A novel adaptation of endoscopic optic nerve decompression in non‐traumatic optic neuropathy: A retrospective case series

Patients with imaging-proven anterior skull base meningioma were offered endoscopic optic nerve decompression if ophthalmological examination demonstrated a relative afferent pupillary defect (RAPD) and visual field deficit, or worsening visual acuity in the affected eye. Surgery aimed to prevent further visual loss due to compression, and protect against deterioration due to radiotherapy treatments. From October 2015 to October 2019, six patients had optic nerve decompression under the care of a multidisciplinary anterior skull base team. Five patients demonstrated both RAPD and visual field defect, with Patient 6 showing reduced visual acuity only. We retrospectively chart reviewed each case. Operative technique and complications were noted, along with change in visual acuity and colour vision. Colour vision was assessed using Ishihara plates, with the retrospective nature of our study, and involvement of multiple clinicians, impacting the standardisation of this assessment across the patient cohort.


| INTRODUCTION
The use of the endoscopic approach in cases of non-traumatic optic neuropathy is well documented in the literature. 1 This approach can also be of benefit in cases of visual loss from skull base meningiomas directly compressing the nerve. Specifically, the transnasal endoscopic approach affords excellent visualisation of the orbital apex and optic canal with minimal patient morbidity.

| Objectives
We intend to highlight the benefits of our technique (which includes a posterior to anterior approach) and discuss the role of optic nerve sheath incision by reviewing visual outcomes and complications.

| Ethical considerations
This was an audit of local practice and therefore did not require ethical approval. All patients involved were appropriately consented for the procedure. We used the CARE checklist when writing our report. 2

| Design, setting, participants and main outcome measures
Patients with imaging-proven anterior skull base meningioma were offered endoscopic optic nerve decompression if ophthalmological examination demonstrated a relative afferent pupillary defect (RAPD) and visual field deficit, or worsening visual acuity in the affected eye. Surgery aimed to prevent further visual loss due to compression, and protect against deterioration due to radiotherapy treatments. We retrospectively chart reviewed each case. Operative technique and complications were noted, along with change in visual acuity and colour vision. Colour vision was assessed using Ishihara plates, with the retrospective nature of our study, and involvement of multiple clinicians, impacting the standardisation of this assessment across the patient cohort.

| Operative technique
Our technique differs from the commonly reported approach in the literature. 3 The sphenoid sinus is accessed with either a transnasal or trans- Initially, the bone over the medial aspect of the optic canal at the level of the sphenoid is thinned using a diamond burr. The remaining bone is flaked off using a curette to allow exposure of the optic nerve sheath (see Figure 2). The dissection is then progressed anteriorly with the bone over

Key Points
• Endoscopic optic nerve decompression is of demonstrable benefit in cases of non-traumatic optic neuropathy.
• Visual improvement can be immediate and continue for 1 year.
• Surgical decompression can prevent visual deterioration following radiotherapy for meningioma compressing the optic nerve.
• Our technique involves posterior to anterior bony decompression of the nerve.
• There were no injuries to the optic nerve, internal carotid or orbital contents in our cohort.
the medial aspect of the orbital apex again thinned and excised with a curette. The periorbita is incised and the condensation of fascia representing the Annulus of Zinn is also incised medially. There can be some prolapse of orbital fat when the periorbita is incised which may otherwise obscure the dissection in the sphenoid had it not been performed first.

| RESULTS
Five females and one male were included, with an average age of 51.5 years (range, 30-66 years). All six had a meningioma (see Table 1).
The average length of stay was 4.3 days (range, 1-11 days).
Six weeks post-operatively three patients (50%) had similar visual acuity scores to baseline. Two patients (33%) had improvement, one of whom had progressed from finger counting to 6/6 vision in the initial post-operative period. Colour vision was also noted to improve in two patients (33%) (see Table 2).
Vision deteriorated post-operatively in one patient (17%) with acuity dropping from 6/9 to 6/30. There was an initial improvement in the immediate post-operative period but their acuity progressively worsened over the next 4 weeks. One year after radiotherapy, visual acuity was better than the preoperative baseline.

| Synopsis of key findings
Our results for visual improvement compare favourably to the 54%-70% shown in reported literature, 3 and show demonstrable benefit in protecting against the visual deterioration typically associated with radiotherapy. 4

| Comparison with other studies
A significant difference between traumatic optic neuropathy and compression from meningioma is that post-operative radiotherapy is a frequent further stage of treatment for the latter, with studies showing local control rates of 85%-95% at 10 years. 5 This report shows that following optic nerve decompression vision is maintained or improved during subsequent radiotherapy. The benefit of optic nerve decompression is therefore not described solely by post-operative vision improvement as in the traumatic nerve compression group, and postradiation visual outcome should also be recorded to fully appreciate the benefits of this surgery.

F I G U R E 1 Elevation of sphenoid mucosal flap (denoted by the black arrow)
F I G U R E 2 Posterior orbital periosteum (denoted by the white arrow) and optic nerve sheath (denoted by the black arrow) are visualised after bony removal.
A metanalysis by de Melo et al. 6 reports that radiotherapy alone may improve vision in cases of optic nerve sheath meningioma; however, the location of meningioma in our cohort is more diverse. Thus, without decompression, the typical outcome of radiotherapy would be visual deterioration. 5  The use of a posterior to anterior approach, starting with drilling of the lateral sphenoid wall to identify the optic nerve canal prior to disruption of the lamina papyracea, is our preferred method of optic canal access. This prevents bulging of the periorbita that may obscure the view of the optic canal-this can be further complicated if the periorbita is torn and allows nasal herniation of orbital fat. This pragmatic "back to front" approach allows the greatest, and safest, exposure of the canal.
Incision of the optic nerve sheath step is a poorly documented, yet controversial, adjunct and we no longer routinely perform this step. A small cadaveric study by Onofrey et al. 7 suggests that opening the optic nerve sheath may damage the nerve. Berhouma et al. 3 also note the risk of CSF leak and ophthalmic artery injury, and after literature review, it is noted that Xu et al. 8 similarly questioned the necessity of this step. In a cohort of patients with traumatic optic neuropathy, they found no statistically significant difference in visual outcomes between those who did and did not undergo this step.
However, Thaker et al. 9 noted the opposite, with 46% undergoing sheath incision showing improved visual outcomes versus 36% for those who did not. This finding, however, was not statistically significant and applied to traumatic cases only. We are unable to identify any published literature discussing the outcomes of optic nerve sheath incision in patients with non-traumatic causes of optic neuropathy that would be considered analogous with our cohort.
In our cohort, the optic nerve sheath was incised with no ocular morbidity, but subsequent CSF leaks occurred. In the 50% of cases where this was identified intra-operatively, it was effectively repaired with a simple sphenoid mucosal flap. This report, therefore, highlights that there is limited evidence for this adjunct and a high rate of CSF leak.

| CONCLUSION
In conclusion, acknowledging the small number of patients involved, this study demonstrates that endoscopic transnasal optic nerve decompression can be beneficial in cases of optic neuropathy from meningioma. We have deployed a posterior to anterior approach for optic canal dissection and have reported benefits for visual acuity.
Such decompression additionally prevented further visual loss following subsequent radiotherapy.