STUDY ON THE OPTIC NERVE VARIATIONS IN RELATION TO POSTERIOR PARANASAL SINUSES USING CT IN TERTIARY CARE CENTRE

Abstract

Background and objectives: Optic nerve has a close relationship with the posterior paranasal sinuses. Depending on the degrees of pneumatization of these sinuses, the optic nerve may indent the sinus wall or protrude into them, predisposing the nerve to injury during sinus surgeries. Aim was to analyse the optic nerve variations in relation to the posterior paranasal sinuses and to categorize the subjects as per DeLano's classification.
 Methods: Cross-sectional study was conducted on 336 patients above 16 years of age who underwent computed tomographic evaluation of head and brain. From volume data, multiplanar reconstructions were made in axial, coronal and sagittal planes and analysed in both bone and soft-tissue windows. Relationship of optic nerve with posterior paranasal sinuses were categorised into 4 types according to DeLano's classification. Bony dehiscence of optic canal and pneumatization of anterior clinoid process were also assessed.
 Results: 672 optic nerves were assessed, the most common optic nerve type identified was type 1 (62.6%), followed by type 2 (18.6%). Types 3 and 4 were seen equally in number (9.6% each). Dehiscence of optic nerve canal and pneumatization of anterior clinoid process (ACP) was seen respectively on 66 (9.8%) and 75 (11.2%) sides. Compared to type 1 and 4 optic nerves, dehiscence of bony optic canal was statistically more significant in type 2 and 3 nerves. 45.5% individuals with anterior clinoid process pneumatization had associated optic canal dehiscence, which was found to be statistically significant.
 Conclusion: Optic nerve variations were identified and classified according to DeLano’s classification. The range was within international limits and type 1 was the most common occurrence. When compared to other similar studies among Indian population, disparity observed in the frequency of optic nerve types and bony optic canal dehiscence in our study, were probably due to differences in ethnicity and size of study group. Association of bony optic canal dehiscence with type 2 & 3 optic nerves and ACP pneumatization with type 3 optic nerve was found to be statistically significant, making them more vulnerable to injury during surgery. Knowledge of optic nerve relation with posterior paranasal sinuses, and its identification in preoperative computed tomography (CT) scan are important to avoid injury to optic nerve.
 Keywords: Optic nerve, sphenoid sinus, posterior ethmoid cell, Onodi cell, anterior clinoid process pneumatization, optic canal dehiscence.