The sphenoid sinus is one of the most morphologically variable and surgically important structures of the skull base. Located below the sella turcica, neighbored by parasellar regions, such as the orbital apex, pterygopalatine fossa and lateral sellar region (cavernous sinus), it is clinically related to these and surgically relevant as corridor for various approaches. Moreover, at the sphenoethmoidal junction, important variations occur, most of these related to the presence of the Onodi cells and the intrasinusal protrusions of the optic nerve. That is why any identified and previously undescribed morphological variation at that level must be added to the well-established protocols, clinical and surgical. During a retrospective CT study of the sphenoid sinus anatomical features a previously unreported morphology was encountered and is reported here. It refers to a unilateral sphenoethmoid cell (SEC), Onodi-positive, not only overriding the superior aspect of the sphenoid but also its lateral side to get intimately related to the maxillary nerve. As that SEC expanded medially to the cavernous sinus apex, it altered the usual endosinusal morphological correlations and also added itself within the limits of the Mullan's triangle. It appears so that such postero-infero-lateral extended pneumatization of an Onodi cell alters the surgical landmarks and also can blur clinical pictures, by adding maxillary and pterygopalatine signs and symptoms.
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