Normal-Tension Glaucoma (NTG)
Chat Highlights
October 3, 2007

On Wednesday, October 3, 2007, Dr. Michael James Pro, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Normal-Tension Glaucoma (NTG)".

Moderator:  Welcome back to chat, Dr. Pro.  Tonight our topic is Normal-Tension Glaucoma (NTG). Wouldn’t it be less confusing to patients if NTG was always called Low-Tension Glaucoma (LTG)?

Dr. Pro:   Good question. Sometimes we call it Low-Tension Glaucoma.  That, however, seems to imply the damage occurs specifically because the pressure is low, whereas NTG implies that a patient is developing glaucoma (damage to the optic nerve) at pressures in the normal range.

P:   Is NTG hard to diagnose?  How is it diagnosed?

Dr. Pro:  Yes, it can be tough to diagnose.  The existence of NTG was debated for years.  Physicians would insist that the patient must have periods when the pressure was above normal.  Eventually, however, the evidence supported the existence of NTG.

In making the diagnosis, the eye doctor must inquire about other conditions. Is the cornea thin, and thus the IOP higher?  Is the optic nerve congenitally abnormal (tilted or imperfectly formed) and perhaps the visual field defect has always been present?  Are there other conditions, such as damage to the nerve from vascular disease?  That is called anterior ischemic optic neuropathy, which seems to be related to hypertension or diabetes.  The clinician must ask many other questions, such as about trauma, blood loss, vasospasm, and snoring.

P:  What IOP readings are considered normal?

Dr. Pro:  Normal IOP readings are consistently below 21 mm Hg, after a correction for corneal thickness.

P:  How does a thin cornea affect NTG?

Dr. Pro:  As I mentioned, a thin cornea can affect the measurement of the IOP.  For instance, you may examine a patient with an IOP of 19 mm Hg and think the patient has NTG until the cornea is measured and found to be thin.  When the IOP is corrected for corneal thickness, the IOP may then be 22 mm Hg or so. It is my opinion, however, that true NTG is seen in those patients with worsening glaucoma visual fields at pressures in the low teens.  The visual fields tend to have a common feature -- defects near fixation (at the center) -- rather than starting in the periphery, as in other glaucomas.

P:  Why does NTG affect central vision first?

Dr. Pro:  We don’t know, but the nerves tend to have damage at the top and bottom.  They have a characteristic appearance, as do the visual fields.

P:  Does NTG afflict a particular age, sex, or race?

Dr. Pro:  Classically, NTG is a disease of the elderly (mean age of patients in studies has been in the 60’s).  Results of studies are mixed about sex, although in my practice I seem to see more women with NTG.  It is much more common in Asians; in fact, in Japan, NTG is the main type of glaucoma.

P:  Should a patient diagnosed with NTG always be evaluated by a neuro-ophthalmologist?

Dr. Pro:  Great question.  That used to be the case, and many NTG patients were screened and imaged.  But there is no evidence that evaluations by a neuro-ophthalmologist are useful in patients who present with glaucomatous optic nerves, visual fields, and a low IOP.  I do not hesitate to involve neurology if the nerves look "weird".  For instance, are the nerves pale or swollen?  Does the patient have an atypical visual field?  Those things can suggest a neurological etiology.  Since NTG can be confused with other diseases of the nerve, we look at pallor and cupping.  If the nerve is pale, with minimal cupping but significant visual field loss, we consider neurological disease.

P:  Have studies found a relationship between blood pressure and NTG?

Dr. Pro:  Yes, some studies found that overnight diastolic "dippers" [patients whose IOP drops during sleep at night] are prone to suffer damage from glaucoma.  Dippers could have a nocturnal diastolic blood pressure as low as 30 mm Hg.  That probably causes an insufficient blood supply to the optic nerve.

P:  I have NTG and low blood pressure.  How can I find out if I’m a dipper?

Dr. Pro:  Getting a 24-hour blood pressure measurement can show whether you are a dipper.

P:  Having NTG and low blood pressure, I’m concerned about damage to my optic nerve.  What can I do to increase my low blood pressure?

Dr. Pro:  First, you should find out if your blood pressure is truly low overnight.  If it is, a cardiologist may be required to help you reduce the dipping of overnight blood pressure.

P:  How can one monitor blood pressure for 24 hours?

Dr. Pro:  Twenty-four hour monitors can be useful in diagnosing low systemic blood pressure as a contributing cause of NTG. Treating low blood pressure, however, can be difficult.

P:  I have high (above 140 mm Hg) blood pressure.  Has high blood pressure (hypertension) been shown to be a risk factor for progression of glaucoma?  Can various cardiac arrhythmias be a risk factor for glaucoma?

Dr. Pro:  Any condition that interrupts normal blood supply to the nerve could be a risk factor.  High blood pressure (hypertension) can be a risk for NTG, if the blood pressure medications lead to a low overnight pressure.  Sometimes 24-hour blood pressure monitoring can be useful.

P:  Is a family history of diabetes a risk factor for NTG?

Dr. Pro:  Diabetes is not generally considered a risk factor for NTG.

P:  I believe that using pilocarpine years ago caused a retinal detachment in my left eye.  Can Lumigan or any other glaucoma medication cause retinal detachment?

Dr. Pro:  Pilocarpine can cause a retinal detachment, but retinal detachment has not been associated with Lumigan.

P:  Both NTG and AMD (age-related macular degeneration) have defects near fixation and loss of central vision.  How are NTG and AMD different?

Dr. Pro:  It is important to examine the optic nerve carefully. In glaucoma, the defects in the visual field can be very specific and outlined, whereas in AMD the visual field can look more indistinct.

P:  Are the medications used for NTG the same as those used in other types of glaucoma?

Dr. Pro:  The medications used are the same. I like to start with Xalatan, Travatan, or Lumigan.

P:  I have NTG and I know that I am not supposed to take steroids, such as cortisone or prednisone.  But what about a local injection of cortisone given for Golfer's Elbow (medial epicondylitis)?

Dr. Pro:  I think that local injections are okay.  Those are deposits that do not get distributed through the blood.  But be sure to let your glaucoma specialist know at the next visit.

P:  I may need a trabeculectomy. My IOP is 12 to 14 mm Hg.  What are my risks for hypotony?

Dr. Pro:  If your vision is getting worse with IOPs of 12 to 14 mm Hg and you need a trabeculectomy, there is a risk of hypotony.  The target pressure may be 8 to 10 mm Hg.  Your risk of hypotony is more than the risk for someone who needs a target pressure of, say, 15 mm Hg or so.

Moderator:  Thank you very much, Dr Pro.  See you in a couple of weeks.

On October 17, Dr. Wilson discussed "Depression in Newly Diagnosed Patients" in the Chat room. Click here for highlights of that meeting.

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