VASCULAR SUPPLY OF THE OPTIC NERVE

(Update)

Guest author

TOOBA HAYAT

Ajmal Khan Tibbiya College

Aligarh

India

The visual pathway receives its blood supply from 2 arterial systems: the carotid and the vertebral which are connected to each other at the base of the brain by the arterial circle of Willis.

On the basis of its blood supply, the optic nerve can be divided into the following parts:

A. OPTIC NERVE HEAD

B. INTRAORBITAL PART

C. INTRACANALICULAR PART

D. INTRACRANIAL PART

A. OPTIC NERVE HEAD

Based on the blood supply, the ONH can be divided into the following four regions (anterior to posterior):

1. Surface nerve fiber layer

2. Prelaminar region

3. Lamina cribrosa region

4. Retrolaminar region

The surface nerve fiber layer is supplied by retinal arterioles. In some cases the temporal area is supplied by the “posterior ciliary artery (PCA)” circulation arising from the deeper prelaminar region. The cilioretinal artery (or rarely a tiny cilio-papillary artery) is occasionally present and supplies the corresponding sector of the surface layer.

The prelaminar region is supplied by fine centripetal branches from the peripapillary choroid.

The lamina cribrosa region is supplied by centripetal branches from the short PCAs, either directly or from the circle of Zinn-Haller formed by the short PCAs. The blood vessels in the lamina cribrosa lie in the fibrous septa and form a dense capillary plexus.

The retrolaminar region has dual blood supply:

Peripheral centripetal vascular supply= This is formed by the pial vascular plexus. This receives supply from recurrent pial branches arising from the Circle of Zinn-Haller (or short PCAs and peripapillary choroid. Pial branches from the central retinal artery and other orbital arteries may also contribute.

Axial centrifugal Vascular supply= It is constituted by 1-8 branches from the central retinal artery during its course in the centre of the optic nerve.

Circle of Zinn-Haller

Haller in 1754 and Zinn in 1755 described this arterial circle, situated in the peripapillary sclera and formed by anastomoses between PCAs.

Circle of Zinn-Haller gives three sets of branches: to the lamina cribrosa; to the Peripapillary choroid and pial branches to the retrolaminar region.

Olver et al described the circle of Zinn-Haller as a microscopic, intrascleral, elliptical, microvascular anastomosis formed by branches of the medial and lateral paraoptic short PCAs. The authors preferred the term “perioptic nerve arteriolar anastomoses” for this structure.

According to them, the complete or incomplete ellipse is divided into superior and inferior parts by the entry points of these branches into the eye and shows morphological variations (interindividual as well as intraocular in the same person) in terms of its form, position and branches.

Of the 18 casts of the circle of Zinn-Haller that they examined, complete anastomosis was found in 8 (44%), complete with narrowed sections in 6 (33%) and incomplete anastomosis in 4 (23%). They found that branches from the circle of Zinn-Haller also formed arteriolar-arteriolar anastomoses.

B. Intraorbital part

This is further subdivided by the point of entry of the central retinal artery in the optic nerve into:

(a) Anterior segment

(b) Posterior segment

Anterior segment

This lies between the ONH and the site of entry of the central retinal artery into the nerve. This part has two vascular systems for its supply=

(i) A peripheral centripetal vascular system

This is present in all cases and is formed by a pial vascular plexus, supplied by multiple pial branches originating from the Peripapillary choroid, circle of Zinn-Haller, central retinal artery, ophthalmic artery and other orbital arteries.

(ii) An axial centrifugal vascular system

This is present in 75% of nerves and supplied by 1-8 intraneural branches of the central retinal artery. The supply by the central retinal artery may extend 1-4 mm behind the site of its penetration into the optic nerve and give centrifugal branches.

Posterior segment

This is primarily supplied by the peripheral centripetal vascular system formed by pial vascular plexus. It is supplied by small collateral arteries usually arising from the ophthalmic artery and less often from other orbital arteries.

In about 10% of optic nerves there may be an axial centrifugal system extending backwards for a variable distance, formed by intraneural branches of the central retinal artery.

C. Intracanalicular part

The major blood supply to the optic nerve within the optic canal is presumably from the periaxial system of vessels (peripheral centripetal vascular system). The pial plexus in this part is fed mainly by branches from the ophthalmic artery.

Wolff has also mentioned the ophthalmic artery as the main vascular supply to this part of the optic nerve.

Behr and also Steele and Blunt have reported small branches from the ophthalmic artery, arising just distal to the optic canal.

However, Magitot and also Francois described its supply from the internal carotid, anterior cerebral and anterior communicating arteries, in common with the supply to the intracranial part.

Isayama found that the intracanalicular part of the optic nerve was supplied by a centripetal vascular system, formed by the pial vessels arising from the first branch of ophthalmic artery.

D. Intracranial part

There is great diversity of opinion about the source of blood supply to this part of the optic nerve. According to some books this part of the optic nerve is exclusively supplied by the periaxial system of vessels.

The pial plexus here is contributed by 4 sources=

• Branches from the internal carotid artery, either directly or through the recurrent branch of anterior superior hypophyseal artery (supplies the inferior aspect of the nerve containing the lower retinal fibers).

• Branches from the anterior cerebral artery (supplies the superior aspect of the optic nerve containing upper retinal fibers).

• Small recurrent branches from the ophthalmic artery.

• Twigs from the anterior communicating artery.

According to Behr and also Francois, the intracranial part is supplied by pial branches from the internal carotid, anterior cerebral and anterior communicating arteries. The nutrient vessels supply the nerve centripetally and are end-arteries.

Magitot described a small artery, usually from the internal carotid artery, as the principal vascular supply to the intracranial portion of the nerve and this artery terminates in the chiasma. He also described a branch of ophthalmic artery, dividing into ascending and recurrent branches which supply the upper surface of the nerve.

Wolff described anterior cerebral and internal carotid arteries supplying the superior and inferior aspects of the nerve respectively, with additional help from the anterior communicating and ophthalmic arteries.

According to Dawson, the “prechiasmal artery”, a branch from the ophthalmic artery near its origin, is the main supply, and it runs backwards on the medial side of this part of the nerve to join its fellow of the opposite side, along the anterior border of the chiasma and supplies the two.

Dedicated to Dr Sohan Singh Hayreh. Without his pioneering efforts our knowledge regarding the ocular vasculature and its diseases would have been so much incomplete: The Glog

In a pleasant surprise and a singular honor we had a response from Dr Hayreh himself regarding our efforts. Our heartiest thanks to Dr Hayreh; he will remain an inspiration to many for his monumental work.