Optic nerve sheath fenestration: A second lease at sight

Abstract

Purpose:

To study the safety and efficacy of optic nerve sheath fenestration surgery in patients with optic disc edema due to different etiologies.

Methods:

Records of 18 eyes of 15 patients who underwent optic nerve sheath fenestration for vision threatening optic disc edema were reviewed retrospectively, and results were analyzed. Improvement of visual acuity was the main measure of outcome. Improved visual fields, resolution of optic disc edema, diplopia, and headache were other benefits that were observed.

Results:

Fifteen patients between 13 and 54 years of age were included in the study. Three patients underwent successive bilateral surgery. Idiopathic intracranial hypertension was the most common cause for optic disc edema and was found in 80% of the patients. Mean preoperative logMAR acuity was -1.9789 ± 1.46270, which improved to -0.9022 ± 1.23181 (p < 0.005) in the operated eye, and mean logMAR acuity of contralateral eye improved from -1.3378 ± 1.50107 to -1.0667 ± 1.33813 (p < 0.05).

Conclusion:

Early optic nerve sheath fenestration is an effective modality for treating optic disc edema due to a wide myriad of causes and helps resolve the associated symptoms.

Papilledema is defined as optic nerve head edema, in patients with raised intracranial pressure (ICP).[1] It can occur irrespective of age, sex, or ethnicity, though relatively rare in infants due to open fontanellae.[1,2] Acute papilledema can present as vision loss and blind spot enlargement, followed by loss of peripheral and then central vision and field loss in later stages. In patients with idiopathic intracranial hypertension (IIH), conservative management includes weight loss, oral carbonic anhydrase inhibitors, diuretics, and topiramate. Surgical options include optic nerve sheath fenestration (ONSF) and neurosurgical shunting procedures like ventriculoperitoneal or lumbo-peritoneal shunt.[1]

Methods

This retrospective study was conducted by reviewing records of 18 eyes of 15 patients who underwent ONSF surgery at a tertiary level eye hospital in Jaipur, India. The patients who underwent surgery were between 13 and 54 years old. All patients with optic disc edema due to IIH and related causes associated with diminution of vision along with visual field defects who underwent surgical management with ONSF were included in the study. Neuro-physician's review was done for all patients, as was neuroimaging, to ascertain the etiology of disc edema. Medical management was continued in all patients. All surgeries were performed by a single, highly experienced surgeon. Optic disc edema grading was done as per modified Frisén scale as shown in Table 1.[3] The pro forma was completed by single researcher. For statistical analysis, Snellen BCVA was converted to logMAR value.

Table 1.

Modified Frisen Scale[3]

Papilledema Grade	Features
Grade 0	Normal disc
	Radial nerve fiber layer striations, without tortuosity
Grade 1	Minimal edema.
	Subtle gray C-shaped halo with a temporal gap obscuring underlying retinal details.
	Temporal disc margin normal
Grade 2	Mild edema with circumferential halo. Elevated nasal border
	No major vessels are obscured
Grade 3	Moderate edema.
	All borders are elevated
	One or more obscured segment of major blood vessels leaving the disc. Circumferential halo with an irregular outer fringe.
Grade 4	Marked edema.
	Total border obscuration.
	Elevated nerve head, including the cup. Totally obscured segment of major blood vessel.
Grade 5	Severe edema
	All vessels on disc and leaving the disc are obscured

*Modified Frisen scale aids in the evaluation of papilledema to confirm the clinical examination, treatment assessment, and follow-up

