Optic Nerve Sheath Fenestration: Indications, Techniques, Mechanisms and, Results Raghu C. Mudumbai, MD
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Introduction
In the late 19th century, DeWecker1 initially described the use of optic nerve sheath fenestration (ONSF) in a case of neuroretinitis at a time when little was known about the pathophysiology of optic nerve swelling. The procedure lay relatively dormant until renewed interest arose from studies investigating the axonal basis of papilledema and its resolution with ONSF. This surgery has been utilized in a variety of other optic nerve conditions not related to papilledema, with largely disappointing results, including the Ischemic Optic Neuropathy Decompression Trial.2 Although prospective clinical trials have not been performed to compare the efficacy of ONSF to other treatment modalities like shunting procedures, many studies have confirmed that ONSF can play a significant role in preventing vision loss in conditions where intracranial pressure (ICP) is elevated, like idiopathic intracranial hypertension (IIH). ’
Benefits of ONSF in IIH
ONSF is a relatively safe procedure that may help prevent visual decline in patients with IIH as demonstrated in several clinical series.3–5 Typically after surgery, there is a corresponding resolution of papilledema in most patients.6 Patients can also experience significant gains in visual field (VF), even if the VF is significantly depressed at the time of surgery.7 Although ONSF is typically reserved for IIH patients who experience vision loss, an additional benefit may be improvement in headache in a majority of patients.8 While the mechanism is not clear, unilateral ONSF can reduce disc swelling in the contralateral optic nerve, improve visual function in both optic nerves, and eliminate the need for bilateral fenestration.9 It should be kept in mind that visual INTERNATIONAL OPHTHALMOLOGY CLINICS Volume 54, Number 1, 43–49 r 2014, Lippincott Williams & Wilkins
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function may deteriorate several years after an initially successful ONSF; careful, serial follow-up is required.10 A Cochrane review concluded that there is a lack of randomized controlled studies for the treatment of IIH; at present, there is insufficient evidence to recommend whether ONSF or lumboperitoneal shunting procedures should be performed in IIH patients.11 ’
Complications
A number of complications have been associated with ONSF, most typically minor and transient. The most serious complication, permanent vision loss, can occur acutely or from failure of the fenestration, even with clinical improvement of the appearance of the optic nerve.12 Ischemic optic neuropathy and arterial occlusions are intraocular causes of vision loss, and retrobulbar hemorrhage can lead to a compressive optic neuropathy.13 Risk of blindness from the surgery in a large series is approximately 1% to 2%.4 The medial approach, because it requires detachment and reattachment of the medial rectus, can be associated with diplopia and adduction deficit from dysfunction of the medial rectus; the double vision is typically transient.5,14 Both medial and lateral approaches to the optic can lead to a tonic pupil because of the close proximity of the posterior ciliary nerves and ciliary ganglion.15,16 ’
Surgical Approaches
The most common surgical approach is transconjunctival medial orbitotomy, which has been utilized since the 1970s.17 The main advantages of this approach include good visualization of the optic nerve including anatomic landmarks, familiarity of technique, and ease. A brief description of the procedure follows. Most surgeons use the operating microscope for the entire procedure, although introducing it after the rectus muscle disinsertion can be done. Typically, the patient is placed under general anesthesia. A nasal conjunctival peritomy is performed for 120 to 180 degrees with oblique relaxing incisions. The medial rectus is isolated with muscle hooks and tagged with standard strabismus suture (5-0 or 6-0 Vicryl) and is disinserted. The muscle stump is tagged with a larger suture, typically 4-0 silk, in a running stitch manner to facilitate lateral retraction of the globe and adequate exposure of the optic nerve. Occasionally, placing traction sutures underneath the superior and inferior recti muscles can facilitate visualization. Thin, small malleable retractors are placed medially along with cottonoids to prevent orbital fat from entering the field of the optic nerve. The eye is rotated laterally, and the long posterior ciliary artery and nerves18 are followed posteriorly toward the insertion of the optic www.internat-ophthalmology.com
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nerve on the globe. Periodic relaxation of the retraction is necessary to evaluate for dilation of the pupil (indicating ischemia to the eye from the retraction). Once the optic nerve has been visualized, a small blade, such as a 15-degree, is used to make slits that run longitudinally to the course of the optic nerve. Care is taken to minimize disruption of the short posterior ciliary vessels, which also serve as a useful landmark in identifying the optic nerve. Commonly, a small egress of CSF will be noted after the incision. After the dura and arachnoid trabeculations have been carefully separated from the underlying nerve by the use of a neurosurgical pick or similar instruments, parallel slits can be connected by long neurosurgical scissors to form a window. Alternatively, these slits can remain unconnected for multiple sites of filtration.19 ’
Alternative Approaches
Although the medial orbital approach is probably the most commonly utilized, the optic nerve can be approached by a variety of techniques, each of which has relative advantages and disadvantages. A lateral orbitomy provides the advantage of excellent exposure with good clinical outcomes reported.20 The lateral approach can be modified to obviate the need for bone removal or disinsertion of extraocular muscle.21 More recently, the upper eyelid approach has generated much interest because of its relative advantage of not requiring muscle detachment, adequate exposure of the optic nerve, and avoidance of critical orbital structures during dissection.22 Superomedially, the optic nerve can be visualized in the medial intraconal space through an upper lid crease incision. This approach provides a quick, relatively wide exposure without requiring muscle disinsertion and limited need for sharp dissection, thereby avoiding orbital blood vessels that lie on the temporal side of the optic nerve.23 Reported complications include transient diplopia, transient ptosis, and tonic pupil. How Does ONSF Help Resolve Papilledema?
