Optochiasmatic Cavernomas: Updated systematic review and proposal of a novel classification with surgical approaches.

Cavernomas are histologically benign vascular malformations found at different sites in the brain. A rare site for such cavernomas, however, is the anterior optic pathway, comprising the optic nerve, chiasma, and optic tract-called optochiasmatic cavernomas (OCC). These lesions usually present with sudden onset or progressive vision loss, headache, and features mimicking pituitary apoplexy. In this paper, we describe a case of OCC operated at our center. We carry out an updated review of literature depicting cases of OCC, their clinical presentation, management, and postoperative complications. We also propose a novel classification system based on lesion location and further analyze these cavernoma types with respect to the surgical approach used and visual outcome. A 30-year-old lady had presented with a 3-week history of progressive bilateral vision loss and headache. Based on imaging, she was suspected to have a cavernous angioma of the chiasma and left optic tract. Due to progressive vision deterioration, the lesion was surgically excised using pterional craniotomy. Postoperatively, her visual symptoms improved, but she developed diabetes insipidus. Clinical and radiological follow-up has been done for 18 months after surgery. A total of 81 cases have been described in the literature, including the present case. Chiasmal apoplexy is the most common presentation. Surgical excision is the standard of care. Our analysis based on lesion location shows the most appropriate surgical approach to be used for each cavernoma type. Visual outcome correlates with the preoperative visual status. Visual outcome is good in patients presenting with acute chiasmal apoplexy, and when complete surgical excision is performed. The endonasal endoscopic approach was found to provide the best visual outcome. In addition to preoperative visual status, complete surgical excision predicts favorable visual outcomes in OCC. Our proposed classification system guides the appropriate surgical approach required for a particular location of the cavernoma.

Microsurgical Resection of a Parachiasmatic Craniopharyngioma via a Left-Sided Pterional Transsylvian Approach.

Craniopharyngiomas are histologically benign tumors that originate from squamous rests along the pituitary stalk. They make up approximately 1.2% to 4.6% of all intracranial tumors and do not show significant differences in occurrence based on sex. Adamantinomatous craniopharyngiomas have 2 peaks of incidence, commonly observed in patients from ages 5 to 15 years and again from 45 to 60 years. In contrast, papillary craniopharyngiomas mainly affect adults in their fifth and sixth decades of life.1 The "malignancy" of craniopharyngiomas is attributed to their location and the challenges associated with achieving complete removal because they can manifest in the sellar, parachiasmatic, and intraventricular regions or a combination of these.2,3 Various approaches have been used to resect these tumors.4,5 Radical resection offers the most promising option for disease control, potential cure, and the ability to transform the disease from lethal to survivable in children, allowing for a functional adult life.2,3 Meticulous evaluation is crucial to determine the appropriate approach and side, with particular emphasis on closely examining the relationship between the tumor and optic pathways (nerve, chiasm, tract), which are frequently involved. This assessment should also include the tumor's relationship with other crucial structures, such as the hypothalamus and adjacent arteries, to ensure that the strategy is adjusted accordingly to further minimize the risk of postoperative morbidity. Video 1 demonstrates a left-sided pterional transsylvian approach to remove a parachiasmatic craniopharyngioma involving the left optic chiasm and tract.

Is the OCT a predictive tool to assess visual impairment in optic chiasm compressing syndrome in pituitary macroadenoma? A prospective longitudinal study.

Visual dysfunction is a prevalent symptom in patients with non-functioning pituitary macroadenoma (NFPM); the role of OCT in such patients has not been yet determined. This is a prospective longitudinal observational study over a period of 6 years, on 20 patients presenting a radiological compression of the optic chiasma without visual acuity (VA) and visual field (VF) disturbances. The primary endpoint was to evaluate the impact of NFPA on neuro-axonal loss by measuring RNFL thickness using OCT at inclusion (T0), 12 months (T1), 24 months (T2), and 36 months (T3), respectively. The secondary endpoint was to monitor the evolution of OCT over time and assess any relationship between the degree of OCT alteration and the degree of radiological and clinical optic chiasm compression syndrome. Among the 20 patients included, eight (40%) showed an altered RNFL-OCT at diagnosis, while the remaining 12 (60%) showed a normal pattern. During a mean ophthalmologic follow-up of 60 months, 4 patients (20%) presented an asymptomatic reduction of RNFL-OCT thickness although all 20 had a VA/VF stable. To our knowledge, this study represents the first attempt to longitudinally evaluate the natural history and evolution of RNFL-OCT in patients with radiologically asymptomatic chiasmatic compression syndrome. The results do not clearly demonstrate the role of the OCT as an early prognostic factor for visual dysfunction.

