The eye works as a camera; it has lenses, a diaphragm and a film. The cornea e and the crystalline lens are the parts that function as focusing lenses. Between them we find the iris, the coloured part of the eye that varies from person to person. At the centre of the iris there is the pupil (diaphragm), which adjusts to admit more or less light. Thanks to the cornea and the crystalline lens (lenses), the image is sharply focused on the retina (film). The retina lines the posterior chamber of the eye, which is filled with a gelatinous substance known as “vitreous body”. Light passes through the cornea, pupil, crystalline lens and vitreous body and hits the retina, thus generating the visual stimuli. The visual stimuli are then transformed into electrical impulses and carried through the optic nerve to the brain, which interprets them by shaping the images. Any alteration affecting one or more of these parts results in a defective perception of the images.
What is the aqueous humour?
Intraocular pressure (IOP)
The eye behaves like a semielastic ball filled with constantly circulating fluids. If the amount of fluid produced does not equal the amount reabsorbed, there is a fluid surplus. Just like in a balloon that is too inflated, this causes an increase in the internal pressure. Thus, the internal pressure of the eye, or intraocular pressure, corresponds to the tension of the eyeball. The fluid flowing inside the eye, which is responsible for the intraocular pressure, is known as the “aqueous humour”.
Where does the aqueous humour drain to?
The aqueous humour is produced behind the iris and flows forward through the pupil; the drainage channels of the aqueous humour are just next to the internal angle between the cornea and the anterior face of the iris. Obviously, they are not visible with the naked eye but may be compared to a microscopic filter located right at the bottom of the angle, in a sort of drainpipe, beyond which the drainage channels direct the flow through the wall of the eyeball where the fluids are reabsorbed. The aqueous humour, in fact, has nothing to do with the formation of tears.
What is glaucoma?
Glaucoma is a disorder of the optic nerve, which is usually damaged by excessively high intraocular pressure. The optic nerve is similar to a transmission cable connecting the retina to the brain and is composed of a bundle of filaments known as “nerve fibres”: more than a million for each eye. The point where this cable connects to the rear part of the eye is known as the “papilla” or optic nerve head. The health of the papilla depends on the circulation of blood and nutrients. This is slowed down and inhibited when the ocular pressure becomes too high. Each fibre of the optic nerve carries a part of the visual message from the retina to the brain, where the images of our surroundings are formed. The “visual field” is the whole image that we can see without shifting our gaze.
Causes and effects
When, because of glaucoma, the fibres of the optic nerve become damaged, some areas known as “scotomas”, where it is no longer possible to see, appear within the visual field. Initially very small, the scotomas are noticed only when they become bigger and there is already extensive damage to the optic nerve. In the meantime, the patient will continue to see clearly at the centre. In the final phases the optic nerve is completely destroyed and blindness occurs. It is for this reason that it is essential to diagnose the glaucoma as early as possible.
How many different types of
glaucoma are there?
1) Chronic open-angle glaucoma
This type of glaucoma, also known as primary open-angle glaucoma, is the most common form of glaucoma and is associated with a kind of ageing of the drainage filter: the capacity to reabsorb the aqueous humour decreases with increasing age and the pressure within the eye gradually increases. We are talking about glaucoma when, because of the high pressure within the eye, the optic nerve becomes damaged and as a consequence the visual field is also damaged. More than 60% of glaucomas in adults are of this type. Unfortunately, this form of glaucoma may gradually and without any symptoms affect vision, to the point where it causes serious damage before being detected. In some cases the glaucomatous damage progresses in spite of the fact that the intraocular pressure remains within apparently normal limits. The treatment of these cases of “normal-pressure glaucoma” can be difficult.
2) Closed-angle Glaucoma
Closed-angle glaucoma is the result of an obstruction in the angle, which closes as if a sheet of paper floating in a sink were to rest on the plug-hole and block it. In the eye it is the iris that can act as such sheet of paper, which can be sucked against the cornea and completely block the passage of the aqueous humour towards the angle. This usually occurs gradually, but if this closure is sudden, we are dealing with an “acute attack” of glaucoma. The latter is accompanied by clear symptoms, which usually prompt the patient to see an ophthalmologist, with great urgency. These symptoms may include:
3) Secondary glaucoma
Secondary glaucoma results from specific causes, such as traumas of the eye, medications, intraocular tumours, inflammation and circulatory disorders. The ocular pressure increases because the drainage system of the fluids is damaged. In these cases the treatment can be very complicated
4) Congenital glaucoma
This is a rare form of glaucoma that is present at birth or early infancy. In these cases the drainage angle has a congenital abnormality. These cases, therefore, will require highly qualified treatment.
How is glaucoma diagnosed?
The most common type of glaucoma occurs in adulthood and is more common in the elderly. It is usually asymptomatic, therefore the diagnosis is often by chance and occurs during an eye examination carried out for some other reasons. For this type of glaucoma age is the most important risk factor, in addition to possible hereditary factors. It is a good idea, even in the absence of any symptom, to undergo a complete eye examination at least once after 40 years of age. Only a physician specialized in ophthalmology can reliably measure the ocular pressure, is trained and authorised to use the appropriate instruments and, above all, is capable of assessing the presence of other important diagnostic signs or identifying relevant risk factors. Not all the tests are essential at each examination and for each patient, although some should be repeated at regular intervals to identify early signs of glaucoma or to follow the evolution
How to recognise it
What is tonometry?
