reducing postoperative morbidity.[2]

Numerous articles have referred to the IPS as a fundamental anatomical landmark for direct and safe access to the ventricular atrium.[10,11,14] Different aspects of this transsulcal corridor have been thoroughly studied in the literature, evaluating critical variables such as the exact distance from the surface and the bottom of the sulcus to the roof of the atrium, the complex architecture of the white matter fascicles that are traversed and must be preserved during the trajectory, as well as the identification of the best anatomical area of the sulcus to make the incision with the lowest possible morbidity.[3,11] However, despite some studies mentioning the need to incline the dissection according to the addressed segment of the sulcus, the dissection angles from multiple topographic points of the IPS toward the ventricular atrium had not been systematically quantified.[11,14] Defining these angles accurately represents a knowledge gap that must be resolved to optimize preoperative planning and the precision of the surgical trajectory.

Consequently, to bridge this gap, the main objective of the present study is to detail the microanatomy and morphometry of the transsulcal approach through the IPS directed to the ventricular atrium. To achieve this, the study aims to evaluate the macroscopic morphology (continuous or interrupted, number of accessory branches) of the IPS along its extension; measure the length of the IPS using the inextensible thread method; define the dissection angle toward the ventricular atrium using the anterior limit, the midpoint, and the posterior limit of the IPS as entry points; measure the depth from these three superficial anatomical landmarks to the ependymal roof of the atrial cavity; and quantify the interindividual variability of the studied variables through the use of statistical measures such as standard deviation (SD) and coefficient of variation (CV%), aiming to establish the predictability of the surgical corridor.

MATERIALS AND METHODS

Study design and anatomical preparation

A descriptive microanatomical and morphometric study was carried out using 9 adult human cerebral hemispheres (6 right and 3 left), previously fixed and preserved in a 10% formaldehyde solution. Dissections were performed at the Microsurgical Neuroanatomy Laboratory of the University of Buenos Aires (UBA), Argentina. Under magnification with a NEWTON brand MEC model microscope, equipped with a DRLED10W model illuminator of the same brand, the arachnoid of the lateral aspect of the parietal lobe was meticulously removed using microforceps and microscissors to precisely delimit the lips of the IPS.

Superficial morphometry

The morphology of the IPS was recorded, classifying it as continuous (single path without interruptions) or interrupted (with one or more gyri crossing its path), and according to the presence or absence of accessory lateral branches (recording the number of branches when applicable). Due to the tortuous and irregular nature of the sulcus, the measurement of its superficial length was performed using a flexible adaptation method. An inextensible thread was placed along the entire path of the sulcus, faithfully following its contours and anatomical variations from its origin to its termination [Figure 1]. Subsequently, the thread was extended and measured in millimeters using a calibrated rigid ruler [Figure 2].[7] This comprehensive evaluation of the contours is fundamental, as anatomical studies have shown that the IPS is not a simple cleft and features transitional gyri and interruptions.[20]

Figure 1:

Measurement of the superficial length of the intraparietal sulcus of a left cerebral hemisphere using a flexible adaptation method. An inextensible thread was placed along the entire path of the sulcus, faithfully following its contours and anatomical variations from its origin to its termination.

Figure 2:

Measurement of the superficial length of the intraparietal sulcus in the left cerebral hemisphere from the previous image. The thread was extended and measured in millimeters using a calibrated rigid ruler. 1L : Left hemisphere number 1. Red line :Thread placed over the contour of the intraparietal sulcus in the previous image.

