Normal-Tension Glaucoma

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Normal-Tension Glaucoma Overview

Glaucoma is usually high pressure inside the eye that damages the optic nerve and can result in permanent vision loss. Normal-tension glaucoma (also called low-tension glaucoma) is a unique condition in which optic nerve damage and vision loss have occurred despite a normal pressure inside the eye.

Eye pressure, called intraocular pressure (IOP), is measured in millimeters of mercury (mm Hg). Normal eye pressure ranges from 10-21 mm Hg. Most people with glaucoma have IOP of greater than 21 mm Hg; however, in normal-tension glaucoma, people have IOP within the normal range.

By definition, people with normal-tension glaucoma have open, normal-appearing angles. In fact, the features of normal-tension glaucoma are similar to primary open-angle glaucoma (POAG), the most common form of glaucoma (see Primary Open-Angle Glaucoma).

  • Although the occurrence of normal-tension glaucoma varies worldwide, it is very prevalent in Japan.
  • In the United States, up to 15-25% of people with open-angle glaucoma experience normal-tension glaucoma.
  • According to the Baltimore Eye Study, 50% of individuals with changes in their optic disc (the front surface of the optic nerve) and in their visual field had an IOP of less than 21 mm Hg on a single visit, and 33% had an IOP of less than 21 mm Hg on 2 measurements.
  • Normal-tension glaucoma is more common in women than in men.
  • Normal-tension glaucoma affects adults, with an average age of 60 years.

Normal-Tension Glaucoma Causes

Although its cause is not completely understood, normal-tension glaucoma is generally believed to occur either because of an unusually fragile optic nerve that can be damaged despite a normal pressure inside the eye or because of reduced blood flow to the optic nerve.

  • Unusually fragile optic nerves may be inherited.
  • Reduced blood flow to the optic nerve can be due to disorders of the blood vessels (called vascular diseases), including vasospasms and ischemia.
    • Vasospasms are spasms or constrictions of the blood vessels.
    • Ischemia is reduced oxygen delivered to the tissue, in this case the optic nerve, because the blood vessels are either narrowed or obstructed.

Normal-Tension Glaucoma Symptoms

Regular eye examinations with an ophthalmologist (a medical doctor who specializes in eye care and surgery) are very important because people with normal-tension glaucoma do not experience any early symptoms of it.

Because of its silent nature, people do not usually have any visual complaints until late in the course of the disease. By the time a person with normal-tension glaucoma would notice vision loss, significant amounts of optic nerve damage and vision loss would have already occurred. The optic nerve damage and vision loss are permanent.

When to Seek Medical Care for Normal-Tension Glaucoma

Regular eye examinations with an ophthalmologist are important to screen for optic nerve damage and vision loss despite a normal eye pressure.

Those who are suspected of having normal-tension glaucoma may also undergo a physical examination with a doctor who is familiar with both cardiovascular diseases (those involving the heart and the blood vessels) and neurologic disorders (those involving the nervous system) because of their association with this type of glaucoma.

Your ophthalmologist will want to know if there is a history of glaucoma or optic nerve abnormalities in your family because these conditions are often inherited.

During your eye examination, your ophthalmologist will also ask you about the following, all of which may be associated with normal-tension glaucoma:

  • Past ocular history
  • Any illnesses
    • Vasospasms, such as Raynaud syndrome
    • Coagulopathies (These diseases affect the ability of the blood to clot.)
    • Previous blood loss or shocklike episodes
    • Nocturnal hypotension (Below normal blood pressure occurs during the nighttime hours.)
    • Autoimmune disorders (Normal tissues no longer function or are destroyed by the body's own immune system.)
    • Vascular diseases, including atherosclerosis (In atherosclerosis, the arteries are blocked or closed.)
    • Thyroid disease
    • Sleep apnea (Breathing is interrupted or momentarily stopped while sleeping, particularly prevalent in people who are overweight.)
    • Alzheimer disease

When Should You be Screened for Glaucoma?

Though glaucoma is a serious progressive disease, catching it early offers the best chance at treatment. If you are under 40 and have no family history of glaucoma, having a regular general eye exam is fine.

Major factors in glaucoma risk include age, race, sex, and family history.

Normal-Tension Glaucoma Exams and Tests

During an eye examination, your ophthalmologist performs tests to measure IOP as well as to rule out ocular hypertension (any situation in which the pressure inside the eye is higher than normal) and early POAG (see Ocular Hypertension and Primary Open-Angle Glaucoma). These tests are explained below.

