Short Sightedness (Myopia)

Myopia is essentially a disorder of eye growth. Young children have small eyes; the eyes grow as the child grows until, in most people, the eye grows to the correct size to give sharp distance vision. This happy state is known as Emmetropia (pronounced ‘emma-tropia’). However, in myopia the eye continues to grow. Usually the earlier someone becomes short sighted, the more short sighted they will be when the process stops. It is not unusual for eyes to continue changing for 10 years or more after myopia is first diagnosed.

Myths:
1. Not wearing glasses will make a short sighted eye more short sighted
2. Wearing glasses will make a short sighted eye more short sighted.

Neither of these commonly held beliefs are strictly true. For most individuals the amount of focussing error their eyes will have is pre-determined and inherited from their parents and is not influenced by spectacle wear. However, recent research has identified some young individuals who become slightly more short sighted the more close work they do, and there is some evidence that maintaining clear vision rather than letting the spectacles become out of date can slightly reduce the overall amount of shortsightedness that will develop.


Long Sightedness (Hyperopia or Hypermetropia)

Longsightedness exists when the eye is too short for its focussing ability and so the image is not in focus by the time it reaches the retina.

In young people the lens in the eye is usually very flexible allowing it to change shape so that the image can be brought forward and focused on the retina. This is known as accommodation and it occurs automatically. So young long-sighted people can see at all distances depending on the amount of accommodation they have. However, abnormally large amounts of long sight may not be overcome and this can lead to headaches, blurred vision and even delayed or suspended development of the visual system (see Amblyopia, next issue).

Myths:
1. Not wearing glasses will make a long sighted eye more long sighted
2. Wearing glasses will make a long sighted eye more long sighted.

Long sightedness is due to the eye being too small or too short, or less commonly, it is due to the lens in the eye not being sufficiently curved to focus the light on the retina. Wearing glasses may be necessary to assist in focussing, but does not influence the amount of long sight. Not wearing glasses may result in delayed or suspended development of vision in young children, but does not influence the overall amount of long sightedness.


Astigmatism

Astigmatism is perhaps the most difficult of the basic focussing errors to explain. Astigmatism exists when the front surface of the eye (the cornea) and/or the surfaces of the lens inside the eye are the shape of the side of a barrel or egg or rugby ball rather than being the spherical shape of a table tennis ball or soccer ball.

If we return to the television pixel analogy (see previous issues), the astigmatic shape causes each pixel on the television screen to be focused not as a circular dot, but as an oval or even stretched into a line depending on the amount of astigmatism. These ovals then overlap each other confusing the image. Unlike long or short sight, astigmatism blurs vision at all distances, though it is usually more troublesome for far distances.

Small amounts of astigmatism are very common, whilst larger amounts are more unusual. Astigmatism is often present along with long or short sightedness.

Astigmatism can be corrected by spectacle lenses, contact lenses and various types of refractive surgery, but not by exercises.

 

Strabismus ( “squint, turn, cast” )

Normally both eyes look at the same thing. To look at a distant object the eyes should both look straight ahead, sometimes known as parallel gaze. To look at a near object, for reading a book for example, both eyes turn inwards or converge but both eyes are still looking at the same thing. If one eye is turned too far inwards, not inwards enough, or outwards compared to the other eye then there is a strabismus or squint.

Most squints arise at birth or not long afterwards and so are known as congenital strabismus or congenital squint. Most squints have one eye turning too far into look at the same object the other eye. This type of squint is called an esotropia. If the squinting eye is turned outwards compared to where the non-squinting eye is looking, then there is an exophoria.

Congenital esophoria, if untreated, can lead to poor development of the sight in the squinting eye, such that the eye is not capable of normal vision even if any focussing error is corrected by lenses. This is often known as ‘lazy eye’, though the correct term is amblyopia (see below).

Some esophorias can be corrected or straightened simply by wearing glasses. Others may require surgical correction. Parents noticing a squint in their infant or toddler should take their child along to see an optometrist or ophthalmologist immediately. The sooner the squint is investigated the better chance there is of ensuring normal development of vision.

If both eyes are looking at different things then the individual may see double. This is usually the situation for squints that occur after the age of about 7 years. Any sudden on or recent onset of double vision should be investigated urgently by an optometrist or ophthalmologist.


Amblyopia (Lazy Eye)

This is a developmental condition. Whilst we are (almost) all born with a pair of eyes, we are not born with good eyesight. The ability to see colours and detail has to develop. This development is not usually complete until at least the 7th birthday. If for some reason vision in an eye does not develop normally (the ability to see detail does not fully develop), then that eye is said to be ‘lazy’ or amblyopic.