Prechiasmatic Transection of the Unilateral Dodge Class â Optic Pathway Glioma without Neurofibromatosis Type 1: Technical Description and Clinical Prognosis.

OBJECTIVE: For unilateral Dodge Class Ⅰ optic pathway glioma (OPG-uDCⅠ) without neurofibromatosis type 1, unilateral isolated optic nerve gliomas before the optic chiasm have been confirmed to possibly cause visual deterioration and poor prognosis. For this type of highly selective localized tumor, we explored surgery as the only treatment method. This article retrospectively analyzed and summarized the clinical data of this case series, with the aim of exploring the main technical details and clinical prognosis. METHODS: Included were patients with OPG-uDCⅠ without neurofibromatosis type 1 and experiencing vision loss on the affected side. The fronto-orbital approach was used, which was mainly divided into 3 parts: intraorbital, optic canal, and intracranial. All patients underwent prechiasmatic resection without any adjuvant treatments. The follow-up period was 3 months after surgery, and magnetic resonance imaging and contralateral visual acuity were reviewed annually after surgery. RESULTS: All OPG-uDCⅠ cases were completely removed without any adjuvant treatments, and there was no recurrence during the follow-up period. Pathological results showed that, except for 1 adult patient with pilomyxoid astrocytoma (World Health Organization grade Ⅱ), the others all had pilocytic astrocytoma (World Health Organization grade Ⅰ). Five patients experienced transient ptosis, and all recovered 3 months after surgery. CONCLUSIONS: For OPG-uDCⅠ without neurofibromatosis type 1, radical prechiasmatic resection of the tumor is possible, without the need for postoperative radiotherapy and chemotherapy.

Resultado a largo plazo de una serie de fetos con agenesia aislada del septum pellucidum / Long-term postnatal outcome of fetuses with isolated agenesis of the septum pellucidum

Objetivo: Reportar el resultado a largo plazo de una serie de fetos con agenesia del septum pellucidum aislada (ASP), con medición de su quiasma óptico mediante neurosonografía fetal (NSG). Método: Se incluyeron todas las pacientes con ASP y NSG evaluadas desde el año 2008 a la fecha y con seguimiento hasta su edad escolar. En todos los casos se consignaron los datos clínicos de NSG y de resonancia magnética (RM), cuando esta se realizó. Se entrevistó telefónicamente a los padres. Resultados: Nueve pacientes cumplieron los criterios: cuatro con displasia septo-óptica (DSO) (rango de seguimiento: 5-14 años) y cinco sin DSO (rango de seguimiento: 7-10 años). Un décimo caso se excluyó por tener solo 6 meses de seguimiento. Ninguna de las ASP tuvo otra anomalía detectada en su seguimiento. Ninguno de los casos con DSO tuvo alteración del tamaño de su quiasma óptico en la NSG ni anormalidad en la vía óptica en la RM. Conclusiones: En nuestra población, el riesgo residual de DSO frente a ASP es del 44,4%. En el seguimiento, nuestra definición de ASP por NSG no tuvo falsos negativos con relación a otras anomalías de aparición posnatal, a excepción de la DSO.

Objective: To report the long-term outcome of a series of fetuses with isolated septum pellucidum agenesis (ASP) with measurement of their optic chiasm by fetal neurosonography (NSG). Method: All patients with ASP and NSG evaluated from 2008 to date and with follow-up until their school age were included. In all cases, clinical, NSG and magnetic resonance imaging (MRI) data were recorded. Parents were interviewed by telephone. Results: Nine patients met the criteria: four with septo-optic dysplasia (SOD) (follow-up range: 5-14 years) and five without SOD (follow-up range: 7-10 years). A tenth case was excluded because only 6 months of follow-up. None of the ASP cases had another anomaly detected in their follow-up. None of the cases with DSO had anomaly of the size of their optic chiasm on NSG or abnormality in the optical pathway in the MRI. Conclusions: In our population, the residual risk of DSO versus ASP is 44.4%. At follow-up, our NSG definition of ASP had no false negatives in relation to other postnatal-onset anomalies, except for SOD.

