A lot of research has been done to understand the reasons behind this gradual loss of nerve fibers. Its still not fully understood, but what we do know is that the pressure inside the eye plays a significant role. And its also a well known fact that reducing the eye pressure can stop or slow down glaucoma.

The front part of the eye is  filled with a clear fluid called ‘Aqueous Humour’ which nourishes the eye and keeps it healthy. Similar to blood pressure, it also exerts a pressure in the eye called Intraocular Pressure (IOP).

Flow of Aqeous Humour

Normal IOP is 10 to 20 mmHg. Excessive pressure within the eye can damage the nerve. This rise typically happens when the drainage of this fluid gets blocked causing accumulation of fluid inside the eye.

In some case however, the nerve gets damaged even at pressure lower than 20 mmHg. This happens due to increased sensitivity of the nerve itself to seemingly normal levels of eye pressure. Therefore eye pressure in itself is not enough to diagnose or rule out glaucoma.

‘Aqueous humor’ is a clear fluid that is constantly formed in the eye, bathes the inside and then drains out through the angle of the eye. Here it flows out through a spongy meshwork (trabecular meshwork) and leaves the eye. Eye pressure rises when this exit passage is blocked.

In ‘Open Angle Glaucoma’, the trabecular meshwork itself doesn't function properly blocking the exit of the fluid.

In ‘Angle Closure Glaucoma’ the angle of the eye itself is blocked and the fluid is unable to reach the trabecular meshwork for drainage.

In ‘Normal Tension Glaucoma’ there is no buildup of eye pressure and the fluid drains properly, but the nerve of the eye itself is too sensitive to even normal pressure and gets damaged

In ‘Congenital Glaucoma’ the trabecular meshwork is defective since birth and the child develops increase eye pressure soon after birth.

Not necessarily. A person has glaucoma only if the optic nerve is damaged. Increased eye pressure means there is an increased risk for glaucoma. An increased pressure without any damage to the optic nerve doesn’t mean glaucoma is present. And this scenario is called 'Ocular Hypertension'

Glaucoma is a very silent disease which allows it to go untreated. By the time a patient notices loss of vision, the eye damage is severe. This makes it essential to have regular screening to detect glaucoma at an early treatable stage. The loss of nerve fibres in glaucoma is irreversible and therefore early detection and treatment is essential. Loss of nerve fibres in the ‘Optic Nerve’ cause loss of ‘field’ of vision. The central vision remains clear till the last stage but there is a gradual loss of peripheral vision. In the later stages only a ‘Tunnel Vision’ may be left.

Some patients with ‘Angle Closure Glaucoma’ have a sudden episode of rise in eye pressure or an attack of glaucoma. In these cases, the eye becomes red and extremely painful. Nausea, vomiting, headache and blurred vision may also occur. These attacks usually occur in the evening time though it can occur at any time of the day.

Sometimes these attacks are preceded by smaller milder attacks which are associated with mild to moderate pain. The patient may also observe rainbow like coloured halos around lights. These attacks usually resolve without any treatment within a few hours and can be missed. Treatment at this stage is very easy and can prevent a full blown attack and need for lifelong medications later.

Everyone over 40 years of age is at risk for glaucoma

Other important risk factors for ‘Open Angle Glaucoma’ include family history of glaucoma, black race, high pressure in the eyes, diabetes, hypertension and near-sightedness (myopia).

‘Angle Closure Glaucoma’ is more common in patient of ‘Asian’ origin including Indians and in people with far sightedness (hypermetropes) or anybody with a narrow angle of the eye.

American Academy of Ophthalmology's recommended frequency of glaucoma screening is

• Age 20-29: People with family history of glaucoma or of African descent should have an eye examination every 3 - 5 . Others at least once during this period.
• Age 30-39: People with familiy history of glaucoma or of African descent should have screening every 2 -4 years. Others at least twice during this period.
• Age 40-64: All people should have a screening eye examination done every 2-4 years.
• Age 65 or older: All people should have a screening eye examination done every 1-2 years.

Screening for glaucoma is done by an ophthalmologist (eye doctor) and involves the following tests.

• Tonometry – This is to check pressure in the eye. Anybody with IOP (eye pressure) greater than 21 is considered at risk and needs more detailed testing. The most common instrument to measure IOP is ‘Applanation tonometer’. The eye is first numbed using anesthetic eye drops followed by placement of the sensor on the eye to measure the “eye pressure”. Another good screening tool is "Noncontact tonometer" which safer as it does not involve touching the eye. The machine just gives out a puff of air and takes the measurement.
  

