What is normal tension glaucoma (NTG)?
Glaucoma is the name for a group of ocular diseases that occur as a result of:
- High eye pressure
- Optic nerve damage
- Loss of vision
There are four main types of glaucoma that affect adults: Primary open angle glaucoma, primary angle closure glaucoma, normal tension glaucoma, and secondary glaucoma.
Each type of glaucoma causes vision loss as a result of damage to the optic nerve. In most cases, this is due to a drainage problem within the anterior angle of the eye that does not allow the aqueous humor (ocular fluid) to effectively flow out of the eye.
The buildup of fluid in the eye causes increased pressure within the eye (intraocular pressure, IOP) which pushes on the optic nerve, and consequently, with time, causes damage to the nerve. Any damage to the optic nerve will result in partial or complete, permanent, vision loss.
Normal tension glaucoma (NTG) is a form of open angle glaucoma, that occurs as a result of optic nerve damage, even when the pressure within the eye is within a normal range (12-22 mm Hg).
Normal tension glaucoma, also called normal-pressure glaucoma, accounts for approximately 30-40% of all glaucoma cases.
Normal tension glaucoma occurs gradually over time, with consistent IOP levels below 21 mm Hg, causing optic nerve head ‘cupping’, thinning of the fiber layer of the retinal nerve, and functional vision loss.
‘The silent thief of sight’
Normal tension glaucoma, being a form of open angle glaucoma, is often referred to as ‘the silent thief of sight’ because by the time the disease is detected, a large amount of irreversible damage to the optic nerve has already occured, causing permanent vision loss.
Similar to open angle glaucoma, NTG generally does not present with any symptoms in its early stages. Over time, as the optic nerve becomes more damaged, your peripheral vision will be affected, and you may notice blind spots or the inability to see something out of the corner of your eye.
As the disease progresses, it will continue to damage more nerve fibers of the optic nerve, and if uncontrolled, can result in tunnel vision— a condition in which you can only see straight ahead, as if you are looking through a paper tube.
Eventually, if uncontrolled, your central vision will be affected as well, causing a total loss of vision.
Symptoms of NTG may include:
- Reduced vision
- Loss of peripheral vision
- Bulging or swollen cornea
- Pupil dilation that doesn’t change with light adjustments
- Redness in the white (sclera) of the eye
Who is at risk for NTG?
The cause of normal tension glaucoma has yet to be discovered, scientists continue to search for answers.
However, certain factors have been associated with a higher risk of NTG:
- Family history of NTG
- Asian descent
- History of systemic heart disease such as irregular heart rhythm
How is NTG diagnosed?
Normal tension glaucoma is diagnosed during a comprehensive eye exam. Your eye doctor will examine the health of your optic nerve by looking for signs of damage, using the following tests:
Tonometry calculates your IOP based on your eye’s response to a puff of air.
Tonometer or tonopen is a tool used to obtain another measure of your IOP. During this test, your eyes will be numbed so that your doctor can gently touch the surface of your eye with either a tonometer or tonopen.
An optical coherence tomography (OCT) scan is a non-invasive, diagnostic tool that uses a laser to provide color-coded, cross sectional images of the retina and optic nerve.
A visual field test produces a computerized map of your visual field and measures the range of your peripheral vision. Your eye doctor will use this test to check for any areas of vision loss that may be caused by optic nerve damage. Loss of peripheral vision that may not even be noticeable to the patient, may be the first sign to indicate optic nerve damage.
Optic disc examination involves a thorough evaluation of the surface of the optic nerves, to examine the quality of the margin, or neuroretinal rim, and optic disc cup.
Your eye doctor may examine your optic disc with direct ophthalmoscopy, a slit lamp, optic disc photography, or through digital imaging devices.
Examination of anterior angle (gonioscopy) enables a thorough evaluation of the anterior angle.
This determines if the angle is open or closed, and can detect the presence of abnormal blood vessels, adhesions, or damage from previous eye trauma.
During this test, a special contact lens prism will be placed on the surface of your eye to enable a clear view of your anterior angle and drainage system.
Please note: It is important to inform your doctor of any other general health problems you may be experiencing, as well as any medication that you take on a regular basis. Normal-tension glaucoma can occur with blood vessel problems that cause low blood pressure, migraine headaches, or cold hands and feet.
How is NTG treated?
Normal tension glaucoma is generally treated the same way as open angle glaucoma— by reducing the IOP to an even lower level than normal. This is usually accomplished through anti-glaucoma eye drops, laser treatments, or surgery.
Anti-glaucoma eye drops
Most cases of NTG can be controlled with eye drops.
Each type of eye drop works in a different manner, in order to effectively control IOP.
Different types of anti-glaucoma eye drops:
- Prostaglandins relax the muscles in the eyes to allow better fluid drainage, reducing build-up of IOP.
- Beta-blockers decrease the amount of ocular fluid production and are typically prescribed in combination with prostaglandins.
- Alpha-adrenergic agonists decrease the rate of fluid production. They may be prescribed in combination with other anti-glaucoma eye drops.
- Carbonic anhydrase inhibitors decrease the rate of fluid production, and are generally prescribed in combination with other anti-glaucoma eye drops.
- Epinephrine decreases the rate of fluid production, while increasing its outflow from the eye.
- Hyperosmotic agents are used in emergencies, for patients with a severely high IOP that must be reduced immediately before permanent damage occurs. They reduce IOP by lowering ocular fluid volume.
Laser surgery may be recommended for some patients with NTG. During a laser procedure, the surgeon creates different avenues to increase fluid drainage, and reduce IOP.
Selective laser trabeculoplasty (SLT) is the most frequently performed laser surgery for open angle glaucoma and NTG. This laser procedure stimulates a biochemical change that improves the fluid drainage from the eye.
Selective laser trabeculoplasty effectively lowers IOP by 20 to 30 percent, and has been proven just as effective as anti-glaucoma eye drops. Improved IOP after an SLT treatment has been shown in about 80 percent of patients, and may last 3 to 5 years. This laser surgery can also be repeated if needed.
Minimally invasive glaucoma surgery (MIGS)
Minimally invasive glaucoma surgery (MIGS) uses microscopic tools and devices during the surgical procedure to lower the risk of surgical complications.
Micro trabeculectomies involve implanting a microscopic tubeinto the eye to enable proper fluid drainage through the anterior angle.
The iStent is the most frequently performed MIGS procedure for the treatment of mild to moderate open angle glaucoma and NTG.
The iStent procedure is generally combined with cataract surgery, and improves IOP levels by creating a bypass through the anterior angle to improve the outflow of fluid from the eye— through its natural drainage system.
Glaucoma filtration surgery
This type of surgery is recommended for mild to moderate stages of glaucoma, if eye drops and laser surgery have not been effective in lowering IOP levels.
During this procedure, a small piece of the eye’s wall is removed, in order to create a new pathway for the fluid to drain through—thereby lowering IOP levels.
When to see your doctor
Normal tension glaucoma can cause permanent damage to your optic nerve and result in irreversible vision loss.
Visit your eye doctor for regular exams to detect any signs of optic nerve damage, before your vision is affected.If you have noticed any changes to your vision, schedule an appointment with your eye doctor as soon as possible.
Early detection of NTG will facilitate earlier treatment— thereby reducing vision loss, and preserving remaining vision.