Optic Nerve Sheath Fenestration and Optic Nerve Biopsy

Abstract

Optic nerve sheath fenestration (ONSF) is an important procedure in the management of patients with idiopathic intracranial hypertension (IIH) who do not respond to medical treatment. Traditional surgical approaches for fenestration have limitations. Furthermore, when tumors in the intraconal space medially and superiorly are encountered, complex surgical approaches with significant morbidity have been used. We present our surgical approach to these conditions with a technique that is quick, safe, and efficient.

Introduction

The medial and superior intraconal space, just posterior to the globe and next to the optic nerve, presents a challenge to the orbital surgeon: the tetrahedral shape of the orbit and the presence of the globe make access to this area difficult. Traditionally, tumors in this space have been approached via a transcranial route or with lateral out-fracturing of the lateral bony orbit and a medial transconjunctival approach. Both these approaches carry significant risks. We developed a surgical approach to this space which is relatively quick and can be performed efficiently and safely. This technique is useful for performing ONSF, obtaining optic nerve biopsies, and removing lesions in the medial and superior intraconal space [1].

Definition of ONSF

Optic nerve sheath fenestration is the opening of the optic nerve sheath with the removal of a window of the sheath. It is usually performed on the orbital portion of the optic nerve, with the aim of releasing the cerebrospinal fluid from the subarachnoid space, which normally extends forward to just behind the optic disc.

Mechanism of ONSF

ONSF is thought to reduce the pressure on the optic nerve head by reducing the subarachnoid pressure. It has been theorized that a cerebrospinal fluid (CSF) filter from the subarachnoid space of the optic nerve into the orbital tissue continues to give a sustained reduction of pressure at the optic nerve head. MRI and ultrasound studies have demonstrated fluid collection around the fenestration site after surgery. The other theory of fibrous tissue proliferation at the incisional site, preventing transmission of elevated CSF pressure to the optic nerve head, seems less likely as one sees the improvement in the visual field within a few days of surgery. However, such fibrosis may well contribute to a sustained improvement in the transmission of pressure to the optic nerve head over the following months [2].

Indications for ONSF

The commonest indication for ONSF is to release pressure around the optic nerve in IIH where there is progressive visual loss which is resistant to medical therapy. In approximately 10% of patients with untreated IIH, visual loss will progress to the point of meeting the legal criteria for blindness [3]. Numerous other conditions have been reported in the literature to have been helped with ONSF (Table 12.1). It should be noted that the second commonest indication for ONSF used to be ischemic optic neuropathy, but the ischemic optic neuropathy trial has shown that not only does ONSF not help but (the medial approach) technique itself may lead to more harm [4]. Certain other indications for ONSF like central vein occlusion and for normal tension glaucoma are reflected in isolated reports but the efficacy has not been.

Table 12.1 Indications for optic nerve sheath fenestration

History of ONSF

It is remarkable that the first optic nerve sheath fenestration (ONSF) was presented as early as 1872 by the renowned Parisian ophthalmologist De Wecker at the International Ophthalmic Congress in London for the treatment of “neuroretinitis” [5]. He described a transconjunctival approach between the lateral and inferior rectus muscles with the fenestrations being performed with longitudinal incisions and being done blindly under the guidance of a finger threaded back to the optic nerve without anesthesia! Henry Power (who became the first ophthalmic surgeon at St. Bartholomew’s Hospital in 1870) used this approach in a 13-year-old girl whose vision was improved from no vision to hand movements.

Carter of London (who helped establish the Nottingham Eye Hospital in 1959) split the lateral rectus longitudinally to perform fenestration of the optic nerve under direct vision and presented four of his cases [6].

Muller in 1916 performed what amounted to a Kronlein procedure with removal of the lateral orbital rim for better exposure of the optic nerve [7]. As is often the case, this was presented as a new procedure with failure to acknowledge the previous techniques. Therefore, this procedure was called “Muller’s operation for nerve sheath trepanation.”

Gomez-Marquez of Barcelona performed a lateral canthotomy and detached the lateral rectus to gain access to the optic nerve in 1935 [8].