Surgical trajectories and angular analysis

To evaluate the surgical corridor through the IPS, three entry points were established on the surface of the sulcus: the anterior limit, the midpoint, and the posterior limit. From each of these landmarks, straight metallic pins were inserted directed toward the ventricular atrium, and a thread was placed connecting the base of each pin with its respective tip, passing through the medial face of the cerebral hemisphere, representing the pin’s superficial trajectory. Once the trajectories were fixed, digital photographs were taken with a Nikon D5100 camera equipped with a Nikon 18–55 mm lens of the medial face of each hemisphere. This ensured a clear perspective that simultaneously visualized the base of the pin on the cortex and its tip emerging into the atrial cavity, angling the camera so that the thread represented the continuation of the pin, connecting its base with its tip in the image, serving as a reference so that all photographs were taken from the same angle [Figure 3]. For the geometric analysis, the images were imported into graphic editing software, where the intercommissural line (joining the anterior and posterior commissures) was manually drawn as the basal reference plane, being a reproducible anatomical axis widely used in neurosurgery and stereotactics. In addition, three lines perpendicular to the intercommissural line directed to the base of each pin were drawn. Finally, the angle formed between each trajectory and its respective line perpendicular to the intercommissural line was measured using the open-source software ImageJ (version 1.53k), developed by the National Institutes of Health (NIH), an internationally validated tool for the precise measurement of anatomical structures [Figure 4].[16] In 3 of the 9 hemispheres (2 right and 1 left), it was not possible to complete the required photographic record for the angular analysis under adequate conditions. These specimens were partially dissected, and identifying the anterior or posterior commissure was not possible, making it impossible to draw the intercommissural line; therefore, these specimens were included exclusively for the other variables.

Figure 3:

Left cerebral hemisphere with straight metallic pins inserted into the anterior limit, midpoint, and posterior limit of the intraparietal sulcus directed toward the ventricular atrium, with a thread connecting the base of each pin with its respective tip, passing through the medial face of the hemisphere. 1L : Left hemisphere number 1. Grey lines : Straight metallic pins inserted into the anterior limit, midpoint, and posterior limit of the intraparietal sulcus directed toward the ventricular atrium. Red lines : Thread connecting the base of each pin with its respective tip, passing through the medial face of the hemisphere.

Figure 4:

Left cerebral hemisphere from the previous image. The intercommissural line (thick yellow) is observed as the basal reference plane, with three lines perpendicular to the intercommissural line directed to the base of each pin (thin yellow). The angle formed between each trajectory and its respective line perpendicular to the intercommissural line (white) was measured, resulting in 4.8° at the most anterior point, 23.2° at the midpoint, and 40.5° at the most posterior point of the intraparietal sulcus. 1L : Left hemisphere number 1. Grey lines : Straight metallic pins inserted into the anterior limit, midpoint, and posterior limit of the intraparietal sulcus directed toward the ventricular atrium. Red lines: Thread connecting the base of each pin with its respective tip, passing through the medial face of the hemisphere. Thick yellow line: Intercommisural line. Thin yellow line:Lines perpendicular to the intercommissural line directed to the base of each pin. 1: Posterior limit of the intraparietal sulcus. 2: Midpoint of the intraparietal sulcus. 3 : Anterior limit of the intraparietal sulcus.

Deep morphometry

To determine the approach depth, the tip of each pin was approximated precisely at the ependyma of the ventricular atrium, and a mark was made with a permanent marker on the body of the pin at the exact point where it passed through the cortex. After removing the pins, the distance between the mark (cortex) and the tip (ependyma) was measured with a calibrated ruler, recording the values corresponding to the anterior, middle, and posterior points of the IPS.

Data recording and analysis

The morphometric and angular data obtained were recorded individually for each anatomical piece. The results were expressed using descriptive statistics and organized in detail in comparative tables using Microsoft Excel (2024 version). For each quantitative variable, the following were reported: mean, SD, and range (minimum–maximum). To quantify interindividual variability, CV% = SD/Mean × 100 was calculated for ratio variables (number of collateral branches, sulcus length, and approach depth). Given that there is no universal scale for interpreting CV% in cadaveric neuroanatomical morphometry, the authors defined a priori the following thresholds based on the expected biological variability range for brain structures: CV% < 15% = high predictability; CV% 15–30% = moderate predictability; CV% > 30% = low predictability.

RESULTS

Morphology and length of the IPS

The IPS presented a continuous morphology in all 9 studied hemispheres (100%). The number of accessory branches showed a mean of 3.22 ± 0.97 (range 2–5 branches; CV%: 30.16%). The length of the IPS, measured by the inextensible thread method, presented a mean of 3.96 ± 0.71 cm (range2.7–5.0 cm; CV%: 17.88%). In the right hemispheres (n = 6), the mean was 2.83 ± 0.75 (CV%: 26.57%) for the number of accessory branches and 3.82 ± 0.74 cm (CV%: 19.5%) for the length of the IPS. In the left hemispheres (n = 3), the mean was 4 ± 1 (CV%: 25%) for the number of accessory branches and 4.23 ± 0.67 cm (CV%: 15.73%) for the length of the IPS [Table 1].

