Hamad Hejazi

Introduction

Between 2021 and 2022, over 6.7 million CT scans were performed in England, most of which contained the orbit within the imaging (1). Emergency CT head and facial trauma radiographs are among the most frequently requested studies in acute medicine, often ordered to exclude intracranial haemorrhage or infarction. In these scans, the orbit is often also imaged but remains a bystander as the focus lies on the intracerebral contents. The orbital contents are consistently included and just as consistently underexamined.

This article was prompted by experiencing clinical encounters in which orbital findings on emergency imaging had not been reported or communicated to the ophthalmology team. It argues that the gap between what emergency CT sees and what ophthalmology is told represents a correctable patient safety failure and that this failure has not yet been addressed in a systematic way.

The Diagnostic Range of the Incidental Orbit

There is a broader range of clinically significant orbital findings than what is conventionally appreciated. These can include enlarged extraocular muscles, significant for thyroid eye disease (TED), which is reliably demonstrated on CT. This radiological finding can precede clinical signs and symptoms and even derangement in thyroid function lab tests (2). Another standard CT head finding relevant to the orbit is optic nerve sheath diameter (ONSD) enlargement, which is a validated surrogate marker for raised intracranial pressure (ICP) (3). In paediatric patients, intraocular calcification on neuroimaging should be considered retinoblastoma until proven otherwise.

MRI remains the gold standard for detailed orbital soft tissue assessment. It enables characterisation of disease activity in TED through STIR sequences and superior optic nerve visualisation. However, CT is performed at volume in the acute setting, and that is why this argument focuses on CT.

The above are high-stakes findings, but even beyond these, findings like globe asymmetry, proptosis, retrobulbar fat stranding, and superior ophthalmic vein engorgement each carry specific diagnostic weight that can materially alter clinical management. These findings often appear on routine scans acquired regularly across NHS acute trusts and are not limited to specialist imaging centres.

The Evidence Gap

The evidence surrounding missing orbital findings is persuasive, though it may be circumstantial. Up to 10% of neuroimaging and up to 30% of body CT scans have incidental findings (4). The Royal College of Radiologists (RCR) acknowledges the absence of agreed protocols governing the management of such findings across most anatomical regions. Time pressure is significant when emergency radiology is required, and reports focus on primary clinical indications aiming to answer the question the requesting team has posed. A radiologist reviewing a CT head following a road traffic collision is not actively searching for extraocular muscle thickening or ONSD asymmetry.

Currently, there is not a specific orbital review checklist within UK emergency CT reporting templates. The RCR maintains robust standards of communication of urgent significant findings [5). but they presuppose that a finding has first been identified. From a practical perspective, a finding that is not looked for cannot be found or communicated. Therefore, the missing of orbital findings, or the ‘orbital blind spot’ on emergency imaging is not a failure of individual clinical or reporting judgement.

Case Studies: ONSD and Thyroid Eye Disease

Optic Nerve Sheath Diameter

Due to anatomical positioning of the optic nerve sheath, rising ICP can be directly transmitted as sheath expansion (6). An ONSD threshold of greater than 5 mm on CT has been validated against direct invasive ICP measurements. Of note, a meta-analysis looking at CT and ultrasound ONSD studies in severe traumatic brain injury reported pooled sensitivity of 0.91 and specificity of 0.77 (7). ONSD measurement only requires callipers alongside the existing CT data and hence does not add to imaging time or additional cost.

Measurement of the ONSD has garnered interest and is increasingly used by emergency doctors and intensivists as an ICP-proxy, and its non-invasive nature makes it favourable in some contexts. However, its rate of adoption remains variable. This is because it is inconsistently documented in radiology reports and there is no agreed threshold at which abnormal ONSD prompts communication to ophthalmology. Ophthalmologists are rarely involved in the escalation pathway whenever this finding is flagged. The orbital window for non-invasive ICP assessment is present on most emergency CT scans and whether anyone looks through it remains an area that can be improved upon significantly.

Thyroid Eye Disease

There exist a few pathognomonic CT findings in Thyroid Eye Disease (TED), namely fusiform extraocular muscle enlargement sparing the tendinous insertion, increased orbital fat volume, and apical crowding (2). What makes this particular finding more

important is that it often precedes the presentation of clinically detectable signs and symptoms, and sometimes even lab results. Hence, incidental bilateral rectus muscle enlargement on a CT ordered for an unrelated indication represents an early detection opportunity (8).

Currently, there are not any established triggers in UK emergency reporting workflows for communicating incidental orbital findings to ophthalmology or endocrinology teams. While it may sometimes appear as an unreferenced observation, the diagnostic opportunity is often lost before it can be capitalized on by early detection.

Lessons from Radiology

Across other specialties, there are structured and effective responses developed by radiology to address the issues of incidental findings. Lung-RADS has come to existence because incidental lung findings were being inconsistently identified and reported (9). Similarly, Li-RADS addresses incidental liver observations on contrast-enhanced imaging. In the above cases, structured reporting frameworks were embedded that enabled identification, communication and safe follow-up of incidental findings.

Currently, the orbit lacks such a framework. A standardization in orbital observation addendums for CT heads and a threshold for communication (in incidental TED changes and ONSD asymmetry) to ophthalmologists would be quite beneficial. Also, a cross-specialty guideline jointly owned by ophthalmology and radiology can also help address this gap. Borrowing from infrastructure used by Lung-RADS and Li-RADS, structured incidental reporting can be applied to the orbit.

Recommendations

There are a few actionable steps that can help close the above gap and get ahead of orbital pathology.

Firstly, structured orbital reviews could be added to CT head reporting templates. This need not be done in an emergency, but most registrar reports are reviewed by a consultant, and a secondary review can have a simple checklist regarding orbital findings. A brief scan for ONSD measurement, extraocular muscle symmetry and globe integrity would go a long way.

Another step that could help is the development of a joint guidance on the pathway of communication of significant orbital findings by the Royal Colleges of Radiologists and Ophthalmologists, respectively. Defining referral thresholds jointly can remove dependence on informal escalation and the ambiguity that comes with it. Structured communication protocols between the two specialties exist for other radiology findings relevant to ophthalmology and the orbit can benefit from the same.

Finally, the training curricula for both radiology and ophthalmology should incorporate orbital education across both specialties. Radiology trainees should be educated on which orbital findings should be routinely escalated to ophthalmology and ophthalmology trainees should be taught what a CT and MRI can or cannot reliably demonstrate. At present, neither curriculum explicitly mandates this and inclusion could improve clinical outcomes.

Conclusion

The orbit appears on millions of CT scans every year. It is imaged in detail, reported by clinicians whose primary focus lies elsewhere, and largely unexamined from an ophthalmological perspective. The ONSD and TED examples illustrate the cost of this inattention. Diagnostic opportunities with patient consequences are being lost on datasets that currently exist.

Radiology has brilliantly demonstrated that structured incidental finding frameworks improve clinical outcomes in other specialties. To extend this success to the orbit, there is not any pertinent requirement of extra scans or technology. Subsequently, this would lead to improved detection rates, better communication to ophthalmologists and higher follow-up rates across orbital pathology.

References

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