CT scans appear unreliable for optic canal fracture diagnosis

This retrospective case series suggests that 1 in 5 cases of optic canal fracture (OCF) in patients with traumatic optic neuropathy are missed on high resolution CT (HR-CT).

The large study adds to the evidence supporting a high false-negative rate with CT imaging. At least 4 smaller case studies have shown false negative rates ranging from 16.1% to as high as 88%, even with clear CT imaging.

Data was drawn from 1,275 patients with indirect traumatic optic neuropathy, including 708 patients who had visible OCF on HR-CT image. During surgery, an additional 187 (20.9%) patients were found to have fractures, of which 73% were non-displaced linear intracanalicular fractures. The remaining 27% of missed OCFs were slightly displaced comminuted fractures without impingement.

Baseline visual acuity was significantly worse in patients with OCF (P<0.01). Surgical efficacy was better in patients without OCF compared to patients with OCF (87.6% vs. 78.4%, P<0.001). However, visual acuity at 3 months was similar between groups.

The authors offer 3 possible reasons for a missed diagnosis:

• Linear fractures can be difficult to detect. While more significant OCT displacement can be easily detected, it is possible that the thinness of the orbital walls and absence of displaced fragments lead to missed fractures. In this study, 136 of the 206 patients without displacement had missed OCFs by HR-CT.
• In fractures with a slight displacement, the bone of the optic canal may be too thin to be seen on HR-CT.
• The optic canal and its adjacent structures may be anatomically abnormal in some patients, making detection more difficult.

Although it remains unclear from this study whether these fractures affect final visual acuity outcome, the authors remain optimistic about the effectiveness of endoscopic transethmoid optic canal decompression surgery for indirect traumatic optic neuropathy.