Myopia and Hyperopia are eye related disorders but knowing the difference between hyperopia vs myopia is what makes you understand much more about these topic.
Myopia (Near Sightenedness)
Origin of the word: It comes from the Greek word ‘miles’ meaning short-sighted. these people cannot see distant objects clearly.
Anatomy of Myopia
Let’s compare the eyeball to a camera for better understanding. when we take a picture light after reflecting from an object, enters the camera and passes through following,
- it passes a transparent glass wanted the aperture of the camera.
- Then it passes a camera lens that helps us in focusing light rays.
- lastly there is a camera film on which light is focused for a clearer picture.
Eye is somewhat similar, as when we see an object light after Reflection from that object enters our eye through,
- it passes cornea the transparent curved front part of the eye.
- Then it passes through the crystalline lens that helps us in focusing light rate.
- lastly there is the retina light-sensitive receptor cells in the posterior part of the eyeball on which light is focused for a clear image.
Definition of Myopia
It is a form of refractive error inability to refract,bend,focus the light rays properly, in which parallel rays of light i.e, coming from a distinct object. After entering the eye, a focused in front of the retina instead of on retina with eye muscles at rest.
Causes of myopia
- Curvature: increase of curvature of corny or lens causes light rays to be focused in front of retina and hence myopia occurs.
- Axial: increasing axial length of eye, increases the converging power and cause light rays to be focused in front of retina.
- Index: increasing refractive index i.e, light-bending focusing power of the lens. For example in Cataract.
- Positional: anterior displacement of lens. For example after trauma. This again causes light rays to be focused in front of retina.
Types of myopia
1) Congenital myopia
Myopia which is present from birth itself.
2) Simple myopia
This one is most common starts from 5 to 10 years till 15 to 20 years and is mostly due to curvature or length problem of eyeball.
3) Pathological myopia
It is hereditary Progressive and is due to degenerative changes in the eyes.
- Blurred vision
- Half shutting of ice
- Parents of child will tell outward deviation of eye (Divergent squint)
- normal patients require to move their eyes inward while focusing near objects. This is not required in myopic patients causing their eye to move out intermittently or constantly.
- Prominent eyeball – as eyes big
- Retinal changes in pathological myopia
- Retinoscopy: a handheld device that throws light in the eye and allowing us to observe its movements and reflection. Hence helping to confirm the refractive error.
- A-scan: A small ultrasound probe that throws ultrasound waves in the eyeball and then detects their Reflections or Echo timings Calculating the eyeball length.
Same as treatment for Hyperopia. (See below)
Hyperopia (Far Sightedness)
Anatomy of Hyperopia
Same as Myopia. (See above)
Definition of hyperopia
It is a form of refractive error, inability to refract, Bend Focus the light rays properly in which parallel rays of light i.e, coming from the distinct object after entering the eye are focused behind the retina with accommodation eye muscles at rest.
This refractive error has 3 key features,
1) Images are formed behind the retina:
In normal people images has to be focused on the retina for it to be clear.
2) Parallel light rays:
The rays that only come from object at 6 meters or beyond must be used for assessment as when objects come closer than 6 meters lightweight start to diverge changing their focus.
This is why we play Snellen’s chart at 6 meters. Eye must be addressed just like a DSLR cameras focusing power can be changed by rotating its zoom lens.Eye changes its focusing Power by changing the shape of its lens with the help of contraction and relaxation of certain muscles located inside the eyeball.
These muscles Must be at rest to correctly access the eyes, focusing power. This is also the reason why we use cycloplegic or muscle paralyzing drops before estimating the amount of refractive error.
Both hyperopia and myopia have abnormal distant vision i.e, they cannot focus distant objects clearly.
But the unique thing about hyperopia is that if these patients increase the power of their lens by contraction of Intra ocular muscles, they can focus the image accurately on their retina. This will result in excessive and continuous use of intraocular ciliary muscles in order to keep the image focused on the retina resulting in pain.
Also when they try to see a near object, there eyes will require even more focusing power, which these muscles can no longer provide. hence causing near objects to become blurry.
So lot of hyperopic patients (Not all) with less degree of hyperopia will say that they see distant objects clearly but they have pain or discomfort in their eyes or their eyes, get tired. And also near vision is mostly not that good. Hence, these patients are mistakenly called farsighted.
Causes of Hyperopia
- Curvature: curvature of cornea or lens is flatter than normal.
- Axial: short axial length of eye.
- Index: Decrease in refractive index i.e, light-bending focusing power of the lens. For example- in cortical cataract.
- Positional: posterior displacement of lens. For example- after trauma.
- Aphakia: absence of lens in eye therefore eye cannot focus light rays on retina.
Types of Hyperopia
1) Total Hyperopia
it is the Refractive error calculated after complete cyclopaedia, intra ocular muscle, paralysis induced by certain drops
2) Latent Hyperopia
Amount of hyperopia corrected by the focusing power produced due to normal tone of intraocular ciliary muscles.It is usually one diopter.
3) Manifest hypermetropia
It is the remaining portion of hyperopia. It is further divided into,
- Facultatitive: amount of hyperopia corrected by the focusing power produced due to active contraction of ciliary muscles.
- Absolute: amount of hyperopia, that cannot be corrected even by contraction of ciliary muscles.
- Eye straining or discomfort
- Asthenopia including eye tiredness, pain, headache or mild photophobia.
- Blurred vision with eye straining when muscle contraction attempts but fails to correct hyperopia.
- Blurred vision only – when refractive error is high and could not be corrected at all by muscle contraction efforts.
- Inward deviation of eye – convergent squint, normal patients require to move their eyes inwards while focusing near objects. This is enhanced in hypermetropic patients causing their eye to move intermittently or constantly
- small eyeball
- corneal diameter may be small
- retinal exam might show false changes due to distortion of image, when hyperopia exceeds 5 diopters.
- Retinoscopy: a handheld device that throws a light in the eye and allowing us to observe its movements and reflection. Hence helping to confirm the refractive error.
- A-scan: a small ultrasound probe that throws ultrasound waves in the eyeball and then detects their reflections or echo time, calculating the eyeball length.
Note: At Birth all eyes are hyperopic + 2.5 to +3 diopter. I.e, light is focused behind the retina as the eyeball grows and add about six to seven years age. eyeball is abnormal sites and light is now crystal on the retina or emmetropic. In some people, it continues to grow causing myopia or light is focused in front of retina.
Unlike the camera maximum 2/3 focusing in an eye is done by transparent front part cornea. while the rest 1/3 is done by the lens. So while treating these patients, we have to increase the focusing power of the eye so that the image is formed on the retina instead of behind it. This can be done by using either a converging lens e.g, non-surgical treatment, glasses, contact lenses or some surgical procedure.
Making the central part of the cornea more curved. This includes different methods that were changed and advanced depending on reduction of side effects and Recovery times and Improvement of degree of vision.
PRK (Photorefractive keratectomy)
Remove corneal epithelial cells by a solution. then use laser to reshape the cornea i.e, making it steeper and then apply a bandage contact lens.
Side effects include pain and recovery is in one week. at least it corrects 2 diopter hyperopia.
LASIK (laser in-situ Keratomileusis)
Correct up to four diopter. a famous and frequently perform nowadays, in this procedure a device, keratome, slices of flap of cornea 160 micrometer, laser is applied to change the corneal shape and then the flap is repositioned.
Avantages: Absence of post-operative pain and More rapid visual rehabilitation.
it involves getting radio frequency energy to corneal stroma causing change in its shape. Can correct low to moderate hyperopia.