Table 1:

Macroscopic morphology (continuous/interrupted), number of accessory branches, and intraparietal sulcus (IPS) length in the 9 cerebral hemispheres studied (6 right and 3 left). Mean, standard deviation (SD), and coefficient of variation (CV%) are presented per hemisphere and globally.

Dissection angles

Angular data were obtained in 6 of the 9 hemispheres (4 right and 2 left). The mean dissection angle relative to the intercommissural line was 4.5 ± 15.9° (range −16.6° to 24.9°) from the anterior limit, 19.33 ± 11.99° (range 3.9°–32.1°) from the midpoint, and 29.33 ± 12.14° (range 14.8°–41°) from the posterior limit of the IPS. The difference in the angular trajectory between the anterior and posterior limits of the IPS toward the atrium in the same hemisphere presented a mean of 24.83 ± 7.67° (range 16.1°–35.7°). In the right hemispheres (n = 4), the means were 1.33 ± 18.88° for the anterior angle, 15.35 ± 12.85° for the middle angle, and 24.5 ± 12.28° for the posterior angle. In the left hemispheres (n = 2), the means were 10.85 ± 8.56° for the anterior angle, 27.3 ± 5.8° for the middle angle, and 39 ± 2.12° for the posterior angle [Table 2]. As an additional finding, in 2 of the 6 hemispheres with angular recording, the trajectory toward the ventricular atrium from the anterior limit of the IPS had to be directed posteriorly, resulting in a negative value [Figure 5].

Table 2:

Dissection angles (in degrees) measured relative to the intercommissural line from the anterior limit, midpoint, and posterior limit of the intraparietal sulcus (IPS) toward the ventricular atrium, and angular difference between the anterior and posterior points of each hemisphere with a valid photographic record (4 right and 2 left). Mean and standard deviation (SD) are presented per hemisphere and globally.

Figure 5:

Right cerebral hemisphere in which the trajectory toward the ventricular atrium from the anterior limit of the intraparietal sulcus had to be directed posteriorly, resulting in a negative value for this angle. 1R :Right hemisphere number 1. Grey lines :Straight metallic pins inserted into the anterior limit, midpoint, and posterior limit of the intraparietal sulcus directed toward the ventricular atrium. Red lines : Thread connecting the base of each pin with its respective tip, passing through the medial face of the hemisphere. Thick yellow line : Intercommisural line. Thin yellow line :Lines perpendicular to the intercommissural line directed to the base of each pin. 1 : Anterior limit of the intraparietal sulcus. 2 : Midpoint of the intraparietal sulcus. 3 : Posterior limit of the intraparietal sulcus.

Approach depths

The approach depth from the cortical surface to the ependymal roof of the ventricular atrium was determined in 8 of the 9 hemispheres (5 right and 3 left). The mean depth was 4.86 ± 0.74 cm (range 3.7–5.7 cm; CV%: 15.27%) from the anterior limit, 5.24 ± 0.79 cm (range 4–6.6 cm; CV%: 15.07%) from the midpoint, and 5.4 ± 0.82 cm (range 3.7– 6.3 cm; CV%: 15.27%) from the posterior limit of the IPS. The intra-specimen mean (average of the three depths per hemisphere) was 5.17 ± 0.75 cm (range 3.8–6.17 cm; CV%: 14.46%), while the intra-specimen range (difference between maximum and minimum depth in each hemisphere) showed a mean of 0.66 ± 0.41 cm (range 0.3–1.2 cm; CV%: 61.41%). In the right hemispheres (n = 5), the depth means were 4.68 ± 0.82 cm (CV%: 17.45%) at the anterior limit, 5.08 ± 0.99 cm (CV%: 19.56%) at the midpoint, and 5.2 ± 0.95 cm (CV%: 18.29%) at the posterior limit, with an intra-specimen mean of 4.99 ± 0.89 cm (CV%: 17.87%). In the left hemispheres (n = 3), the depth means were 5.17 ± 0.61 cm (CV%: 11.83%) at the anterior limit, 5.5 ± 0.2 cm (CV%: 3.64%) at the midpoint, and 5.73 ± 0.55 cm (CV%: 9.61%) at the posterior limit, with an intra-specimen mean of 5.47 ± 0.38 cm (CV%: 7.03%) [Table 3].