Humphrey's visual field analysis was performed wherever possible. Patients aged less than 12 years and more than 60 years were excluded from the study. Also patients with history of chronic use of systemic steroids and major systemic co-morbidities making them unfit for general anesthesia were not included in the study. Patients with incomplete documentation were also excluded. All surgeries were done under general anesthesia. Detailed informed consent was taken from the patients and guardians in case of minors and dependent patients. Preoperatively, moxifloxacin eyedrop was given to all patients. Surgeries were performed under magnification through the medial transconjunctival approach for exposure of optic nerve. After the eye was painted, draped and lid speculum applied, a limbal peritomy was done, and medial rectus muscle identified, secured, tagged, and transected from the globe [Fig. 1a]. A bridle suture was applied to muscle stump to improve exposure [Fig. 1b]. Malleable retractors were applied, and blunt dissection was carried out, till the “pearly white” optic nerve was visualized, and muscle hook and lens hook were used to aid exposure of optic nerve sheath as and when needed [Fig. 1c]. A sharp paracentesis knife was used to make an opening in optic nerve sheath, with a gush of CSF indicating successful full-thickness penetration [Fig. 1d]. A small area of optic nerve sheath tissue (approx. 3-5 mm) was removed with the help of Vannas scissors, leaving behind a window, to ensure continued drainage in the postoperative period [Fig. 1e]. Tissue retractors were removed, and intraoperative moxifloxacin 1 mg solution was spread over the site. The muscle was reattached, and conjunctiva closed, eye patched, and anesthesia reversed [Fig. 1f-h]. Postoperatively, the patients were started on moxifloxacin–dexamethasone combination drops four times a day, brimonidine 0.15% eyedrops three times a day, and chloramphenicol-polymyxin B ointment at night along with lubricant drops four times a day. A broad spectrum oral antibiotic was given for five days, anti-inflammatory–analgesic combination for seven days, and oral steroid deflazacort in tapering dose for three weeks. Oral acetazolamide 250mg was continued in preoperative dose for three weeks and then tapered. Patients were reviewed on postoperative day one, day seven and after one month, and monthly thereafter till the completion of one year. Patient's preoperative best corrected visual acuity and visual acuity at two months postoperative were compared once the vision and disc edema had stabilized. In all patients, oral acetazolamide was stopped between three months to one year. The described research adhered to tenets of the Declaration of Helsinki. IRB approval was received for the study.

Figure 1.

(a) Medial rectus tendon is disinserted. (b) Posterior ciliary vessels identified. (c) “Pearly white” optic nerve sheath identified. (d) Optic nerve sheath incised. (e) Small area of tissue removed leaving behind a window for drainage. (f) Medial rectus muscle sutured at insertion. (g) Peritomy closed. (h) Round-reacting pupil

Statistical analysis

The data was collected, coded, and fed in SPSS (IBM version 23, IBM Corp., Armonk, NY, USA) for statistical analysis. The descriptive statistics included mean and standard deviation. The inferential statistics included paired t-test, ANOVA test, independent t-test, and Pearson's correlation test for the comparison. The level of significance was set at 0.05 at 95% confidence interval.