There are several different causes of amblyopia. The most common cause is strabismus (described above), but it can also be caused by uncorrected refractive error (moderate to large amounts of long/short-sightedness or astigmatism as well as trauma and various disease processes that are beyond the scope of this article.

Amblyopia in one of a pair of eyes is quite common, and often preventable. Amblyopia in both eyes is much less common and may be preventable, and of course it is much more of a disability. Children with delayed or arrested development of vision almost never complain; their vision seems perfectly normal to them. Not only that, but they can be perfectly normal and healthy in all other respects.

Infants should have a basic eye check by a paediatrician within 24 hours of birth, and small children should have their eyes checked by an optometrist/ophthalmologist or appropriately trained nurse at age 3 and before starting school. Also, if parents or guardians notice odd eye movements or behaviours such as ‘screwing up’ of the eyes to see, excessive eye rubbing or eye watering, or have any concerns whatsoever, then the child should be examined by an eye-care professional.

Children DO NOT have to be able to read numbers or letters, or even be able to talk before they can have their eyes examined. They can have their eyes examined at any age, though the older they are, the longer they can concentrate (usually) and the more information can be gained at one appointment.

Pterygium

This is a wing-like flap of tissue from the white of the eye, growing across the cornea and heading for the centre of the pupil. Pterygia are caused by irritation from dust, sand, UV light from the sun and other irritants. They most commonly occur on the white of the eye (the ‘sclera’) on the side between the eye and the nose. This is thought to be due to there being more reflected light onto that area by the bridge of the nose and more irritants finding their way on to the ‘nasal’ side of the eye compared to the other (‘temporal’) side.

If there is thickening or yellowing of the tissue in the 3 o’clock and 9 o’clock positions on the white of the eye, but no invasion of tissue into the cornea, the lesions are called pingueculae (singular = pingueculum).

Pterygium (and occasionally Pingueculum) can be treated surgically, but it is far better to prevent them in the first place. Avoid dusty, sandy conditions (difficult in our environment!) and wear appropriate non-prescription or prescription sunglasses.
Floaters

Floaters in the vision or muscae volitantes (flitting flies) in the vision are very common. Many people complain of being able to see ‘threads’, ‘tadpoles’, or ‘spots’ floating around, particularly when looking at a plain background such as a blue sky or a white wall.

Most floaters are annoying but entirely harmless and may even be considered normal. They are caused by discontinuities or strands of collagen fibres in the vitreous humour. The vitreous is the gel-like substance that occupies most of the space behind the lens in the eye. This may be disturbed by a blow to the head or by sudden deceleration such as found in bungee jumping. The last patient I saw with sudden onset of floaters had been ‘dumped’ off a surf board at the Gold Coast. The trauma of her head hitting the sand had been enough to disturb her vitreous.

Most of us have collagen fibres and other impurities suspended in our vitreous. As long as they are stationary our brains can filter them out of our vision and so we are not aware of them. Unfortunately at around age 35 (give or take 10 years or so) our vitreous begins to shrink and liquefy – a process known as vitreous syneresis. This allows anything suspended in the vitreous to move around and so we experience floaters.

The vitreous gel is usually attached to the retina around the head of the optic nerve at the back of the eye (the ‘posterior pole’) and often in a few places elsewhere on the retina. When the surface of the vitreous shrinks away from the places where it is attached to the retina, the individual may experience a sudden onset of floaters (just to the right of straight ahead for the right eye and just to the left of straight ahead for the left eye) which may be accompanied by flashes of light if the retina is disturbed when the vitreous contracts. This is known as a ‘posterior vitreous detachment’ or ‘PVD’. When this occurs there is a 1% risk of a retinal tear, break or detachment for about 60 days after the event. Since a retinal detachment may cause permanent damage to the sight of the eye, it is recommended that people experiencing symptoms of vitreous detachment should consult their optometrist or ophthalmologist urgently (remembering to bring along sunglasses and someone to drive you – the eye drops used to dilate your pupils so your retina can be checked will make you more light sensitive for a few hours and may blur your vision considerably).

Unfortunately there are many other causes of retinal detachment. Anyone experiencing a sudden onset of floaters/flashes/shadows/veils/cobwebs or other disturbances in their vision should consult their optometrist or ophthalmologist urgently. Most retinal detachments require treatment within 24 hours to ensure the best outcome and reduce the chances of permanent vision loss.

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