Table 3:

Approach depths (in centimeters) from the anterior limit, midpoint, and posterior limit of the intraparietal sulcus (IPS) to the ependymal roof of the ventricular atrium in 8 cerebral hemispheres (5 right and 3 left). Mean, standard deviation (SD), and coefficient of variation (CV%) are presented per hemisphere and globally.

DISCUSSION

The present study is an effort to systematically quantify the dissection angles from the anterior limit, midpoint, and posterior limit of the IPS toward the ventricular atrium, establishing a three-dimensional guide to optimize the safety and precision of this surgical corridor. The complexity of this cavity has been described in classic studies by Rhoton, who emphasized that any approach to this region must navigate around a dense network of fascicles.[17] To achieve an adequate understanding of this subcortical architecture, the method described by Klingler, based on the freezing and thawing of brain tissue, remains the gold standard for studying white matter.[1,19] In general, our results demonstrate that the macroscopic morphology of the IPS is predominantly continuous in the studied specimens, which directly conditions the planning of the cortical incision. It features an average superficial length of 3.96 ± 0.71 cm and a mean depth to the ependymal roof of the ventricular atrium of 5.17 ± 0.75 cm, presenting morphological variations depending on the selected entry point.

First, the macroscopic evaluation of the IPS revealed that the sulcus was present continuously in all studied hemispheres, and an average of 3.22 ± 0.97 accessory lateral branches was recorded. This superficial morphological uniformity contrasts with what was previously reported by Koutsarnakis et al., as well as Zlatkina et al., who, when mapping sulcal morphology, emphasized that interruptions of the IPS are frequent and can disorient the intraoperative perception of this surgical corridor.[11,20] Furthermore, recent morphological investigations demonstrate that the interrupted pattern of the IPS is a common variant, especially in the right hemisphere, and that this morphological interruption is linked to a larger volume and diameter of the underlying white matter tracts, demanding greater caution during transsulcal dissection.[15] Regarding linear morphometry, our analysis yielded an average length of 3.96 ± 0.71 cm using the flexible adaptation method with an inextensible thread, a value that contrasts with the 8.05 cm average described by Gargurevich et al. in their dissections.[7] These metric discrepancies reaffirm that presurgical planning must be strictly individualized due to the inherent irregularities of the parietal cortex.

The finding with the most significant applicability in our research lies in the systematization of the angles and depths for the surgical trajectories. Historically, authors such as Ribas et al. and Kawashima et al. defined the IPS as an exceptional anatomical landmark to reach the atrium while minimizing the disruption of fascicles; particularly, the preservation of optic radiations represents one of the greatest challenges.[10,14] Mahaney and Abdulrauf demonstrated through anatomical dissection that the lateral wall and the roof of the atrium are intimately lined by these visual fibers.[12] Today, the integration of diffusion tensor tractography allows confirmation of this complex spatial layout, revealing the close relationship between the surgical corridor and the tracts before making the cortical incision.[13] However, there was a lack of geometric guidelines indicating the required inclination according to the addressed segment. Our analysis determined that the required dissection angle from the anterior limit of the IPS is 4.5 ± 15.9° (from −16.6° to 24.9°) relative to the intercommissural line, requiring a depth of 4.86 ± 0.74 cm (CV% 15.27%). In contrast, the midpoint requires 19.33 ± 11.99° (from 3.9° to 32.1°) and 5.24 ± 0.79 cm (CV% 15.07%), while the approach from the posterior limit demands 29.33 ± 12.14° (from 14.8° to 41°) and 5.4 ± 0.82 cm (CV% 15.27%). This marked variation in the obtained angles demonstrates the high interindividual variability and the impossibility of standardizing them, contrary to what is observed in the depths, which present moderate predictability. This becomes relevant when selecting the entry point; Capilla-Guasch et al. proposed the posterior third of the IPS as the “safe entry zone” as it allows avoiding the most eloquent tracts, while Koutsarnakis et al. pointed out the midpoint as the most optimal region, because it is directly above the atrium, guaranteeing the most direct route.[3,11]