Predicting visual recovery in pituitary adenoma patients post-endoscopic endonasal transsphenoidal surgery: Harnessing delta-radiomics of the optic chiasm from MRI.

OBJECTIVES: To investigate whether morphological changes after surgery and delta-radiomics of the optic chiasm obtained from routine MRI could help predict postoperative visual recovery of pituitary adenoma patients. METHODS: A total of 130 pituitary adenoma patients were retrospectively enrolled and divided into the recovery group (n = 87) and non-recovery group (n = 43) according to visual outcome 1 year after endoscopic endonasal transsphenoidal surgery. Morphological parameters of the optic chiasm were measured preoperatively and postoperatively, including chiasmal thickness, deformed angle, and suprasellar extension. Delta-radiomics of the optic chiasm were calculated based on features extracted from preoperative and postoperative coronal T2-weighted images, followed by machine learning modeling using least absolute shrinkage and selection operator wrapped with support vector machine through fivefold cross-validation in the development set. The delta-radiomic model was independently evaluated in the test set, and compared with the combined model that incorporated delta-radiomics, significant clinical and morphological parameters. RESULTS: Postoperative morphological changes of the optic chiasm could not significantly be used as predictors for the visual outcome. In contrast, the delta-radiomics model represented good performances in predicting visual recovery, with an AUC of 0.821 in the development set and 0.811 in the independent test set. Moreover, the combined model that incorporated age and delta-radiomics features of the optic chiasm achieved the highest AUC of 0.841 and 0.840 in the development set and independent test set, respectively. CONCLUSIONS: Our proposed machine learning models based on delta-radiomics of the optic chiasm can be used to predict postoperative visual recovery of pituitary adenoma patients. CLINICAL RELEVANCE STATEMENT: Our delta-radiomics-based models from MRI enable accurate visual recovery predictions in pituitary adenoma patients who underwent endoscopic endonasal transsphenoidal surgery, facilitating better clinical decision-making and ultimately improving patient outcomes. KEY POINTS: • Prediction of the postoperative visual outcome for pituitary adenoma patients is important but challenging. • Delta-radiomics of the optic chiasm after surgical decompression represented better prognostic performances compared with its morphological changes. • The proposed machine learning models can serve as novel approaches to predict visual recovery for pituitary adenoma patients in clinical practice.

Investigation of Demyelination and Remyelination Processes of the Compressed Optic Chiasm: An Experimental Model.

AIM: To demonstrated demyelination and remyelination of the optic nerve histologically by electron microscopy in an experimental model similar to the compression of pituitary adenomas on the optic chiasm. MATERIAL AND METHODS: The rats were fixed to a stereotaxic device under deep anesthesia, and a balloon catheter was placed under the optic chiasm through a burr hole which was in front of the bregma in accordance with the brain atlas of rats. The animals were divided into five groups (n=8): control, mild compression demyelination, severe compression demyelination, mild compression remyelination, severe compression remyelination. The fine structures of the tissues obtained were evaluated using electron microscopy. RESULTS: We found a significant difference in the severity of degeneration when comparing group 1 with group 5 (p < 0.001); there was no degeneration in group 1 rats and severe degeneration in all of the group 5 rats. Oligodendrocytes were found in all rats in group 1 and none of the rats in no group 2. The nuclei were preserved in the group 1 rats but damaged in all of the group 5 rats. There were no lymphocytes or erythrocytes in group 1 and all positives in group 5. CONCLUSION: This technique, which induced degeneration without causing damage to the optic nerve with toxic or chemical agents, revealed Wallerian degeneration similar to tumoral compression. After compression relief, the optic nerve remyelination process can be better understood, particularly for sellar lesions. In our opinion, this model may guide future experiments to identify protocols to induce and accelerate remyelination.

Endoscopic Endonasal Transtuberculum Approach for Pediatric Tuberoinfundibular Craniopharyngioma: 2-Dimensional Operative Video.