Results

Eighteen eyes of 15 patients were included in the study, and out of which 12 patients underwent surgery on one eye, while three patients underwent bilateral optic nerve fenestration surgery for ongoing disease in the contralateral eye, albeit at different times. The male-to-female ratio was one to four. The youngest patient was 13 years old, the oldest being 54 years old, with average age of population being 32.73 ± 12.22 years. The time of presentation after onset of symptoms ranged from one day to three years, mean time being 6.37 ± 11.02 months. In our study, 14 (93.33%) patients presented with chief complaints of diminution of vision. Of these, one patient had an additional complaint of diplopia, one patient had swelling in orbital region, one patient had a history of road traffic accident one day prior, and one patient had cortical venous thrombosis few days prior. One patient (6.66%) presented with chief complaint of headache. On examination, 12 (80%) patients were found to have bilateral optic disc edema, two (13.33%) had optic atrophy, and one (6.66%) patient with a history of road traffic accident was found to have unilateral optic disc edema as a result of traumatic optic neuropathy. Among the 12 patients who had bilateral optic disc edema, six (50%) patients had grade 1 disc edema, three (25%) patients had grade 2 disc edema, one (8.3%) patient each had grade 3, 4, and 5 disc edema, preoperatively, in the eye that was operated. One patient who underwent surgery on both eyes had grade 0 disc edema at the time of presentation for second eye surgery. In the other eye, one (8.3%) patient had grade 0 disc edema, six (50%) had grade 1, two (16.6%) had grade 2, and one (8.3%) each had grade 3, 4, and 5 disc edema. After surgery, there was an average improvement of 1.85grades of disc edema. On investigating to look for cause of disc edema, 12 (80%) patients were diagnosed with IIH, of these two already had ventriculo-peritoneal shunts in situ. One was diagnosed to have Wegener's granulomatosis-related orbital inflammatory disease along with chronic disc edema, one had cortical venous thrombosis, and one had developed traumatic optic neuropathy with doubtful optic sheath hematoma. Humphrey's visual field analysis was performed on both eyes of six (40%) patients and one (better) eye of three (20%) patients. Visual field analysis was not possible in six (40%) patients. Among the patients who underwent visual field analysis in both eyes, three had severely depressed fields, one had an enlarged blind spot in one eye, with scattered non-specific defects in the other eye, one had generalized peripheral constriction of visual fields, and one patient had normal visual fields. Among patients who underwent visual field analysis in only one eye, one had a small remaining temporal island, one had peripheral constriction of visual fields, and one had grossly defective visual fields. Out of these, five patients (55.56%) with IIH showed improvement in visual fields after surgery. Improvement in visual acuity was seen in the operated eyes of all the patients. On comparing logMAR visual acuity in the operated eye, before and after surgery, mean visual acuity before surgery was -1.9789 ± 1.46270, and after surgery it had improved to -0.9022 ± 1.23181. On applying paired t-test, P value was found to be 0.000, signifying that change in vision was highly significant. On similar assessment of contralateral eyes, mean preop vision was -1.3378 ± 1.50107 and mean postoperative vision was -1.0667 ± 1.33813 [Fig. 2]. On paired t-test, P-value was found to be 0.011, signifying that this change was also statistically significant, though less than change found in the eye that was operated upon. The improvement in number of lines read on chart ranged from 0.5 lines to 14 lines in the operated eye, mean improvement being 5.40 ± 4.43 lines. In case of contralateral eye, 10 eyes showed no change in the visual acuity after surgery, while the best improvement was seen in one case where the patient gained 7.25 lines of vision in non-operated eye. The mean improvement was 1.22 ± 1.96 lines. Pearson's correlation coefficient was calculated between the duration of symptoms, in months, and number of lines of vision that was gained, in case of both operated eye and contralateral eye.[4] Pearson's correlation was -0.368 for operated eye and -0.270 for other eye as shown in Table 2. Similar analysis was done between the gain in number of lines and age of the patients in years. The Pearson's correlation coefficient in case of operated eye was 0.518, and that for other eye was -0.153 as shown in Table 3. The correlation between age and amount of vision gained in the operated eye was found to be statistically significant with P-value of 0.028. The same in case of contralateral eye was not significant, with P value of 0.543. Three eyes (16.67%) developed atonic pupil in the immediate postoperative period.

Figure 2.

Improvement in logMAR visual acuity after optic nerve sheath fenestration in the eyes that underwent surgery and in the contralateral eye

Table 2.

Comparison of visual acuity preoperatively and postoperatively

	Mean	Standard deviation	Pearson’s correlation‡	P
Duration	6.3683	11.34035	- 0.368	0.133 (N.S)
Line improvement in operated eye	4.5972	4.39011
Duration	6.3683	11.34035	- 0.270	0.279 (N.S)
Line improvement in contralateral eye	1.0833	2.06867

^‡Pearson’s correlation was calculated between the duration of symptoms in months and the improvement in visual acuity in both eyes

Table 3.

