What is Normal Tension Glaucoma?
Glaucoma is a condition that progressively damages the optic nerve (the part responsible for transmitting visual signals to the brain), usually due to high pressure in the eye. However, there’s a form of glaucoma called Normal-tension Glaucoma (NTG) where the pressure in the eye is normal but the damage still occurs. Its origins are not well understood and there’s a debate on whether it’s simply a subtype of typical glaucoma or a group of different diseases.
NTG affects people in varying degrees, from causing no symptoms to leading to complete blindness. Losing vision can have major life impacts, including not being able to drive. When considering treating NTG, it’s important to balance the side effects and costs of treatment against the risk of losing sight. It’s also worth noting that NTG can mimic other nerve-related eye conditions — so if NTG is suspected, it’s essential to rule out other possible causes.
What Causes Normal Tension Glaucoma?
There are several theories for what causes Normal Tension Glaucoma (NTG). NTG is distinct from other glaucoma conditions because it represents a diverse range of causes, but eventually leads to the same result – the loss of nerve cells in the retina. Even though the eye pressure in NTG patients is within a normal range, it’s believed that pressure-related factors still play a role in causing the condition.
Other theories suggest that things like poor blood supply to the optic nerve, metabolic and nerve cell disorders, oxidative stress, and unusual structural characteristics at the optic nerve could be involved. Genetics is also thought to be a significant factor because NTG is often seen in family clusters and is more common in certain ethnic groups, even after they have moved to new geographical locations. There are three specific genes linked with NTG, but these are rarely found even among NTG patients and are more prevalent in those who develop the condition early or have a strong family history of NTG. The genes are called Optineurin (OPTN), TANK binding kinase (TBK1), and Myocilin (MYOC).
There are several risk factors associated with NTG, including age, family history, being female, higher-than-average eye pressure, thin cornea, high blood pressure, low blood pressure during sleep, migraines, Raynaud’s phenomenon (a condition that affects blood flow to the extremities), dementia, and obstructive sleep apnea.
Risk Factors and Frequency for Normal Tension Glaucoma
How common Normal Tension Glaucoma (NTG) is can vary a lot depending on your ethnicity. For example, in East Asian countries, Primary Open-Angle Glaucoma (POAG) – a type of glaucoma – is found in 1 to 4% of the population. Of these, up to 95% are NTG cases. But in white European populations, while around 2% have POAG, only about one third of these are NTG cases. NTG is more common in sub-Saharan African populations where it’s found in about 50% of the 3 to 8% of the population with POAG. As people generally live longer, we expect the number of people with both POAG and NTG to go up.
The range of ‘normal’ Intraocular Pressure (IOP), which is the pressure inside your eye, is decided based on what’s normally found in healthy people. This is usually 2 standard deviations from the average. For about 95% of healthy people, this means an IOP between 11 and 21mmHg. So, often an IOP of 21mmHg is used as the cut-off to diagnose NTG.
Signs and Symptoms of Normal Tension Glaucoma
Glaucoma generally doesn’t cause symptoms in the early stages, and many people only discover they have it during routine eye examinations. However, as the disease progresses, patients might start noticing problems such as lower quality vision, difficulty adapting to different levels of light, and trouble seeing in low contrast environments. In advanced stages, changes in the field of vision become noticeable.
If a person has family members with glaucoma or a history of blindness, this should be disclosed to the eye specialist. It’s also important to discuss any potential risk factors, like steroid use, past eye injuries or surgeries. Any allergies should be mentioned as they could affect the course of treatment. Information about current medication use is crucial as well.
Once glaucoma is suspected, several tests are performed. They include examining the pupil’s response to light, checking the color vision, and measuring the eye’s internal pressure. Techniques like slit-lamp examination and gonioscopy are used to observe the structure of the eye and rule out other causes of glaucoma. A dilated fundus examination allows the doctor to detect changes in the optic disc – a common indicator of glaucoma. The progression of the disease can be seen in the enlargement of the cup-to-disc ratio, which is the proportion of the optic disc’s ‘cup’ (where the nerves exit the eye) to the overall size of the disc.
Advanced glaucoma may also cause optic disc changes like pale spots (pallor) and small blood spots (hemorrhages) on the edges. Doctors also check for peripapillary atrophy, a type of tissue damage around the optic nerve head, although this is not specific to glaucoma. In the early stages of the disease, changes in the optic disc often occur before vision loss.