Creation of an on-going filtration site through the creation of a fistula may relieve the optic nerve from cerebrospinal fluid pressure.24 Alternatively, filtration may still occur through a scarred bleb that has formed.25 It is also possible that CSF pressure is prevented from distally affecting the intraocular optic nerve by ONSF, creating a fibroblastic scar in the subarachnoid space.26 ONSF in Other Optic Neuropathies
ONSF has been touted as a surgical option for a variety of optic nerve disorders that are not related to raised ICP, including progressive www.internat-ophthalmology.com
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nonarteritic ischemic optic neuropathy, low-tension glaucoma, and acute-retinal necrosis involving the optic nerve.27–29 None of these proposed indications have gained general acceptance, primarily because the mechanism of optic nerve injury differs substantially from papilledema.30 How elevated ICP leads to the development of axonal stasis and papilledema has been studied extensively.31 Optic disc edema from elevated ICP is predominantly due to intra-axonal swelling32 with minimal collection of interstitial fluid.33 Although optic nerve head swelling is seen in a multitude of optic neuropathies, the fundamental pathophysiological basis of vision loss is different between papilledema and other optic neuropathies like NAION34 or hypertensive disc edema.35 In nonarteritic anterior ischemic optic neuropathy (NAION), where the main insult is acute ischemic damage to the optic nerve, a large, multicenter randomized trial was terminated early as patients treated with ONSF for NAION (not just progressive cases) had a significantly greater risk of losing at least 3 lines of vision compared with the carefully observed patients at 6 months.2 This worsening vision may be due to further compromise of optic nerve circulation from either direct damage to vasculature or from surgical manipulation. ONSF currently should be reserved for conditions where vision loss has resulted from optic disc edema from raised ICP. There have been no randomized clinical trials so far to evaluate the role of medical or surgical management (ONSF or intracranial shunt placement) in the treatment of papilledema.
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Special Considerations Pregnancy
As IIH is a condition that most commonly affects young women in their child-bearing years, pregnancy requires special consideration. Pregnancy rates for IIH patients are similar to the general population with generally similar visual outcomes.36 Most pregnant patients can be treated conservatively and followed up closely until delivery.37 Some patients do experience worsening severity of vision loss that can rapidly progress, requiring ONSF.38 There are potential risks to the mother and fetus from general anesthesia from changes in ICP from induction.39 ONSF can be performed under local anesthesia if the risk of general anesthesia is determined to be too high. The shorter operative times required for ONSF as compared with shunting procedures can also be a consideration for pregnant patients with IIH, as is the absence of a need to instrument the peritoneal cavity.40 www.internat-ophthalmology.com
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Pediatric IIH
Small studies have indicated that ONSF may be safely utilized in the pediatric population with similar outcomes to adults.41,42 The surgical considerations and approach are not appreciably different from adult patients. Algorithms for Utilization of ONSF in IIH
At present, evidence-based guidelines for when ONSF should be utilized in the management of vision loss in IIH do not exist.11 When visual loss is noted at presentation, Corbett et al43 have recommended consideration of ONSF early in the clinical course. Banta and Farris3 recommend ONSF when progressive visual loss, as defined by loss of >2 lines of Snellen acuity or new onset or progression of VF defect, occurs despite initial medical management. Pineles and Volpe44 have reported that earlier ONSF surgery may result in better visual outcomes. Neither are there any prospective studies to guide surgical decision making, nor are there data comparing ONSF visual outcomes to CSF diversion or medical therapy. ’
Conclusions
Visual loss can occur as a result of raised ICP and papilledema, of which IIH is a common cause. Although evidence-based guidelines for its use do not exist, there is substantial evidence that ONSF can help prevent vision loss in IIH and is generally a safe procedure. ONSF is contraindicated in NAION and leads to worse visual outcomes. Multiple surgical approaches can be utilized, the most common of which are medial orbitotomy and upper eyelid crease. A prospective trial comparing all modalities of treatment of IIH is required to further clarify the particular role of each intervention, including ONSF.
The author declares that there is no conflicts of interest to disclose.
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References
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