nvi-the-problems

[5.1] What is myopia?

Also called shortsighted, or nearsighted. Myopia is the inability to see distant object clearly. The physiology is that the image produced by the eye focuses just short of the retina, leaving a blurred image to fall on the macula.

Textbooks say the physiology can vary, the axial length from the lens to the retina is too long, or the lens is over accommodating, or the curvature in the cornea has changed, or the lenticular curvature has changed. The lens itself may by displaced of its own accord or the inter occular pressure is too great. Diabetes is known cause of myopia and general ill health; stress, emotional problems can bring on temporary conditions. Some very rare cases result from a genetic pigmentation problem with the retina.

Conventional optometry regards myopia as a hereditary problem contrary to evidence to suggest a strong environmental factor. Conventional Optometry treats myopia by the use of stronger and stronger prescriptions.

Behavioral optometry regards myopia as a symptom of vision imbalance which is often responsive to vision therapy.

Mypoia is measured in negative spherical diopters. ie -1 D. one diopter is the power of a lens which will focus parallel rays at a distance of one meter.

[5.2] What is astigmatism?

Astigmatism is the condition where a point focus can not be formed on the retina. The refraction of light is unequal in different meridians. This means lines in one one direction may be blurred while others are not. Astigmatics often report double vision (diplopia) or even multiple images (polypopia) in both eyes or even a single eye.

The physiology is as uncertain as that of myopia. The cornea or lens may not be rotationally symmetrical, or the eye itself by be asymmetric. The extraocular muscles are sometimes responsible for deforming the shape of the eye. Conventional optometry treats astigmatism through the prescription of lenses. Behavioral optometry regards astigmatism as correctable with vision therapy.

Elliot Forrest O.D. found a relationship between head movement, posture and visual scanning. If you use a computer or read a lot then you will tend to scan from along the horizontal meridian and neglect the vertical meridian, with a resultant astigmatism in that meridian.

Astigmatism may also result from a twisting of the spine, pelvis or neck. Astigmatics often have a tight or restricted posture and movement.

You can quickly tell if you are astigmatic with the astigmatic mirror section [9.21]

[5.2.1] What is the notation for astigmatism?

Astigmatism is measured in cylindrical diopters. Three numbers are needed. The first number is the power of a spherical lens. The second is the power of cylindrical lens and the last number is the principal meridian in degrees for that lens. Facing a patient: 0 degree or 180 degree is horizontal, 90 degree is vertical, 45 is up right to down left, and 135 degree is up left to down right.

Optometrist usually use “minus” notation eg +2.25 -0.75 x 175 and ophthalmologist use “plus” notation eg +2.25 +0.75 x 5 These are the same measurement.

so is -1.00 +1.00 x 135 and -1.00 -1.00 x 45

The average astigmatic usually has the horizontal meridian flatter than the vertical meridian.

[5.3] What is hypermetropia?

Also farsighted or longsighted or hyperopia. Hypermetropia is the condition where the eye tends to focus behind the retina. Hypermetropia is usually not a problem as small amounts can be overcome by accommodation. Indeed the majority of people are born hypermetropic.

The physiology is as poorly understood as with all vision problems. The eye may be considered too long, the retina may have detached, accommodation may be insufficient. etc etc etc.

[5.4] What is presbyopia?

Also old age sight. Presbyopia is a failure in accommodation with the onset of age. It mostly affects near vision, but distance vision is also sometimes affected.

The physiology of presbyopia is considered to be from change in the shape and or structure of the lens are commonly blamed. Less commonly the strength of the ciliary is at fault.

As with all sight problems this is poorly understood.

[5.5] What is emmetropia?

Emmetropia is the ideal eye where a relaxed flattened lens focuses exactly on the retina. Commonly emmetropia is another word for normal sight. The emmetropic eye is fully rested and display no signs of stress when looking at the far point or the near point.

No eye is perfectly emmetropic all of the time.

The opposite of emmetropia is called ammetropia.

[5.6] What is strabismus?

Also “cross-eyes” or, less commonly “squint”. A rare condition where the eye turns in, out, up or down. That is the eyes are no longer working as a team. Behavioral optometry beleives that the cause being at a cortical level is treatable. Untreated the condition lead to many emotive dificulites.