Interest in optic nerve sheath surgery was revived by Professor Hayreh’s investigations into the pathogenesis of optic nerve edema. He demonstrated the reversal of papilledema with ONSF and also showed that papilledema could be prevented with ONSF. He was the first to show bilateral improvement after unilateral ONSF and also showed decreased intracranial pressure following ONSF [9, 10].

Smith et al., in 1969, used a Kronlein approach with lateral retraction of the eye and a medial approach to ONSF on a patient with field changes, visual obscurations, and disc swelling caused by an arachnoidal cyst of the optic nerve sheath [11]. The patient recovered her vision.

Sydney Davidson of Liverpool presented results of ONSF in five patients with unresectable intracranial tumor in 1969 [12]. He showed reduction of headaches and disc edema in all his patients and vision improvement in four. He used Berke’s 1953 modification of Kronlein’s 1888 lateral approach [13] which improved upon the semicircular temporal scar by performing a lateral canthotomy, undermining and cantholysis of the common canthal attachment to the lateral orbital rim. Berke’s modification [14] was a major advance in orbital surgery. Davidson, however, described the dissection to expose the optic nerve “difficult and tedious.” I participated in this approach with Mr. Davidson in the mid-1980s in Liverpool, and, true enough, each procedure took us around 3 hours. I have a distinct memory of the consultant taking a couple of breaks during the procedures while the senior house officer continued to mind the patient. We even performed two fenestrations with the help of neurosurgeons at Walton Hospital via a transcranial approach, which, not surprisingly, took us several hours.

The next major advance was by Galbraith and Sullivan in 1973 when they described their medial trans-conjunctival approach with detachment of the medial rectus muscle [15]. This approach, which is familiar to ophthalmologists, continues to be the most commonly used one for ONSF although other techniques (see below) are beginning to get accepted as they reduce morbidity and surgical time.

Tse et al. in 1988 described the lateral approach to the optic nerve via a lateral orbitotomy and removal of the lateral orbital rim [16]. This is similar to Mr. Davidson’s Berke modification and removal of the lateral orbital rim. In their approach, a large skin incision with bone removal and manipulation of the temporalis muscle was needed.

Kersten and Kulwin in 1993 [17] described a lateral canthotomy and cantholysis approach to the optic nerve without removal of the lateral orbital rim, which was similar to that of Gomez-Marquez in 1935 except they did not detach the lateral rectus muscle.

Patel and Anderson in 1994 described a lateral upper eyelid approach to the optic nerve without a canthotomy and without detachment of the lateral rectus muscle [18]. Their procedure allowed better exposure of the optic nerve with reduced complications and was quicker to perform than other procedures described until then. They entered the muscle cone via the periosteum and between the superior rectus and lateral rectus muscles.

Blessing and Tse have recently described a modified lateral transconjunctival approach to the optic nerve performed in combination with a lateral canthotomy and retraction of the lateral rectus without detachment of the muscle [19].

We presented our superomedial approach to the optic nerve and intraconal space in 2000 but have been using it since 1995 [1].

Development of the Superomedial Approach to the Optic Nerve

The development of the superomedial approach to the optic nerve, as with some advances in medicine, was based upon a fortuitous observation. When my kids were messing about in a hot tub, as kids are wont to do, I heard a scream. My daughter had a hand over her right eye: she complained that her brother had poked her in the eye. An abrasion was present over the superomedial upper eyelid where a bruise was developing but of greater concern was her claim that she could not see. She could see my hand moving but she claimed she could not see as well as with the opposite eye. I dilated her and examined the optic nerve and the fundus and found nothing untoward. By this time, she had an impressive bruise on the upper eyelid, and her vision was back to what she called normal. There was no limitation of ocular movement and no proptosis. This reminded me of a case of bilateral auto-enucleation that I had seen in London, where the patient had enucleated both her globes with impressive lengths of optic nerves attached to the globes by insinuating a finger under the upper eyelid medially. The upper and lower eyelids were intact, and the sockets almost looked like there had been surgical enucleations, except for the bleeding that was present. Finally, while performing fat decompressions with or without bone removal for thyroid-related proptosis, I had regularly made upper eyelid incisions not only to access the levator aponeurosis and Müller’s muscle but also to remove intraconal orbital fat. I had often commented on the impressive view of the optic nerve that I obtained during this dissection. Based upon these observations, I went on to use this approach to obtain optic nerve biopsies, perform optic nerve sheath fenestration, and remove intraconal orbital tumors from 1995. In 1999, we carried out cadaver studies to measure the angles, the exposure, and the distance to the optic nerve of the four commonly used approaches for optic nerve fenestrations. These studies clearly showed the superiority of the superomedial approach to the optic nerve for fenestration but also for obtaining biopsies and for removing medial, superomedial, and superior intraconal tumors. We reviewed our results of fenestrations and removal of intraconal tumors which we published together with details of the surgical technique [1].

