Chat Highlights
Genetics and Glaucoma
June 27, 2001

Norma Devine, Editor

On Wednesday, June 27, 2001, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Genetics and Glaucoma." 

Moderator:  Tonight we will be discussing "Genetics and Glaucoma" with Dr. Rick Wilson.

P:   Have clear hereditary patterns been identified for pseudoexfoliation glaucoma?  

Dr. Rick Wilson:   Not yet,  to my knowledge.  This is obviously a field that is advancing rapidly.

P:  Is the TIGR/MYOC gene believed to be the main one responsible for primary open-angle glaucoma (POAG) or are there others?

Dr. Rick Wilson:   The TIGR gene is one of many genes at this time that have been identified. Most have been associated with juvenile glaucoma, because the genetics are more clear-cut.

P:  The TIGR/MYOC affects drainage.  What can you tell us about genetics and aqueous production?

Dr. Rick Wilson:  Not much.  I should have suggested that we put off this discussion until Doug Rhee joins us from the National Eye Institute.  He has the latest information on genetics.  Unfortunately, we have no medicines that are good at increasing aqueous production and know little about the genetics of production.

Moderator:  How are the genetics of juvenile glaucoma more clear-cut, and what are the implications of that for detection and treatment?

Dr. Rick Wilson:   Some of the congenital glaucomas, like Axenfeld's, are autosomal dominant.  It is much easier to track that gene than when four or five genes are working in concert to produce the glaucoma.  Primary open-angle glaucoma seems to be a type of disease that takes more than one gene to cause it. 

Moderator:  What does autosomal dominant mean?

Dr. Rick Wilson:  Autosomal dominant means that the dominant gene is not on the X or Y (male or female) chromosome, and overpowers the like-positioned gene on the other part of the pair, so that whatever it codes for (e.g., brown hair) is always carried out.

Moderator:  What is a gene?

Dr. Rick Wilson:  We inherit 23 pairs of chromosomes.  One of each pair is contributed by the mother, and one of each of  pair is contributed by the father at conception.  Each chromosome is composed of  many, many genes.  The genes are a code written in four amino acids that tell our cells how to act and what to produce.

P:  What pathology is usually associated with eyes that have inherited glaucoma?  That is, what is not functioning, or over-functioning, to cause glaucoma?

Dr. Rick Wilson:  Great question.  I wish somebody knew.  All of us have debris settling into drain of the eye.  Patients with primary open-angle glaucoma seem to collect more of this debris, or the cleaning apparatus of the drain does not work in these affected people as it does in normal patients.

P:  Is that why some people with high IOP (intraocular pressure) don't develop glaucoma?

Dr. Rick Wilson:  Yes, some people are much more resistant to the damaging effects of elevated pressure than others.  That does seem to be genetic.  However, we are now learning that perhaps these patients have thicker corneas than normal, so we falsely test them as having higher IOP than they really do.  Therefore, they are not more resistant.  They just have normal pressure that we perceive as high.  Interestingly, African-Americans seem to have thinner corneas than average and test as having lower IOPs than they really have.

P:  Does corneal thickness account for more than a 2 or 3 mm Hg of error on a Goldmann tonometer?

Dr. Rick Wilson:  In my experience, if  the cornea is perhaps 20% thicker than normal, it may cause an error of 5 to 8 mm Hg. 

P:  How many corneas are THAT thick? Is that rare?

Dr. Rick Wilson:  I don't think we have large enough studies to be sure.  It is rare, but may be more common than we presently realize.

P:  In one study I saw, the IOP was measured from INSIDE the eye, and corneal thickness didn't seem to make that much difference.

Dr. Rick Wilson:  The corneas in that study must have been in the normal range.  

Moderator:  If Africa-Americans have thinner corneas and a genetic predisposition to higher rates of glaucoma, and people with thick corneas seem to have more resistance, is that a genetic link:  thin corneas and more glaucoma, thick corneas and less glaucoma?

Dr. Rick Wilson:  I don't know if patients with thicker corneas have more resistance.  What I was suggesting was that the people we thought had resistance to elevated IOP may just have had thicker corneas and really had normal IOP. 

P:  How is the accuracy of  IOP for an eye with a thick cornea ascertained?

Dr. Rick Wilson:  There is a nomogram that can help to adjust for the difference in corneal thickness.

P:  Does the nomogram compensate for a thicker cornea automatically, or does the doctor have to manually adjust it?

Dr. Rick Wilson:  At present, the doctor  has to look it up and mentally subtract or add the number to the reading.  Two new tonometers are close to the market.  One takes the IOP through the eyelid, and corneal thickness is not an issue, and the other one automatically adjusts.  

P:  How is a cornea with greater than three diopters tested? 

Dr. Rick Wilson:  The best way is to take the IOP with the axis of the tonometer (the prism that the blue light lights up) half way between the two axes of astigmatism.  You can also take the IOP twice with the axis of the tonometer at 90 degrees and 180 degrees.  Remember,  astigmatism is having a cornea more the shape of the side of a football than a beach ball.  That is, the curve of the cornea is different on one axis, compared to 90 degrees away.  It is unusual to have more than three diopters of astigmatism, unless you have had a corneal transplant.

P:  Is there a correlation between the size and shape of the eye and the interrelationship of its various components with the development of primary open-angle glaucoma? 

Dr. Rick Wilson:  Yes.  Patients with small, far-sighted eyes are much more prone to narrow-angle glaucoma.  Highly nearsighted eyes may be more prone to primary open-angle glaucoma.

P:  So the size and shape of the eye would be a genetic factors?

Dr. Rick Wilson:   Usually.  

P:  In a prescription for spectacles, what number relates to the thickness of cornea?

Dr. Rick Wilson:  None of the measurements have anything to do with corneal thickness.   In my refraction, -7.00 +1.75 x 178, the +1.75 is the amount of astigmatism I have. The higher the minus number, the bigger the eye and the thinner the sclera or white coat around the eye.  Corneas with three or more diopters of astigmatism (the second "D." number in your glasses' prescription) have to be tested in a certain way for the reading to be accurate.

Moderator:  Thank you for being here, Dr. Rick.  Enjoy your trip to Scotland.

Dr. Rick Wilson:  Dr. Myers will be here on July 11th.  Ask lots of questions and have a great Fourth of July everyone.

End of highlights for June 27, 2001.

On July 2, Dr. Henderer met with the Monday night support group.  Click here for highlights of that meeting.

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