What Is Normal-Tension Glaucoma?
Glaucoma is a disease affecting the optic nerve and can result in permanent, irreversible vision loss. While most cases of glaucoma are associated with higher than average eye pressures, normal-tension glaucoma (and low-tension glaucoma) is a unique condition in which glaucomatous optic nerve damage (optic neuropathy) occurs despite an average or below average eye pressure.
Eye pressure, called intraocular pressure (IOP), is measured in millimeters of mercury (mm Hg). Population-based studies show that most eye pressures fall within the range of 10 to 21 mm Hg. Many people with glaucoma have IOP of greater than 21; however, in normal-tension glaucoma, IOP can run below 21 or even below 10.
By definition, people with normal-tension glaucoma have open, normal appearing anterior chamber angles. In fact, the features of normal-tension glaucoma are similar to primary open-angle glaucoma (POAG), the most common form of glaucoma.
- In the United States, roughly half of all glaucoma patients have normal-tension glaucoma, with eye pressures below 22, according to the Baltimore Eye Study.
What Causes Normal-Tension Glaucoma?
Although its cause is not completely understood, normal-tension glaucoma (and low-tension glaucoma) is generally believed to occur because of an unusually susceptible optic nerve or reduced blood flow to the optic nerve, causing damage despite a normal intraocular pressure.
Research is ongoing to better understand its cause with the hope that treatments that are more effective will be available in the future.
What Are Normal-Tension Glaucoma Risk Factors?
Normal-tension glaucoma (NTG) may run in families and be inherited. Increasing age is also a risk factor for most forms of glaucoma. Additional risk factors may include anatomical abnormalities of the thin sieve-like bony structure that the nerve traverses (the lamina cribrosa), irregularities in blood flow to the nerve (both high blood pressure and episodes of low blood pressure, particularly during sleep), diabetes, vasospastic conditions such as migraine syndromes and Raynaud's phenomenon, and sleep apnea.
What Are Normal-Tension Glaucoma Symptoms?
In the early stages, there are typically no symptoms of glaucoma. The optic nerve damage can result in permanent vision loss that is so gradual that a patient may not be aware of it. This is true of all forms of glaucoma: high-tension, normal-tension, and low-tension alike. For this reason, regular eye examinations with an eye doctor (ophthalmologist or optometrist) to screen for the presence of glaucoma are very important. The screening should include not just the eye pressure check but also close examination of the optic nerve to look for signs of early damage.
When to Seek Medical Care for Normal-Tension Glaucoma
Currently, the recommendation is to begin undergoing routine screening at age 40, though you might consider earlier screening if you have relatives with glaucoma. A family history of glaucoma is a risk factor, along with higher eye pressure and increasing age.
Those suspected of having normal-tension or low-tension glaucoma should consider undergoing a complete physical examination to treat any cardiovascular diseases or diabetes, as any medical problems that impair good blood flow to the nerve may hasten the optic nerve damage.
What Exams and Tests Diagnose Normal-Tension Glaucoma?
As part of the complete eye examination, your ophthalmologist or optometrist will begin with reviewing any family history of glaucoma, and reviewing your medical history for conditions that often appear in conjunction with normal-tension glaucoma that are thought to be risk factors, such as cardiovascular disease, high or low blood pressure, diabetes, migraines, and Raynaud's syndrome.
It will also be important to review any history of neurologic disorders, head and eye trauma, stroke, blood loss requiring transfusions, and other conditions that could have resulted in IOP-independent, non-glaucomatous optic neuropathy.
The exam will include a vision check and measurement of the baseline eye pressure (IOP). Regardless of whether the eye pressure is high, low, or average, it is the examination of the optic nerve itself that determines if glaucoma is present.
- A special microscope called a slit lamp examines the front of your eyes, including your cornea, anterior chamber, iris, and lens. With a slit lamp examination, the ophthalmologist looks for signs of other causes or risk factors of glaucoma.
- Tonometry is a method used to measure the pressure inside the eye.
- Medical professionals will take measurements for both eyes on at least two to three occasions. Because IOP varies from hour to hour in any individual, measurements may be taken at different times of day.
- Pachymetry is a measurement of the corneal thickness. Thinner than average corneas carry a higher risk for glaucoma.
- Medical professionals perform gonioscopy to check the angle of your eye. This is the area between the peripheral iris and the peripheral cornea, where a circular sieve-like structure called the trabecular meshwork sits. A fluid inside the eye called aqueous flows through the trabecular meshwork, but if it is blocked partially or completely, eye pressure will build up. To examine the angle, an eye doctor will place a special contact lens called a gonio prism on the eye. By definition, people with normal-tension glaucoma have open, normal-appearing angles. Gonioscopy will allow the examiner to confirm that the angles are open, versus being narrowed, damaged, scarred, or blocked (closed), as seen in other forms of glaucoma (for example, narrow-angle or closed-angle glaucoma, traumatic angle-recession glaucoma, and congenital glaucoma).
- An eye doctor will examine each optic nerve for any damage or abnormalities; this may require dilation of the pupils to ensure an adequate view. The portion of the optic nerve that enters the back of the eye is visible to the examiner. This is the optic nerve head or the disc. An optic nerve with glaucomatous damage has a characteristic cupping within the disc (an enlarging indentation seen in the center of the optic nerve head). The presence of cupping is a risk factor for all forms of glaucoma. The eye doctor will also look for other clues such as pallor (pale color), notches in the rim of the optic nerve head, small disc hemorrhages (blood on the edge of the nerve), and thinning of the tissue surrounding the nerve (peripapillary atrophy).
