Endoscopic surgery of orbit

Introduction:

With the advent of use of endoscopes in nasal surgeries, its use for other surgical modalities are on the increase. The major advantage of using the endoscope is its excellent resolution, image quality, improved visualisation of key areas.

Kennedy etal first described endoscopic orbital decompression during the early 1990's. Since then endoscopes have been widely used in orbital decompression procedures for managing Proptosis due to Graves disease (Grave's orbitopathy). It is also being increasingly used for optic nerve decompresssion procedures.

Endoscopic orbital decompression:

In this procedure the whole medial wall of orbit along with the medial portion of the orbital floor is removed. The use of endoscope allows for unmatched visualization of various critical anatomical regions like skull base, and orbital apex. Major advantage is the avoidance of skin incisions.

Figure showing the areas of bone removed in orbital decompression

Procedure:

This procedure is performed under general anesthesia. The use of image guidance system helps in avoiding complications and injury to vital structures.

1. The nasal mucosa is decongested using cotton pledgets soaked in 4% xylocaine with 1 in 100000 units adrenaline.

2. Incision is sited just behind the maxillary line, and through the uncinate process. The whole of the uncinate process is medialized and removed.

Lamina papyracea being curretted out

Removal of uncinate process enables better visualization of the maxillary sinus ostium. The ostium is opened widely up to the floor of the orbit superiorly, the wall of the maxillary

sinus posteriorly, and the inferior turbinate inferiorly. Care is taken not to extend the maxillary sinus ostium anteriorly beyond the frontal process of maxilla to prevent

injury to the naso lacrimal duct. A 30 degree endoscope is used to visualize the infra orbital nerve through the maxillary sinus ostium as the nerve courses along the floor of the orbit.

Lamina papyracea after removal

Now endoscopic sphenoidotomy is performed. After this process the anterior and posterior ethmoidal arteries can be identified as it courses through the skull base. The middle turbinate is removed to enable optimal visualization of the medial orbital wall. An image guidance system may be used at this point for intranasal navigation.

After complete exposure of the medial orbital wall, a curette is used to penetrate the thin bone of lamina papyracea. The thin bone is carefully elevated exposing the underlying periorbita. Bone removal is proceeded superiorly towards the roof of ethmoid, inferiorly up to the orbital floor and anteriorly up to the maxillary line. Bone over the frontal recess area is left intact, this is done to prevent prolapsing orbital fat from obstructing frontal sinus drainage.

Removal of orbital floor could be the most demanding task. Only that portion of the floor of the orbit medial to the infraorbital nerve is removed. A spoon curet could be used for this purpose. A sickle knife may be used to open up the periorbita. The periorbital incision is started at the posterior limit of decompression and brought anteriorly in order to prevent orbital fat from obscuring the vision. Parallel incisions are made along the ethmoid roof and orbital floor. To reduce the risk of diplopia post operatively a sling of fascia overlying the medial rectus may be preserved. On completion of this procedure fat could be seen prolapsing into the nose and maxillary sinus. Nasal packing should be avoided to enable maximum decompression.

Complications:

1. Diplopia

2. Post operative bleeding

3. Epiphora due to injury to nasolacrimal duct

Endoscopic optic nerve decompression:

The most common indication for optic nerve decompression is traumatic optic neuropathy. Endoscopic decompression of optic nerve has many advantages, which include excellent visualization, preservation of olfaction, with rapid recovery time. It also has the added advantage of lack of external scars.

Anatomy of optic nerve:

The optic nerve per se can be divided into three segments: intraorbital, intracanalicular, and intracranial. The main goal of optic nerve decompression is to releive the compressive forces within the intracanalicular portion of the nerve. The optic nerve canal is formed by two struts of the lesser wing of sphenoid. This canal carries the optic nerve and ophthalmic artery. Within the optic canal the nerve is ensheathed by three meningeal layers. At the orbital apex is the fibrous annulus of Zinn. This thick fibrous layer is the least expandable portion of the fibrous tissue around the optic nerve. This area has been postulated to be the most susceptible site for pathologic compression of the optic nerve.

Procedure:

Patients are prepared in the same way as for any other orbital surgeries described. A standard sphenoethmoidectomy is performed.

The anterior face of the sphenoid is opened widely, and the bulge of the optic nerve canal is identified along the lateral wall of the sphenoid sinus. After complete sphenoethmoidectomy, a spoon curette is used to fracture the lamina papyracea approximately 1 cm anterior to the optic canal. The lamina is removed carefully in a posterior direction to expose the annulus of Zinn. When the optic canal is reached the thin lamina is replaced by thick bone of the lesser wing of sphenoid. This bone must be thinned with a diamond burr before removal. It is recommended that the optic sheath is also incised to enable better decompression.

Complications:

1. CSF leak

2. Meningitis

3. Visual decompensation