Volumetric segmentation in the context of posterior fossa-related pathologies: a systematic review.

BACKGROUND: Segmentation tools continue to advance, evolving from manual contouring to deep learning. Researchers have utilized segmentation to study a myriad of posterior fossa-related conditions, such as Chiari malformation, trigeminal neuralgia, post-operative pediatric cerebellar mutism syndrome, and Crouzon syndrome. Herein, we present a summary of the current literature on segmentation of the posterior fossa. The review highlights the various segmentation techniques, and their respective strengths and weaknesses, employed along with objectives and outcomes of the various studies reported in the literature. METHODS: A literature search was conducted in PubMed, Embase, Cochrane, and Web of Science up to November 2023 for articles on segmentation techniques of posterior fossa. The two senior authors searched through databases based on the keywords of the article separately and then enrolled joint articles that met the inclusion and exclusion criteria. RESULTS: The initial search identified 2205 articles. After applying inclusion and exclusion criteria, 77 articles were selected for full-text review after screening of titles/abstracts. 52 articles were ultimately included in the review. Segmentation techniques included manual, semi-automated, and fully automated (atlas-based, convolutional neural networks). The most common pathology investigated was Chiari malformation. CONCLUSIONS: Various forms of segmentation techniques have been used to assess posterior fossa volumes/pathologies and each has its advantages and disadvantages. We discuss these nuances and summarize the current state of literature in the context of posterior fossa-associated pathologies.

[Endoscopic transnasal resection of clival meningiomas]. / Rezul'taty endoskopicheskogo transnazal'nogo udaleniya meningiom oblasti skata.

BACKGROUND: Surgical treatment of ventral and ventrolateral meningiomas of posterior cranial fossa is difficult in modern neurosurgery. This is due to peculiarities of approach to these areas and concentration of critical structures (cranial nerves and great vessels). Currently, endoscopic transnasal approach to these meningiomas allows partial, and in some cases, total resection. However, this technique is not widespread. OBJECTIVE: To analyze the world literature data on postoperative outcomes in patients with clival and petroclival meningiomas after endoscopic transnasal resection. MATERIAL AND METHODS: We analyzed 22 articles representing treatment of 61 patients with clival and petroclival meningiomas. RESULTS: Total or near-total resection was achieved in 22.9% of cases, subtotal resection - 40.9%, partial resection - 26.2% (data were not provided in other cases). Even partial and subtotal resection leads to significant regression of symptoms. CONCLUSION: Endoscopic transnasal surgery is a full-fledged alternative to transcranial approaches in surgical treatment of clival meningiomas. It is also an additional option for patients with petroclival meningiomas after ineffective transcranial approaches. Transnasal tumor shrinkage and devascularization lead to brainstem decompression, regression of hydrocephalus and baseline clinical symptoms.

Red flags alerting a posterior cranial fossa tumor from audiovestibular perspectives - a review.

BACKGROUND: There is no comprehensive and up-to-date overview of audiovestibular approach to the posterior fossa tumors in the literature. OBJECTIVE: This paper reviewed the literature relating to tumors at the posterior cranial fossa to find red flags alerting a posterior fossa lesion from audiovestibular perspectives. METHODS: This review was developed from articles published in those journals listed on the journal citation reports. Through the PubMed database, Embase, Google Scholar, and Cochrane library, 60 articles were finally obtained based on the PRISMA guidelines for reporting reviews. RESULTS: The presence of one red flag indicates a positive predictive value of 33% for detecting a posterior fossa lesion. Clinical features, namely, 1) mid-frequency sudden sensorineural hearing loss (SNHL), 2) bilateral sudden SNHL, and 3) rebound nystagmus may indicate a posterior fossa lesion, representing one, two, and three red flags, respectively. CONCLUSION: Those with 1) mid-frequency sudden SNHL, 2) bilateral sudden SNHL, and 3) rebound nystagmus trigger one, two, and three red flags, respectively, alerting clinicians the possibility of a posterior fossa lesion, which warrant MR imaging to exclude life-threatening or treatable conditions. SIGNIFICANCE: Patients with posterior fossa tumors may have potential life-threatening outcome.

