Primary open-angle glaucoma, the most common form of glaucoma, develops slowly and has symptoms that often go unnoticed. Raised IOP is a hallmark of most forms on glaucoma. However, in certain conditions, optic nerve damage may occur even when the IOP is below the normal level of 21 mm Hg. This form of glaucoma is commonly referred to as normal-tension glaucoma (NTG).
NTG is associated with glaucomatous optic nerve damage, progressive thinning of the retinal nerve fiber, and visual field defects.
What are the risk factors for normal tension glaucoma?
Some factors that predispose persons to increased risk of developing NTG are
- Family history of NTG - The TANK-binding kinase 1 gene has been found to be associated with NTG development.
- Japanese ancestry –The condition occurs most commonly among Japanese persons.
- Persons with a history of heart disease such as irregular heart rhythm, vascular dysfunction and ischemia.
- Female patients – The condition is more prevalent in females than male.
- Persons with hypertension and impaired glucose tolerance.
How is normal-tension glaucoma diagnosed?
A complete medical history is taken with special emphasis on history of head injury. Past episodes of migraine, shock, neurological symptoms, and Raynaud's phenomenon are also investigated.
The doctor may also inquire regarding the use of medications such as systemic steroids and beta blockers.
Visualizing the optic nerve damage
NTG is diagnosed by observing the optic nerve for signs of damage. This is done using an ophthalmoscope or by visual field testing.
The ophthalmoscope is a non-invasive test used to examine the shape of the optic nerve. A nerve that is cupped and not a healthy pink color suggest abnormality.
A visual field test provides a map of the patient’s complete field of vision. The test can detect damage to the optic nerve and is useful to detect dysfunction in central and peripheral vision.
Other diagnostic tests
- Blood pressure should be monitored for at least 24 hours to exclude the possibility of nocturnal systemic hypotension, which may cause NTG
- Certain blood tests are carried out to further confirm the diagnosis.
Tests to detect the levels of vitamin B12, folate, erythrocyte sedimentation rate, C-reactive protein (CRP) and serum angiotensin converting enzyme (ACE) are conducted.
- Cranial imaging is done using magnetic resonance imaging (MRI) to identify any lesions that may be present in the intracranial space.
- To detect blood flow abnormalities, capillaroscopy is performed.
How is normal-tension glaucoma treated?
As per the Collaborative Normal Tension Glaucoma Study Group, an IOP reduction of 30% can help to slow the progression of normal-tension glaucoma.
Doctors treat normal tension glaucoma by using pharmacologic agents, laser treatment, or conventional surgery.
Pharmacologic agents, such as prostaglandin analogs, alpha-2 agonists, carbonic anhydrase inhibitors, and miotics are normally used.
Surgery is an option if medications fail to achieve the target IOP or if the patient is non-adherent to treatment. Deep sclerectomy, laser trabeculoplasty, and filtration surgery are safe and effective options. They are recommended in patients who show disease progression despite maximum tolerated medical therapy.
Another important part of NTG management includes identification and treatment of underlying medical conditions such as migraine, anemia, hypotension, cardiac arrhythmia, hypothyroidism, and autoimmune diseases.
Neuroprotection with a calcium channel blocker helps by increasing blood flow to the damaged optic nerve.
Accurate diagnosis and effective management of NTG can be challenging; however, effective IOP control can help to minimize disease progression.