What is glaucoma?
Glaucoma is the term used to refer to a group of conditions that are characterised by specific damage to the optic nerve. The optic nerve is essentially what connects the eye to the brain. It transmits visual information in the form of nerve impulses all the way from the retina (i.e. the light-sensitive layer at the back of the eye) to the primary visual (striate) cortex located in the occipital lobe (i.e. the back) of the brain where it is then processed and interpreted.
When the optic nerve becomes damaged in glaucoma it leads to a progressive loss of nerve fibres within the retina. This in turn leads to a progressive loss of the visual field (i.e. peripheral vision) and if left untreated will result in blindness.
What causes the optic nerve damage?
The main cause of the optic nerve damage that occurs in glaucoma is an increased intraocular pressure (IOP). This is essentially the fluid pressure inside the eye and it is determined by the balance between the production and drainage of this said fluid. The fluid in question is referred to as the aqueous [humour] and it is mainly found within the anterior (i.e. front) chamber of the eye. It is therefore not connected to our tears in any way.
The easiest way to think about this is to imagine that there is a ‘tap’ inside the eye. This ‘tap’ is essentially a group of cells located just behind the iris (i.e. the coloured part of the eye) and is responsible for producing the aqueous. At the same time, there is a ‘drain’ through which the aqueous can exit the eye. This ‘drain’ is located within the anterior chamber, in the angle that is naturally formed between the cornea (i.e. the transparent front surface or ‘window’ of the eye) and the iris. Therefore, any increase in the pressure of the aqueous will be caused by an increase in production from the ‘tap’ and/or a decrease in outflow through the ‘drain’.
How does the increased fluid pressure damage the optic nerve?
The mechanism by which the increased fluid pressure damages the optic nerve is still unclear but several theories have been put forward. For example, it has been suggested that the increased fluid pressure compromises the blood supply to the optic nerve and it is the reduced blood flow that results in the damage and subsequent loss of the nerve fibres. Alternatively, some believe that it is the increased fluid pressure itself that directly damages the nerve fibres as they enter into the optic nerve.
However, regardless of the mechanism, it is a known fact that reducing the fluid pressure in these circumstances delays the progression of the optic nerve damage and consequently delays any further vision loss from occurring.
Is there just one type of glaucoma?
As it happens, there are many different types of glaucoma and also many conditions that are related to glaucoma in some way. Examples include:
- Primary Open-Angle Glaucoma (POAG)
This can also be referred to as Chronic Simple Glaucoma (CSG) and is the most common type of glaucoma. It is characterised by a higher fluid pressure inside the eye, an open ‘drain’ and glaucomatous damage (i.e. optic nerve damage and visual field loss).
- Primary Angle-Closure Glaucoma (PACG)
This refers to glaucoma caused by primary angle closure (i.e. when the ‘drain’ becomes blocked and the aqueous is unable to exit the eye). Sometimes this can present as a sight-threatening emergency, whereby the ‘drain’ suddenly becomes completely blocked. This is referred to as Acute Congestive Angle Closure or, more simply, Acute Glaucoma.
- Normal-Tension Glaucoma (NTG)
This can also be referred to as Low-Tension Glaucoma (LTG) and is a variant of POAG/CSG. It is when the fluid pressure inside the eye is normal but there is evidence of glaucomatous damage (i.e. optic nerve damage and visual field loss).
- Ocular Hypertension (OHT)
This is when there is a higher fluid pressure inside the eye but no signs of any glaucomatous damage (i.e. no optic nerve damage and no visual field loss). Anyone with OHT is considered to be a glaucoma suspect and therefore at an increased risk of developing glaucoma themselves.
- Secondary Glaucoma
This is when the glaucoma occurs secondary to (i.e. as a result of) some other condition or cause, examples of which include inflammation (i.e. Inflammatory Glaucoma), cataract (i.e. Lens-related Glaucoma) or trauma (i.e. Traumatic Glaucoma).
- Primary Congenital Glaucoma (PCG)
This is a very rare form of glaucoma that occurs early on in life and sometimes even in utero.
What are the risk factors for POAG/CSG?
There are several risk factors for the development of POAG/CSG and some examples of these are listed below:
POAG/CSG is more common with advancing age, affecting 1% of people over the age of 40 years and 5% of people over the age of 65 years. It is uncommon for it to occur before the age of 40 years.
People of African descent are at an increased risk of developing this type of glaucoma.
- Family history
There is a genetic component to this type of glaucoma and generally it is the first-degree relatives (siblings more so than offspring) who are at an increased risk of developing it. This is why it is important that all people over the age of 40 years with a first-degree relative who has/had glaucoma have their eyes checked at least once every 12 months in order to detect any changes as early as possible (should they ever occur). The NHS recognises the importance of this, hence why they will cover the cost of the sight test for these people.