'Choroid bar': easy-to-seek marker of normal posterior fossa at 12-14 weeks' gestation.

OBJECTIVES: Our objectives were: (1) to assess the visualization rate of the choroid bar in a consecutive series of 306 first-trimester scans; (2) to verify, in this cohort of fetuses, the normality of the posterior fossa later in pregnancy; and (3) to confirm the non-visualization of the choroid bar in a retrospective series of fetuses with posterior fossa malformations. METHODS: This study included a prospective and a retrospective series. The former comprised 306 fetuses undergoing routine obstetric ultrasound at our unit in both the first and second trimesters over a 6-month period, while the latter comprised 12 cases of posterior fossa malformations. In the prospective study, the presence of the choroid bar, which is defined as a visually continuous, homogeneously hyperechogenic, thick structure bridging the cisterna magna from side to side, was evaluated at the end of the first-trimester nuchal translucency scan. In the retrospective study, previously acquired three-dimensional volume datasets were processed in order to assess whether the choroid bar could be visualized in cases of open spinal dysraphisms and vermian cystic anomalies. In the prospective study, confirmation of a normal posterior fossa was based on the sonographic features of this anatomical region at the time of the second-trimester anomaly scan at 19-21 weeks' gestation, while, in the retrospective study, it was based on autopsy results, when available, or further direct imaging of the defect later in pregnancy. RESULTS: In the prospective study, the choroid bar could be visualized in all 306 fetuses, on transabdominal ultrasound in 287 (93.8%) cases and on transvaginal ultrasound in 19 (6.2%) cases. The choroid bar was displayed with a ventral/dorsal approach in 67 (21.9%) cases, with a lateral approach in 56 (18.3%) cases and with both in 183 (59.8%) cases. All 306 cases were confirmed to have a sonographically normal posterior fossa at 19-21 weeks. On the other hand, in the retrospective study, it was not possible to visualize the choroid bar in any of the fetuses with posterior fossa malformations. CONCLUSIONS: We have described a new sign, the choroid bar, consistent with a normal posterior fossa at 12-14 weeks' gestation. The choroid bar provides the option of screening for major abnormalities of the posterior fossa, since its absence raises suspicion of both open spinal dysraphisms and posterior fossa cystic malformations. At the same time, it is easy to visualize, as it can be seen with all lines of insonation. © 2023 International Society of Ultrasound in Obstetrics and Gynecology.

The Effect of Posterior Fossa Decompression Surgery on Brainstem and Cervical Spinal Cord Dimensions in Adults with Chiari Malformation Type 1.

OBJECTIVE: Posterior fossa decompression (PFD) surgery creates more space at the skull base, reduces the resistance to the cerebrospinal fluid motion, and alters craniocervical biomechanics. In this paper, we retrospectively examined the changes in neural tissue dimensions following PFD surgery on Chiari malformation type 1 adults. METHODS: Measurements were performed on T2-weighted brain magnetic resonance images acquired before and 4 months after surgery. Measurements were conducted for neural tissue volume and spinal cord/brainstem width at 4 different locations; 2 width measurements were made on the brainstem and 2 on the spinal cord in the midsagittal plane. Cerebellar tonsillar position (CTP) was also measured before and after surgery. RESULTS: Twenty-five adult patients, with a mean age of 38.9 ± 8.8 years, were included in the study. The cervical cord volume increased by an average of 2.3 ± 3.3% (P = 0.002). The width at the pontomedullary junction increased by 2.2 ± 3.5% (P < 0.01), while the width 10 mm caudal to this junction increased by 4.2 ± 3.9% (P < 0.0001). The spinal cord width at the base of second cervical vertebra and third cervical vertebra did not significantly change after surgery. The CTP decreased by 60 ± 37% (P < 0.0001) after surgery, but no correlation was found between CTP change and dimension change. CONCLUSIONS: The brainstem width and cervical cord volume showed a modest increase after PFD surgery, although standard deviations were large. A reduction in compression after PFD surgery may allow for an increase in neural tissue dimension. However, clinical relevance is unclear and should be assessed in future studies with high-resolution imaging.