[5.7] What is amblyopia?

Also “lazy eye”. A condition where one eye apparently looses clear sharp vision which cannot be compensated for with corrective lenses. A condition best diagnosed by a professional.

[5.8] How accurate are visual measurements?

Not very. Vision changes continuously and the autorefraction and subject refraction tests performed by optometrists and opthalmologists give only a “snapshot” of your current acuity. Autorefraction gives a sort of “at rest” measurement and subjective refraction measures your ability (some people say “under pressure to perform”) to discern what the eyes present to your mind. A score of 20/20 does not include your acuity at the near point.

Many other measurements are not covered by a standard optometry test.

Eye-movement skills, the ability to quickly focus from one object to another.

Eye-teaming skills, converging and diverging, the ability to turn the eyes inward and outward in a coherent way.

Eye focusing skills, the ability to make rapid and accurate shifts in visual inspection with instantaneous clarity and at any distance. Also the ability to maintain focus on a an object. Focusing is about expanding the point of vision not about forcing or trying harder. Effects/reflects concentration abilities.

Dynamic visual acuity, the ability the see sharply while an object or person is in motion.

Peripheral vision, the ability to use side horizontal and vertical vision. Used to position yourself in space and time. Improvements result in better balance and movement.

Binocular fusion, the ability to coordinate the eyes precisely so the brain can “fuse” the input. Dyslexia is a catch all term to label crossed and left/right reversal dificulties. It is about a lack of co-ordination between the left and right hemispheres of the brain and body. Fusion creates depth perception.

Eye-hand co-ordination. Combining the visual and motor systems, if the eye movements are jerky then the hand movements will be too. Effects sports performance and even handwriting.

Visual form perception. Becoming aware of the shapes features of sensory information. The ability to organize images on the printed page into letters and/or words.

Visualization and Visual memory. How you make and store and recall visual images and information. Effects your ability to learn.

[5.9] How do I measure my own acuity?

To measure acuity with an eye chart: Stand back from an eye chart. Determine the smallest row you can read. Take the distance you are standing from the chart (normally 20 feet or 6 meters) and divide that by the number printed next to the line. This ratio is your acuity. The ability to read black letters on a white background from twenty feet (6m) is known as “Snellen acuity” named after the inventor of the eye chart, and is usually represented with “20″ in the numerator. Expressed as a percent, it is your “percent of normal vision”. This percent must not be taken as an absolute measurement, since one will have different acuities at different distances.

To measure amount of myopia in diopters: Measure the furthest distance you can see the letters absolutely clear and black (not just readable). This is your focal length. If you measure your focal length in meters, you can compute your refractive error in diopters by taking the negative reciprocal. For example, if things start to blur out at half a meter, you have a refractive error of -2.00 D. The greater the magnitude of this number, the more your myopia. The number you come up with by this means should represent more myopia than what the eye doctor would give you as a prescription, since eye doctors are trained to “under- prescribe”.

[5.10] What is average daily vision?

A subjective measurement of how your vision faired during the day. It is easy to get a good idea of the state of your vision by using a set of daily landmarks or signs to judge its current state. You will also find vision varies quite a bit during the day, so you need to take this in consideration when making your evaluation. Many people find that vision is directly related to their current state of alertness and health. As you become tired during a typical day or are ill your vision reflects this.

[5.11] What is peak daily vision?

Peak daily vision is defined as the “best” subjective point of clarity throughout the day. Most people are surprised to find just how good the peak point of vision during the day actually is.

[5.12] Should I keep track of peak and average daily vision in a diary?

Yes. While this is not mandatory, it helps you see long term trends, and gives more reliable feedback as you experiment with changes to your lifestyle and undertake various remedies for your vision. Changes over weeks and months are difficult to notice until they reach a large threshold. It is much easier to note down the state of your vision each day, whether you had any clear flashes and what duration they were, rather than to rely on your memory. Include as much detail as you feel necessary. You would also be surpised at how quickly you adjust to better eyesight and forget what poor vision was like. Some people improve without realizing they have improved.