Anatomy

The superomedial orbit is a “busy” area but with natural planes which allow relatively easy access to the medial and superior intraconal orbital space. The medial eyelid skin and orbicularis muscle make the anterior wall of the superomedial space. Superiorly behind the orbicularis lies the trochlea and the superior oblique tendon which traverses medially and posteriorly to the superior globe. The superior rectus muscle marks the inferolateral aspect of the space. The medial rectus marks the inferior boundary, while the medial levator aponeurosis and the superomedial globe mark the inferior border of the entry to the superomedial space. The medial border is composed of the nasal and ethmoid bones (Fig. 12.1). The supraorbital, the supratrochlear, and infratrochlear vessels and nerves are along the roof and the medial wall of this space. The space behind the medial orbital septum leads to the intraconal fat which is divided by major and minor fascial planes of Koornneef [20]. The superior ophthalmic vein is in the superior part of this space and can be traced by following branches of the supraorbital vein. The superior ophthalmic vein will generally be superior to the plane of dissection and is rarely encountered during the dissection. Moving through the orbital fat between the medial rectus and superior rectus, one gets to the space medial to the optic nerve where one finds the medial posterior ciliary arteries as they enter the globe. A variable vortex vein may be encountered along the superomedial globe. The short and long posterior ciliary nerves enter the globe usually at the 3 and 9 o’clock positions. The Tenon capsule covers the posterior globe separating the globe from the intraconal fat. Posterior ciliary arteries and short posterior ciliary arteries mostly enter the sclera medial and lateral to the optic nerve with only 9% of eyes showing a superior posterior ciliary artery [21]. The central retinal artery enters the optic nerve 8–15 mm behind the globe, inferior, inferolateral, or lateral to the optic nerve.

Fig. 12.1

The anatomical borders of the superomedial space used for the approach to the optic nerve and the superomedial intraconal orbit

The orbital surgeon needs to be familiar with the different sections of the optic nerve: it is divided into four sections.

• A. The optic nerve head (also called the optic disc) is where the nerve inserts into the eye. 1.2 million retinal ganglion cells converge to form the optic nerve head, which is 1.5 mm in diameter and 1 mm in length. Blood supply is from the circle of Zinn-Haller and the posterior ciliary arteries (branches of the ophthalmic artery).

• B. The intraorbital optic nerve which is 28 mm long (the distance from the back of the eye to the orbital apex is 15 mm, which allows proptosis of the globe without affecting the optic nerve, up to a certain point): the retinal nerve fiber layer exits the eye at the lamina cribrosa, which is a portion of the sclera made up of sheets of elastic fibers with fenestrations. The axons are covered by myelin after exiting the lamina cribrosa which increases the diameter of the optic nerve to 3 mm. The nerve acquires a covering of pia mater, arachnoid mater, and dural sheath. The pial vessels derived from the ophthalmic artery provide the blood supply to the orbital optic nerve.

• C. Intracanalicular optic nerve: the periorbita of the orbit and the dura mater of the optic nerve fuse at the orbital apex, where the nerve is also encircled by the annulus of Zinn which is made up of the insertions of the four rectus muscles. The intracanalicular optic nerve is relatively fixed within the optic canal which measures 8–10 mm in length and 5–7 mm in width. Anteriorly, the nerve is supplied by the ophthalmic artery, posteriorly from the internal carotid artery branches.

