Introduction:
Glaucoma is a group of diseases causing long-standing progressive optic nerve damage. Glaucoma is thus a sort of generic name, referring to the plethora of diseases causing optic nerve damage with one common feature, a higher intraocular pressure (higher pressure inside the eye than the normal limits). A few patients belong to a lesser-known, smaller subgroup among the broader classification of glaucoma called normotension glaucoma. This peculiar disease is one where, as the name suggests, the intraocular pressure is normal. But, is classified as a type of glaucoma as it produces eye changes that are characteristic of glaucomatous damage. This article describes the essential features of this disease, how the disease progresses, and what measures need to be taken to detect and manage this problem.
What Is Normotension Glaucoma?
Glaucoma is a disease that causes damage to the optic nerve, which connects the eye to the brain and allows it to see. The disorder is typically brought on by too high eye pressure. The "normotension" kind, however, is distinct. The area around the front of the eye usually contains fluid. That fluid does not drain as well as it should in many forms of glaucoma. As a result, it backs up like water in a blocked drain. As a result, the pressure inside the eye increases. The optic nerve begins to suffer damage over time.
However, even when the eye pressure remains normal, the optic nerve is destroyed in normal-tension glaucoma. It is also called "low-tension" or "normal-pressure" glaucoma.
What Are the Causes of Normotension Glaucoma?
The cause of normotension glaucoma is proposed to be due to blood flow changes causing damage to the optic nerve (the nerve of sight), from blood vessels that are presumed to be more sensitive to blood flow changes than normal. This damage, just like in all glaucomas, affects the field of vision (refers to how much of the whole world around us we can see with our eyes regarding size). Thus, only the extent of vision and not the clarity of vision is affected, even in advanced cases.
To give the readers a sense of perspective, a patient with advanced optic nerve damage due to normotension glaucoma will be able to see as clearly as any normal person, but he will only be able to perceive the things around him as if he is looking through a tiny hole (also called in medical terms as tubular vision), because of massive constriction of his visual field.
What Are the Symptoms in a Patient with Normotension Glaucoma?
A patient with normotension glaucoma is usually asymptomatic, and the condition will only be detected by screening.
What Are the Risk Factors of Normotension Glaucoma?
There are groups of people who have a higher risk of developing glaucoma, the most common being genetic. A few races are known to have a higher incidence of normotension glaucoma than the global average. The number is steadily increasing with awareness about the disease and adequate screening modalities being made available. The second group is relatives of known glaucoma patients and patients with hypertension and disease related to blood vessels (stroke, heart disease, diabetes). It is recommended to visit an ophthalmologist for getting screened for glaucoma.
What Are the Investigations Available to Detect Normotension Glaucoma?
During the screening, a preliminary examination of visual acuity (when the doctor or optometrist asks individuals to read a few letters which decrease in size as each line progresses and checks the individuals for glasses) is performed. Then the examination continues with a check of the color vision using Ishihara's charts, and an estimation of the intraocular pressure using applanation tonometry. The examination then includes a thorough examination of the eyes using a slit lamp with special emphasis on the angle of the anterior chamber and optic disc.
The next most important test is the visual field test. This test is the main “treatment-deciding” factor in all glaucomatous diseases. This test, when performed correctly by patients, enables doctors to get an idea of the exact extent of optic nerve damage. Hence even if the pressure-lowering drugs are working and reducing the intraocular pressure, the optic nerve damage does not go down. Then, the treatment must be considered inadequate and must be reviewed.
Since the examination of the retina can be confused as it is a subjective assessment, to maximize effectiveness, it is recommended to supplement the examination with objective investigations. A few of these tools are stereoscopic optic disc photographs, these are just objective supplements to the appearance of the disc and have supportive value with the findings of the other tests.
Central corneal thickness measurement is also done because any variation in the thickness of the cornea can have a corresponding variation in intraocular pressure. Retinal nerve fiber layer analysis is also an objective assessment of optic nerve fiber layer thickness in comparison to age and race-matched normative data. This imaging is done with a machine called optical coherence tomography (OCT). A similar imaging method used to estimate optic nerve fiber thickness is the GDX VCC (glaucoma detection with variable corneal compensation), although it operates on a different mechanism.
In earlier days, doing a diurnal variation test was in vogue. In this test, a patient has to stay overnight at the hospital, and his eye pressure would be measured at regular intervals throughout the night. This is still considered an inexpensive and excellent test to detect normotension glaucoma.
These tests and investigations may not all be required, and they may be adjusted based on different patient-related aspects. As previously mentioned, the reason for this extreme attention and high volume of testing is that the condition has no symptoms, thus the earlier a patient can be diagnosed, the earlier they can begin therapy and prevent optic nerve damage.
What Are the Treatment Methods Available for Normotension Glaucoma?
Treatment modalities available mainly include eyedrops which reduce the pressure of the eyes (antiglaucoma drugs). A few cases may also be prescribed drugs that alter blood flow such as calcium channel blockers. It is quite rare for a patient to require surgery, but that may be necessary depending on the optic nerve damage progression.
Once a patient has been diagnosed to be glaucomatous, he has to be on follow-up until there are no visual field changes for a prolonged period. So more often than not, the treatment is started and progression monitored by visual field test results, and this makes follow-up essential as the disease is not one where the damage is caused and can be prevented in one go. The entire process of follow-up, monitoring, and personalizing treatment takes years, with follow-ups every six months to assess progress.
Thus, the best way to deal with normotension glaucoma is screening and early diagnosis coupled with prompt and regular follow-ups.
Conclusion:
Unfortunately, glaucoma cannot be prevented. However, if the illness is identified and treated at an early stage, blindness from it is frequently avoidable. Therefore, continue to schedule frequent eye checkups with the eye doctor.