Growth patterns and ratios of posterior cranial fossa structures in the Japanese pediatric population: a study utilizing CT scans.

PURPOSE: The changes in the proportion of posterior cranial fossa structures during pediatric development remain unclear. This retrospective study aimed to investigate the growth patterns and ratios of these structures using CT scans. METHODS: Head CT scans of pediatric patients with minor head trauma from Osaka Women's and Children's Hospital between March 2006 and May 2023 were analyzed. The study segmented the intracranial volume (ICV), posterior cranial fossa volume (PCFV), cerebellum volume (CBMV), and brainstem volume (BSV). Correlation coefficients were calculated among the parameters. Patients aged 0 to 10 years were divided into 15 age-related clusters, and mean and standard deviation values were measured. Growth curves were created by plotting mean values sequentially. Ratios such as PCFV/ICV and (CBMV + BSV)/PCFV were examined. Statistical analyses, including unpaired t tests and logarithmic curve fitting, were performed. RESULTS: A total of 234 CT scans (97 from females, 115 from infants under 1 year of age) were analyzed. Positive correlations were observed among the parameters, with the strongest between PCFV and CBMV. The growth curves for ICV, PCFV, CBMV, and BSV exhibited a two-phase process, with rapid growth until approximately 4 years of age, followed by stabilization. The ratios PCFV/ICV and (CBMV + BSV)/PCFV showed increasing trends from birth onwards, stabilizing by 4 and 1 years of age, respectively. CONCLUSION: This study provides insights into the growth patterns and ratios of posterior cranial fossa structures in the pediatric population. The findings demonstrate a two-phase growth process and increasing trends in the examined ratios.

Full-Endoscopic Technique for Posterior Fossa Decompression in Chiari Malformation.

BACKGROUND AND IMPORTANCE: Full-endoscopic techniques are well-described for spinal procedures. Although endoscopic-assisted techniques are reported for posterior fossa decompression (PFD) in Chiari malformation (CM), a full-endoscopic technique is yet to be reported in these patients. The aim of this study was to present and describe a full-endoscopic technique for PFD in patients with CM. CLINICAL PRESENTATION: Two patients diagnosed with CM were operated on by the full-endoscopic PFD technique. The patients consented to the procedure and to the publication of their image. An endoscope with an oval shaft cross-section with a diameter of 9.3 mm, a working length of 177 mm, a viewing angle of 20°, and a working channel of 5.6 diameters were used. Operative videos were recorded. The surgical steps were easily applied after the clear anatomic landmarks, such as the C1 posterior tubercle and the rectus capitis posterior minor muscles. The patients were followed up for 6 months. Both patients were symptom-free with a significant decrease in Visual Analog Scale score and a good functional outcome assessed by Chicago Chiari Outcome Scale after surgery without any complications. CONCLUSION: All the steps of the full-endoscopic technique for PFD described by the authors in their previous human cadaveric study were also feasible on patients with CM.

Combined retrosigmoid and middle fossa approach for a small, superiorly located petroclival meningioma: how I do it.

BACKGROUND: Petroclival meningiomas pose significant surgical challenges because of their deep location and complex surrounding neurovasculature. The use of multiple surgical approaches can optimize safe tumor removal from multiple anatomic compartments. METHOD: We describe a patient with a growing superior petroclival meningioma centered at the posterior clinoid with extension into Meckel's cave that was successfully removed with a combined retrosigmoid and subtemporal middle fossa approach. This strategy avoided the need for anterior petrous bone drilling and tentorial splitting. CONCLUSION: A combined retrosigmoid and subtemporal middle fossa approach can provide safe access to tumors spanning the supra- and infratentorial compartments.