Comparison of visual acuity preoperatively and postoperatively

	Mean	Standard deviation	Pearson’s correlation‡	P
Age (in years)	33.2778	12.63425	0.518	0.028 (S)
Line improvement in operated eye	4.5972	4.39011
Age (in years)	33.2778	12.63425	- 0.153	0.543 (N.S)
Line improvement in contralateral eye	1.0833	2.06867

^‡In the study population, as age increased, the visual acuity improvement increased, and this was found to be statistically significant in case of the operated eye

Discussion

Optic disc swelling can be a characteristic presentation of a wide range of intrinsic ocular, intracranial, and systemic diseases.[5] A study conducted in Japan by Hata and Miyamoto, found that the most common cause for bilateral optic disc edema was papilledema and that of unilateral optic disc edema was anterior ischemic optic neuropathy. Bilateral optic disc edema included pseudopapilledema, small disc size, optic disc drusen, diabetic papillopathy, etc., Other conditions leading to unilateral optic disc edema were optic neuritis, optic disc tumor, compressive and infiltrative neuropathies, true and pseudopapilledema, diabetic papillopathy, and neuroretinitis. Over one-third of cases of papilledema were found to be due to IIH and cerebral venous thrombosis.[5] Diminution of vision in papilledema is thought to occur due of disruption of axonal transport, which causes compression of small arterioles supplying the nerve, giving rise to intraneuronal ischemic damage to optic nerve.[1,6] IIH, however, occurs typically in obese women of childbearing age. In cases where a diagnosis of IIH is made in children, men, or elderly, aggressive evaluation becomes imperative to avoid missing a different etiology.[1] The Monro–Kellie doctrine suggested that total sum of volume of the brain, CSF, and blood in the cranium remains constant, and thus if one of these components increases, the others should decrease; however, when this balance is lost, it leads to raised ICP.[1,7] Visual field defects in papilledema occur due to nerve fiber bundle defects, with sparing of the papillomacular bundle, and hence, central vision is spared till late stages.[1,8] Eventually, chronically raised ICP with disc edema can give rise to optic nerve atrophy and profound vision loss.[8] The original diagnostic criteria for IIH was first proposed by Dandy and later modified, as our understanding evolved over the years.[9] The approach to managing a case of disc edema depends on the symptomatology, grade of disc edema, and amount of vision loss. First step is to treat the cause. Surgical intervention is reserved for patients who are noncompliant, or intolerant to medical management, have acute fulminant edema of disc and field defects, in order to prevent irreversible vision loss.[1] The surgical options available for treating raised ICP include ONSF and shunting procedures. Out of these, the first is relatively safer, has shorter surgical and postoperative recovery time, technically less challenging, and has no mortality from the procedure itself.[10] In 1872, DeWecker first described ONSF as a procedure involving making a small incision in the meninges surrounding optic nerve, which helps relieve elevated ICP.[11] It decompresses peri-optic subarachnoid space and induces scarring of surgical site, thus prevents further accumulation of CSF.[12] It diminishes associated visual sequelae of elevated ICP.[8] It usually relieves pressure around the ipsilateral optic nerve, although sometimes may have a filtration effect, resulting in improvement in headache and contralateral disc edema also.[10] ONSF is indicated in patients with vision loss associated with raised ICP causing disc edema as is seen in IIH, cerebral venous thrombosis, intracranial tumors with secondarily raised intracranial tension due to decreased CSF outflow, optic nerve sheath hemorrhage, cryptococcal meningitis with papilledema, and intracranial breast cancer metastases with papilledema.[8,13-15] It is also indicated in certain local diseases of optic nerve including traumatic optic neuropathy, optic nerve tumors, and optic nerve infiltration with Non-Hodgkin's Lymphoma, and a few studies have shown vision gain in patients with optic nerve head drusen.[8,16] A small study on children showed that efficacy of the procedure is similar to that in adults.[17] The typical complications of ONSF include diplopia, sudden vision loss, and pupillary abnormality resulting from damage to parasympathetic nerve fibers. The previously suggested mechanisms in literature, which help in resolution of optic disc edema post optic nerve sheath fenestration, include direct drainage through the fistula, and fibrous scarring prevents transmission of increased pressure to optic nerve head, which is most sensitive to ischemic damage.