INDICATIONS CORRIDOR AND LIMITS OF EXPOSURE: The endoscopic endonasal transtuberculum approach grants access to suprasellar and retrochiasmatic lesions with hypothalamic involvement. Here, we present a case of a 13-year-old boy with a history of stunted growth, decreased vision, headaches, and low energy with a tuberoinfundibular craniopharyngioma. The patient consented to the procedure. ANATOMIC ESSENTIALS NEED FOR PREOPERATIVE PLANNING AND ASSESSMENT: Evaluation of the sphenoid sinus pneumatization, internal carotid artery disposition, presence of clinoidal rings, variations of the infrachiasmatic corridor (optic chiasm location, height of dorsum sella), and location of the pituitary stalk are crucial for surgical strategy. ESSENTIALS STEPS OF THE PROCEDURE: Harvesting of nasoseptal flap and access to the sphenoid sinus; drilling the sella, tuberculum, and chiasmatic sulcus up to the limbus sphenoidalis and laterally exposing the clinoidal carotid artery segment; wide dural opening to the level of distal rings inferolaterally and falciform ligaments superolaterally; identification and coagulation of superior hypophyseal branches providing tumor supply; intracapsular dissection and debulking and subpial sharp dissection at the hypothalamic tumor interface to achieve complete removal; and reconstruction with inlay collagen, fascia lata, and nasoseptal flap. PITFALLS/AVOIDANCE OF COMPLICATIONS: Preservation of the superior hypophyseal arteries and stalk is essential for preventing pituitary dysfunction. Preoperative reckoning of hypothalamic invasion and identification of adequate interface aids in avoiding complications. To reduce CSF leak risk, multilayer reconstruction was performed and lumbar drain placed postoperatively. VARIANTS AND INDICATIONS FOR THEIR USE: For retroclival extension, intradural pituitary transposition should be considered to expand the corridor; in patients with preoperative hypopituitarism, pituitary sacrifice is most effective to increase retroclival access.

Portable MRI to assess optic chiasm decompression after endoscopic endonasal resection of sellar and suprasellar lesions.

OBJECTIVE: Low-field portable MRI (pMRI) is a recent technological advancement with potential for broad applications. Compared with conventional MRI, pMRI is less resource-intensive with regard to operational costs and scan time. The application of pMRI in neurosurgical oncology has not been previously described. The goal of this study was to demonstrate the efficacy of pMRI in assessing optic nerve decompression after endoscopic endonasal surgery for sellar and suprasellar pathologies. METHODS: Patients who underwent endoscopic endonasal surgery for sellar and suprasellar lesions at a single institution and for whom pMRI and routine MRI were performed postoperatively were retrospectively reviewed to compare the two imaging systems. To assess the relative resolution of pMRI compared with MRI, the distance from the optic chiasm to the top of the third ventricle was measured, and the measurements were compared between paired equivalent slices on T2-weighted coronal images. The inter- and intrarater correlations were analyzed. RESULTS: Twelve patients were included in this study (10 with pituitary adenomas and 2 with craniopharyngiomas) with varying degrees of optic chiasm compression on preoperative imaging. Measurements were averaged across raters before calculating agreement between pMRI and MRI, which demonstrated significant interrater reliability (intraclass correlation coefficient [ICC] = 0.78, p < 0.01). Agreement between raters within the pMRI measurements was also significantly reliable (ICC = 0.93, p < 0.01). Finally, a linear mixed-effects model was specified to demonstrate that MRI measurement could be predicted using the pMRI measurement with the patient and rater set as random effects (pMRI ß coefficient = 0.80, p < 0.01). CONCLUSIONS: The results of this study suggest that resolution of pMRI is comparable to that of conventional MRI in assessing the optic chiasm position in relation to the third ventricle. Portable MRI sufficiently demonstrates decompression of the optic chiasm after endoscopic endonasal surgery. It can be an alternative strategy in cases in which cost, scan-time considerations, or lack of intraoperative MRI availability may preclude the ability to assess adequate optic nerve decompression after endoscopic endonasal surgery for sellar and suprasellar lesions.

[Opportunities and challenges in the diagnosis and treatment of optic chiasm lesions: a clinical and research perspective].

The optic chiasm is a critical component of the visual pathway, and lesions in the pituitary and sellar regions can cause irreversible damage to a patient's visual function, resulting in a significant decrease in their quality of life. As a result, neuro-ophthalmology evaluation is a crucial part of the multidisciplinary treatment of pituitary diseases. However, due to the significant variation in the anatomical structure of the optic chiasm and the sellar region, as well as the complexity of the injury mechanism, chiasm injury can result in diverse manifestations and severity levels, which can make it difficult to correlate with anatomical parameters. In recent years, research has increasingly focused on the early recognition of optic chiasm compression, the prediction of visual function after intervention, and the long-term neurodegenerative effects, while optical coherence tomography (OCT), electrophysiological examinations, and functional magnetic resonance imaging are currently the most commonly used methods for evaluating sellar region lesions. However, the role of these methods, represented by OCT, in clinical diagnosis and treatment, still lacks high-level clinical evidence support, and the evaluation and prediction of optic chiasm function remain key areas for further study. In addition to compression lesions, lesions such as inflammation, infiltration, and demyelination in the sellar region, caused by systemic multi-system diseases, can also lead to visual function damage and require recognition in clinical practice.