  Non-contact Tonometry

• Pachymetry : Measurement of the eye pressure (IOP) can be affected by the thickness of the cornea (transparent front part of the eye). Pachymetry is the measurement of the corneal thickness and thus helps in calculating the "real " or “corrected IOP”.
• Gonioscopy: This test is done in patients with narrow angles who are at a higher risk of ‘Angle Closure Glaucoma’. The test involves placing a special type of thick contact lens on the eye which allows the doctor to view the interior angle and the drainage of the eye.
• Ophthalmoscopy is examination to evaluate the optic nerve (seen as the optic disc) at the back of the eye. Damage to the optic nerve is seen as increased cupping of the disc (a depression in the center of the disc). All eyes have this depression but it becomes larger in glaucoma patients. An eye with suspected disc changes (even with normal IOP) should be evaluated further
  

  Enlarged central depression (cupping)

If the initial screening process raises suspicion of glaucoma, further testing is required to confirm the diagnosis. Since glaucoma is such a silent disease, absence of symptoms is not a justification to avoid these.

Automated Perimetry

The purpose of these tests is to find evidence of damage to the optic nerve. The most important of these is visual field assessment or "perimetry". It maps the visual field to assesses for any functional damage of the nerve.

Another is OCT which uses laser beam to assess for any fine structural damage of the nerve.

The nerve damage and visual loss from glaucoma cannot be reversed. However further damage can be stopped and that is the main aim of glaucoma treatment. It involves reducing the IOP to a level which stops further nerve damage and visual loss.

Reduction of eye pressure can be achieved with eye drops, pills (rarely), laser, surgery or a combination of these.

Eye Drops – This the most common treatment modality. The drops work either by reducing the production of the 'aqueous' fluid or by increasing the drainage of the fluid out of the eye. In severe cases, pills may be given along with the eye drops for adequate control of IOP but these are only used as temporary measure in severe cases.

Laser – Various kinds of lasers have been used to treat glaucoma but the most important of these is the YAG laser used in ‘Angle Closure Glaucoma’, This is usually the first line of treatment in this type of glaucoma. The laser (YAG Laser) is used to create small holes (Perpiheral Iridotomy) in iris (the part of the eye which gives it the colour). The holes forms alternate channels for flow of fluid and prevents closure of the angle and rise of eye pressure. If done early enough, the laser can prevent an acute attack of glaucoma and need for lifelong medications and surgery.

Surgery – Most people believe that surgery in glaucoma is to "cure" the disease or to improve vision. Unfortunately that's not true. As mentioned previously, the vision loss due to glaucoma is irreversible. Glaucoma surgery is to reduce eye pressure in cases where medicines and lasers have failed. It involves creating an artificial drainage passage for the fluid to come out of the eye. The oldest and still the most common procedure is Trabeculectomy. 

Aqueous outflow made in 'Trabeculectomy' surgery

In some cases a valve or a tube is added to help keep the passage open and control the flow of fluid.

Cilioablative Therapy – This is usually the last desperate measure in severe cases of glaucoma. The most common reason for doing this procedure is to relieve the patient of pain in a blind eye. The treatment involves destroying a part of the 'ciliary body' - the structure which forms the aqueous fluid. This in turn reduces eye pressure. This is done using either laser or cryo (cold) therapy. The main disadvantage is the uncontrolled  nature of the procedure frequently reducing the eye pressure to very low levels.

The answer is "frequent" and "lifelong"

Treatment for glaucoma is lifelong and requires frequent monitoring. Checkups are planned every few months for adequate reduction of eye pressure. This is required even in patients who've undergone glaucoma surgery.

Apart from pressure, other tests (perimetry and OCT) are also repeated to check for ongoing optic nerve damage inspite of a low IOP.

Just an eye pressure less than 20 mmHg is not good enough. Every patient has a different target IOP which should be low enough to halt optic nerve damage. If the optic nerve damage continues, the target IOP is revised to a lower level and the treatment increased to achieve it.

Frequent checkups

The only way to prevent vision loss due to glaucoma is frequent checkups. Since the damage due to glaucoma is permanent, it is very important to detect the disease early.

YAG laser iridotomy (holes in iris) can also be used as a preventive treatment in patients with a tendency to develop ‘Angle Closure Glaucoma’.

Some drugs especially steroids can cause glaucoma as a side effect. Patients who are using these medications for any other disease should be frequently screened for glaucoma.