- Imaging studies may be conducted to document the contour and thickness of your optic nerve and to detect changes over time, including fundus photographs to document the size of the cup within the optic nerve head and any disc hemorrhages or notches, and/or NFA (nerve fiber analysis) using OCT (optical coherence tomography) to measure the thickness of the nerve fibers at the optic nerve head, as well as the retinal nerve fiber layer (RNFL).
- Visual field testing checks your central, paracentral, and peripheral (or side) vision, typically by using an automated visual field machine. Areas of visual field loss or dimming of the vision can be picked up, often long before the patient is aware of them. Certain patterns of visual field defects are characteristic of glaucoma.
- This test may confirm that glaucoma is present. However, an absence of visual field defects does not ensure the absence of glaucoma. Visual field defects may not be apparent until as much as 50% of the optic nerve fibers have been damaged.
- An eye doctor will repeat your visual field test over time to look for visual field progression. More aggressive treatment is often indicated if there are signs of worsening field loss.
- If your visual field test reveals defects that appear uncharacteristic of glaucoma, then your ophthalmologist will perform additional tests to look for other causes of optic nerve disease and vision loss.
What Is the Treatment for Normal-Tension Glaucoma?
Regardless of whether the eye pressures typically fall within the average range or even if they tend to run low, the treatment for glaucoma is the same: Lower the eye pressure further with medication, laser, and/or surgery. Once the pressure has been lowered (ideally by 30% initially), tests are repeated to determine if the optic nerve has stabilized. If over time the nerve continues to show glaucomatous atrophy (thinning) and/or if the visual fields tests show visual field progression with worsening field loss, additional treatment is aimed at lowering the eye pressure even further, until the glaucoma is controlled.
Currently, much of the ongoing research in glaucoma aims to find other ways to protect the nerve (neuroprotection).
What Are Medical Treatment Options for Normal-Tension Glaucoma?
Medical treatment focuses on lowering the pressure inside the eye by affecting the flow of aqueous fluid within the eye. These medicated eyedrops work by either decreasing the production of aqueous fluid or by easing the outflow of aqueous fluid, thus reducing the eye pressure. The initial goal is to reduce the pressure by 30% then reassess. The goal is to keep the IOP low enough that no further optic nerve damage or vision loss occurs.
Some glaucoma patients have side effects from the medications. It is important for the physician to review the medical and ocular history to anticipate possible adverse effects from the medications, and it is equally important for the NTG patient to alert the physician if any adverse effects or allergic symptoms develop.
Several studies have shown that if an eye doctor prescribes a beta-blocker eyedrop for lowering eye pressure, it could be safer to limit its use to mornings, as bedtime doses can lower blood flow to the optic nerve during sleep in NTG patients.
Punctal occlusion can minimize most side effects of glaucoma drops (and all medicated eyedrops for that matter). Following the instillation of the eyedrop, you close your eye and apply gentle pressure with your finger to the side of the nose right next to the eye. This is the location of your nasolacrimal duct. By closing off the duct with pressure, you minimize the amount of medication that might flow into the nose or into the back of your throat through the duct. If you can taste the eyedrops after instilling them, then some of the drop is passing from your eye's surface to the back of your throat via the nasolacrimal duct. Be sure to ask your doctor to demonstrate punctal occlusion for you.
Is Surgery an Option for Normal-Tension Glaucoma?
Selective laser trabeculoplasty (SLT) is a laser procedure for lowering eye pressure. The ophthalmologist applies a laser beam to the trabecular meshwork, producing changes that allow the fluid (aqueous humor) to flow more easily from the eye, thus lowering the IOP.
The entire procedure usually takes 30 minutes or less and is relatively painless.
Though SLT usually reduces IOP, unfortunately, this decrease in IOP is not always permanent. Many NTG patients still require medication, and some will require surgery.
For some NTG patients who continue to show visual field progression in spite of maximal medical therapy, surgery may be recommended. The aim of these procedures is to create an alternate pathway (or drainage channel) in the eye to increase the passage of fluid (aqueous) from the eye, which helps in lowering IOP. A trabeculectomy procedure can achieve this, in which the surgeon creates a small channel for the aqueous to drain into a small pocket between the white part of the eye (sclera) and the outer layer (conjunctiva). Various implantable stents are also available that help channel aqueous out of the eye. And finally, there are procedures that directly open the trabecular meshwork further.
Follow-up for Normal-Tension Glaucoma
Normal-tension glaucoma patients require regular follow-up visits to monitor for progression and to make sure there are no side effects from the treatments. Medical professionals typically schedule follow-up visits every three to six months initially, but they can be spaced further apart once good control is achieved.
Is It Possible to Prevent Normal-Tension Glaucoma?
At this time, we know of no way to prevent normal-tension glaucoma. This is an area of ongoing research. It does appear to run in families, so there are genetic components, but anyone can develop NTG. It is important to remember too that if you are diagnosed with NTG, in addition to being compliant with your treatment, it's also important to take steps to keep your general health optimal. If you smoke, quit. Smoking accelerates optic nerve damage. If you have diabetes, talk to your doctor about what you can do to control it better. Blood pressure is a trickier issue. High blood pressure is associated with worse NTG, but so is very low pressure, especially during sleep (nocturnal hypotension). Your doctor may need to adjust the time of day that you take your medication.
What Is the Prognosis for Normal-Tension Glaucoma?
With early diagnosis and treatment, it's possible to prevent optic nerve damage and/or vision loss, if already present, may be slowed or stabilized. Keep in mind that once glaucomatous vision loss occurs, it is permanent and irreversible.
Research is ongoing to understand the causes and to develop better, more effective treatments, and some day it is hoped that damaged optic nerves could be repaired to regain lost vision.