All about eyes | Redlands Optometry

All About eyes

It is probably best to start with a brief explanation of how eyes work. The eye itself is much like a camera or video camera. It has a body (Stroma) to hold all its elements, a series of lenses (Cornea and Lens) to focus light, an aperture (Pupil) to limit the entry of light, a photosensitive film to receive projected images and convert them to an electrical signal (Retina), and a cable network (Optic Nerve) to send these electrical signals away for further processing and interpretation.

Once the electrical signals leave the back of the eye, they pass via the Optic Nerve, through the Optic Chiasm, down the Optic Tract, through the Lateral Geniculate Nucleus, down the Optic Radiations and finally end up at the Visual Cortex, just inside the back of your head.

The visual cortex is arguably the most important element of the visual system as this is the structure that gives rise to our perception of vision. This includes our sense of light and dark, colour, movement, distance, size, orientation, binocularity, balance and a whole range of other visual perceptual phenomena.

In order to see clearly, the images an eye collects at the retina must be in focus; much like a projector, need to be in focus to produce clear pictures. This is achieved by the eye’s 2 lens system: the cornea and the natural lens. The cornea is the clear, front structure of the eye. It is a non-adjustable but very powerful lens of about 45 dioptres (unit of lens power). The natural lens can change shape to vary its power in order to fine tune our focus. In a young eye its power range is approximately 15 dioptres in the non-focused state and over 30 dioptres when fully focused. Unfortunately it loses elasticity with age and becomes very hard, thus preventing us from having any focusing adjustability, typically after the age of 55.

For those wanting more detail, there is a far more technical side: The natural lens is located just behind the iris and it is suspended in place by very thin fibres known as zonules. These zonules are attached to the ciliary muscle which is part of the ciliary body, located circumferentially around the lens zonules. The natural shape of the lens is curved, resulting in the greatest power generation (most focused). In the normal, unfocused state, the shape of the ciliary muscle exerts a tension on the zonules which pulls the lens into a flattish shape. When the ciliary muscle contracts, the effective diameter becomes smaller. This takes the tension off the zonules and the lens is allowed to collapse into its focused state.

Short sighted/myopia short sightedness or myopia is where the uncorrected eye typically only sees short distances clearly. Technically it is where light from a distant object (>6 meters away), enters the eye (with a relaxed focusing system) and is focused in front of the retina. Depending on how short sighted the eye is, reading books/tablets or phones may be quite clear but when looking to the distance eg. driving, it will always be blurry without visual aids. Most myopic eyes have a longer axial length than normal.

Long sighted/hypermetropia long sightedness/hypermetropia or hyperopia is confusing! An uncorrected long sighted eye may see both distance and near clearly, just distance clearly or nothing clearly, depending on the age of the focusing system and the degree of long sightedness. Technically it is where light from a distant object (>6 meters away), enters the eye (with a relaxed focusing system) and is focused behind the retina. For an older eye, with no focusing capacity, this means everything is blurry but near objects are more blurry. A young eye may be able to (and often does) focus to make everything clear, however this requires more effort and may lead to eyestrain, especially for near tasks. Most hypermetropic eyes have a shorter axial length than normal.

Astigmatism is where an uncorrected eye will not see objects at any distance clearly, regardless of how much the eye is or is not focusing. It is typically where the eye is slightly egg shaped. People will sometimes feel uncomfortable or weird for a short period when they wear corrective astigmatic lenses (especially for the first time) as it can initially change their visual perspective of sizes and shapes.

Technically it is where light enters the eye and is focused on 2 different planes: either both in front (myopic astigmatism), both behind (hypermetropic astigmatism) or one in front and one behind the retina. A distant point source, instead of focusing to a point, will focus to 2 lines at 90° to each other and at different focal lengths. If we focus in or out the separation of the two focal lines will remain on different planes. It is typically caused by having a cornea with different curvatures along 90° axes: much like an AFL football.

