Orbital Fat Density as a Diagnostic Tool in Pre-Septal and Orbital Cellulitis.

Purpose: To evaluate the usefulness of measuring orbital fat density in identifying post-septal involvement when initial differential diagnosis between orbital and periorbital cellulitis (OC and POC) is unclear.

Materials and methods: Retrospective study of patients with clinical diagnosis of OC or POC who underwent contrast-enhanced computerized tomographic scans over a span of 10 years. Intraconal orbital fat density was measured with Hounsfield units (HU) in six areas on axial scans consisting of nasal and temporal intraconal sites. These measurements correlated with the initial and final diagnoses. Main outcome measures were HU values at the initial and final diagnoses.

Results: Fifty-seven patients were included. Mean HU measurement was -52 ± 18 HU for the involved side vs. -63 ± 13 for the uninvolved side (P < .001). The values were higher in cases of a final diagnosis of OC in the involved side (P < .001). The HU values were significantly higher in the nasal vs. the temporal locations of each orbit bilaterally (P < .001). The initial POC diagnosis of 20 patients (35%) was revised to OC.

Conclusion: Intraconal fat density measurements can assist in the primary assessment of orbital involvement in patients with an uncertain initial diagnosis, with a HU value higher than -50 is suggestive of orbital involvement.