• D. Intracranial optic nerve: measures 8–12 mm before joining the optic chiasm. The nerve exits the canal under the anterior clinoid process with the ophthalmic artery under it. The internal carotid artery, the superior hypophyseal artery, the anterior cerebral artery, and the anterior communicating artery supply this portion of the optic nerve.

The subarachnoid space of the optic nerve is continuous with the subarachnoid space of the brain. The CSF circulates freely between the optic nerve and the brain. Raised intracranial pressure is transmitted to the subarachnoidal space within the optic nerve sheath, leading to optic nerve head edema (papilledema). The axoplasmic transport system is disturbed, leading to axonal swelling at the optic nerve head, leading to nerve fiber function loss: loss of central vision, peripheral vision, and eventual optic atrophy occur. It has been shown that with raised intracranial pressure, sequestration of CSF at the optic nerve head may cause optic neuropathy [22, 23].

Preparation

Instruments (Fig. 12.2)

Besides the basic plastic surgery instrument set with the standard orbital instruments, certain additional instruments are needed for this procedure. Yasargil and Rhoton bayonet scissors come in various lengths and may be straight or curved. We keep several different lengths and curves available. Rhoton style micro dissectors come in a set with round, curved, angled, sharp, blunt, and small, medium, and large tips: these are useful when accessing the optic nerve sheath, for picking up the sheath prior to fenestration and to undermine the dura when necessary. Neurosurgical cottonoids are useful to keep the orbital fat away from the optic nerve once exposure has been obtained. Sewell orbital retractors are essential for exposing the optic nerve.

Fig. 12.2

Yasargil curved and straight bayonet microscissors, Rhoton sharp, and blunt picks and dissectors

Patient Positioning

We place the patient in a moderate Trendelenburg position as this allows a better angle of approach to the optic nerve and the medial intraconal space. We use this position when operating with operating loupes with a surgical headlight (our preferred technique), or with an operating microscope (which we use when teaching).

Anesthesia

Although we have performed several of these procedures under sedation anesthesia and local anesthesia, general anesthesia with endotracheal intubation is preferred. 1 cc of 0.5% bupivacaine hydrochloride with 1:100,000 epinephrine solution is injected subcutaneously with a 30-gauge 1 cm needle. The injection is administered immediately after anesthesia has been induced and prior to sterile preparation of the surgical area to give time for vasoconstriction.

Surgical Technique (Figs. 12.3a, b and 12.4)

Incision

An incision between 6 and 10 mm is made at or just above the medial upper eyelid skin crease. We make the incision through the skin and use Stevens scissors to separate the orbicularis fibers. There will be oozing from the skin edges and from just under the orbicularis muscle: it is important to control this oozing with bipolar cautery at this point to prevent the blood from obscuring the view posteriorly as we dissect further.

Fig. 12.3

(a, b) Surgical approach is via a natural space that exists between the superior oblique tendon, the medial levator and medial levator horn, and the superomedial orbital rim. The dissection direction is below the superior oblique, above the medial canthal tendon and lateral to the superior rectus muscle and above the medial rectus muscle. (a: Reprinted from Pelton and Patel [1]. With permission from Wolters Kluwer Health, Inc. b: Courtesy of Bhupendra C. K. Patel)

Fig. 12.4

Surgical sequence (a) incision marked over the medial third of the upper eyelid skin crease or just above it (b) blunt dissection through the orbicularis oculi muscle after the skin incision is made (c). SO is superior oblique, OF is orbital fat, G is globe and LP is medial levator palpebrae aponeurosis. This is the entry point. (d). Exposure of the optic nerve with Sewell retractors (e). Fenestration of the optic nerve sheath with removal of a rectangle of the sheath 2 × 3 mm (f). Closure with interrupted 6–0 catgut sutures. (Reprinted from Pelton and Patel [1]. With permission from Wolters Kluwer Health, Inc.)

