Diagnosis: Pneumosinus dilatans (PSD) of the sphenoid sinus
Although its precise definition has been disputed,1PSD is characterized by an abnormal dilation of an air-filled paranasal sinus2that expands beyond its normal anatomical boundaries without thinning of its bony walls.3The first reported case was described in 1918.2The frontal and sphenoid sinuses are most commonly involved,3but the maxillary sinuses, ethmoid air cells, and even a concha bullosa may be affected. The entire sinus or only a part of it may be involved.4The age at presentation varies, but PSD frequently occurs between the ages of 20 and 40 years, with a male predominance.5
Pneumosinus dilatans can be differentiated from a simple pneumocele by the integrity of its bony sinus walls. In a pneumocele, there is thinning of the surrounding bone, which is not seen in PSD.3However, some authors have used both terms interchangeably.3Pneumosinus dilatans should be included in the differential diagnosis of expansile processes of the paranasal tract.6
The pathogenesis of the abnormal sinus dilation is poorly understood.2,3Theories include abnormalities in bony remodeling secondary to neoplastic processes; 1-way-valve mechanism with increased intrasinus pressure2; developmental disorders in which the normal pneumatization process fails to stop7; gas-forming organisms; spontaneous drainage of a mucocele; hormonal factors; and disturbances of osteoblastic and osteoclastic activity.5Antral studies have demonstrated abnormally high pressure in PSD; however, the presence of normal ostia in some cases contradicts this theory.7Some authors believe that if an ostium is small enough to create air trapping, it should also cause problems with mucus drainage,7which is not seen in PSD. Furthermore, blockage of a paranasal sinus ostium will more likely result in negative pressures and effusion rather than expansion.8Chronic inflammatory changes are sometimes observed on histologic examination, as in this case, but are not seen uniformly.8The finding of chronic inflammation and the absence of the intersinus septum may suggest that the dilated sinus was the result of spontaneous drainage of a sphenoid mucocele, but the bilateral patent ostia do not support this theory.
Pneumosinus dilatans has been reported in patients with conditions such as von Recklinghausen disease, fibrous displasia, prolonged cerebrospinal fluid shunting, meningiomas, and arachoid cysts,5but these associations are from case reports and are most likely incidental. There is an even rarer variant of PSD, called pneumosinus dilatans multiplex, in which all the paranasal sinuses and mastoid air cells are involved.5
The expansion of the sinuses in PSD can cause both aesthetic and functional problems, and the symptoms of PSD are related to the site of expansion.5For example, PSD of the frontal sinuses generally presents as a slow change in the external frontal facial countours, pain, diplopia, or other pressure-related symptoms.4On the other hand, if the maxillary sinuses are involved, it may present with nasal obstruction and pain elicited by changes in atmospheric pressure.6
The treatment of PSD is based on patient symptoms, including cosmetic deformities.5If PSD is thought to be the result of pressure disequilibrium, osteotomy should be considered.6Many different treatment modalities have been proposed, including sinus puncture and open or endoscopic sinus surgery. Interestingly (and for unknown reasons), in most cases, the abnormal enlargement stops with surgical treatment.4
In conclusion, PSD is a rare disorder for which there is no consensus regarding pathogenesis or treatment. In this case, the radiographic findings showed that the PSD was an air-filled, expanded sinus without thinning of the sinus wall that occurred in a patient without previous sinus surgery.
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Radiology Quiz Case 1: Diagnosis. Arch Otolaryngol Head Neck Surg. 2010;136(11):1142. doi:10.1001/archoto.2010.189-b