Glaucoma: causes, diagnosis and course
Glaucoma is a collective term for various eye diseases that damage the optic nerve and retina. Glaucoma usually does not appear until after the age of 40, the frequency increases with increasing age. However, glaucoma can also be congenital. If left untreated, glaucoma leads to blindness – it is therefore particularly important to recognise the warning signs of the disease at an early stage. Here you can find out everything important about glaucoma.
Glaucoma is a group of eye diseases that, in an advanced stage, damage the nerve cells of the light-sensitive retina and the optic nerve. So the name of the disease has nothing to do with the bird world. The old term “glaucoma” describes on the one hand the frequently observed (blue-)greenish shimmering of the iris in advanced glaucoma, and on the other hand the “stare” when the eye is blind.
This is how glaucoma develops
In the majority of cases glaucoma is accompanied by increased pressure in the eyeball. This occurs when more aqueous humor is produced in the anterior chamber of the eye, where the lens is located, than can be drained off via the drainage system in the chamber angle. The constant exchange of aqueous humor is important for the function of the eye. The aqueous humour carries nutrients and oxygen to the lens and cornea, which do not have their own blood vessels. The aqueous humor also serves as an optical medium. If it accumulates in the anterior chamber of the eye, the pressure in the eye increases.
The intraocular pressure is measured with a so-called applanation tonometer. The device determines how much pressure is required to deform a specific area of the eye’s curved cornea (the clear layer of skin in front of the pupil). The higher the intraocular pressure, the greater the force that has to be applied. The unit of measurement for the pressure in the eyeball is “millimetres of mercury” (mmHg), the same unit that is also used for blood pressure, for example. Normal values for intraocular pressure are between 10 and 21 mmHg and can fluctuate by about five mmHg during the course of the day – the highest values occur at night and in the early morning hours. In glaucoma, the pressure in the eyeball can rise to values of 40 to 60 mmHg.
Due to the increased pressure, the blood supply and nutrition of the sensitive nerve cells are disturbed in glaucoma. Visual disturbances, so-called visual field failures, are therefore among the typical signs of glaucoma. If glaucoma goes unnoticed or is not treated sufficiently medically, the affected person may even lose his or her vision completely.
Glaucoma can lead to blindness
Glaucoma is one of the most common causes of blindness. In the industrial nations, glaucoma is the third leading cause of blindness.
However, the diagnosis is only known in about two-thirds of cases – in many cases the affected persons know nothing about the disease. If the affected person perceives the visual disturbances himself, the damage to the retina and/or optic nerve is often already far advanced. If damage has already been caused by glaucoma, it is usually irreversible.
Glaucoma is more common with increasing age. Beyond the age of 75, seven to eight percent of people are affected, and after the age of 80 glaucoma even affects between 10 and 15 percent.
Four basic forms of glaucoma
Glaucoma can occur in different forms. Ophthalmologists divide the variants into four large groups according to anatomy and glaucoma causes:
Primary open angle glaucoma (sometimes called “wide angle glaucoma”) occurs when the outflow of aqueous humor is disturbed by deposits (plaques). Open-angle glaucoma is the most common form of glaucoma in older people. Normal pressure glaucoma is a subtype of open angle glaucoma, in which, however, the outflow of aqueous humor is not disturbed and thus the pressure in the eyeball is not pathologically increased.
In narrow-angle glaucoma, the anterior chamber of the affected eye is so flat that the iris of the iris narrows or even blocks the angle of the chamber. This happens especially when the pupil is dilated by darkness or by the effect of medication (or drugs) and the iris “unfolds” in the chamber angle. The outflow of the aqueous humor is thus hindered or even completely prevented (angle block glaucoma). If this flow disorder occurs in attacks, it manifests itself as a glaucoma attack – an ophthalmological emergency. The pressure in the eye can increase so much that the retina and nerves are immediately and permanently damaged.
If glaucoma occurs in a newborn or infant, it is usually a primary congenital glaucoma – an abnormal development of the anterior chamber angle, which therefore cannot adequately drain the aqueous humor formed in the eye.
Secondary (“acquired”) glaucoma is called a glaucoma when other diseases, inflammations or injuries are responsible for the outflow disturbance of the aqueous humor. For example, altered blood vessels, scarring or inflammatory cells can partially or ultimately completely block the chamber angle.
