Arrested Pneumatization of the Sphenoid Sinus in the Skull Base

We report 2 cases of arrested pneumatization of the sphenoid sinus, a normal variant commonly misdiagnosed as a serious condition of the skull base. A 65-year-old man visited a local clinic for regular checkups. Brain CT showed a non-expansile lesion of the soft tissue with a well-defined sclerotic margin in the clivus but without destruction or mass effect on the surrounding bony structures. Subsequent brain MRI revealed that the lesion within the clivus was a high-signal lesion on the T2-weighted image, containing a low-signal round mass seen on a high-intensity signal on the T1-weighted image without contrast enhancement. Thus, the lesion was considered to contain internal fat. A 70-year-old woman diagnosed with a tumor in the greater sphenoid bone visited our hospital. Her brain CT revealed a non-expansile lesion of mixed density and a well-demarcated lesion and internal curvilinear calcification in the left greater wing of the sphenoid bone. The margin was osteosclerotic and the adjacent bony structure was intact. Her brain MRI showed that the lesion within the greater sphenoid bone had multiple low-signal lesions within a high-signal lesion on the T2-weighted image, suggesting internal fat contents. The lesions were diagnosed as arrested pneumatization of the sphenoid sinus and no further examination or treatment was performed. Arrested pneumatization of the sphenoid sinus should be considered in the presence of non-expansile lesion with an osteosclerotic boundary and internal fat component in the skull base.