Astigmatism can and does change throughout people’s lives. This may be due to changes in the corneal curvature, changes in the eyes length and shape, and changes in the natural lens shape over time.

As we age we lose the ability to draw our focus in. This is why normal sighted people need to wear reading glasses, usually in their 40’s and beyond. Technically it is where a loss of compliance of the natural lens means we cannot adjust its shape to give it more dioptric power, hence we cannot focus as close. It is due to the enlargement and hardening of the natural lens; not a muscle weakness. Typically, humans are born with 15-18 dioptres of accommodation (focusing capacity) which we lose all of by the age of 55.

Anisometropia is where we have a different prescription in each eye. While this is easy to correct optically, initially it will often result in a person feeling ‘weird’ because they will get different magnifications in each eye and their brain is trying to match objects of two apparently different sizes. If we need to correct each eye with a significantly different powered spectacle lens, this will result in different magnification of images when they reach the retinae. The area of light that each image projects on the retina is different, and the brain perceives this as two similar shaped images, but of unequal size. In trying to match these two images, especially if there has been no adaptation time, the visual cortex overlays both images so their features match giving rise to skew distortions. This can be quite disconcerting initially as perspective is disrupted, but with time the brain adapts and the world looks normal

People will sometimes report that they feel strange in their new spectacles. Usually this will be: with a change in their prescription, the first time spectacle wearer, with a change in their multifocal design, with an increase in reading prescription, or they have had an increase in their astigmatism.

It is always they aim of the optometrist to make the world as clear as possible for their patients but clarity and perspective are 2 different things! Any change in prescription means a change in magnification. This may be small (or large) but if it is different in each eye, different at different axes around the clock or even just different to what they have adapted to previously, it can certainly upset their perspective and make them feel ‘funny’.

The good news is that humans can also adapt to these alterations quite well. In fact, a Swiss-Austrian psychologist, Theodor Erismann, did many experiments last century with inverting goggles which turned a subjects world upside down and back to front. He consistently found subjects, whilst initially were quite disorientated, quickly learned to function and even ride a bike after 10 days. One even rode a motorbike though Innsbruck! The point is, in most cases our brain can make sense of these changes given time. So if your glasses feel weird, don’t panic; as long as they are clear, you should feel fine in time. (Also see anisometropia and astigmatism).

Floaters

In order to have clear vision, the pathway through the eye must be clear. When this pathway is interrupted it can result in floaters. They appear as spots, threads or webs that move within your vision. It is often noticed when looking at a blank wall or blue sky.

What causes floaters?

The content of the eye is predominantly made up of the vitreous, which is a jelly like substance that supports the globe structure. It is natural for this jelly substance to shrink and pull away from the retina overtime, resulting in debris otherwise known as floaters.

How long do floaters last?

Floaters can be distracting but will usually disappear from view within days to months as the debris settles to the bottom of the eye and is consumed by other cells.

When to see an optometrist?

If the floaters are very large or there are a lot in view it could be a sign of something more serious, especially if you are experiencing flashes.

What are flashes?

Flashes are bright lights that randomly appear in your vision, and the symptom is referred to as ‘photopsia’. They often occur at the edge of your vision, possibly in a ‘C’ shape and may seem like a sudden reflection or light. Some liken the experience to a torch turning on and off in the side/peripheral vision. You should immediately see your optometrist about flashes.

What causes flashes?

The retina is usually stimulated by photons or light ‘packets’, but you can also elicit a retinal response through mechanical stimulation; that is pulling or pushing on the retina. Flashes are usually caused by traction or something ‘pulling’ on the retina. This is usually the vitreous gel pulling on the retina and will often result in a posterior vitreous detachment (PVD) (which also usually gives rise to floaters). A more serious complication of this is a retinal detachment or retinal tear. In many cases the vitreous pulls away from the retinal cleanly but sometimes the pulling can tear the retina or cause it to detach from the globe. This is usually a medical emergency and requires prompt retinal surgery.