Surgical Dissection

The orbital septum is opened medially to expose the medial horn of the levator aponeurosis inferiorly and medially and the medial fat pad centrally. Stevens scissors are used to make an opening in the medial fat pad by using an opening dissection technique only. The scissors are aimed inferiorly, posteriorly, and only slightly laterally. When teaching, we find surgeons aim too far posteriorly. It is best to aim for the back of the globe at this stage. Sewall retractors are used to separate the tissues medially and laterally. If the superior ophthalmic vein is encountered (which is uncommon), it is gently teased superiorly. The retractors which are inserted are placed so that the tips allow for separation of the deeper orbital tissues with little tension placed at the incision. The orbital septae will be encountered which are opened, again using a blunt “scissor-opening” technique. It is sometimes necessary to reposition the retractors and bluntly open the fascial planes several times. A surgical aid at this point is to aim as if one is dissecting toward the medial rectus muscle. This will expose the vortex vein, and a number of posterior wiggly ciliary vessels which are usually medial to the optic nerve but may be in the superomedial quadrant. If such vessels are encountered, long Q-tips or a beaded neurosurgical probe are used to gently push them out of the way. Sharp dissection is not used nor is any electrocautery. If one encounters any gentle oozing (again, this is rare), simple pressure with a moist Q-tip is applied.

Overweight patients have “fat orbits.” If one is fenestrating an optic nerve sheath in benign intracranial hypertension, one may encounter a surfeit of fat at this stage. A good technique is to insert mildly moist neurosurgical cottonoids and use the Sewell retractors over these cottonoids to hold the fat out of the way. This will allow about 6–10 mm of the optic nerve to be exposed quite effectively (Fig. 12.5a–c).

Fig. 12.5

(a–c) Three examples of the optic nerves being exposed (blue arrows) to varying distances. Note the epidural vessels in Fig. 12.5c: these can be gently moved with a moist Q-tip or a blunt Rhoton hook

Fenestration of the Optic Nerve Sheath (Fig. 12.6a, b)

Because the nerve sheath is bulbous just behind the globe, we used to, when we first designed this surgical approach, use neurosurgical forceps to pick up the sheath and then fenestrate it with bayonet curved microscissors. A safer approach is to use a neurosurgical pick which allows the sheath to be tented: a safe incision is made with the microscissors (when egress of the cerebrospinal fluid is seen) and extended, all the while keeping the sheath tented. About 3 mm × 2 mm of the dural sheath is resected to make a window. We have recently designed punches (“Patel Punches”), which allow the surgeon to safely remove a portion of the optic nerve sheath with minimal risk. Although we do bluntly “dissect” the tissue under the sheath with a blunt probe, we have not noted any adhesions in the subdural space.

Fig. 12.6

(a) The dural sheath is being lifted with a sharp, angled Rhoton pick which allows the sheath to be tented whilst a Yasargil curved microscissors is used to fenestrate the sheath. (b) The optic nerve is visible under the sheath fenestration

Effect of Unilateral ONSF

Although it has been observed that the unoperated eye may also show an improvement in the papilledema and vision with unilateral ONSF, this is variable [3]. Some patients show an excellent response, others a partial response, and a significant number will show no improvement at all. Trabeculations in the arachnoid space are thought to reduce the transmission of pressure along the nerve, which explains why only some patients will get an improvement in the eye opposite to the one that is fenestrated. We generally follow the request of the neuro-ophthalmologists who determine if they want unilateral or bilateral surgery, based upon the severity of the disease and urgency for intervention.

Removal of Intraconal Tumors

Medial, superomedial, and superior intraconal tumors have traditionally been approached via a medial conjunctival approach and without fracturing of the lateral orbital wall to allow displacement of the globe. Neurosurgeons often access these tumors that are between the globe and the medial optic nerve via a transcranial approach through the orbital roof.