Combined and staged retrosigmoid, extended middle fossa, and endoscopic transnasal approach to a petroclival chondrosarcoma: how I do it.

BACKGROUND: Tumors of the petroclival region with multicompartment extension can be difficult to address with a single surgical approach. METHOD: We present the case of a patient with a large chondrosarcoma centered at the right petroclival fissure with extension into the cavernous sinus, the region beneath the cavernous sinus, cerebellopontine angle with deformation of the pons, and prevertebral space. A staged complete resection was performed using a stage 1 single-incision combined right retrosigmoid craniotomy and extended middle fossa craniotomy, followed by a stage 2 endoscopic transnasal approach. CONCLUSION: A combined approach to selected petroclival tumors can maximize safe resection.

Basilar tubercles and eminences of the clivus: Novel anatomical entities.

BACKGROUND: The clivus forms the central skull base between the dorsum sellae and the foramen magnum. Although bony variations of the inferior surface of the clivus are well-recognized and have been well studied, studies of bony variations of the basilar (superior) surface of the clivus are scarce. Therefore, the present study was performed to investigate bony anatomical variations on the basilar part of the clivus. METHODS: Computed tomography scans belonging to 407 Indian subjects from the CQ500 open-access dataset were retrospectively reviewed. RESULTS: Bony tubercles on the basilar surface of the clivus were found in 40 cases (9.83%). They were classified into three types including single, double and triple. A single tubercle was found in 35 cases (8.60%) including 12 on the left (2.95%), 10 on the right (2.46%) and 13 in the center (3.19%). The tubercles were doubled in four cases (0.98%) and tripled in one case (0.25%). The average width and height of the tubercles were 4.4 ± 1.5 mm (range 1.4-7.9 mm) and 1.7 ± 0.7 mm (range 0.8-4.2 mm), respectively. Ninety-five (95%) percent of the tubercles were located on the lower half of the clivus. CONCLUSIONS: To our knowledge, these tubercles have not been previously described. Therefore, we suggest the terms "basilar tubercles of the clivus" and "basilar eminences of the clivus", depending on their sizes. Knowledge of these newly described structures is important when interpreting radiological images of the skull base.

Ecchordosis Physaliphora: Does It Even Exist?

The term ecchordosis physaliphora (EP) has been used historically to describe a benign notochordal remnant with no growth potential, most commonly occuring in the central clivus. Unfortunately, the radiologic appearance of EP overlaps considerably with the appearance of low-grade chordomas, which do have the potential for growth. In this article, we review new pathologic terminology that better describes this family of diseases, and we propose new radiologic terms that better address the uncertainty of the radiologic diagnosis. The surgical importance of accurate terminology and the implications for patient care are discussed.

Posterior Fossa Horns in Hurler Syndrome: Prevalence and Regression.

Posterior fossa "horns" caused by internal hypertrophy of the occipitomastoid sutures are one of the more recently defined cranial abnormalities described in mucopolysaccharidoses, especially in Hurler Syndrome. However, details of this finding, including the development and natural history, are not well-understood. Two hundred eighty-six brain MR imaging studies of 61 patients with mucopolysaccharidosis I-Hurler syndrome treated at single institution between 1996 and 2015 were studied. Posterior fossa horn height was measured as the perpendicular distance from the tip of the horn to the expected curvature of the occipital inner table. Fifty-seven of the 61 patients (93.4%) had evidence of posterior fossa horns on at least one occasion. The initial average height of the right horn was 4.5 mm, and the left horn, 4.7 mm. Most of the posterior horns regressed before transplantation in our cohort, though the exact age was variable among the patients. Nearly all patients in our cohort had posterior fossa horns, and these horns regressed with age. The regression of the horns frequently started before transplantation. This trend has not been previously described, and it may suggest unknown effects of mucopolysaccharidosis on skull development.

