Normal-tension glaucoma (NTG) is a form of open-angle glaucoma characterized by damage to the optic nerve, which can lead to irreversible vision loss. This damage occurs even though the pressure inside the eye remains within the normal range of 10 to 21 millimeters of mercury (mmHg). In cases of NTG, individuals show signs of glaucoma despite their eye pressure consistently measuring at or below 21 mmHg.
This condition, sometimes called low-tension glaucoma, is a relatively common form of the disease. The progressive optic nerve damage occurs without the warning sign of elevated intraocular pressure (IOP), making its detection and management a distinct challenge.
Theories on Causation and Risk Factors
The exact cause of normal-tension glaucoma is not fully understood, but research points toward two primary theories. One theory suggests that reduced blood flow, or ischemia, to the optic nerve is a contributing factor. The optic nerve requires a constant supply of blood to function properly, and any impairment to this circulation could make it more vulnerable to damage. This is supported by NTG’s association with conditions that affect blood flow, such as systemic low blood pressure, migraines, or Raynaud’s phenomenon.
Another prominent theory focuses on a structural weakness of the optic nerve head itself. This hypothesis posits that some individuals may have an inherent susceptibility in the cellular structure or support tissues of their optic nerve. This could make the nerve more prone to damage from eye pressures that would be considered safe for the general population. Genetics are also believed to play a role, as having a family history of glaucoma increases an individual’s risk.
Diagnosis of Normal-Tension Glaucoma
Diagnosing normal-tension glaucoma can be complex because a standard pressure check may not raise any alarms. The process often becomes one of exclusion, where clinicians must rule out other potential causes of optic nerve damage before confirming NTG. A diagnosis relies on findings from several specific tests that assess the health of the optic nerve and a patient’s vision.
Key diagnostic procedures include:
- A comprehensive dilated eye exam to directly observe the optic nerve for characteristic signs of glaucomatous damage, such as “cupping,” or thinning of the nerve tissue.
- A visual field test, also known as perimetry, to map out any potential vision loss, which often begins in the periphery and goes unnoticed by the patient.
- Optical Coherence Tomography (OCT), which provides a highly detailed, cross-sectional image of the retina and measures the thickness of the retinal nerve fiber layer.
- Multiple IOP readings at different times of the day to confirm that the eye pressure is consistently within the normal range, as pressure can fluctuate.
Primary Treatment Goals and Methods
The primary goal in treating normal-tension glaucoma is to lower the intraocular pressure, even though it is already within the “normal” range. The Collaborative Normal Tension Glaucoma Study demonstrated that reducing the existing IOP by at least 30% can slow the progression of vision loss in a significant number of patients. This approach establishes a new, lower target pressure for the individual’s susceptible optic nerve.
The first line of treatment is typically prescription eye drops, but other interventions may be considered.
- Prescription eye drops, with prostaglandin analogs often being a primary choice, work by increasing the outflow of fluid from the eye or reducing its production.
- Laser therapy, specifically Selective Laser Trabeculoplasty (SLT), is a non-invasive procedure that uses a focused laser to improve the drainage of fluid from the eye.
- Surgical procedures may be recommended when other treatments are ineffective. Trabeculectomy creates a new drainage channel, while micro-invasive glaucoma surgery (MIGS) involves implanting a device to improve fluid outflow.
Long-Term Outlook and Disease Management
Normal-tension glaucoma is a chronic condition that requires lifelong management. While there is no cure, its progression can be controlled to preserve vision for most patients. The long-term outlook is positive for individuals who adhere to their treatment plan and maintain regular follow-up appointments.
Consistent use of medication is fundamental to keeping intraocular pressure at the target level, as missing doses can cause pressure fluctuations that may lead to further optic nerve damage. Regular monitoring allows the ophthalmologist to track the condition of the optic nerve and visual field over time. If signs of progression are detected, the treatment plan may be intensified by adding more medications or considering laser or surgical options.