The anatomic variations and surgical windows among optic chiasm/nerves and carotid arteries in the sellar region play a role in choosing the best surgical approaches: A Cadaveric study

Abstract

Objective

Understanding the relationship between the carotid artery, optic nerve and the anterior clinoid process is the basis of surgical approaches performed in the subchiasmal region. The location of the optic chiasm, the length of the optic nerves, and the distance and angle between the optic nerves determine the route of surgical approaches. We have determined the types of optic chiasm to study the relationship between vascular and neural structures in this region.

Materials and methods

Thirty autopsy specimens were investigated at the Bursa Forensic Medicine Institute for optic chiasm types and the relationship between the neural and vascular anatomical structures of the sellar–parasellar and subchiasmal region was examined between June 2016 and November 2016.

Results

In this study, 4 prefix types (13%), 6 postfix types (20%), and 20 central types (67%) of chiasm were defined. Furthermore, we measured this angle between two optic nerves, which indirectly shows the location of chiasm according to the diaphragma sellae, and then detected the mean value of this angle as 87.1 ± 11.6°. The “limit” value to designate a chiasm as prefix was measured in the current study as ≥101.1°. The angle between optic nerves ranged from a mean value of 69.9 ± 3.7° in 6 cases with postfix chiasm, to a mean value of 104.0 ± 2.1° in 4 cases with prefix chiasm and a mean value of 88.8 ± 6.7° in 20 cases with central chiasm.

Conclusion

In this study, we showed that the relationship among optic chiasma types, optic nerves and bony and vascular structures around the sellar area was effective at determining the surgical approach to this region.

Resumen

Objetivo

Comprender la relación entre la arteria carótida, el nervio óptico y la apófisis clinoides anterior es la base de los tratamientos quirúrgicos realizados en la región subquiasmática. La ubicación del quiasma óptico, la longitud de los nervios ópticos, y la distancia y el ángulo entre dichos nervios determinan la vía de acceso quirúrgico. Hemos determinado los tipos de quiasma óptico para estudiar la relación entre las estructuras vasculares y neurales en esta región.

Materiales y métodos

Entre junio y noviembre de 2016, se analizaron 30 muestras de autopsia en el Instituto de Medicina Legal de Bursa (Turquía) para determinar los tipos de quiasma óptico, y examinar la relación entre las estructuras anatómicas neurales y vasculares de las regiones selar-paraselar y subquiasmática.

Resultados

En este estudio, se definieron 4 casos de quiasma prefijado (13%), 6 de quiasma posfijado (20%) y 20 de quiasma central (67%). Además, medimos el ángulo entre 2 nervios ópticos que muestra indirectamente la ubicación del quiasma según el diafragma selar, y luego detectamos el valor medio de este ángulo (87,1 ± 11,6°). El valor «límite» para designar un quiasma como «prefijado» se midió en el estudio actual como ≥ 101,1°. El valor medio del ángulo entre los nervios ópticos osciló entre 69,9 ± 3,7° en los 6 casos de quiasma posfijado, 104,0 ± 2,1° en los 4 casos de quiasma prefijado y 88,8 ± 6,7° en los 20 casos de quiasma central.

Conclusión

En este estudio, pusimos de manifiesto que la relación entre los tipos de quiasma óptico, los nervios ópticos y las estructuras óseas y vasculares alrededor del área selar fue eficaz para determinar el acceso quirúrgico en esta región.

Introduction

Sellar and parasellar region is a critical anatomical structure in the middle of skull base, which is limited to optic nerve, chiasm and Willis polygon superiorly, cavernous sinuses and internal carotid artery (ICA) laterally, and brainstem and basilar artery posteriorly.¹ In this region, the most easily affected neural structure by the pathologies is optical apparatus. Anterior cerebral arteries and anterior communicating artery are located superiorly to the chiasm. Tuber cinereum and infundibulum are located postero-inferiorly to the optic chiasm, internal carotid arteries are laterally, and diaphragma sellae and pituitary gland are inferiorly.¹

Most pathologies in sellar and parasellar region are located inferiorly to optic nerve and chiasm, and just posteriorly to chiasmatic sulcus. Chiasm is located centrally (normal location) in 70% of humans, and overlies diaphragma sellae. Optic chiasm might be in 3 different locations as prefix chiasm, central chiasm and postfix chiasm, according to diaphragma sellae, pituitary gland and pituitary stalk. This location variety determines clinical symptomatology and surgical approaches significantly.¹

Understanding the relation between carotid artery, optic nerve and anterior clinoid process is the basis of surgical approaches performed in subchiasmal, sellar–presellar regions. Surgical approaches of pathologies located in sellar–parasellar, subchiasmal region differ according to the region where pathology extends, and to the structures affected by the pathology. Although approaches according to the type of pathology are mostly clear nowadays, it is significant in each case to determine the structures which were affected by the pathology, and to choose the best surgical approach without damaging the surrounding tissues. The location of the optic chiasm, the length of the optic nerves, the distance and angle between the optic nerves determine the morbidity and mortality of surgical approaches. Approach to subchiasmal and parachiasmal region might be challenging due to the critical anatomy of optic nerves, optic chiasm, carotid arteries. Although transnasal endoscopic approaches to this region increased recently, transcranial approach should be considered evidently in resistible lesions.

Albeit some studies on cadavers related to the anatomy of sellar–parasellar region, cavernous sinus, and diaphragma sellae are present, there are few studies in literature where neural and vascular anatomy of subchiasmal–parachiasmal region were compared in terms of transcranial and transnasal approaches.2, 3, 4, 5 In the current study, the relationship of anatomic structures with one another was investigated in order to prefer surgical approach to optico-chiasmatic region in such a way that it does not harm anatomic structures.