Posterior Vitreous Detachment Retinal Detachment

Optometrist get a lot of questions about screen based devices and potential damage to the eyes. This area is somewhat controversial and there is even some disagreement amongst eye care professionals. The main concerns arise from whether the close working distance of these screens causes changes in the eye optics and whether the type of luminance entering the eye affects the eye health and/or optics.

Close working distances have long been cited as the cause of myopia however there are generally few studies which support this in humans. On the other hand, population studies have shown that westernised Indigenous Australians and Inuits are, on average, more myopic than those with a more traditional lifestyle. If there is a risk it seems this is higher the younger you are (we can assume adults have virtually no risk of prescription changes due to screen based work). Likewise, virtually no studies have shown the luminance entering the eye from screens actually damages eye, although many individuals report eye strain after working with such devices. This is probably due to the effort of accommodation and convergence for sustained periods, or eyes drying out.

The Australian Department of Health recommends:

Children 0-2 years of age 0 hours/day

Children 2-5 years of age 1 hour/day

Children 5-18 years of age 2 hours/day

There are many good reasons to limit screen time which are not just limited to vision: screens promote a sedentary lifestyle, they usually mean less exposure to sunlight and there are significant social consequences of burying your head in a screen. With modern lifestyles it is convenient to use electronic devices as baby sitters but we probably should resist the temptation to do so, as it is easy to observe behaviour changes in children after a relative short exposure to such devices.

People often refer to laser surgery with the assumption that there is only one kind. There are in fact several types of lasers used in several types of eye procedures, and these are chosen according to their wavelength emitted by the LASER and the target tissue. Various elements and compounds are used to generate the laser including: YAG (Yttrium-aluminium-garnet), Krypton-green, Krypton-Blue and Argon.

Lasers can be used for refractive surgery, SLT (Selective Laser Trabuculoplasty), Iridotomy, Lens Capsulotomy, retinal photocoagulation, Photogrid Laser, corneal incisions, Capsulorhexis, IOL adjustment and Dispersion of floaters. So when someone says they have had LASER surgery it can be for a range of different issues.

Anti-reflective coatings (often referred to as Multi-coats meaning multiple layers of coating are applied to the lens surface to make one) have long been used in lens based industries and they are typically an add-on in optical dispensing.

What are they and what do they do?

In simple terms, anti-reflective coatings cut down reflections from the surface of a lens. This is helpful for reducing glare reflected off the back surface of a lens into the wearer’s eye. It also significantly improves the cosmetic appearance of glasses by allowing onlookers to see straight through to the wearer’s eyes instead of seeing reflections. In fact, anti-reflective coatings do a lot more than that. By cutting down reflections, they improve light transmission through the lens. Typical light loss through a non anti-reflective coated lens ranges from about 6% to 12%, whereas with a good anti-reflective coating this can drop to below 2%. This can make a significant difference, especially with progressive media cloudiness of the eye, so cataract sufferers will certainly benefit from having an anti-reflective coating. Such coatings also reduce glare from screens and reduce star bursting off car head lights.

So how does an Anti-reflective coating work?

This is probably for the more technical minded but here goes anyway. Visible light ranges from about 400 to 700nm in wavelength. When light hits the surface of a very smooth and polished object, some of it is reflected. If the surface of the object in question is coated with a ¼ of a wavelength thick, light transmissible material, the resultant reflection with be exactly 180° out of phase with the reflection of the parent material, thus cancelling the reflection entirely. Of course this only works perfectly at one wavelength, so wavelengths either side of this determined value will be progressively reflected more the further the departure from the chosen wavelength. This is why anti-reflective lenses will often show subdued reflected ‘blooms’, typically in green or purple hues.

Not all are created equal

Unfortunately not all coatings are the same. Just like paint, the quality of coatings varies widely in terms of how well they work as an anti-reflective coating, how easy they are to keep clean, how chemically and thermally stable they are and how hard they are to scratch. I remember a time when I was locuming at a chain practice and 4 patients in a row said they definitely didn’t want ‘that extra coating’. When I asked why, the responses were invariably “it was too hard to keep clean” or “it peeled off in places”. When I asked the practice manager what they were using as an anti-reflective coating it turned out it was an out-dated product that was probably not great even when it was just released!