You can read everything important about the typical signs of glaucoma in the article Glaucoma symptoms.
Glaucoma: causes and risk factors
Glaucoma means that the visual cells of the retina and/or the head of the optic nerve located in the eyeball have been damaged. The damage is caused by a disturbed blood supply to the sensitive cells, which leads to a lack of oxygen and nutrients.
However, the decisive factor for the development of glaucoma is not the absolute pressure inside the eye, but the difference between intraocular pressure and the pressure in the blood vessels of the retina and the optic nerve, the so-called perfusion pressure. If the pressure inside the eye rises close to, or even exceeds, the perfusion pressure, the fine blood vessels are literally pinched off – blood can no longer flow.
Recent research has shown that only about half of people with glaucoma actually have abnormally high intraocular pressure. In about 50 percent of those affected, the intraocular pressure is thus below the critical limit of 25 mmHg. However, the blood circulation in these patients is already disturbed. Reasons for the imbalance between intraocular pressure and perfusion pressure are therefore not always obstacles to the outflow of aqueous humor, but possibly also changes in the blood vessels or disturbances of the general circulatory function.
The most important causes of glaucoma at a glance:
- Deposits, so-called plaques, which prevent the outflow of aqueous humor into the fine network of the connective tissue balls (trabecula) and the “Schlemm’s canal” in the chamber angle (primary open angle glaucoma). The most important “risk factor” for this is old age.
- In normal pressure glaucoma, the perfusion pressure in the blood vessels of the retina is not sufficient to overcome the actually normal pressure inside the eye. This creates a supply bottleneck in the visual and nerve cells. Triggers or risk factors:
- Cardiovascular diseases such as coronary heart disease (CHD), cardiac insufficiency, arteriosclerosis or peripheral arterial occlusive disease (pAVK) of the carotid or cerebral arteries
- generally low blood pressure or a very low second blood pressure value (diastolic blood pressure), as can occur, for example, with heart valve defects or certain disturbances of vascular function (especially if there is an additional drop in blood pressure during the night)
- chronically high blood pressure (hypertension), which damages the blood vessel wall
- chronically elevated blood fat levels (hypercholesterolemia), which leads to deposits in the blood vessels (arteriosclerosis)
- Diabetes mellitus and other metabolic diseases that alter the inner wall of blood vessels and impede blood flow
- Autoimmune diseases involving blood vessels
- Smoking, because nicotine constricts the blood vessels – including those in the eye. Tobacco consumption is also considered an important risk factor for arteriosclerosis
- (temporary) convulsive narrowing of the blood vessels in Raynaud’s syndrome, migraine or tinnitus
- severe inflammation of the eye, which can lead to scarring or deposits in the chamber angle (inflammatory cells, protein) (secondary glaucoma)
- long-term cortisone treatment
- Drugs that dilate the pupil, because if the eye chamber is already flat, the iris can block the angle of the chamber (narrow-angle glaucoma)
- Severe myopia or hyperopia beyond four dioptres in which the shape of the eyeball and the anterior chamber of the eye is altered
- familial clustering: cases of glaucoma in the family are considered one of the most important risk factors
- black skin colour: people with dark skin have a multiple increased risk of disease
Glaucoma: examinations and diagnosis
Glaucoma is damage to the visual sense cells in the eye or to the optic nerve head located at the back of the eye. It occurs when the blood flow to the cells is restricted or collapses completely because the pressure difference between blood vessels and the eyeball is not large enough. Therefore, examinations of the blood supply and pressure conditions in the eye are among the most important diagnostic measures in glaucoma. In addition, the condition of the retina and optic nerve head and the drainage conditions for the aqueous humor are important for the diagnosis of glaucoma. The most important glaucoma examinations:
Measurement of intraocular pressure (tonometry)
The pressure in the eyeball can be quickly measured with the so-called applanation tonometer. The measuring plate of the device presses on the cornea of the eye (in the area of the pupil) from the front and determines the pressure required to deform a defined area (applanation = flattening, flattening; tonus = tension, pressure). As the cornea of the eye is very sensitive to touch, it is anaesthetised with a local anaesthetic for the examination. The unit of measurement for intraocular pressure is “mmHg” (millimetres of mercury), the upper limit for normal pressure in the eye is 21 mmHg. In the case of glaucoma, the values for the majority of those affected are above this value, in extreme cases (glaucoma attack) sometimes even more than twice as high. In older people, the pressure in the eye is often higher, without this necessarily meaning that glaucoma is present. The measurement result is also influenced by the thickness of the cornea, which should therefore be determined by a further examination.