Onset of Chiari type 1 malformation: insights from a small series of intrauterine MR imaging cases.

PURPOSE: Morphometric studies on idiopathic Chiari malformation type 1 (CM1) pathogenesis have been mainly based on post-natal neuroimaging. Prenatal clues related to CM1 development are lacking. We present pre- and post-natal imaging time course in idiopathic CM1 and assess fetal skull and brain biometry to establish if clues about CM1 development are present at fetal age. METHODS: Multicenter databases were screened to retrieve intrauterine magnetic resonance (iuMR) of children presenting CM1 features at post-natal scan. Syndromes interfering with skull-brain growth were excluded. Twenty-two morphometric parameters were measured at fetal (average 24.4 weeks; range 21 to 32) and post-natal (average 15.4 months; range 1 to 45) age; matched controls were included. RESULTS: Among 7000 iuMR cases, post-natal scans were available for 925, with postnatal CM1 features reported in seven. None of the fetuses presented CM1 features. Tonsillar descent was clear at a later post-natal scan in all seven cases. Six fetal parameters resulted to be statistically different between CM1 and controls: basal angle (p = 0.006), clivo-supraoccipital angle (p = 0.044), clivus' length (p = 0.043), posterior cranial fossa (PCF) width (p = 0.009), PCF height (p = 0.045), and PCFw/BPDb (p = 0.013). Postnatally, only the clivus' length was significant between CM1 cases and controls. CONCLUSION: Pre- and post-natal CM1 cases did not share striking common features, making qualitative prenatal assessment not predictive; however, our preliminary results support the view that some of the pathogenetic basis of CM1 may be embedded to some extent already in intrauterine life.

Establishing proof of concept for sonolucent cranioplasty and point of care ultrasound imaging after posterior fossa decompression for Chiari malformation.

BACKGROUND: Evaluation after posterior fossa decompression for Chiari malformation can require repeated imaging, particularly with persistent symptoms. Typically, CT or MRI is used. However, CT carries radiation risk and MRI is costly. Ultrasound is an inexpensive, radiation-free, point-of-care modality that has, thus far, been limited by intact skull and traditional cranioplasty materials. Ultrasound also allows for imaging in different head positions and body postures, which may lend insight into cause for persistent symptoms despite adequate decompression on traditional neutral static CT or MRI. We evaluate safety and feasibility of ultrasound as a post-operative imaging modality in patients reconstructed with sonolucent cranioplasty during posterior fossa decompression for Chiari malformation. METHODS: Outcomes were analyzed for 26 consecutive patients treated with a Chiari-specific sonolucent cranioplasty. This included infection, need for revision, CSF leak, and pseudomeningocele. Ultrasound was performed point-of-care in the outpatient clinic by the neurosurgery team to assess feasibility. RESULTS: In eight months mean follow up, there were no surgical site infections or revisions with this novel sonolucent cranioplasty. Posterior fossa anatomy was discernable via transcutaneous ultrasound obtained point-of-care in the clinic setting at follow up visits. CONCLUSION: We demonstrate proof of concept for ultrasound as a post-operative imaging modality after posterior fossa decompression for Chiari malformation. With further investigation, ultrasound may prove to serve as an alternative to CT and MRI in this patient population, or as an adjunct to provide positional and dynamic information. Use of sonolucent cranioplasty is safe. This technique deserves further study.

Rapidly progressive cerebral atrophy following a posterior cranial fossa stroke: Assessment with semiautomatic CT volumetry.