Testing for Normal Tension Glaucoma
The Humphrey 24-2 visual field, conducted through automated perimetry, is a common way to monitor glaucoma, including a version called NTG. We can also use other methods like 10-2 fields, mainly in serious situations, and wide-field exams like 30-2 and Estermann. There are quick options like the SITA fast and faster algorithms that deliver similar results.
Using 3D photos of the back of the eye gives us a reference to compare with future eye exams. These photos can be taken again and again to see if there have been changes over time.
OCT technology has done wonders in the management of glaucoma. There are many methods available, but they all work similarly in comparing measurements of nerve cell layers in the eye with a standard database. This technology allows us to evaluate the health of the optic nerve or to track changes over time.
We use a method called corneal pachymetry to measure the thickness of the center of the cornea. If that area is thin, it puts the patient at a higher risk for glaucoma progression. Thinner corneas can also result in underestimating eye pressure when measured with a device called an applanation tonometer. But other factors like corneal hysteresis can also affect the reading on this device.
People with POAG and NTG types of glaucoma may see more significant changes in their eye pressure throughout the day, usually peaking in the early morning. Doctors may recommend repeat eye pressure measurements, taken either throughout the day at the office or at home over 24 hours, to monitor changes. These measurements can be very helpful in patients with fast-progressing disease, even when their eye pressure appears controlled during office visits. Another way to simulate these changes is by carrying out a water-drinking test in the office.
Treatment Options for Normal Tension Glaucoma
When it comes to normal-tension glaucoma (NTG), not every case will result in significant loss of vision. A past study found that 65% of untreated cases presented no progression over a five-year period, showing that monitoring without treatment can sometimes be a suitable first strategy. However, such an approach should always be discussed with the patient.
Treating normal-tension glaucoma is typically done by lowering intraocular pressure (IOP), especially when optic disc or visual field changes have been confirmed to likely progress to vision loss. Existing conditions, like hemorrhages on the optic disc, moderate or advanced disease, and risk factors for blindness, may also necessitate immediate treatment. In these situations, aggressive IOP lowering has been shown to slow down the rate of vision loss. However, it’s important that the benefits of aggressive treatments are weighed against potential side effects.
How can we lower IOP? We can utilize eye drops, laser treatments, or drainage surgeries. Prostaglandin analogs are typically used as a first measure. Comparing the use of eye drops (brimonidine and timolol), it was found they can reduce IOP equally, but brimonidine had less progression. It’s thought that brimonidine may also protect the nerve cells in the eye. Selective laser trabeculoplasty (SLT), a type of laser treatment, can also be effective. In complicated cases, with very low pressure or progression despite the treatment, surgery might be necessary.
As of now, treatments not focused on IOP have not proven effective to alter disease progression, but studies are underway to understand their potential. Several additional risk factors are being examined, such as nocturnal low blood pressure, diet, fitness, obstructive sleep apnea, smoking, and even meditation. While exercise is usually beneficial for overall health, certain yoga positions that position the eye below the rest of the body can significantly raise intraocular pressure. Meanwhile, a study has shown that mindfulness meditation could help lower IOP.
The use of cannabinoids isn’t currently recommended as a treatment for glaucoma, despite it being shown to reduce IOP, as the reduction isn’t clinically significant. It’s crucial that patients discuss treatment options with their doctors before attempting any new treatments to manage normal-tension glaucoma.
What else can Normal Tension Glaucoma be?
When diagnosing eye conditions, doctors think about different types of glaucoma that can show up as normal eye pressure. These include:
- Angle-closure glaucoma
- Glaucoma caused by inflammation due to high blood pressure (Hypertensive uveitis)
- Glaucoma caused by pigment dispersion (Pigmentary glaucoma)
- Glaucoma caused by flaky material blocking the eye’s drainage system (Pseudoexfoliative glaucoma)
- Primary open-angle glaucoma that may be underestimated due to thin central corneas
Doctors also consider whether there have been past instances of high eye pressure, whether from eye injury, blood in the eye, steroid use, or a surgery involving vitreous, a jelly-like substance that fills your eye.
They would then look into previous blood vessel damage, which can create an image similar to non-progressive glaucoma. This can occur after retinal vascular occlusions, optic nerve damage due to lack of blood supply, or extremely low blood pressure circumstances such as post-operation, trauma, or severe infection.