All good modern anti-reflective coatings are both hydro-phobic and lipo-phobic, which means they repel both water and oil. Cleaning has never been easier and usually a quick wipe with a microfiber cloth is all that is needed. Occasionally frames and lenses will develop an oil build up due to secretions from the skin over time, and this often results in lenses that are hard to keep clean as wiping just moves the oil around. Fortunately a quick clean in our ultra-sonic cleaner will restore them back to their ‘easy to clean’ state.

What about Scratches?

This is a bit of a minefield. They is no real industry standard for the ‘scratchability’ of lenses. And this also varies from country to country. The Bayer test, the Mohs scale, the Tumble test, the Eraser test and the Taber test are some of the scratch assessment test or surface hardness scales sometimes referred to in the industry.

In Australia, the Bayer standard is probably more useful and accurate when talking about optical lenses. It compares the ease of scratching a lens surface with steel wool, compared to the that of scratching standard, uncoated resin (CR39) in the same manner. If the net result was 5 times less scratches, the Bayer scratch rating would be 5.0. In general terms, a lens which is 4 times less ‘scratchable’ than a standard uncoated lens is considered to have an anti-scratch coating or treatment. But this is a minimum. Some lenses just meet this standard and others exceed it significantly.

Glass is still the most scratch resistant material for ophthalmic lenses but since it makes up less than 1% of spectacle lenses we will deal with resin lenses. Currently the best anti-scratch treatments offer about 15 to 20 times more scratch resistance than uncoated CR39. Typically these are used in conjunction with or as part of an anti-reflective coating.

Accommodation: Adjusting focus from further to closer. Most human eyes can not focus after the age of 55.

Ametropia: Having a refractive error.

Aniseikonia: The difference in magnification perceived by one eye compared with the other.

Anisocoria: A difference in pupil sizes.

Anisometropia: The difference in prescription size from one eye to the other.

Anti-reflective Coating: A multi-layer of thin coatings applied to the surface of a lens to cut down reflections but setting up destructive interference of visible light’s wavelengths.

Aqueous Humor: Nutrient rich fluid made by the ciliary body, used to nourish various internal components of the eye.

Astigmatism: A different prescription at different axes around the clock in one eye.

Atropine: A chemical agent used to cause temporary mydriasis and cyloplegia. It can be used to treat amblyopia and in very low doses it can be used to treat progressive myopia.

Barrier Laser: Retinal laser used to treat around an area of ‘weakness’ or potential detachment.

Bifocal: A lens with 2 different powers, separated by a visible line (either curved or straight).

Blue Blocker: Additional filtres applied to a lens to exclude short wavelength visible light.

Capsulotomy: A hole blasted in the posterior lens capsule in order to create a clear window through to the retina. Often needed after cataract surgery where the capsule has become fibrosed or ‘frosty’.

Conjunctiva: The outer clear membranous layer over the white part of the eye.

Contact Lens: A prescription or cosmetic lens which sits directly on the surface of the eye.

Convergence: The process of turning eye inwards, usually to view a closer object.

Cornea: The clear curved layer at the front of the eye, over the iris.

Cycloplegia: Rendering the ciliary body unable to act on the lens’ shape, thus disabling the focussing capacity.

Cyclopentolate: A moderately lasting mydriatic and cylopleging agent used to commonly find children’s prescriptions.

Diabetes: A systemic condition where blood sugar levels are not controlled well or at all.

Dioptre: The unit of lens power strength.

Electromagnetic Radiation: The spectrum of photons made up of different wavelengths where visible light (for humans) lies between infra-red and ultra-violet.

Emmetropia: Having no distance prescription.

Extended Reader: A multifocal spectacle lens which is designed to correct near and slightly further tasks.