Slit lamp examination
The slit lamp is probably the most important diagnostic tool of the ophthalmologist. With it he can direct a sharply limited beam of light to the eye. Depending on the bundling and direction of the light beam, different structures become visible or are particularly emphasized. Using a microscope, even the most subtle changes can be detected – for example in the cornea, the anterior chamber of the eye and the chamber angle, the eye lens, but also the retina. The slit-lamp examination takes place in a darkened room and is completely painless for the patient. If glaucoma is suspected, the ophthalmologist evaluates in particular the space available at the entrance to the chamber angle and the depth of the anterior chamber of the eye. He also pays attention to changes in the iris or unusual pigmentation of the cornea.
Examination of the chamber angle
If the suspected diagnosis is glaucoma, the chamber angle is examined with the so-called gonioscopy. It is normally not visible from the outside because of its peripheral position in the anterior eyeball. With a special lens, the ophthalmologist can “see around corners” so to speak. He places the gonioscope directly on the cornea, which has previously been locally anaesthetised. A flat chamber angle (narrow-angle glaucoma), discharge blockages through the iris as well as possible age-related plaques (for example in open-angle glaucoma), adhesions and discolorations can indicate glaucoma.
Visual field measurement
An important examination to detect already existing retinal or nerve damage is the visual field measurement (perimetry). During this examination, the patient is presented successively with optical stimuli at different locations in the room without being allowed to look directly at them. The perception of these stimuli, their location and strength are recorded. The outline and possible failures in the visual field allow conclusions to be drawn about possible disturbances of the visual cells or nerve tracts. However, glaucoma is not the only cause of a restricted field of vision. In addition, visual field failures (scotomas) in glaucoma usually occur late, when more than 30 percent of the nerve fibers are damaged.
With a reflection of the back of the eye (ophthalmoscopy), the ophthalmologist can assess the condition of the retina, its blood vessels and the optic nerve head. The ophthalmoscope is a mixture of magnifying glass and light source. To enable the doctor to view as large a section of the back of the eye as possible, the patient receives special eye drops shortly before the examination, which dilate the pupil. Ophthalmoscopy is particularly informative for the diagnosis of glaucoma, because with this examination glaucoma damage and the stage of the disease are directly visible.
Measurement of blood flow
Various examinations can determine the blood flow to the retina and the optic nerve. Commonly used methods are fluorescence angiography (X-ray contrast examination of the blood vessels in the eye), thermography (recording the heat emission of the eyeball as a measure of blood flow) or capillary microscopy (observation of the finest blood vessels of the retina in magnification). Since the relationship between intraocular pressure and the pressure in the blood vessels of the eye is not correct in glaucoma, a blood pressure measurement is also part of the routine examinations.
For people with pre-existing glaucoma or known risk factors for glaucoma (e.g. narrow chamber angle, age over 40 years, glaucoma in the family, black skin, diabetes mellitus, Raynaud’s syndrome or long-term use of certain medications), regular check-ups by the ophthalmologist are particularly important. This is the only way for the ophthalmologist to detect impending eye damage early on and initiate appropriate treatment in good time.
You can read about how glaucoma is treated in the article Glaucoma treatment.
Glaucoma: course of disease and prognosis
Without treatment, glaucoma leads to blindness because it causes increasing damage to the visual cells of the retina and optic nerves. The disease accelerates the longer it exists. It is important to know that once damage has occurred, it cannot be reversed.
This makes it all the more important to detect glaucoma at an early stage, to avoid risk factors and to consistently continue an initiated treatment. The good news is that glaucoma can be stopped and vision maintained with suitable medication or surgery. Often, medication alone can sufficiently reduce the intraocular pressure in glaucoma.