  • The front of your eyes, including your cornea, anterior chamber, iris, and lens, are examined using a special microscope called a slit lamp. 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.
    • Measurements are taken for both eyes on at least 2-3 occasions. Because IOP varies from hour to hour in any individual, measurements may be taken at different times of day (eg, morning and night).
    • If you are suspected of having normal-tension glaucoma with normal IOP but a suspicious looking optic nerve, your IOP may be checked several times during a single day (called a diurnal assessment or diurnal curve).
  • Gonioscopy is performed to check the drainage angle of your eye; to do so, a special contact lens is placed on the eye. By definition, people with normal-tension glaucoma have open, normal-appearing angles; therefore, this test is important to determine if your angles are open, narrowed, or closed.

Each optic nerve is examined for any damage or abnormalities; this may require dilation of the pupils to ensure an adequate examination of the optic nerves.

  • Different imaging studies may be conducted to document the status of your optic nerve and to detect changes over time. A Doppler ultrasound may be used to monitor blood flow to the eye, including the optic nerve.
  • Fundus photographs, which are pictures of your optic disc (the front surface of your optic nerve), are taken for future reference and comparison. In certain people, ophthalmologists obtain this documentation yearly for detailed comparison.
  • The retina is examined for any defects. This may also require dilation of the pupils to ensure an adequate examination of the retina.
  • Visual field testing checks your peripheral (or side) vision, typically by using an automated visual field machine. Loss of peripheral vision is associated with normal-tension glaucoma.
    • This test is done to rule out any visual field defects due to glaucoma. 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 fiber layer has been lost.
    • If your visual field defects seem to progress in a manner that is uncharacteristic of glaucoma, then your ophthalmologist will perform additional tests to look for other causes of vision loss.

Normal-Tension Glaucoma Treatment

If your ophthalmologist prescribes medicines to help in lowering the pressure inside your eye, properly applying the medication and complying with your eye doctor's instructions are very important. Otherwise, your condition may worsen.

Normal-Tension Glaucoma Medical Treatment

Medical treatment is aimed at lowering the pressure inside the eye. IOP-lowering medications, which come in the form of medicated eyedrops, are used to reduce the pressure by at least 30% (see How to Instill Your Eyedrops). By keeping the IOP low, normal-tension glaucoma can be stabilized, meaning no further optic damage or vision loss occurs.

Surgery for Normal-Tension Glaucoma

During a trabeculoplasty, the ophthalmologist uses a laser beam to place small spots on the trabecular meshwork, which further opens the holes in the trabecular meshwork, allowing the fluid (aqueous humor) to flow better out of the eye, which, in effect, lowers IOP.

You will sit at a slit lamp while the ophthalmologist performs the procedure. A special contact lens (called a goniolens) is placed on your eye so your ophthalmologist can view the trabecular meshwork.

A full treatment generally consists of 100 spots placed over the entire trabecular meshwork. This may be divided between 2 sessions consisting of 50 spots over each half of the trabecular meshwork. The entire procedure usually takes 30 minutes or less and is relatively painless.

IOP is usually reduced, but, unfortunately, this decrease in IOP is not usually permanent. It may last up to 3-5 years following a trabeculoplasty, if successful.

Following the trabeculoplasty, your eye doctor will prescribe medicine to prevent inflammation. You will also continue with your glaucoma medicine.

In severe cases with progressive vision loss, trabeculectomy may be recommended. During trabeculectomy, your ophthalmologist creates an alternate pathway (or drainage channel) in the eye to increase the passage of fluid (aqueous humor) from the eye, which helps in lowering IOP.

Follow-up for Normal-Tension Glaucoma

If you have normal-tension glaucoma, you will have regular follow-up visits with your ophthalmologist to monitor for progression of this condition. Follow-up visits are typically scheduled every 3-6 months.

Normal-Tension Glaucoma Prevention

Normal-tension glaucoma cannot be prevented; however, with regular eye examinations by an ophthalmologist, any further progression can be avoided.

Normal-Tension Glaucoma Outlook

With early diagnosis and medical treatment, further optic nerve damage and/or vision loss may be prevented. If this condition is not detected early, permanent loss of vision can occur.

Support Groups and Counseling for Normal-Tension Glaucoma

Educating people with normal-tension glaucoma is essential for successful medical treatment. The person who understands the chronic (long-term), potentially progressive nature of glaucoma is more likely to comply with medical treatment.

Sources: References
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