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Optic nerve sheath fenestration treats idiopathic intracranial hypertension
Surgical treatment is indicated if lifestyle modification and medical treatment fail to prevent a progression of signs or symptoms.
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Idiopathic intracranial hypertension — also known as pseudotumor cerebri, benign intracranial hypertension or empty sella syndrome — is a rare disorder involving elevated intracranial pressure, which causes daily headaches, nausea and vomiting. Untreated, it can result in substantial vision loss with visual field defects, and hence the treatment focus is often primarily directed toward vision preservation. One of the most commonly affected population segments is obese women of childbearing age, who often present with chronic headache, visual disturbances and papilledema. Less frequently it can occur in healthy-weight children and in men.
The etiology of idiopathic intracranial hypertension is currently unknown. Associated risk factors may include pregnancy, corticosteroid therapy, Cushing’s disease and aldosteronism. Patients with idiopathic intracranial hypertension may also have bilateral transverse sinus stenosis. Being a diagnosis of exclusion, other causes of elevated intracranial pressure must be ruled out by history, imaging studies including magnetic resonance venogram and examination of cerebrospinal fluid.
Cerebrospinal fluid usually has a normal composition, and the increased intracranial pressure occurs in the absence of any underlying mass lesion or hydrocephalus. Untreated chronic papilledema can result in permanent vision loss and optic nerve atrophy. The treatment spectrum encompasses weight reduction, medical treatment and surgical treatments, including cerebrospinal fluid shunting, optic nerve sheath fenestration and endovascular stenting in patients with bilateral transverse sinus stenosis.
In this column, Jacob and Agarwal describe the surgical technique for optic nerve fenestration.
Thomas John, MD
OSN Surgical Maneuvers Editor
Soosan Jacob
Amar Agarwal
In 1872 de Wecker first described optic nerve sheath fenestration. It has been used as a treatment for benign intracranial hypertension and acts by decreasing the cerebrospinal fluid pressure in the subarachnoid space.
The first step in the treatment of benign intracranial hypertension remains lifestyle modification, including weight reduction and restriction of salt, as well as medical management with diuretics and steroids in collaboration with a neurologist. Any invoking or causative factor, if determined as the etiology for the condition, should also be addressed immediately.
Optic nerve sheath fenestration (ONSF) is indicated if despite of all these measures, there is either a progression in symptoms such as a decrease in visual acuity, diplopia, transient visual obscurations or headache or a progression in signs such as an increase in visual field defects, lateral rectus paresis or papilledema.
Surgical technique
The challenge in performing an ONSF is that the operating field is very small and difficult to access. Surgery is ideally performed under the operating microscope to gain good visualization and a magnified view. A medial or lateral approach may be utilized, but the medial approach is the commonly preferred one due to quicker access and the absence of skin incisions.
A 270° peritomy is performed at the superior, medial and inferior limbus (Figure 1). Relaxing cuts are made at both ends of the peritomy. The conjunctiva and Tenon’s capsule are dissected posteriorly, baring the sclera in order to gain good posterior visualization. The superior and inferior recti muscles are hooked, and traction sutures are passed. The medial rectus muscle is also hooked and imbricated 3 mm from its insertion in a double whip-lock manner with a double-armed 6-0 polyglactin suture. The ends of the suture are left uncut.
Figure 1. A 270° peritomy is performed at the superior, medial and inferior limbus.
Figure 2. The medial rectus muscle is hooked and imbricated 3 mm from its insertion with a double-armed 6-0 polyglactin suture. The ends of the suture are left uncut. The muscle is then cut, leaving behind an adequate stump.
Images: Agarwal A, Jacob S
The muscle is then cut, leaving behind an adequate stump (Figure 2). A baseball traction suture is then passed from one end of the cut medial rectus muscle stump and out through the other. This is done using a 6-0 polyglactin suture, which is passed multiple times in a posterior to anterior direction, taking partial-thickness bites in each pass through the sclera just anterior to the muscle insertion (Figure 3). The ends are left uncut. The three traction sutures on the superior, inferior and medial recti are used for pulling on the globe outwards and laterally in order to achieve an adequate operating field.
Figure 3. A baseball traction suture is taken on the cut stump of the medial rectus muscle.
Figure 4. The optic nerve is visualized by following the long ciliary neurovascular bundle posteriorly. An MVR blade is used to create a 2-mm longitudinal nick in an avascular portion of the dura.
A malleable retractor is passed in the space created between the globe and the disinserted medial rectus. The long ciliary neurovascular bundle is identified running in a posterior direction. This is used as a landmark and followed posteriorly to locate the optic nerve. The optic nerve is surrounded by intraorbital fat, and this needs to be gently separated to visualize it.
Pulling on the globe and opening the surgical space both exert pressure on the globe and compromise arterial inflow as well as venous outflow. Hence, after every 60 seconds of dissection, all instruments should be removed from the orbit and traction on the globe should be released in order to allow normal perfusion to be restored. Throughout the surgery, the pupil should be monitored for dilatation of the lateral half because this could imply a compromise of the central retinal artery circulation. The surgeon should also watch for shallowing of the anterior chamber along with acute angle-closure glaucoma, implying venous congestion.
Once the optic nerve is identified, the short ciliary vessels and nerves are seen surrounding it. An avascular portion of the optic nerve should be chosen for fenestration. The optic nerve is held tethered between two cotton-tipped applicators, and a microvitreoretinal blade is used to make a 2 mm long linear nick on the dura (Figures 4 to 6).
Figure 5. The fenestration is then extended until it is about 4 mm to 6 mm long.
Figure 6. Intraoperative photograph showing a fenestration (black arrow) being created on an avascular visible portion of the optic nerve dura with an MVR blade.
As soon as the subarachnoid space is breached, a gush of cerebrospinal fluid escapes. The nick is then gently enlarged until it is about 4 mm to 6 mm long, and a nerve hook is gently swept under the subarachnoid space to lyse any adhesions. Two more such nicks are made on the visible, avascular portion of dura and connected to each other using the nerve hook.
Posterior fenestrations should never be made, for fear of damage to the central retinal artery. The nerve hook should be passed very gently and atraumatically, taking care not to sweep posteriorly and taking care to avoid the entry of the central retinal artery on the posteromedial aspect of the optic nerve (Figure 7). The gush of cerebrospinal fluid is visible only with the first entry into the subarachnoid space and may not be seen with subsequent fenestrations.
Figure 7. Intraoperative photograph showing a nerve hook being swept gently under the fenestration to lyse subarachnoid adhesions. Posterior dissection should never be done, for fear of damaging the central retinal artery.
Figure 8. Fundus photographs at 6-month postop visit show regression of papilledema in both eyes following ONSF done in the right eye.
Finally, the medial rectus muscle is reattached to its cut stump. The traction sutures are removed from the superior and inferior recti, and the peritomy is closed. The optic disc and retina are evaluated to check perfusion to the retina.
Complications
Complications that can be anticipated include transient or permanent loss of vision, central retinal artery occlusion, lateral rectus palsy, pupillary dilatation, orbital hemorrhage and angle-closure glaucoma. Careful and meticulous surgery under the operating microscope can usually prevent these complications.
ONSF can give gratifying results not just for the ipsilateral eye but also sometimes for halting progression in the contralateral eye (Figure 8).