Tonometry is nothing more than the measurement of the ocular pressure, and may reliably be performed only by an ophthalmologist. After the eye is anaesthetised with a drop of local anaesthetic and a yellow–orange dye is applied, a small cone of plastic material attached to the “biomicroscope” is placed on the cornea. Illumination with a blue light optimises the measurement of the pressure. This is a quick, absolutely painless test that carries no risks. The other instruments for measuring the ocular pressure are the indentation tonometer, the pneumotonometer and the noncontact (air puff) tonometer, but they are usually less precise.
What is gonioscopy?
Gonioscopy is the examination of the drainage angle of the eye. After administering a drop of local anaesthetic, a special contact lens is applied to the eye. This technique, which may be performed only by an ophthalmologist, is completely painless and gathers important information on both the anatomy of the anterior part of the eye being examined and the type of glaucoma that might be present. Lenses of the same type are used to highlight the parts that are likely to be subjected to the laser treatment.
What is ophthalmoscopy?
Ophthalmoscopy i s the examination of the posterior structures of the eye. It is performed with an instrument that shines a light to illuminate the inside of the eye. Thanks to a system of lenses it is thus possible to directly observe the retina and the point where the optic nerve connect to the eyeball, known as “papilla” or “optic nerve head”. In the presence of glaucoma, the damaged optic nerve head appears flattened by the excessive pressure, turns pale and shows different types of alterations, depending on the individual.
What is perimetry?
Perimetry is the technique that examines the visual field. It uses both computerised and manual instruments and is capable of assessing the sensitivity of each eye to homogeneous light stimuli projected inside a special dome. The test usually maps out the boundaries of the field of vision where the glaucomatous defects called scotomas are represented by darker than normal areas, or by numerical values that the specialist is then capable of interpreting.
How can glaucoma be treated?
People suffering from glaucoma must undergo specialised regular examinations throughout their entire life. This is because glaucoma is a complex disease which requires more than just monitoring the ocular pressure. The same pressure value may produce different effects in different patients, consequently each person must be assessed individually. Once the damage has occurred it is usually irreversible. The purpose of treatment is to prevent further worsening and it is based on the use of medication, laser treatments and surgery.
Medication is usually administered locally as eye-drops or gel. These substances are aimed at reducing the production of aqueous humour or improving its drainage. They are to be taken regularly and continually, and should not be interrupted without the doctor’s permission. Some may have unpleasant side effects: some of them may cause burning or redness of the eye, others may temporarily cloud the eyesight and cause headache. Some can also interfere with the cardio-respiratory activity, and it is therefore essential to inform the eye specialist/ophthalmologist about any personal health problems and any other therapy one may be undergoing. When eye-drops alone or even a combination of them are not sufficient, tablets may be prescribed in addition. These may also cause side effects, such as tingling fingers, gastrointestinal problems and appetite loss. In people who are subject to kidney stones oral medications may cause a recurrence. This explains why tablets are not widely used.
When medication is not enough
When the use of eye-drops and/or tablets is not sufficiently effective or poorly tolerated, one can consider laser treatments, which vary depending on the type of glaucoma. In the case of “closed-angle glaucoma” the treatment used is Yag Laser Iridotomy. The procedure used for “chronic open-angle glaucoma” is Laser Trabeculoplasty. Laser treatment may also be used as a first choice approach instead of the pharmacological treatment.
How is the treatment
After an eye-drop of local anaesthetic is applied to the eye, the treatment is performed by placing on the patient’s eye a special contact lens that enables the specialist to focus the laser beam. The procedure is usually painless even though at times there is some discomfort (needle prick sensation) and the patient can be bothered by the flashes of light. The treatment takes only a few minutes.
The Yag laser iridotomy consists in making a tiny hole in the iris thus allowing the chambers of the eye in front and behind the iris to connect. This procedure favours the forward circulation of the ocular fluids towards the filter and can prevent the sudden closure of the drainage angle. The iridotomy is used as treatment for closed-angle glaucoma and for prophylaxis in patients at risk of the onset of an acute attack. It is also used as preventive treatment for the “other” eye of someone who has had an acute glaucoma attack in one eye. It may be useful to avoid or postpone the surgical treatment.
WHAT IS A LASER?
A laser is a beam of homogeneous and powerful light. Different types of laser lights are used in ophthalmology in order to take advantage of the peculiarities of each beam. The effect of the laser radiation on the target can be tissue evaporation, incision or heating.