Once the intraconal space is reached and the tumor identified, traction on tumors may be provided with a 4-0 silk or the use of a cryoprobe. Gentle “Ngorongoro Crater” circular dissection with a freer elevator which gently circles the tumor while separating any surrounding attachments is performed. Patience is needed here to continue this careful circular dissection as this leads to a gentle delivery of tumors like hemangiomas, hemangiopericytomas, and lymphangiomas with minimal traction being applied to the surrounding structures. If a cryoprobe is used, it is not used to apply strong traction but just a gentle hold on the tumor while this dissection is performed. This principle should apply to the removal of any orbital tumors as injury to surrounding structures is caused by blind dissection, aggressive manipulation, severe traction, and sharp dissection. Even large intraconal tumors can be safely delivered with excellent cosmetic and functional results (Figs. 12.7a–g and12.8).

Fig. 12.7

(a) 23-year-old female presents with an 18-month history of worsening double vision, pressure, and pain on movement of the right eye. Patients have right proptosis, hypoglobus, and exoglobus. Vision is 20/20 with no visual field defect (b). Evidence of a large slow-growing superomedial orbital mass with molding of the medial wall of the orbit (c). Superomedial skin incision approach with gentle dissection around the tumor (d). Delivery of a large tumor via this incision (e). Appearance of the closed incision at the end of the procedure (f). Tumor measures 26 mm in diameter. Histopathology shows it to be a cavernous hemangioma (g). Six months after surgery. No residual ptosis, double vision, or globe malposition. Vision is 20/20

Fig. 12.8

The superomedial approach allows the removal of tumors in the medial, superomedial, and superior intraconal space as marked. ON: optic nerve

Bleeding

Other than cauterizing the oozing at the incision site, we generally never use any bipolar cautery within the orbit with these procedures. If ciliary vessels are encountered, the retractors and the orbital cottonoids can be removed and replaced: this will generally move the vessels out of the way. Otherwise, gentle teasing of the vessels away from the optic nerve can be performed with a moist Q-tip or blunt Rhoton dissectors. Neither cryotherapy nor cautery should be used on or close to the optic nerve.

Closure

The cottonoids are removed, and a simple skin closure is performed with interrupted 6-0 catgut sutures. Local anesthesia is administered around the incision site at the end of the procedure. Most of our patients do not need prescription pain medication after these operations.

Postoperative Care

Immediately after surgery, the eye and vision are examined. Pain is usually mild. Prescription opiates are rarely required. We generally do not apply a patch on the operated eye. Erythromycin eye ointment is applied twice a day for a week.

Complications

Complications have been minor with this surgical approach. This report is based upon 168 optic nerve sheath fenestrations and 27 medial, superomedial, or superolateral tumor removals by the author. Vertical diplopia was noticed in the immediate postoperative period in a few patients, which did not last more than a few days except in seven patients who had vertical diplopia for up to 2 months, but none had permanent double vision. Mild medial ptosis for 7–14 days is common because of the surgical approach. To date, we have not had to perform ptosis surgery on any of the patients. It is possible that mild residual ptosis may remain in some patients, but, again, we have not had occasion to address this in any of the patients. A tonic pupil developed in six patients. There were no cases of hematoma, vision loss, permanent ptosis, or permanent strabismus.

Comparison to Other Surgical Approaches to ONSF (Boxes 12.1, 12.2, and 12.3)

Box 12.1 Complications seen with medial transconjunctival ONSF (reported in 4.8% to 45% of cases with a mean of 12.9%)

Pupillary dysfunction (16.3%), strabismus, lost medial rectus muscle, damage to third nerve branch to medial rectus, conjunctival bleb, globe perforation, chemosis, Tenon’s cyst, dellen formation, corneal ulcer, microhyphema, angle closure glaucoma, pupil dysfunction, chorioretinal scar from globe traction, branch arterial obstruction, central artery occlusion, choroidal ischemia, traumatic optic neuropathy, optic nerve cyst formation, orbital apex syndrome, orbital infection, orbital hemorrhage, lost muscle, conjunctival abscess, optic disc hemorrhage. Cilioretinal and long posterior ciliary retinal artery occlusions

Based on data from Ref. [24]

Box 12.2 Complications seen with superomedial ONSF

Medial ptosis (usually transient), vertical diplopia (always transient), pupillary dysfunction (less than 2%), visible upper eyelid scar (rare)

Based on data from Ref. [1]