Floaters: Debris in the vitreal chamber which are visualised as ‘things’ drifting around in space.

Fundus: The inside of the posterior (vitreal) chamber of the eye.

Glaucoma: A condition where the optic nerve fibres are progressively damaged, often due to high pressure in the eye.

Globe: The body of the eye.

Graduated: A term used to describe the variation of either power or tint across a lens.

Hypermetropia: Longsighted.

IOL: Intra-ocular lens.

IOP: Intra-ocular pressure.

Iridotomy: A YAG laser ‘punched’ hole in the peripheral iris to form an addition communication to Schlemm’s canal for aqueous humour drainage in people with elevated IOP risk.

Iris: The coloured part of the eye.

Lens: Part of the optics of the visual system, located just behind the iris.

Luminance: The intensity of light emitted from a surface per unit area in a given direction.

Macula: The central part of the retina which usually sees things in great detail.

Macular Degeneration: Damage to the macula, usually due to age, which results in a loss of central vision.

Mydriasis: Dilation of the pupil.

Migraine: Either in visual or headache form; a visual migraine is where a subject sees non-real visual scintillations which may move and change in size and last up to about 20 minutes. This can be followed by a migrainous headache where the sufferer is very sensitive to light, sound and smells and suffers an extreme headache. Both can occur independently of each other but often suffered of both types will stop getting the headache component as they age.

Multicoat: Another common term for anti-reflective coating.

Multifocal: A variable focussing lens, either spectacle or contact lens.

Mydriacyl: A short term mydriatic and cyclopleging agent, usually used to make pupils larger for better fundus examination.

Myopia: Short sightedness.

Optic Nerve: The electrical network which carries messages from the photoreceptors to the brain for further processing. It has about 1.5 million nerve fibres at birth.

Ortho-K: Ortho-keratology; a process of temporarily changing the corneal shape through the use of reverse geometry RGP contact lenses. These are usually worn overnight and removed on waking to give the wearer good daytime vision without other correction means.

Photochromatic: The term to describe lenses which react to varies wavelengths of light to change their visible light transmissibility; ie they go dark in sunlight and clear indoors.

Photocoagulation: Burning of the retinal tissue to seal off vessels or to ‘spot weld’ tissue in place.

Photon: The unit of electromagnetic radiation.

Photoreceptor: The light sensitive elements in the eye.

Photopsia: The sensation of seeing flashing lights.

Polarised: Light which is only propagated at one axis plane. Lenses which polarise light completely cut out a lot of reflected glare which is not propagated at the same axis as the polarised filter.

Presbyopia: The loss of accommodation of the natural lens.

Pupil: The usually circular hole in the middle of the iris.

Retina: The photosensitive part of the eye.

Retinal Detachment: The separation or peeling of the retina from the globe of the eye.

Refraction: The change in direction of light accompanied by a change in material or composition of the medium the light moves through.

RGP: Rigid Gas Permeable. Refers to hard contact lenses which are permeable to oxygen.

Schlemm’s Canal: The circular collection ring just outside the iris which receives the excess circulating aqueous humour.

Single Vision: Lenses which have only one focal length, ie distance or reading.

SLT (Selective Laser Trabeculoplasty): Laser used to injure several sites around the trabecular meshwork in order to facilitate better outflow of aqueous humour for people with high intra-ocular pressure.

Stereo-Vision: The ability to process images from both eyes simultaneously, enabling an integrated, 3 dimensional image.

Trabecular meshwork: Spongy tissue around the outer anterior segment of the iris where aqueous humour drains from the eye.

Tri-focals: lenses which have 3 distinct focussing zones separated by 2 lines. They are becoming far less common and are usually set up for distance, intermediate and near viewing.

Transitions: The premier brand of photochromatic resin lenses.

UV: Ultra-violet or the part of the electromagnetic spectrum where wavelenghths are a fraction to short to be seen by humans.

Vitreous: The gel compartment between the iris and the back of the eye.

Redlands Optometry

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