The Optic Nerve
The optic nerve is the second of the cranial nerves and is the nerve of vision.
It is about 5 cm long and is divided into three segments. About 3 cm of the nerve is in the orbit cushioned by the orbital fat around it. Another 1 cm is intracranial or with the brain cavity. 1 cm of the nerve is enclosed in a bony canal on the lateral wall of the sphenoid sinus.
Visual loss may occur from compression of the nerve from injury (hemotoma) or mucoceles of the sphenoid sinus or the posterior ethmoids. Blunt and penetrating trauma may cause bleeding within the protective dural sheath that the nerve is encased in. Compression of the nerve from the bleeding may result in visual loss.
Surgical decompression of the nerve is an option when the subject has not responded to mega doses of steroids which is the standard treatment for traumatic optic neuropathy. Other indications for optic nerve decompression include mucocele, tumours etc.
Before considering surgery it is essential to consult an ophthalmologist to document the visual loss. Visual field and colour vision is tested and documented. A fundoscopic examination is done to rule out optic nerve atrophy. In the presence of optic nerve atrophy the patient may not benefit from optic nerve decompression.
The surgery is usually carried out with minimally invasive techniques.
Using the endoscopes to visualise the sinuses the sphenoid sinus is opened and the nerve identified.
A diamond burr is then used to drill away the bone overlying the nerve down to the nerve sheath. Caution is exercised not damage the surrounding structures. The sheath of the nerve is sometimes incised to “decompress” the nerve. This is practised when the cause of the visual loss is trauma and bleeding within the sheath is suspected.
The technique is minimally invasive, the endoscopes provide excellent visualization, the morbidity is lesser and an external scar is avoided.
Outcome however can be unpredictable. Most studies report improvement in vision in 30 % of the cases but there is no guarantee.
The video clip below shows the surgical technique