People who are myopic (i.e. short-sighted) are considered to be at greater risk of developing this type of glaucoma.
How is POAG/CSG detected?
There are several checks that can be carried out in order to help us look for any signs of POAG/CSG and all of these are generally included in a routine sight test anyway. Similarly, we can also pick up on any features that may suggest an increased risk or predispose a person to developing POAG/CSG or other types of glaucoma later on.
These checks would include:
- Measuring the fluid pressure inside the eye
This is achieved by using a device called a tonometer. Generally we tend to use a non-contact tonometer (i.e. where it does not involve touching the eye) and the most common type of non-contact tonometer that people will be familiar with is the infamous ‘puff of air’. However, sometimes it is necessary to measure the fluid pressure using a contact tonometer (i.e. where it does involve touching the eye). This involves instilling a drop of local anaesthetic to numb the cornea and then a probe is placed momentarily onto the numbed cornea in order to take the measurement.
- Assessing the visual field
This involves looking into a machine and responding to a series of dots being presented at random points in your vision.
- Examining the optic nerve
When we look inside the eye to assess the health of the retina, we can see where the optic nerve enters into the eye. This is referred to as the optic nerve head or optic disc and by looking carefully at this area and comparing between the two eyes we can determine whether there is anything suspicious about them.
Can POAG/CSG be treated?
The simple answer is yes and it is important to stress that POAG/CSG is a very manageable condition once diagnosed. Any treatment of the condition is with a view to reducing the fluid pressure inside the eye. This is on the basis that if the fluid pressure is reduced then this will in turn delay any further progression of the glaucomatous damage and thus reduce the risk of any further loss of vision.
In most cases, POAG/CSG is treated by means of eye drops that have been specifically formulated to reduce the fluid pressure inside the eye. There are many different types of these eye drops and they all act in different ways to have the desired effect. Sometimes if a certain type of eye drop is not satisfactory then another may be substituted in or a combined preparation may be necessary. This is why people who have been diagnosed with POAG/CSG will be monitored regularly at the hospital eye department in order to ensure that the eye drops are having and maintaining the desired effect.
If the eye drops are not working for whatever reason then a surgical procedure may be indicated in order to significantly reduce and stabilise the fluid pressure in the long-term.
What happens if POAG/CSG is left untreated?
The key point about POAG/CSG is that generally it has no symptoms. There is no pain associated with it and similarly vision can seem to be very much the same as normal. However, silently the vision is slowly deteriorating and all the while the person in question is completely unaware of it. This is because POAG/CSG typically affects peripheral vision first and so is not noticed by most. It is only when peripheral vision has become significantly compromised that people may then realise that something is not right (e.g. they may start bumping into things more so than usual or be slower to see oncoming vehicles when driving).
People do not generally come in with symptoms of POAG/CSG and so it is not a condition that can be diagnosed from symptoms. It can only be diagnosed by measuring the fluid pressure inside the eye, assessing the visual field, examining the optic nerve and by looking for the signs. This is why it is so important that people have their eyes checked regularly because the sooner it is diagnosed the sooner it can be treated and therefore the better the visual prognosis will be.
What is acute glaucoma?
Acute glaucoma, also known as acute congestive angle closure, is a form of glaucoma where the fluid pressure inside the eye shoots up very suddenly. It comes about as a result of a sudden and complete blockage of the ‘drain’ located within the anterior chamber of the eye, in the angle that is naturally formed between the cornea and the iris. This prevents the aqueous from being able to exit the eye and so the fluid pressure increases rapidly. It is a sight-threatening emergency.
What are the symptoms of acute glaucoma?
Acute glaucoma typically only occurs in one eye at a time and the symptoms associated with it are as follows:
- Severe pain
This occurs in and around the eye and may wake the person up at night because it is so severe.
- Reduced vision
The person may report seeing haloes around bright lights.
- Nausea and vomiting
This may only occur in severe cases.
The affected eye will be red and its pupil tends to be vertically oval, unresponsive and fixed in a semi-dilated position.
How is acute glaucoma treated?
Acute glaucoma is initially treated with strong medication in order to rapidly reduce the elevated fluid pressure inside the eye. Subsequently, depending on the nature of the attack it may be that eye drops, laser treatment and/or surgery are indicated to further reduce and stabilise the fluid pressure. The same management, although usually it is just the laser treatment, applies to the unaffected eye in order to prevent a similar attack from occurring in the future.
If you have any queries or concerns regarding anything glaucoma-related then please do not hesitate to contact us.