The CT Prevalence of Arrested Pneumatization of the Sphenoid Sinus in Patients with Sickle Cell Disease

Discussion

The normal pneumatization process of the sphenoid sinus begins after birth and usually ends at 10–14 years of age with the formation of a fully pneumatized sinus lined by respiratory epithelium.^3,4 This process can be interrupted for unclear reasons, especially in the sphenoid sinus. The most familiar etiology is chronic inflammation, as seen in patients with cystic fibrosis,^7⇓–9 but other factors can produce a similar result. It is also unclear why arrested pneumatization has been shown to occur most often in the sphenoid sinuses, though it may be because of greater variation in the extent of aeration in the sphenoid sinus compared with other paranasal sinuses.⁶ Regarding arrested pneumatization, there is a theory that aeration triggers the fatty marrow conversion in the sphenoid sinus,¹ whereas another theory suggests that the ratio of trabecular to cortical bone is the driving mechanism.¹⁰ Yonestu et al⁵ suggested a subsequent theory of regional blood flow changes as the reason for atypical fatty marrow that persists into adulthood.

It is important to note that the theory of regional blood flow has not been proved. Using MR signal intensity changes on T1WI, Yonetsu et al ⁵ showed sphenoid marrow conversion before 1 year of age, when aeration of the sphenoid bone does not occur.^4,11 This suggested that factors other than air supply, such as blood supply, play an important role in conversion of the marrow.⁵ Humoral factors also may play a role in arrested pneumatization of the sphenoid sinus, considering that serum erythropoietin levels have been shown to be elevated in patients with SCD.¹² The elevated erythropoietin may interrupt the conversion of hematopoietic to fatty marrow.

The phrase “arrested pneumatization” makes an assumption about the etiology of the sphenoid lesions that has not been proved. In fact, radiologists may be uncomfortable using the phrase because it is only one of several theories for the etiology of these fibro-osseous lesions. Our study supports this terminology and the theory of regional blood flow anomalies or increased serum erythropoietin as a potential cause of arrested sinus pneumatization.

The differential diagnosis for a mass in a normal sphenoid sinus pneumatization site includes fibrous dysplasia, clival chordoma, chondrosarcoma, intraosseous lipoma, intraosseous hemangioma, hamartoma, ossifying fibroma, enchondroma, and fibrous osteoma.^{6,13⇓⇓–16} Arrested pneumatization may be distinguished from these entities as a nonexpansile lesion with a thin cortical margin, internal fatty content, and curvilinear internal calcifications (Figure).⁶ These calcifications differ from the “ring and arc” calcifications seen with chondroid tumors.¹³ Intraosseous lipomas share some imaging characteristics with arrested sinus pneumatization,¹⁴ leading to the theory that these entities are of similar etiology. However, our findings do not support this theory. In particular, arrested pneumatization should not be confused with ossifying fibroma. Whereas the internal matrix of arrested pneumatization is characterized by curvilinear calcification and fatty contents, ossifying fibromas exhibit ground-glass marrow on CT, often with an overlying radial pattern of calcification.¹⁷ Ossifying fibromas are also expansile and may be exophytic into the sinus itself.¹⁸

Our study has some important limitations. The study was retrospective, and all the patients in the study were imaged for various clinical reasons. The number of patients was too small to break down our results by age to determine when these lesions formed and determine whether age is predictive of prevalence. Although the control patients were matched for age and sex, patient race was not reliably available to us, so we could not account for this potential confounder. Serial CT scans could provide more information about the usual development of arrested sphenoid pneumatization, but in our series, no patients with lesions had serial scans. Although the total number of patients in our series who had arrested pneumatization was small, the statistical results comparing the 2 groups were robust, with a P value less than .001.

Our study included some patients younger than 14 years of age, when the development of the sphenoid sinuses is still ongoing. Although these patients may be less likely to have fibro-osseous lesions, we felt justified to include these patients because 2 patients with SCD (ages 8 and 14 years) had evidence of arrested sphenoid pneumatization. Additionally, the case-control format of our study would prevent this from biasing our overall results.