Specific Imaging Findings

Skull radiographs show a “growing” or expanding fracture accompanied by a soft tissue subcutaneous mass. CT shows a defect in calvarium that is filled with a cyst of CSF density and may also contain brain tissue. MRI shows the cyst to be isointense to CSF in all sequences, while the contained brain is usually of bright T2 signal due to encephalomalacia and gliosis. The adjacent brain may be atrophic and the ventricles dilated. There is no abnormal contrast enhancement. In the past, a follow-up skull radiograph was obtained in all young children to exclude this complication; however, nowadays this complication is known to be so rare that this is no longer indicated.

Pertinent Clinical Information

Leptomeningeal cysts present as slowly growing, soft calvarial subcutaneous masses at the site of known (and sometimes unknown) fractures or craniotomy defects. On physical examination, patients classically demonstrate a cranial defect with a bulging and pulsatile center. Because there generally is underlying malacia of the brain, seizures may accompany the cyst. The cyst will grow up to a size and then remain stable for years, but it generally becomes apparent within one year of the initial trauma. However, onset of seizures has been described even more than two decades after the initial trauma. Thin bones, such as those surrounding the orbits, are preferentially involved. If the cyst projects into an orbit, proptosis ensues. If the cyst projects into the nasal cavities, CSF leak may be the initial presentation with or without meningitis.