Optic Nerve Sheath Fenestration
- Author: Adam J Cohen, MD; Chief Editor: Hampton Roy, Sr, MD more...
Optic nerve sheath fenestration (ONSF) procedures were first described by De Wecker in 1872 as an incision in the meninges surrounding the optic nerve in order to relieve elevated intracranial pressure.[1, 2] The procedure is typically performed in instances of papilledema due to idiopathic intracranial hypertension (IIH), in which the main symptom is rapid and/or progressive vision loss rather than headache. The effect is normally limited to the ipsilateral optic nerve, although occasionally the procedure appears to have a filtration effect, which is propagated along the entire system, resulting in improvements in headaches and contralateral disc edema as well. Typically however, the effect is limited to the ipsilateral optic nerve.
The image below depicts optic nerve sheath fenestration.
Although the procedure is most commonly performed for vision loss due to idiopathic intracranial hypertension, some cases exist of successful stabilization or reversal of vision loss due to optic nerve sheath hemorrhage, cryptococcal meningitis with papilledema, and intracranial breast cancer metastases with papilledema.[4, 5, 6] This procedure has not proven to be helpful in treating vision loss due to anterior ischemic optic neuropathy.[7, 8]
A small study in the pediatric population showed efficacy of optic nerve sheath fenestration similar to that in adults.
IIH is also referred to as pseudotumor cerebri. It is a disorder of unknown etiology that must meet the following criteria:
Signs and symptoms of increased intracranial pressure (headache, nausea, transient visual obscurations lasting seconds, double vision, dizziness, emesis)
Elevated cerebrospinal fluid (CSF) pressure (>200 mmHg in nonobese adults and >250 mmHg in obese adults)
Normal neuroimaging studies
Normal neurologic examination (with the exception of papilledema and/or cranial nerve palsies)
No other identifiable cause such as medications (including vitamin A, tetracycline, oral contraceptive pills, nalidixic acid, lithium, steroid use, or withdrawal)
IIH usually occurs in obese females 20-45 years of age.
Patients must undergo a complete ophthalmic history, neurologic examination, and an ophthalmic examination including visual acuity, color vision, pupillary assessment, motility, visual field testing, and funduscopic examination to evaluate for presence and extent of papilledema (swelling of optic nerve). Emergent neuroimaging (CT scan or MRI) should be performed to rule out any intracranial processes such as dural sinus thrombosis, which can result in papilledema. Lumbar puncture must be done to check the CSF composition and opening pressure. Blood pressure should also be recorded.
Medical treatment of idiopathic intracranial hypertension should be instituted, including weight loss, acetazolamide or furosemide, and systemic steroids if needed. If vision loss continues despite these measures, ONSF should be considered. Preoperative discussion should include reviewing the risks of orbital surgery in such a proximity to the optic nerve and the need for continued treatment of the underlying disease process, as this procedure is not a cure for the disease. If bilateral vision loss is present, the eye with more severe vision loss should undergo the procedure first.
Indications include idiopathic intracranial hypertension with rapid or progressive vision loss.
General anesthesia is recommended for this procedure. If necessary, due to patient health, and in experienced hands, it may be performed under local anesthesia.
Equipment for optic nerve sheath fenestration includes the following:
Wire lid speculum
Ophthalmic fine-toothed forceps
Jameson muscle hook (x2)
Double armed 5-0 Vicryl suture with spatulated needle
4-0 silk suture
Schepens orbital retractor
Cotton tip applicators
Long neuro-bayonet toothed forceps and scissors
Long-handled 15º blade or Supersharp blade
Trans-sphenoidal or Bellucci scissors
The patient should be positioned in a supine position.
Patients should avoid anticoagulants perioperatively.
Approach and technique for an optic nerve sheath fenestration varies. This article describes a transconjunctival medial orbitotomy approach. Through this approach, after decompression is achieved, an optic nerve sheath biopsy may also be performed.
After general anesthesia is performed, the eye and eyelid are prepped and draped in a standard ophthalmic surgical fashion. An eyelid speculum is first placed. Peritomy at the nasal limbus is performed with relaxing incisions and dissection down to bare sclera. The medial rectus muscle is isolated with a muscle hook. Next a double-armed 5-0 Vicryl suture is passed through the insertion of the medial rectus with locking bites at the inferior and superior aspects of the muscle. The muscle is then disinserted with Westcott scissors. 4-0 silk suture is passed in a running stitch fashion through the muscle stump to be used as a traction suture.
Hemostasis, if needed, may be maintained with either bipolar or needle-point cautery. Using the 4-0 silk traction suture, the globe is rotated laterally, occasionally releasing it to avoid ischemia to the globe. A careful retraction of the orbital soft tissue is undertaken with a malleable retractor, until exposure of the optic nerve sheath is achieved. A Schepens orbital retractor and cotton-tipped applicators can be used to retract the prolapsing orbital fat surrounding the nerve.
Once adequate exposure is achieved, a marking pen may be used to mark the planned incision point on the sheath. This point should be in the distal aspect of the nerve sheath, in order to avoid damage to the nerve itself, but at least 2-3 mm from the globe in order to avoid the vascular supply, a plexus of short ciliary vessels. A long 15 º blade, or a Supersharp blade, similar to the one used for paracentesis formation in cataract surgery, is used to make an incision longitudinally along the nerve sheath, away from the nerve in an upward manner in order to avoid injury to the nerve itself (see the images below).
An egress of fluid may be observed, which indicates release of the cerebrospinal fluid under pressure. The edge of the incision should be everted with fine-toothed neurosurgical forceps and Bellucci scissors introduced to extend the incision to a length of 5-7 mm. Again, care should be taken to stay superficial, with the inferior blade tenting the dura and arachnoid layer to avoid contact with deeper tissue. A second incision, parallel to the first may be created in the same fashion. A superior oblique hook may be used to elevate the dura and arachnoid through the initial incision to assist in subsequent incisions. The introduction of the hook will also release any adhesions.
Another option may be to excise this entire window of tissue between the incisions. Occasionally, this may be indicated for biopsy purposes if the etiology is in question. Hemostasis in the orbit is performed with cottonoids and cotton-tipped applicators, as well as light bipolar cautery if needed. The traction suture is then removed. The medial rectus muscle is reattached to its insertion point using the 5-0 Vicryl suture. The conjunctiva is then reapproximated with an absorbable suture.
Patients should avoid anticoagulants in the immediate postoperative period. Systemic antibiotics should be given perioperatively and overnight. Intravenous steroids may also be given perioperatively and overnight. Visual acuity and pupil check should be performed a few hours after surgery, and the patient may be admitted overnight for observation. The patient is usually given an antibiotic-steroid drop for 1 week. A tapered oral steroid regimen may be given on a case-by-case basis. Typically, recovery is relatively pain-free, without the need for narcotic pain medicine. At 1-2 weeks postoperatively, the patient should be examined, and the optic nerve function should be assessed. If visual recovery occurs, it is often rapid with optic nerve function and appearance improving over the first several weeks postoperatively.
Potential complications are as follows:
Diplopia, which is typically temporary
Sudden loss of vision (either from a vascular occlusive event, direct injury to nerve, or a hemorrhage)
Orbital or intrasheath hemorrhage resulting in vision loss
Pupillary abnormality from damage to parasympathetic nerve fibers
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