Normal-tension Glaucoma
Chat Highlights
June 21, 2006

On Wednesday, June 21, 2006, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Normal-tension Glaucoma."

Moderator:    Welcome back to chat, Dr. Wilson.  Tonight the topic concerns normal-tension glaucoma (NTG).  That sounds like an oxymoron.  What does NTG mean?

Dr. Rick Wilson:    It means the typical changes that glaucoma causes in the optic nerve and visual field, but the changes occur in the 22 mmHg IOP (intraocular pressure) range or under.

P:    Are there any known causes for NTG?

Dr. Rick Wilson:   NTG may well be a wastebasket term for different optic nerve diseases that cause the typical kind of glaucoma damage.  Decreased circulation to the eye, usually systemically, often has been linked to NTG.  Low blood pressure throughout the body makes it harder for the heart to pump blood into the eye against the IOP and can cause NTG.  Low blood pressure during the sleeping hours (nocturnal hypotension) has also been linked to NTG.

P:   When is blood pressure the lowest, and does that affect IOP?

Dr. Rick Wilson:   There is a diurnal curve of blood pressure similar to the diurnal curve of IOP. As with eye pressure, the blood pressure is at its lowest during the early morning hours.  In patients with hypertension (high blood pressure), the lowest pressures are between 2:00 and 4:00 a.m.  Two-thirds of the normal population will have a blood pressure drop of greater than 10% during that period.  These people are termed "dippers."

Patients with systemic hypertension usually show evidence of a much greater swing in systolic and diastolic blood pressure, with an average drop in pressure of 26% from day to night.  Hypertensives treated with beta blockers can have diastolic blood pressures during sleep of 50 mmHg or less and rarely down to 30 mmHg or less.  An abnormally deep dip in pressure may compromise local vascular supply.  Dr. Stephen Drance found a much higher incidence of POAG (primary open-angle glaucoma) progression among dippers than among non-dippers.

P:   Why is NTG sometimes referred to as a vasospastic disease?

Dr. Rick Wilson:   The incidence of migraines is much higher in NTG patients than in the general population, which suggests a vasospastic component to NTG.  We theorize that many people may have a decreased ability to increase or decrease blood flow in response to local metabolic requirements and changes in systemic blood pressure, circulating hormones, etc.  

An MRI (magnetic resonance imaging) shows a greater prevalence of diffuse, cerebral, small-vessel, ischemic changes in NTG patients compared to controls.  That suggests vaso-occlusive disease, usually related to cholesterol, or other narrowing of the arteries blocking blood flow.

P:   Could vaso-occlusive problems cause graying out of vision?

Dr. Rick Wilson:   Yes, that's a frequent sign of transient ischemic attacks (TIAs), but it can also be seen with migraines.

P:   Hasn't a connection been found between NTG and autoimmune disease?

Dr. Rick Wilson:   Yes, autoimmune disease has been linked to NTG.  Serum from patients with NTG has been found to contain high titers of antibodies against retinal proteins.

Moderator:   What is the typical type of damage seen in NTG patients?

Dr. Rick Wilson:   The typical type of glaucoma damage seen with NTG is very similar to that seen in POAG.  The rim of the optic nerve is slightly thinner, and there is more atrophy (loss of tissue) around the nerve than is seen with POAG.  On average, the corneas of NTG patients are thinner; the central cornea thickness is about 510 - 520 microns.

P:   Is NTG hard to detect in typical glaucoma screenings?

Dr. Rick Wilson:   Yes, because the only sign would be an abnormal appearing optic nerve, not an elevated IOP.

P:   Are there any early symptoms an NTG patient can notice?

Dr. Rick Wilson:   No, there are no more early symptoms than with POAG (primary open-angle glaucoma).  Only when NTG becomes advanced do the patients notice a blind spot (scotoma) close to the center of their vision.

P:   Does it help to know the cause of NTG?

Dr. Rick Wilson:   If the cause is treatable, it would help to know the cause.  I have been finding quite a few patients with low systemic blood pressure, but it is not always possible to raise the systemic blood pressure with present treatments.

P:   Couldn't doctors drive themselves crazy trying to find the causes of patients' NTG?

Dr. Rick Wilson:   Yes, there is a great deal that needs to be discovered before we can understand this disease.

P:   Is there any relationship between sleep apnea and NTG?

Dr. Rick Wilson:   An article stated a definite relationship, but since I have heard contradictory information I do not regard it as a definite cause at this time.  Sleep apnea does seem to elevate the IOP, so it would probably be a regular POAG, but not detected because the patient is asleep when the IOP is elevated.

P:   What is the course of treatment for NTG?

Dr. Rick Wilson:   It is the same as with POAG, except the target IOP is lower.  Usually, the doctor wants to lower IOP by 35 to 40% from a level at which the patient is sustaining damage and watch carefully.

P:   When should a NTG patient see a neurologist or a neuro-ophthalmologist?

Dr. Rick Wilson:   Usually when the visual field, the optic nerve, or the patient's symptoms are not typical of NTG.

P:   Is there any research currently being done to find a cure for NTG?

Dr. Rick Wilson:   Yes, all kinds of research is being done to identify people genetically who are subject to NTG and those who are most at risk for progressive disease.  Stem cell research is moving forward, though probably more quickly overseas where there are not the restrictions we have here.  At Johns Hopkins, they are implanting electronic chips in the early phases of developing artificial retinas.

P:   Is it true that a vaccine is being used in Israel to treat glaucoma?

Dr. Rick Wilson:   Yes, they have a vaccine in Israel that seems to offer some form of neuroprotection.

Moderator:   Do you have anything you would like to add to the discussion before you leave?

Dr. Rick Wilson:   It is apparent that many kinds of insult can injure the optic nerve, and the nerve responds in the same way.  Until we understand how all these different entities cause their damage, we have a hard time sorting out the underlying causes of glaucoma.

Moderator:   Thank you, Dr. Wilson.

On June 28, Dr. Wilson discussed "Glaucoma and Dry Eyes" in the Chat room. Click here for highlights of that meeting.

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