Optical coherence tomography detects early optic nerve damage before visual field defect in patients with pituitary tumors.

Optical coherence tomography (OCT) is a useful tool for predicting visual recovery after the removal of pituitary tumors. However, the utility of OCT in patients with pituitary tumors and a normal visual field is unclear. We aimed to analyze OCT features in pituitary tumors without visual field defects. Pituitary tumors without visual field defects were selected. A total of 138 eyes from 69 patients, assessed by the Humphrey visual field test and OCT, were enrolled in this study. Using preoperative coronal sections of MR images, patients were divided into chiasmal compression (CC) and non-chiasmal compression (non-CC) groups, and OCT characteristics were examined. The CC and non-CC groups consisted of 40 and 29 patients, respectively. There were no differences in age, sex, tumor type, or degree of visual field testing, but the tumor size was different between the two groups. On OCT, macular thickness ganglion cell complex (mGCC) was significantly thinner in the CC group than that in the non-CC group (112.5 vs 117.4 um, P < 0.05). Based on a database of healthy participants, 24% and 2% of eyes in the CC and non-CC groups had abnormal mGCC thickness (P < 0.01), respectively. In a sub-analysis of the CC group, patients with an abnormal mGCC thickness were older than a normal one (58.2 vs 41.1 years, p < 0.01). OCT can detect early optic nerve damage due to optic CC by pituitary tumors, even in normal visual fields. The degree of mGCC thinning may provide an appropriate surgical timing for pituitary tumors that compress the optic chiasm.

Preoperative volume of the optic chiasm is an easily obtained predictor for visual recovery of pituitary adenoma patients following endoscopic endonasal transsphenoidal surgery: a cohort study.

BACKGROUND: Predicting the postoperative visual outcome of pituitary adenoma patients is important but remains challenging. This study aimed to identify a novel prognostic predictor which can be automatically obtained from routine MRI using a deep learning approach. MATERIALS AND METHODS: A total of 220 pituitary adenoma patients were prospectively enrolled and stratified into the recovery and nonrecovery groups according to the visual outcome at 6 months after endoscopic endonasal transsphenoidal surgery. The optic chiasm was manually segmented on preoperative coronal T2WI, and its morphometric parameters were measured, including suprasellar extension distance, chiasmal thickness, and chiasmal volume. Univariate and multivariate analyses were conducted on clinical and morphometric parameters to identify predictors for visual recovery. Additionally, a deep learning model for automated segmentation and volumetric measurement of optic chiasm was developed with nnU-Net architecture and evaluated in a multicenter data set covering 1026 pituitary adenoma patients from four institutions. RESULTS: Larger preoperative chiasmal volume was significantly associated with better visual outcomes ( P =0.001). Multivariate logistic regression suggested it could be taken as the independent predictor for visual recovery (odds ratio=2.838, P <0.001). The auto-segmentation model represented good performances and generalizability in internal (Dice=0.813) and three independent external test sets (Dice=0.786, 0.818, and 0.808, respectively). Moreover, the model achieved accurate volumetric evaluation of the optic chiasm with an intraclass correlation coefficient of more than 0.83 in both internal and external test sets. CONCLUSION: The preoperative volume of the optic chiasm could be utilized as the prognostic predictor for visual recovery of pituitary adenoma patients after surgery. Moreover, the proposed deep learning-based model allowed for automated segmentation and volumetric measurement of the optic chiasm on routine MRI.

Diencephalic syndrome-like presentation of brainstem tumor: a series case-based review.

