Optic Nerve Canal Relation to Posterior Paranasal Sinuses in Indian Ethnics: Review and Objective Classification

Posterior paranasal sinuses consisting of posterior ethmoid and sphenoid sinuses show varying pneumatization and relationship to Optic Nerve Canal (ONC). Most commonly ONC is superolateral to sphenoid sinus, however varied protrusion of ONC into the sinuses may occur. Rarely it may pass completely through the sinus. Optic Nerve (ON) is covered by a thin bony layer or by periosteum and sinus mucosa without bone. Extensive pneumatization of sphenoid sinus and bony dehiscence predisposes to ON injury. The posterior ethmoid cell namely the Onodi cell is of great surgical importance when the ON is along its lateral wall and surrounds the nerve. These varied relations imply that a detailed knowledge of the intimate relationship of ONC with the posterior paranasal sinuses is necessary to avoid unintentional complications during the surgeries involving the ONC region.

To assess relationship of ONC to the posterior paranasal sinuses in Indian Ethnic study group.

Retrospective review of 100 paranasal sinus Computed Tomography (CT) scans were done using 64 row Multi Detector Computed Tomography (MDCT). Multiplanar reformation images were assessed for ONC relation to posterior ethmoid and sphenoid sinuses, wall dehiscence, and Pneumatization of Anterior Clinoid Process (PACP).

200 optic nerve canals were assessed and grouped into four types based on the modification of Delano et al., classification. The most common OCN was Type-1 (60%), followed by Type-2 (15%), Type-3 (14%) and Type-4 (11%). Dehiscence was seen in 35(17.5%) mostly in Type-3 canals. PACP was seen in 30 (15%).

Knowledge of ON relationship to posterior paranasal sinuses will reduce the devastating complications during sinus surgeries. Our modified ONC classification is based on the bony canal morphology and the degree of protrusion into sinuses. As with other reported studies worldwide, Type-1 ONC is the commonest among the Indians also. Type-2 and 3 ONC with sinus wall dehiscence and PACP are the most important anatomical variations which may potentially predispose to risk of surgical injury to the optic nerves.