WHAT IS GLAUCOMA?
Glaucoma is an optical condition, in which the optic nerve (nerve of the eye) gets damaged because of increase in eye pressure(IOP-intra ocular pressure). High pressures are caused by accumulation of excess fluid in the eye. Early detection, accurate diagnosis and timely treatment are imperative in improving or preserving vision.
The eye can be likened to a basin where the faucet is ever running and the drain is open always. The fluid called aqueous humour constantly circulates throughout the anterior chamber of the eye. The aqueous humour is produced by a small gland called the ciliary body, which is located at the rear of the iris. It flows between the lens and the iris and after nourishing the lens and the cornea, seeps out through a sponge-like tissue termed the trabecular meshwork. Its width is just one-fiftieth of an inch and works like the drain of the eye.
Nature has placed the trabecular meshwork at the angle where the cornea and iris meet. If the meshwork gets clogged, the aqueous humour cannot drain out of the eye in its normal course and builds up within. Be the ‘sink’ that is the eye is a closed chamber and the ‘water’ cannot overflow. So, the built-up fluid increases pressure in the eye. This is called wide or open angle glaucoma. If excessive air is blown into a balloon, it bursts, but the eye is too strong to burst. The pressure instead, damages the weakest point, a site in the sclera where the optic nerve exits the eye.
The optic nerve carries visual information to the brain. Though it is made up one million nerve cells, it is extremely small in diameter-roughly twenty-thousandth of an inch. When the fluid builds up the pressure, the cells of the optic nerve are compressed. It can result in the death of the nerve cells, which leads to permanent blindness. It cannot be overemphasised that early diagnosis and timely medical intervention can prevent this. The doctor measures the IOP to check for glaucoma.
A tonometer measures eye pressure. In a procedure known as applanation tonometry, local anaesthesia is administered with eye drops. Then the patient sits at a slit-lamp where a plastic prism is gently pushed against the eye to measure IOP. Air tonometry is an investigation, in which a puff of air is directed onto the cornea to measure IOP. No anaesthesia is required here, as the instrument does not come into eye contact.
A visual field test tells your eye doctor if and to what extent glaucoma has affected your range of vision. The visual field test is an important indicator of the extent of damage IOP has inflicted on the optic nerve. There are various field tests a patient may be subjected to. In a computerised visual field test, the patient’s chin is placed on a stand attached to a computerised screen. Whenever the patient sees a flash of light, she/he is asked to press a button and the doctor takes a printout of your field vision. The Goldmann perimeter test is similar, but no computer is used. The examiner notes down whenever the patient indicates that the light is visible
The examiner can look directly at the optic nerve through the pupil with an ophthalmoscope. The appearance and colour of the optic nerve shows whether damage is there and to what extent.
Gonioscopy is the process of examining the intraocular fluid outflow drainage angle. Eye constantly produces fluid and there is a drainage angle for it to flow out. This test can be used to determine whether the High eye pressure is caused due to closed/blocked angle (angle closure glaucoma) or the angle is open or not functioning well(open angle glaucoma). This is highly important as the management each sub-type is slightly different. In this test, a mirrored lens is placed on the surface of the eye after instilling local anaesthesia containing eye drops.
Diagnosis of glaucoma would involve medication and vision possible surgical intervention to preserve vision and quality of like.
Educating the patient involves:
- To understand what the disease is.
- The importance of the treatment.
- The risk to first degree relatives.
- Optical Coherence Tomography (OCT) is used to measures the retinal nerve fiber layer thickness in glaucoma which is used to the detect damage.
- Heildelberg Retinal Tomography (HRT) is used to observe the changes in the structure of the optic nerve and used to detect early progression of glaucoma
- Diagnosing Glaucoma with imaging (GDx) enables to take exact measurement of retinal fibers which would help in the diagnosis of glaucoma.
How the Eye Works
To properly understand glaucoma, one needs to know a few elementary facts about the eye and its functioning. This can help you better understand your condition and can take better decisions at the time of consulting and choosing treatment options. Your eye is a camera that captures data about the shape, colour and movements of physical objects and relays that information to the brain, which then processes that information into the images we see. Now let’s look at the different structures of the optical apparatus.
The outer layer of the eye is the sclera. It is a thin but tough protective leathery shell. It is what we call the ‘white of the eye’. The outer layer of the sclera is the cornea, a transparent tissue through which rays of light enter the eye. It can be likened to the lens of a camera and enables the eye to focus on the light.
The iris is the colored part of the eye. It determines the colour of the eyes and functions as like a camera’s diaphragm. The iris has muscles that control the size of the pupil, which in turn regulates the amount of light that enters the eye. The pupil is the dark-coloured disc in the centre of the iris. It opens and closes according to the intensity of the light. If you are out on a bright, sunny day, when there is plenty of sunlight, the iris constricts the pupil. This limits the amount of light that can pass through the pupil to the retina at the back of the eye. When there is less light, the pupil is widened so that more light can pass to the retina. The retina is like the film of a camera; the lens behind the iris adjusts its thickness and shape to focus the image on the retina, which delivers the image to the brain via the optic nerve.