Box 12.3 Complications seen with lateral ONSF

Pupillary dysfunction (up to 50%), vision loss, retrobulbar hemorrhage, double vision, ptosis

Based on data from Ref. [25]

The immediate retrobulbar optic nerve is bulbous because of the larger subarachnoidal space and the loose arachnoidal attachments in this region. This makes this portion of the optic nerve the safest to fenestrate. In cadaver studies, we showed that the angle of access to this portion of the optic nerve through the medial transconjunctival approach is acute (25 degrees) as compared to the superomedial approach which had an average of 38 degrees. Although the angle was larger via the lateral approach without a bone flap (54 degrees) and largest with a bone flap (72 degrees), the lateral approach encounters the ciliary ganglion and the associated ciliary nerves. Indeed, Henry Stallard, in his description of the Stallard lateral orbitotomy approach, noted that “...most of the important blood vessels and nerves lie on the lateral side of the optic nerve.”

In the medial transconjunctival approach, traction must be placed on the globe to move it laterally to allow the nerve to be fenestrated, especially now that the medial approach is generally used without out-fracturing the lateral orbital wall.

The average distance from the skin incision to the nerve in the superomedial approached was 25 mm. Although the distance was shorter with the medial transconjunctival approach 20 mm, this distance of 25 mm is only slightly longer and allows the surgeon to work safely in the confines of limited space. The increased horizontal space available to the surgeon with gentle traction in the superomedial approach makes this extra 5 mm of little significance.

Medial Transconjunctival Approach (Galbraith and Sullivan Procedure)

The main disadvantage of this approach is the acute angle of approach to the optic nerve and the need to disinsert the medial rectus muscle. Whereas the distance to the optic nerve is the shortest (20 mm), there is risk to the extraocular muscle, potential for pupil dysfunction and need for substantial traction to the globe. In our hands, these procedures took 45–60 minutes of operating time.

Lateral Orbital Approach with Bone Flap (Tse et al. Procedure) (Fig. 12.9)

This approach gives the most en face approach to the optic nerve, but the distance to the optic nerve is the longest at 36 mm. There needs to be significant traction applied to the lateral rectus muscle and extensive dissection of intraorbital tissues to reach the optic nerve. There is also a very high chance of pupil dysfunction. The width of the surgical field is good. Operating times are about 45–60 minutes.

Fig. 12.9

Access to the lateral portion of the optic nerve is hindered by the short posterior ciliary vessels, nerves, and ciliary ganglion, and the retinal artery also enters the optic nerve inferolaterally. This increases the risk of complications with this approach

Lateral Orbital Approach Without Bone Flap (Patel and Anderson Procedure)

This approach has the same advantages and disadvantages as the lateral orbital approach with a bone flap, except the distance to the optic nerve is a little shorter at 30 mm. The exposure of the optic nerve is more limited, and significant traction needs to be applied. Pupil abnormalities are common with this approach. The surgical time with this technique was 15–30 minutes. Even though we were able to reduce the surgical time by designing a lateral eyelid crease approach to the optic nerve without a canthotomy or bone removal, we realized that the lateral approach to the optic nerve for fenestration is just unwise because of the many important structures that crowd this surface of the optic nerve: central retinal artery, ciliary ganglion, short posterior ciliary nerves, and short posterior ciliary vessels. We abandoned this approach because of the high incidence of pupillary abnormalities and the potential for more severe complications. Pupillary abnormalities, whether full-blown Adie’s pupils or just sectorial iris involvement, should be taken seriously as most of these patients that we operate upon are young and such pupillary abnormalities cause visual problems.

Advantages of the Superomedial Approach to the Optic Nerve and Intraconal Tumors

In all the other procedures, there is significant manipulation of one or more extraocular muscles with resultant potential problems. The superomedial approach, on the other hand, allows access to the intraconal space anterior and below the superior oblique tendon, medial to the superior rectus, and superior and medial to the medial rectus muscle. Mild retraction needs to be applied to the region of the medial horn of the levator aponeurosis only. With the understanding that the retraction is provided deeper within the wound with angling of the Sewell retractors, little traction needs to be applied to the aponeurosis. Inevitable postoperative swelling does give temporary double vision in a small number of patients, but this resolves within a few days.