PURPOSE: Russel described a rare clinical entity known as diencephalic syndrome (DS) in 1951, which was traditionally caused by a neoplasm in the hypothalamic-optic chiasmatic region. DS is characterized by severe emaciation despite adequate or slightly reduced caloric intake, locomotor hyperactivity, euphoria and other minor features. Current evidence suggests that a rare population of children with a similar phenotype may have their tumor located in the posterior fossa instead, defining the DS-like presentation, a rare entity with few cases reported in the literature. METHODS: A thorough search of three databases (PubMed, Ovid Medline, and Ovid Embase) was conducted to identify relevant papers reporting children with DS associated with brainstem tumors. To our knowledge, only seven cases have been documented in the literature. Moreover, we present four of our own cases, focusing on the unusual clinical presentation, the diagnosis process, and the lag time between the initial symptoms and the definitive diagnosis. RESULTS: In this review, the mean lag time between the onset of symptoms and diagnosis was 20.9 months (median: 16 months; range: 1.5-72 months), whereas in our series of cases, the time was 32.5 months (median: 33 months; range: 7-57 months). CONCLUSION: Despite recent significant advances in neuro-oncology diagnostic tools, this mean lag time did not improve when compared with the previous literature review from 1976. Throughout these data, we aim to raise awareness in the hopes of detecting intracranial neoplasms earlier in cases of children with profound emaciation of unknown cause.

Predicting visual outcomes after decompression of pituitary tumours based on stratified inner-retinal layer thickness and age.

PURPOSE: The purpose of the study was to evaluate the potential role of stratified preoperative optical coherence tomography (OCT) measurements and age in predicting postoperative visual field (VF) improvement among adult patients with chiasmal compression due to pituitary tumours after decompression surgery. METHODS: Postoperative visual outcomes were analysed using mean deviation of the VF test. Eyes were divided into three groups based on preoperative OCT parameters including peripapillary retinal nerve fibre layer (pRNFL) and ganglion cell-inner plexiform layer (GCIPL) thickness: groups 1, 2 and 3 with pRNFL thickness <65 µm, 65 µm or more but <85 µm, and 85 µm or more, respectively. The GCIPL thickness was also divided into three groups based on values ranging from 55 to 77 µm. Each group was further categorized according to age: 20 years or older but younger than 40 years, 40 years or older but younger than 60, and 60 years or older. RESULTS: This study included 197 eyes of 197 patients with chiasmal compression due to pituitary tumours. No patient showed complete VF recovery in group 1 with a preoperative pRNFL thickness <65 µm or a GCIPL thickness <55 µm regardless of age. These groups showed the worst VF outcome (pRNFL, p = 0.0001; GCIPL, p < 0.0001). However, a significant recovery in VF (greater than 2 dB) was observed in 45% of group 1 patients based on pRNFL thickness and in 61.54% of group 1 patients according to GCIPL thickness. In groups 2 and 3 with a preoperative pRNFL thickness of 65 µm or more and a GCIPL thickness of 55 µm or more, the rate of complete VF recovery decreased as subjects' ages increased. Group 3 with a preoperative pRNFL thickness of 85 µm or more and a GCIPL thickness of 77 µm or more were 2.5-fold and 4.0-fold more likely to completely recover VF, respectively, compared with group 2. CONCLUSIONS: Stratified preoperative pRNFL and GCIPL thicknesses measured via OCT in different age categories are effective biomarkers for predicting visual functional outcomes.

Decussating axons segregate within the anterior core of the primate optic chiasm.

BACKGROUND: The axons of ganglion cells in the nasal retina decussate at the optic chiasm. It is unclear why tumours cause more injury to crossing nasal fibres, thereby giving rise to temporal visual field loss in each eye. To address this issue, the course of fibres through the optic chiasm was examined following injection of a different fluorescent tracer into each eye of a monkey. METHODS: Under general anaesthesia, cholera toxin subunit B-Alexa Fluor 488 was injected into the right eye and cholera toxin subunit B-Alexa Fluor 594 was injected into the left eye of a single normal adult male rhesus monkey. After a week's survival for anterograde transport, serial coronal sections through the primary optic pathway were examined. RESULTS: A zone within the core of the anterior and mid portions of the optic chiasm was comprised entirely of crossing fibres. This zone of decussation was delineated by segregated, interwoven sheets of green (right eye) and red (left eye) fibres. It expanded steadily to fill more of the optic chiasm as fibres coursed posteriorly towards the optic tracts. Eventually, crossed fibres became completely intermingled with uncrossed fibres, so that ocular separation was lost. CONCLUSIONS: A distinct, central compartment located within the anterior two-thirds of the optic chiasm contains only crossing fibres. Sellar tumours focus their compressive force on this portion of the structure, explaining why they so often produce visual field loss in the temporal fields.