Orbital morphology in exophthalmos and exorbitism

Exophthalmos is a protrusion of the eyeball due to an increase in orbital contents in a normal bony orbit. Exorbitism is a protrusion of the eyeball due to a decrease in capacity of the orbital container, with a normal orbital content volume such as seen in a congenital form termed nonsyndromic exorbitism. High myopia can enhance proptosis. The purpose of this study was to provide values for orbital measurements from computed tomography and to suggest computed tomography criteria for nonsyndromic exorbitism.

Seventy-three computed tomography scans were collected (57 of Graves' ophthalmopathy and 16 of nonsyndromic exorbitism). Thirty-two scans from nonproptotic patients constituted a control series. Nine measurements and two indexes, performed on a reference axial computed tomography slice transecting the neuro-ocular plane, were obtained from each scan.

The angle between the sagittal axis and the lateral orbital wall, as well as the width of the ethmoid bone (midinterorbital distance), was found to be more open in the nonsyndromic exorbitism population. A lateral orbital wall angle greater than or equal to 42 degrees and a midinterorbital distance greater than 30 mm were chosen as cutpoints. The association of these two criteria allowed the authors to obtain a sensitivity of 62 percent, a specificity of 78 percent, a positive predictive value of 80 percent, and a negative predictive value of 86 percent for nonsyndromic exorbitism.

The different mechanisms of globe protrusion have to be taken into account before an orbital expansion/decompression procedure is planned. Only a preoperative morphological analysis of the orbital shape permits a precise analysis of the relative position of the ocular globe and orbital structures, in addition to clinical examination.