The posterior ciliary nerves and vessels enter the globe at the 3 o’clock and the 9 o’clock positions. Therefore these structures are most vulnerable using the medial and lateral approaches. Adie’s pupil is a significant risk with the lateral approach. The superomedial approach, by contrast, allows the surgeon to approach the optic nerve at the 11 to 2 o’clock positions where there are the least number of vessels and nerves.

Other advantages of the superomedial approach include very little bleeding, a wider surgical view, little pressure on the globe, an excellent cosmetic outcome, and minimal surgical time. The eyelid anatomy is very familiar to oculoplastic and orbital surgeons. Lastly, no cutting of normal tissues is needed as the approach is more of a “star wars” gliding through anatomical planes. It is a truism about orbital surgery that the risk of complications rises with increased surgical dissection and with longer surgeries. Certainly, speed alone should never be a reason to choose a particular surgical approach. However, with experience comes efficiency, and our average skin-to-skin operating time over the last 50 cases for ONSF is just under 10 minutes (with the fastest being 3 minutes 40 seconds, a little quicker than the world mile record).

Surgical Tips for Successful ONSF with the Superomedial Approach

Whenever a new surgical procedure is introduced into the literature, the usefulness of the procedure is judged by how successfully the technique is performed by surgeons other than the inventors. To that end, after presenting the procedure at major meetings and after publishing it, I received multiple calls from experienced orbital surgeons. Based upon these calls and also upon the experience of teaching the technique to fellows and residents, the following surgical tips may be useful.

The simple advice of ensuring that the surgeon had absolute hemostasis before entering the orbital space cannot be stressed enough. If there is a continuous ooze from the vessels at the edge of the incision and below the orbicularis muscle, this will spread into the surgical site, which, being effectively at the end of a tunnel, will make it difficult to know where the blood is coming from.

When performing orbital dissection once the orbital septum has been opened, only blunt dissection with the opening of Stevens scissors should be employed. There should be little, if any, cutting of tissues. The sequence is blunt separation of the fat lobules and Koornneef’s septae followed by repositioning of the Sewell retractors. This has to be done several times.

Several surgeons called saying they had a hard time finding the optic nerve. The commonest mistake was pointing the dissection too posteriorly. It is surprising how anteriorly and inferiorly the dissection needs to be to reach the optic nerve within a few seconds. A good technique that our fellows have found useful is to pretend that you are aiming for the medial rectus muscle.

Initially, we used to lift the sheath with neurosurgical microforceps to allow us to fenestrate the sheath. Of course, patients do not always have a large amount of CSF around the nerve which carries the risk of causing injury to the optic nerve. A much better approach is to tent up the sheath with an angled Rhoton sharp-tipped pick and cut the sheath with curved Yasargil bayonet microscissors. Once the opening is made and CSF has been released, a blunt hook may be used to tent up the sheath, thereby allowing one to create a fenestration. It is easy to create a fenestration measuring 2–4 mm square. In order to create fenestrations easily, we have designed modified surgical punches (Patel Punch, 1 mm and 2 mm in diameter) which have been used successfully on a few patients.

As has been discussed, “fat orbits” will allow floppy orbital fat to tumble into your surgical site. Mildly moist cottonoids are very useful. Dry cottonoids are sticky to the orbital fat. We usually place two, one superolaterally and the other inferomedially, and use the Sewell retractors to hold these cottonoids and fat back.

A good assistant is essential. We designed some self-retaining retractors for this operation but found that there needs to be dynamic retraction, and the use of Sewell retractors and a good assistant are better than any self-retaining retractors. I have learned to resist the temptation to be too helpful.

Conclusion

A review of the current preference of orbital surgeons for optic nerve sheath fenestration has shown a rapid increase in the uptake of the superomedial approach, although the medial approach is still the procedure most familiar to surgeons and used most often [26]. We believe that with careful study of the orbital anatomy will allow most surgeons to successfully undertake this approach for fenestration as well as for the removal of orbital tumors.