[18,19] In the current study, all surgeries were performed by a single, highly experienced surgeon, thus ensuring no procedural bias, and observations were made by a single researcher, which helped avoid any observational bias. Most studies in literature have assessed the effect of ONSF on disc edema associated with IIH. Our study has also included patients with disc edema associated with traumatic optic neuropathy, cortical venous thrombosis, and Wegener's granulomatosis along with IIH. Efficacy of procedure was assessed based on the change in visual acuity after surgery in both operated as well as contralateral eyes. For the purpose of statistical analysis, visual acuity was converted from Snellen's values to logMAR equivalents using a conversion chart. Negative value was given to visual acuity less than 6/6, and positive value was given for visual acuity better than 6/6, zero being the logMAR equivalent for visual acuity of 6/6. The average age of our study population was 32.73 years (SD ± 12.56). This was similar to a study conducted in Pakistan, wherein the average age of the study group was 36.23 ± 5.19 years and another study conducted at the University of Iowa, wherein mean age of population was 32 years.[10,20] Our study population had four times as many females as males, which was in accordance with previous studies conducted in different parts of the world, most probably due to much higher incidence of IIH in females in reproductive age group.[10,19,21] Optic nerve head edema can affect macula, due to swelling of retinal nerve fiber layer and presence of exudates and subretinal fluid, secondary to disturbed axoplasmic flow. Thus, resolution of disc edema should improve vision in patients undergoing optic nerve sheath fenestration.[22] In our study, 93.33% patients presented with diminution of vision, with mean visual acuity of the eye being operated being logMAR -1.9789 ± 1.46270, which improved to -0.9022 ± 1.23181 showing definite improvement, which was also highly statistically significant. The mean paired difference was logMAR -1.0767 ± 0.90153. Improvement was also seen in visual acuity of opposite eye, though less than the eye that underwent surgery, with mean paired difference being -0.2711 ± 0.40054. This change was also found to be statistically significant. In a retrospective study conducted by Buzzacco and Katz at The Ohio State University Medical Center, mean paired difference before and after surgery was logMAR 0.001, P = 0.74.[19] In a study conducted on 578 eyes of 331 patients, improvement or stability was seen in 94.4%.[23] In our study, improvement in vision after surgery was noticed in all 18 eyes (100%) that underwent surgery, and eight contralateral eyes (44.44%) also showed improvement. No patient showed worsening of vision after surgery. This was better than results in a study conducted by Fonseca et al.,[21] in which only 50% of the eyes that underwent surgery, showed improvement in vision, while remaining showed stable vision, and three out of 14 patients showed worsening of visual acuity. In our study group, 12 patients presented with bilateral optic disc edema and one with unilateral optic disc edema due to traumatic optic neuropathy. All of them showed improvement in the grade of edema, with average improvement of 1.85 grades. These results were in tune with previous studies that showed improvement of papilledema in 71–100% eyes.[18,20,21] In two patients, there was continued visual acuity loss in spite of previously implanted ventriculoperitoneal shunts, prior to ONSF surgery, suggesting need for a second procedure post shunt placement. On looking for an association between duration of symptoms and improvement of vision after surgery, P-value was found to be 0.133 and Pearson's correlation coefficient-0.368, thus, though no statistically significant correlation was found, an inverse association was found, showing that as duration of symptoms increased, gain in visual acuity decreased. This suggests that overall visual prognosis improves with earlier intervention, in patients requiring surgery, and overall, the procedure is a safe and effective way of managing optic disc edema, due to a number of causes, including pseudotumor cerebri, traumatic optic neuropathy, cortical venous thrombosis, and inflammatory conditions affecting orbit.

Our study was limited by small sample size, short average follow-up time, and variability in visual field assessment. Use of modified Frisen scale to evaluate papilledema was also a limiting factor as it allows for variability due to subjective nature of the evaluation. This can be avoided by quantification of disc edema, using optical coherence tomography.

Conclusion

The study proved that early surgical intervention for ONSF is a safe and effective method of treating optic disc edema associated with IIH & related causes and reversing vision deterioration and other symptoms like headache and nausea associated with it. We also concluded that bilateral optic nerve sheath fenestration is not always required, as improvement in visual acuity was also seen in the contralateral eye in 12 out of 15 patients.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.