Laser trabeculoplasty improves the functioning of the drainage filter of the aqueous humour by widening its pores and thus reducing the ocular pressure (Fig. 1 and 2). The actual target of the treatment is the filter of the drainage angle, located between the iris and the cornea, which fails to work correctly in patients with openangle glaucoma. Since this filter is known as “trabeculum”, this laser technique is named “trabeculoplasty”.
n most cases a decrease in ocular pressure is obtained. Since the amount of this decrease depends upon many factors, it is impossible to predict either the final effect or whether it will be sufficient to prevent further damage by the glaucoma, and for how long. For successfully treated cases this technique enables the postponement of surgical treatment for a long time and it may enable the reduction of pharmacological treatments, even though in most patients the use of anti-glaucoma eyedrops is still needed. Laser treatment is applied as an “additional option” for controlling a chronic disease lasting one’s entire life. For this reason, even when a remarkable reduction in pressure with the laser treatment is obtained, it is important to continue to undergo examinations as planned.
Before it is too late
Why and when glaucoma surgery
henever incisional surgical intervention becomes necessary to keep the glaucoma under control, the eye specialist/ophthalmologist has to chisel a new drainage channel to allow the aqueous humour to escape from the internal part of the eye. The surgical intervention is recommended to avoid damage to the optic nerve, damage that would otherwise inevitably progress. Surgical treatment is, therefore, indicated when either the laser treatment or medication is not effective in lowering the intraocular pressure. The complications from modern anti-glaucoma surgery are fortunately rare. It is important to remember that there is no “ideal” pressure value in an absolute sense and all the efforts made are always aimed at preventing worsening of the damage. Surgical treatment may have specific uses in different patients with the same intraocular pressure but with different degrees of severity of their disease.
How does glaucoma surgery
The objective of the surgical treatment is to create an efficient drainage channel that is an alternative to the “diseased” ones, thus linking the space between the iris and the cornea and the one situated under the membrane, called the conjunctiva, lining the eyeball. An operation of this type is known as trabeculectomy or “filtering operation”.
This operation is performed under local anaesthetic and can be carried out at day hospital. It consists in the removal of a tiny block of tissue from the eye wall. This creates a channel that allows the fluid to drain from the drainage angle, not directly to the outside but to a space within the ocular wall. The effective functioning of the valve made by the surgeon results in the formation of a swelling like a vescicle, called by doctors “filtering bleb”, where the aqueous humour accumulates under the conjunctiva before spreading backwards. Sometimes, because of the natural scarring, the valve may close and the ocular pressure may again increase. It is for this reason that the use of substances capable of reducing the scarring activity, known as “antimetabolites”, has been introduced. These substances are applied during the operation or during post-operative examinations to maintain effective filtration.
During glaucoma surgery the most relevant complication i s haemor rhage. On the other hand, during the postoperative period the following problems may occur:
These events are not very frequent and can usually be resolved. However, the eye specialist will recommend the surgical treatment only when all other treatments are not effective or well tolerated.
Other surgical operations
In certain cases an operation may be necessary to insert a small plastic valve. The function of these “draining implants” is to allow the escape of the aqueous humour through the wall of the eyeball by spreading i t ba ckwa rds , unde r the conjunctiva, thus lowering the ocular pressure. In other patients where a drainage operation is not practicable, it is possible to attempt to reduce the formation of the aqueous humour by selectively damaging parts of the “ciliary body”, i.e. the structure behind the iris that produces it. Operations of this type may be carried out using a freezing probe or a laser beam.
lthough most treatments effectively lower the ocular pressure, there is no definitive cure for glaucoma and it is not possible to restore the visual function already damaged by the disease. The treatment can prevent further worsening, especially if carried out in the early phases of the disease.
Questions & Answers
Ten suggestions to follow
1 - Is it necessary to get up at night to carry out the treatment?
Not usually. It is best to organise the times of treatment in such a way that you can rest quietly.
2 - How long and where should you keep eye-drops from the time they are opened?
Usually for a month, preferably in a cool dark place. If the eye-drops change colour then they must be discarded.
3 - What happens if you forget to administer the eye-drops?
If there is a long delay the ocular pressure increases, it is therefore best to stick to set times even though, occasionally, a variation is allowed.
4 - Should you apply eye-drops before the eye specialist examination too?
Unless there are precise indications from the doctor, the treatment should continue as normal.
5 - If two types of eye-drops have to be administered at the same time, can you apply them together?
It is necessary to wait a few minutes between the first and the second eye-drops.
6 - If you are not sure that you have applied the first eye-drop correctly, is it dangerous to apply a second one?
No, although it is best to dry the eye immediately, to prevent the extra eye-drop from flowing into the tear duct.
7 - Is it dangerous to use the eye-drops more frequently than prescribed?
It is not dangerous, but it has no positive effects on the progression of the disease, and it could lead to undesired effects.
8 - Why are some patients treated with both eye-drops and tablets?
In some cases it is necessary to take both types of medication to strengthen the therapy.
9 - Is it possible to use other medication in addition to glaucoma medication?
It is very important to tell the family doctor about the diagnosis of glaucoma and the medications being used. For instance, you should be careful when taking some tranquillisers, anti-asthmatics or medication for the digestive system because they could have an effect on the ocular pressure.
10 - How long does it take for the eyedrops to be absorbed?
A few minutes, after that it is O.K. to have a shower, a bath or go swimming.
Glaucoma, the silent theft of sight