The diagnosis was incidental intraosseous pneumatocyst of the ilium. A gas-containing, cyst-like lesion within the medullary cavity of the bone, intraosseous pneumatocyst is a rare benign lesion reported primarily in the adult population. The lesion has been described most frequently adjacent to joints, especially the sacroiliac joint.
Intraosseous pneumatocysts related to the sacroiliac joint have a predilection for the iliac side of the joint, theoretically because of differences in the characteristics of the surrounding cartilage. The iliac bone is lined by fibrocartilage that is thin compared with the almost twice as thick sacral hyaline cartilage,1 facilitating diffusion of gas into the iliac bone. Intraosseous pneumatocysts have been reported in many sites, including vertebral bodies—especially cervical vertebrae—as well as the clavicle, rib, acetabulum, and scapula.
The cause of pneumatocysts is unclear, but several theories have been suggested. Ramirez and colleagues2 proposed that there is spontaneous development of intraosseous air or gas accumulation within a preexisting simple fluid-filled cyst or ganglion. Yamamoto and associates3 described a case of a cervical pneumatocyst in which an air-filled cyst became a fluid-filled cyst and subsequently was replaced with granulation tissue, as shown by MRI at 40 weeks’ follow-up. Laufer and colleagues4 suggested that the collection of gas is actually nitrogen and is released from the adjacent joint.
Vacuum phenomenon in and of itself refers to the presence of nitrogen gas; typically, it is noted in the intervertebral disks, where it occurs in conjunction with degenerative disease. Vacuum phenomenon also can be induced in normal joints with traction, which creates a negative pressure state in the joint and thereby elicits the release of nitrogen gas from adjacent soft tissues into the joint.
Although acquired benign air cysts in bone have been reported in proximity to joints with degenerative changes and vacuum phenomenon, no underlying joint disease was present in other reports of cases. Berenguer and coworkers5 reported cases of sacral pneumatocysts in patients in whom there were findings of degenerative changes of the joint along with intra-articular gas.
These gas collections in the bone may progress in size or to fluid-filled cysts.3 Hall and Turkel6 observed an adult patient who had a normal ileum on CT examination before decompression of an S1 nerve root and L4, L5, and S1 fusion. A follow-up study performed 2 years later incidentally revealed a pneumatocyst that enlarged on a follow-up study 6 years later. This case illustrates the acquired nature of pneumatocysts. Other reports in the literature describe pneumatocysts that did not change in size on follow-up imaging 6 months to 2 years after the initial imaging.
Intraosseous pneumatocyst is a rare finding in the pediatric population. Kota and Coleman7 described well-defined intraosseous gas with sclerotic margins in the ileum of a 9-year-old boy in whom a CT scan was obtained because of a history of blunt trauma and abdominal pain. In our present case, however, the patient had no recent or remote history of trauma.
Another case report described an intraosseous pneumatocyst in a right cervical rib in a 17-year-old girl who had Klippel-Feil syndrome.8 There was no history of trauma, and a lack of weight bearing associated with this bone makes the pathogenesis of this lesion less likely to be associated with degenerative joint disease. As in our case, these lesions often are asymptomatic and usually are an incidental finding on imaging performed for another disease process.