BACKGROUND: The effect of posterior cranial fossa stroke on changes in cerebral volume is not known. We assessed cerebral volume changes in patients with acute posterior fossa stroke using CT scans, and looked for risk factors for cerebral atrophy. METHODS: Patients with cerebellar or brainstem hemorrhage/infarction admitted to the ICU, and who underwent at least two subsequent inpatient head CT scans during hospitalization were included (n = 60). The cerebral volume was estimated using an automatic segmentation method. Patients with cerebral volume reduction > 0% from the first to the last scan were defined as the "cerebral atrophy group (n = 47)," and those with ≤ 0% were defined as the "no cerebral atrophy group (n = 13)." RESULTS: The cerebral atrophy group showed a significant decrease in cerebral volume (first CT scan: 0.974 ± 0.109 L vs. last CT scan: 0.927 ± 0.104 L, P < 0.001). The mean percentage change in cerebral volume between CT scans in the cerebral atrophy group was -4.7%, equivalent to a cerebral volume of 46.8 cm3, over a median of 17 days. The proportions of cases with a history of hypertension, diabetes mellitus, and median time on mechanical ventilation were significantly higher in the cerebral atrophy group than in the no cerebral atrophy group. CONCLUSIONS: Many ICU patients with posterior cranial fossa stroke showed signs of cerebral atrophy. Those with rapidly progressive cerebral atrophy were more likely to have a history of hypertension or diabetes mellitus and required prolonged ventilation.

The Management of Hydrocephalus in Midline Posterior Fossa Cystic Collections: Surgical Outcome From a Retrospective Single-Center Case Series of 54 Consecutive Pediatric Patients.

BACKGROUND: Hydrocephalus frequently occurs with midline posterior fossa cystic collections. The classification of this heterogeneous group of developmental anomalies, including Dandy-Walker malformation, persisting Blake's pouch, retrocerebellar arachnoid cysts, and mega cisterna magna, is subject of debate. The absence of diagnostic criteria is confusing regarding the ideal management of PFCC-related hydrocephalus. OBJECTIVE: To decipher the surgical strategy for the treatment of children with PFCC-related hydrocephalus through a retrospective analysis of the surgical outcome driven by their clinical and radiological presentation. METHODS: This study enrolled patients operated of symptomatic PFCC-related hydrocephalus. Clinical and MRI features were examined, as well as the surgical outcome. Unbiased subgroup classification of the patients was performed with multiple component analysis as a function of imaging characteristics and hierarchical clustering on principal component. Outcome was assessed with binomial logistic regression and Kaplan-Meier analysis. RESULTS: Fifty-four patients were included between 2007 and 2021. Multiple component analysis suggested that cerebellar and vermian hypoplasia, vermian rotation, basal-tentorial angle, and fastigial angle were strongly correlated. Hierarchical clustering and the distribution of the patients in the bidimensional plot showed the clear segregation of 3 major clusters, which correlated with the radiological diagnosis ( P < .01). Binomial logistic regression and survival analysis showed that endoscopic third ventriculostomy was an effective treatment for patients with persisting Blake's pouch, while failing to control hydrocephalus in most of patients with Dandy-Walker malformation. CONCLUSION: Preoperative MRI in patients with PFCC-related hydrocephalus is essential to better define the diagnosis. The choice of treatment strategy notably relies on correct radiological diagnosis.

Sella Volume and Posterior Fossa Morphometric Measurements in Chiari Type 1.

