The Optic Nerve, also known as the ‘second cranial nerve’ is responsible for transmitting visual information from the eye (retina) through electrical impulses to the vision centers of the brain. It has 1.2 million nerve fibers which carry the messages. This nerve consists of the axons of retinal ganglion cell and supporting cells.
This nerve is responsible for giving us vision, colour vision, contrast sensitivity and side vision. Any damage to this nerve can lead to loss of vision. Glaucoma is one of the relatively common chronic eye condition affecting the optic nerve which preferentially damages side vision first. Loss of central vision occurs late in the disease.
What is Glaucoma and why does it happen?
Glaucoma is a name for a group of conditions in which optic nerve is damaged where it leaves the eye ie at the optic nerve head (see figure). This damage then leads to very specific type of defects when testing of side vision called visual fields is done.
Glaucoma is considered one of the leading causes of irreversible vision loss or blindness in the world. Often, it is caused due to the increased pressure in the eye, but not exclusively so. Glaucoma can also occur when eye pressure is normal.
The intraocular pressure can increase when the aqueous humour that is present in the anterior chamber (front part of the eye) fails to drain away properly.
The eye pressure deemed high for a susceptible optic nerve, causes slow damage to it. Firstly, the side vision is affected; in case glaucoma is untreated, the damage progresses to tunnel vision and eventually, it can lead to loss of central vision also.
Types of glaucoma
Based on the appearance of the drainage angle, when viewed with gonioscopy, glaucoma is divided into ‘open angle’ and ‘angle closure’ glaucoma (see figures below).
Primary Open angle Glaucoma or POAG and Primary angle closure glaucoma or PACG are ‘chronic’ in nature, ie they can take many months to years to develop.
However Angle closure disease has an ‘acute’ variety which can lead to loss of vision in hours, if help is not available immediately.
Although increased ocular pressure is the most common and important risk factor, some other risk factors are also known.
Glaucoma Risk Factors
Glaucoma can affect anyone, but certain groups of people are at higher risk. The glaucoma risk factors include:
- Age more than 35 years
- A family history of glaucoma
- High degree of myopia (short-sightedness)
- Eye Injury
- Usage of steroid medication, either systemically (injected, orally or inhaled) or in the form of eye drops or ointment.
Initial Hints and Precautions
Usually, Glaucoma does not show any early symptoms. It develops slowly over a period and it causes visual field loss or side vision loss first. Hence, many people fail to realise that they have Glaucoma. Majority of the people who have Glaucoma feel fine. Glaucoma patients do not find any change in their vision as their central vision is maintained until very late. By the time patients become fully aware of their vision loss, glaucoma would have usually advanced.
Rarely, some people experience vision haziness or see bright halos around the light, owing to the fluctuating levels of the intraocular pressure. However,this asymptomatic disease is often detected during a routine eye test. Hence, it is crucial to schedule regular eye examinations. During an eye examination, if glaucoma is picked up, then the eye doctor can prescribe some preventive treatment to protect the vision.
Loss of vision because of glaucoma is non-reversible with any treatment or even with surgery. Hence, it is best to visit an eye doctor or Ophthalmologist regularly as a precautionary measure.
Rarely, glaucoma can develop suddenly in an ‘acute’ fashion and cause:
- Intense pain in the eye
- Blurred vision, etc.
Clinical tests to Diagnose Glaucoma
Often before glaucoma shows any noticeable symptoms, it is detected during a routine eye test.
Some clinical tests that are specific to glaucoma include:
- Applanation Tonometry (measures the pressure in the eye)
- Gonioscopy (viewing the drainage angle with a special lens)
- Bio-microscopic fundoscopy (view the optic nerve head in 3-D with a special lens)
Investigations for glaucoma
If the Optometrist/ Ophthalmologist has a suspicion of glaucoma, then generally a perimetry (visual field test) is carried out to check for side vision. Perimetry is repeated annually, or sooner, in diagnosed cases.
If glaucoma is suspected to be in the early stages, imaging (scans) may also be carried out.
As glaucoma affects the nerve, any damage is permanent. Hence, it is not possible to reverse or cure any vision loss that occurred before the diagnosis of glaucoma. But treatment can help prevent further worsening.
Currently, glaucoma treatment is directed towards controlling of pressure in the eye. For this, eye drops are used for a lifetime; sometimes tablets are used for a short duration. In most of the chronic open-angle glaucoma cases, the eye pressure is controlled with the help of eye drops. Laser or surgery is scrutinised only when eye drops fail to give good results.
For all angle-closure glaucoma, laser is done first. With Laser Peripheral Iridotomy, a ‘hole’ is created in the iris of the eye (peripheral part of the coloured portion), to try and open up the drainage angle. This treatment is a minor procedure performed in OPD and patients recover very quickly. After this, it is treated like open-angle glaucoma.
Since surgery carries a higher risk of complications, Ophthalmologist reserves it as the last form of treatment.
Once the treatment is started, it has to become a part of daily routine for a lifetime. However, glaucoma can progress even on treatment. So, follow-up at regular intervals is also a necessary step in Glaucoma treatment. This way one can control the disease and help maintain whatever vision is left.