The Optic Nerve
Chat Highlights
June 11, 2003

Norma Devine, Editor

On Wednesday, June 11, 2003, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "The Optic Nerve."

Moderator:  Tonight's topic is the optic nerve.  Dr. Wilson, perhaps you can start by defining the optic nerve.  What and where is it?

Dr. Rick Wilson:  The optic nerve carries the nerve impulses of sight from the retina back to the brain.  It enters the back of the eye just nasal to the anterior-posterior axis of the eye.

P:  Please define what "cupping" means. 

Dr. Rick Wilson:  When the ganglion cells are killed in the retina by the elevated IOP (intraocular pressure), there is a proportionate loss of nerve fibers.  The nerve starts out with about 1.2 million fibers.  When enough fibers have been lost, the decrease of fibers results in a depression in the surface of the optic nerve. 

P:  Is the optic nerve the largest nerve in the body?

Dr. Rick Wilson:  Heavens, no!  The spinal column holds the largest nerve, with the two branches going into each leg being much larger.

P:  How many optic nerve fibers have to be lost to create significant cupping?

Dr. Rick Wilson:  We think 50% or more of the total fibers have to be lost to get recognizable cupping.  There is so much redundancy in the supply to the retina that much of the nerve can be lost before the effect is that apparent.

P:  Why does the loss of optic nerve fibers result in a depression in the surface of the optic nerve?  Do the fibers add a structural element?

Dr. Rick Wilson:  The nerve fibers are the optic nerve.  When they are lost, they atrophy or disappear, leaving only the hole in the white scleral coat in the back of the eye, the scleral canal, and the mesh-like layer at the outside of the canal.  When there are many nerve fibers, the cup is small.  When many fibers are lost, their loss shows up as a bigger depression in the middle of the nerve, since the remaining fibers move to the walls of the canal.

P:  What percentage of the optic nerve is required to retain useful sight?

Dr. Rick Wilson:  It's mostly hypothetical, but I would think that 50,000 fibers would leave a central island of relatively decent vision, with some vision off to the temporal side.

P:  What is the best way to evaluate the optic nerve?

Dr. Rick Wilson:  With a magnified, stereo view and a good, controllable light source.  That can be obtained with the slit lamp microscope and a Hruby or Volk lens.

P:  Why is it that one optic nerve may be affected and not the other, even though pressures may be almost the same?

Dr. Rick Wilson:  We aren't sure.  There could be a difference in the blood circulation to the nerves, or a difference in the support that the nerve gets as it goes through the meshwork at the back of the eye.

P:  How does glaucoma destroy the optic nerve?  Does increased pressure press the vitreous against the optic nerve, thus cutting off blood supply?

Dr. Rick Wilson:  Again, we are not sure.  One view is that as sheets of lamina cribrosa are bowed backward under increased IOP, they crimp nerve axons, interrupting axoplasmic flow, leading to cell death (horseradish peroxidase studies in Theories of Optic Nerve Damage in Glaucoma: Mechanical).

According to Quigley, the superior and inferior poles have the least support from the lamina cribrosa and therefore suffer damage first.  Vascular damage may be due to interference with blood flow, either from IOP-induced changes to the prelaminar and laminar flow, which may cause a disorder in autoregulation, or to mechanical closure of the capillaries in the intralaminar nerve, which lie in connective tissue sheets.  Distortion of these sheets from bowing of the lamina may decrease blood flow to the axons, halting axonal transport, which requires energy. 

The present thinking is that a variety of vascular and mechanical factors combine to damage the optic nerve at the lamina.   Alternatively, damage may come from direct mechanical injury to the retinal ganglion cells, with secondary axon degeneration.  In other words, the causes of damage are complex, with the injured retinal ganglion cells sending out chemicals that then damage cells around them, even if the IOP has been lowered.

P:  I've read that the cup-to-disc ratio can actually improve after successful reduction of IOP.  Does that mean that before the ganglion cells actually die, they can merely be dysfunctional; that is, cell dysfunction is a precursor to cell death?

Dr. Rick Wilson:  Very perceptive.  Yes, that is correct.

P:  The medical approach to treating open-angle glaucoma and controlling damage to the optic nerve is to lower intraocular pressure.  Has any research been done regarding the chemical character of the vitreous itself, and the possibility of defective fluid affecting the health of the optic nerve?

Dr. Rick Wilson:  Studies of the composition of the vitreous have been done and resulted in some controversy.  A lot of work is being done in the area of growth factors, nitric oxide, glutamate, and other mediators and how they affect the retinal ganglion cells.  If we can figure that out, perhaps we can protect the nerve against elevated IOP.

P:  Can the optic nerves cup slightly even though there is no disease? 

Dr. Rick Wilson:  A slight cupping occurs with aging from the loss of optic nerve fibers, just as we lose brain cells.

P:  Could a lot of the cupping effect merely be stretching, and not be damaging? 

Dr. Rick Wilson:  Yes.  In babies with their elastic sclera, the increase in the size of the scleral canal from stretching due to increased IOP is marked.  That can result in much decreased cupping when the IOP is reduced to normal.

P:  How does asymmetry of the optic nerves help in diagnosing glaucoma?

Dr. Rick Wilson:  Almost everyone has 0.1 or less difference in cupping.  Therefore, if there is more difference than that, an unnatural cause is suspected.  

P:  What is your opinion of the Heidelberg Retinal Tomography (HRT) test as a means of assessing the optic nerve?

Dr. Rick Wilson:  It is moderately good at detecting progression in glaucoma damage, although not as good as a glaucoma specialist. It is almost worthless at diagnosing glaucoma, unless the glaucoma is advanced.  

P:  What treatment can be used for an optic nerve that has been swollen and inflamed for several months?

Dr. Rick Wilson:  In some neurologic diseases, systemic steroids are used to reduce the swelling and inflammation in the optic nerves.

P:  If the swelling and inflammation of the optic nerve are a complication from glaucoma surgery, what then?

Dr. Rick Wilson:  That would be a very unusual situation, unless the needle from the local anesthetic hit the nerve.  Nerve swelling can come from too low an IOP. 

P:  What causes the nerve to swell?  

Dr. Rick Wilson:  It may be that the meshwork at the back of the scleral canal is bowed into the eye, causing the meshwork to bow forward.  That's just the opposite of when the IOP is high in the eye and bows the meshwork back.  But the effect is the same.  When the meshwork is bowed in or out, it crimps the nerve fibers going through it.

P:  Is there a difference in the appearance or structure of the optic nerve in different kinds of glaucoma?  If so, why?

Dr. Rick Wilson:  It is thought that normal-tension glaucoma may have more notches in the cup of the nerve, rather than expanding concentrically, and is accompanied by tiny hemorrhages at the edge of the optic nerve.  It is also thought that normal-tension glaucoma may be more related to circulation problems than to elevated IOP.

P:  What's the largest cup size you've ever seen?

Dr. Rick Wilson:  1.0 or total cupping.  That is, no nerve fibers, and a blind eye.

P:  Is the shape of the cup in any way related to the degree of damage to the optic nerve?  I'm thinking of an elongated cup.

Dr. Rick Wilson:  It is quite usual in glaucoma to have a vertically oval cup.

Dr. Rick Wilson:  I must leave now, as I have to be at the New Jersey office by 7:15 a.m.  See you in a couple of weeks.  Please welcome Dr. Doug Rhee next week and have lots of questions for him.

End of highlights for June 11, 2003.

On June 18, Dr. Rhee discussed "Unconventional Treatments" in the Chat room. Click here for highlights of that meeting.

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