Another diagnostic consideration would be naturally occurring changes or birth abnormalities in the optic disc which could be mistakenly diagnosed as glaucoma. These include natural hollowing of the optic disc, titled discs, abnormal enlargement of the back of the eye due to nearsightedness (myopic fundus), calcium deposits on the optic disc (disc drusen), a small hole or depression in the optic nerve (optic disc pit), or birth defects like optic disc coloboma or underdevelopment of the upper segment of the optic nerve.
Next, they look into other optic nerve disorders which could be genetic, caused by toxins, injury, or inflammation.
Lastly, they check for tumors in or around the brain, including those that affect the pituitary gland. Indicators of tumorous growth could be young age without family history, fast progression of the condition, symptoms only in one eye, bulging eye, coordination problems, unusual optic disc findings such as disc pallor without cupping, visual problems that are worse on one side of vision. In such cases, an MRI with contrast of the front part of visual pathway is necessary to rule out tumors.
What to expect with Normal Tension Glaucoma
Normal-tension glaucoma (NTG) can, in severe cases, lead to loss of vision in both eyes, even with treatment. However, not everyone with NTG will experience significant vision problems. Generally, this condition progresses more slowly than primary open-angle glaucoma (POAG).
Certain people are at a higher risk of worsening vision, including females and those with a history of migraines, the presence of bleeding in the optic disc, or notable daily variations in intraocular pressure (the pressure inside the eyes).
Those who already have advanced NTG or those who lose vision rapidly are at greatest risk of becoming blind due to this condition. It’s vitally important to identify and treat these so-called ‘rapid progressors’ as aggressively as possible.
Possible Complications When Diagnosed with Normal Tension Glaucoma
Normal Tension Glaucoma (NTG) can significantly impair a person’s ability to perform day-to-day activities due to vision loss. Activities that involve visual searching can be affected, and the risk of falls increases. On the other hand, if the disease is asymmetric, that is, if one eye is less affected than the other, the person may still be able to carry out their daily activities, even with severe vision loss in one eye. In NTG, central vision defects are more common, increasing the risk of developing a central blind spot. This can be seen when both eyes’ vision is tested simultaneously. Due to vision loss, people with this condition might not meet the vision standards required to obtain a driving license in many countries. This can also affect professional drivers or pilots, who might lose their licenses even if they have a relative mild condition in one eye.
All classes of eye pressure-lowering medications can cause side effects such as eye redness and discomfort in the eye surface. Prostaglandin analogs, a group of these medicines, can cause excessive hair growth and changes in the color of the iris of the eye. Long-term use can lead to what is known as prostaglandin-associated periorbitopathy, a complicated condition that causes eye socket fat reduction, drooping of the eyelid,chronic inflammation of the eyelids, and skin discoloration. This is most commonly seen with the drug bimatoprost. Moreover, surgical procedures to filter eye fluid may not be as successful in these patients. Additionally, topical beta-blockers, another group of eye pressure-lowering drugs, should not be used in patients with lung diseases such as asthma and Chronic Obstructive Pulmonary Disease (COPD) because they can cause disorders of the heart and blood circulation, vivid dreams, erectile dysfunction, and mood changes. Drugs like carbonic anhydrase inhibitors must be carefully used in patients allergic to sulfa drugs.
Another group of medications called alpha-agonists should not be given to young children.
Complications from Selective Laser Trabeculoplasty (SLT), a procedure performed to lower eye pressure, are rare but can include inflammation inside the eye or a sudden increase in eye pressure after the procedure.
Complications from glaucoma surgery can include:
- Failure of the surgery
- Abnormally low eye pressure
- Cataract development
- Blebitis, an inflammation in the area of the surgery
- Erosion of the tube inserted for the surgery
- Endophthalmitis, a serious eye infection
- Corneal decompensation, a state in which the cornea cannot perform its functions
Preventing Normal Tension Glaucoma
Glaucoma is a serious eye condition, recognized as the second leading cause of permanent blindness globally. It’s a significant health concern in both advanced and developing nations. Detecting glaucoma early can be difficult, as it often shows no symptoms. Because of this, several public health strategies, like widespread screenings, are being considered to lessen the impact of patients being diagnosed with severe stages of the disease. Public education about the importance of regular eye check-ups can help in identifying glaucoma and other eye problems earlier.
If a person is diagnosed with glaucoma, it’s crucial that they inform their close family members about getting regular eye exams. This awareness could lead to early detection of the disease, improving the overall outcome.