Finally, the comparative analysis of the morphometry between the cerebral hemispheres provided crucial data regarding the asymmetry and predictability of the approach. We observed hemispheric differences, recording a mean length of 3.82 ± 0.74 cm (CV 19.5%) on the right side and 4.23 ± 0.67 cm (CV 15.73%) on the left side, as well as a mean intra-specimen depth of 4.99 ± 0.89 cm (CV 17.87%) on the right side compared to 5.47 ± 0.38 cm (CV 7.03%) on the left. This asymmetry in lengths and depths is not an incidental finding; it supports the established concept of structural cerebral asymmetry, in which sulcal depths and cortical folding patterns vary between hemispheres due to differential cortical expansion and the lateralization of cognitive and language functions.[9] Consistent with the studies by CapillaGuasch et al., who evaluated the complex three-dimensional relationships of white matter using transillumination, our morphometric parameters allow us to determine that, provided the individual angular trajectories are respected, the transsulcal approach through the IPS is a reproducible technique for accessing deep within the parietal lobe and reaching the ventricular atrium. This is because the degree of variability suggests moderate to high predictability regarding the length of the sulcus and the depth of the surgical path.[3]

Multiple clinical series have reaffirmed that intraparietal transsulcal approaches, sometimes assisted by tubular retractors or endoscopic optics, have been shown to reduce postsurgical morbidity compared to classic transcortical approaches.[5] While some authors note that transcortical incisions remain a viable option with a manageable risk of postoperative seizures, recent literature reviews emphasize that choosing the natural corridor significantly decreases the rate of permanent visual field deficits, in addition to preventing damage to functional cortex and tracts.[4,8] This confirms the viability of the IPS not only for pure atrial pathology but also as an access route to the CCC.[2] Figures 6-8 show an illustrative case where this approach is utilized.

Figure 6:

Illustrative case. A 56-year-old female patient presenting with headache. A brain magnetic resonance imaging was performed, revealing an intraventricular lesion at the level of the left atrium. (a) Sagittal slice; (b) axial slice; (c) coronal slice.

Figure 8:

Postoperative brain magnetic resonance imaging 3 months after the previous case. No tumor remnant is evident, with satisfactory evolution and no neurological deficit after the procedure. (a) Sagittal slice; (b) axial slice; (c) coronal slice.

Figure 7:

Intraoperative images of the previous patient. An incision was made through the intraparietal sulcus to approach the lesion located in the left ventricular atrium, allowing for its complete resection. Pathology reported a WHO grade I meningioma. (a) Initial dissection of the intraparietal sulcus; (b) deepening of the sulcal opening; (c) entry into the atrium through the ventricular roof; (d) exposure of the intraventricular tumor; (e) dissection of the tumor within the atrium; (f) final surgical cavity after tumor resection.

Limitations

This study presents a limitation that must be considered when interpreting its results. The sample size was small (9 hemispheres), with even smaller subsamples for angular (6 hemispheres) and depth variables (8 hemispheres), which limits the generalization of the findings and does not allow ruling out the influence of chance on the differences observed between hemispheres.

CONCLUSION

According to what was observed in the available sample for this study, the transsulcal approach through the IPS constitutes an anatomically constant and predictable access route to the ventricular atrium in terms of its superficial morphology and working depth, presenting low to moderate interindividual variability. However, the dissection angles required to reach the ependymal roof from the different topographic points of the sulcus exhibit high interindividual variability and marked hemispheric asymmetry. Therefore, although the IPS represents a reproducible natural corridor that facilitates the preservation of white matter tracts, it is imperative to carry out strictly individualized angular presurgical planning to maximize the safety of this surgical corridor when approaching atrial lesions and deep areas such as the CCC.

Ethical approval:

This is a microanatomical study conducted on pre-existing, anonymized, and formalin-fixed human brain specimens belonging to the permanent educational collection of the Microsurgical Neuroanatomy Laboratory at the University of Buenos Aires. As the research did not involve living human subjects, patient data, or the procurement of new biological tissues, formal Institutional Review Board or Ethics Committee approval was not required.

Declaration of patient consent:

Patient’s consent is not required as there are no patients in this study.

Financial support and sponsorship:

Nil.

Conflicts of interest:

There are no conflicts of interest.

Use of artificial intelligence (AI)-assisted technology for manuscript preparation:

The authors confirm that there was use of artificial intelligence (AI)-assisted technology. During the preparation of this work, the authors used Gemini to assist in drafting and editing the manuscript, as well as to aid in the initial literature search. After using this tool, the authors thoroughly reviewed and edited the content, manually verified the accuracy of all references, and take full responsibility for the final content of the publication.

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