AIM: To retrospectively evaluate the relationships between morphometric measurements in craniovertebral junction, and the possible genetic basis of the disease in Chiari 1 malformation patients undergoing surgical treatment. MATERIAL AND METHODS: We included patients who underwent surgical treatment for Chiari 1 malformation with unknown etiology between 2013-2018 in the Neuosurgery Clinic of Eskisehir Osmangazi University. Morphometric and volumetric measurements of the posterior fossa from cranial and cervical radiological imagings of patients with Chiari 1 malformation were performed by a single experienced observer. The results were independently reviewed by two experienced observers who supervised the process and verified the calculations. RESULTS: It was observed that basal angle was greater in the group of patients with Chiari 1 malformation (125.29° ± 9.522° Vs. 112.33° ± 9.09°), and a significant difference was observed (p=0.000). In patient group, it was also found that the basion-dens distance (3.51 mm ± 1.695mm Vs. 5.27 ± 1.695 mm) (p=0.000), and cerebellar hemispher height were shorter (58.47 ± 5.143 mm Vs. 52.05 ± 5,008 mm) (p=0.000). Tentorium angle was greater in the patient group (97.95° ± 11.321° vs. 87.79° ± 9.891°) (p=0.034). In patient group, dens height was found to be longer (13.9 ± 2.46 mm vs. 14.79 ± 1.631mm) (p=0.041). Sella volume was found to be smaller (41.4 mm3 Vs. 53.3 mm3) in the patient group compared to the control group (p=0.034), and foramen magnum plane was larger (64.6° vs. 31.1°) (p=0.000). CONCLUSION: It is concluded that many parameters including BA, FMP, BDD, CHH, TA, DH, and cellar volume are abnormal, suggesting the need for focussing to the hormonal metabolism - related synchondral changes.

A new supportive approach in the diagnosis of Chiari malformation type 1 in pediatric patients.

PURPOSE: Chiari malformation type 1 (CM-1) is a posterior fossa anomaly characterized by herniation of the cerebellar tonsils from the foramen magnum (FM). This study compares FM, medulla spinalis (MS), and herniated cerebellar tonsils ratios by making area measurements from axial plane MRI in CM-1 patients and the control group. METHODS: Our study evaluated 30 pediatric patients with CM-1 and 30 people in the control group. The lengths of the McRae line, twining line, and clivus line were measured on the posterior cranial fossa evaluation. The areas of FM (AFM), MS (AMS), and herniated cerebellar tonsils (ATONSILS) were measured by axial images. RESULTS: As a result of area measurements obtained from axial cross-sectional MRI, a statistically significant difference was found between CM-1 patients and the control group. According to the results of the ROC analysis, if an individual's AMS/AFM value is above 17.9% or the ATONSILS/AFM value is above 18.4%, it can be interpreted as a CM-1 patient. CONCLUSION: It will be easier to diagnose the patient with the new approach we obtained from axial MR images in addition to sagittal MR images. This method can be a guide in some cases when the surgeons are undecided.

Defining the Clival Recess Surgical Corridor and Clival Classification System for Approach to Sellar Pathology.

BACKGROUND: Sellar masses within the pars intermedius, bordered anteriorly by normal pituitary gland/stalk, and/or with ectatic cavernous carotid anatomy are challenging and high risk when approached through the endonasal standard direct/anterior sellar approach. This approach portends itself to a higher risk of pituitary gland/stalk injury and subtotal resection with the aforementioned anatomic variants. OBJECTIVE: To describe the indirect clival recess corridor approach to sellar lesions. This corridor is a "silent" point of access to lesions in this region endoscopically. While skull base teams may have used this approach to some degree, it has not yet been described in the literature to our knowledge. METHODS: We defined the clival recess surgical corridor with skull base craniometric measurements and use a case example with aberrant anatomy to illustrate the approach. We cross-sectionally reviewed 42 patients with sellar and suprasellar masses. To describe the approach's anatomy, we devised and defined the terms dorsum sella plumb line, anatomic corridor, angle of osseous, and operative corridor. RESULTS: Created novel clival aeration grade informing surgical planning. Classified clival aeration as Grade 1 (100%-75% aeration), Grade 2 (75%-50% aeration), Grade 3 (50%-25% aeration), and Grade 4 (25%-0% aeration). This classification system determines extent of drilling of the clivus required to optimize the clival recess corridor approach and its limitations. CONCLUSION: The clival recess surgical corridor is effective for accessing pituitary lesions within the sella. Consider the indirect approach when a standard direct/anterior sellar approach has high risk for vascular injury and/or endocrinological dysfunction.