Letter to the Editor: Letter about the Article Entitled “Optic Nerve Sheath Fenestration: A Five Year Audit. Neuro-Ophthalmology 2005;29:173–177”

Dear Editor

I read the article by Knapp and Sampath¹ in a recent issue of this journal (Neuro-Ophthalmology 2005;29:173–177) with interest. The optic nerve sheath fenestration (ONSF) technique used by those authors is different in some respects from that used in previous studies. Optic nerve sheath fenestration is a surgical procedure performed in a critical and narrow area and in a limited time. Therefore, all the details related to the surgical technique are important.

Two main surgical techniques have been described for decompression of the optic nerve. In the first technique, about 4 × 4 mm of the dura and arachnoid layers are excised and a meningeal window is created.² The other technique comprises multiple longitudinal incisions with arachonidal dissection in the dura.³ In the second technique, it is assumed that multiple trabecular compartments of cerebrospinal fluid (CSF) are decompressed and that the orbital fat will not occlude the dural slits. The optic nerve can be reached by either medial or lateral orbitotomy. Knapp and Sampath made a fenestration of 0.5–1 mm in the nerve sheath through a medial conjunctival approach. Another specific aspect of their technique is that the bilateral surgery was performed simultanously in those patients with significant visual loss in each eye. Knapp and Sampath's published study does not provide sufficient information about the details of their surgical technique.

For the success of the ONSF, both the dura and the underlying arachnoid should be opened. There are two signs showing that the first incision is performed to a proper depth: a gush of CSF and the collapse of the dural sheath. After this step, the main sign showing that the dura and arachnoid have been excised completely is the yellowish color of the optic nerve. In the window technique, removal of the nerve sheath should expose the yellowish optic nerve covered with the pia. In the slit technique, arachnoid trabeculations and adhesions are lysed with a tenotomy hook introduced in the subdural space. I think that if a fenestration of 0.5–1.0 mm is made, these technical details might be difficult to monitor.

It has been proposed that ONSF relieves papilledema either by creating a CSF drainage or by scarring at the excision site.⁴ A relatively small opening may produce limited drainage or scarring. It is interesting that even a small fenestration can yield very good results. How can this observation be explained?

One finding supporting the filtration hypothesis is the improvement of the papilledema in both eyes after unilateral ONSF. This pleasant finding has been observed in 36–52% of the patients.², ³ Therefore, it might be monitored whether or not the first operation is effective on the contralateral eye. In the study by Knapp and Sampath, 10 of 13 patients underwent bilateral surgery and three underwent unilateral surgery. The rate of bilateral improvement after the unilateral operation and the number of patients who underwent simultaneous bilateral operations were not reported. If an effective surgical technique is used, ONSF may not be necessary for both eyes.