Wearing contact lenses for a too long period of time can cause irreversible damage the cornea.
This is the warning by scientists of the università Cattolica of Roma during the XI national congress of the Congresso Nazionale della Società Italiano Trapianto di Cornea. The lenses can aneasthesize the cornea, incapable to warn that something is wrong through itching and reddening.
“People must understand that contactlenses are not toys”, says Emilio Balestrazzi, director of the ocular clinic of the Roman Polyclinic.
Archive for March, 2008
Prolongued use of contact lenses can damage the cornea
Sunday, March 30th, 2008Choosing the right frame for you (1/5)
Thursday, March 27th, 2008
General Guidelines
Certain shapes will complement your facial characteristics, other don’t…
This series of articles will help you to choose the best shape for your facial characteristics.
Although frame styles change frequently, these orientations will always apply:
Eye Position Regardless of the frame’s shape your eyes should be centered within the lens opening.
Width: The frame should be wide enough to leave a small gap between the frame’s temples and the sides of your head.
Proportions: The size and the weight of your glasses must be proportional to your dimensions and weight. Consider a heavier frame if you have heavy eyebrows. If you are slender, choose a delicate frame.
Brow: The top of the frame should follow your brow line and should not be significantly higher or lower than the brow line. For thick eyebrows, consider using thicker and/or darker frames.
Lenses: Thinner, lighter, non-reflective lenses are always the best choice. Choose high-index plastic lenses with anti-reflective coating for a pleasing appearance. If bifocals are prescribed, choose progressive lenses to avoid the old-fashioned bifocal lines.
Continues…
Recurrent symptoms in hypermetropia
Monday, March 24th, 2008
The eyesight In absolute hypermetropia is scarse but not painful. The scarse eyesight is accepted as a normal condition and the vision of nearby object, e.g., reading a written text, is obtained holding the written text close to the eye, thus creating a large, although confused, image on the retina. Reading so close to the eye can look like an absolute hypermetropia instead of a myopia.
The slight facultative hypermetropia, if present in young subjects and with a normal eyesight, normally is not painful but in time it will cause an astenopia and, at a certain age, absolute hypermetropia.
The effort of the eyelash muscles causes a headache in the frontal area. The delaying action on the excessive convergence by the straight external eyemuscles manifests itself, especially in women, as nevralgic pain close to the temples. The necessity of excessive accomodation causes also an excessive convergence which often transforms itself first in exophoria, then in exotropia.
Other symptoms may be: clumsiness working with close-by objects, “heavy” eyes, the eyesight becomes darker as if the illumination is reduced and other symptoms when working on close-by objects. The eyes and eyelashes have a tendency to redden and inflamed, wind, dust and a too strong illumination can cause significant disturbances.
One can say that, if there are inconveniences in longsightedness, the eyes become tired, opaque, when working on nearby objects.
The first reaction of the patient is to close the eyes and to rub them, massaging the eye muscles. Also the presence of wrinkles between the eyebrows and close to the temples can be a symptom of an uncorrected hypermetropia.
From Optometria e Oftalmologia by Prof. Sergio Villani (Volume One).
Related article: Vision of the hypermetrope
The prescription of corrective lenses: Secondary effects of the optical correction of ametropy (2/2)
Friday, March 21st, 2008
3) Variation of the dimensions of the images on the retina
The corrective lenses produce a variation of the dimensions of the images on the retina: with negative lenses the images are smaller, with positive lenses instead are larger than without lenses. The variation of the dimensions of the images increases with the power of the lenses and the distance from the eye.
When starting to wear lenses one notices the variation of the dimensions of the images of the objects, that they seem smaller (with negative lenses) or larger (with positive lenses). This effect disappears quickly with the use of glasses, because a new relation between the dimensions of the images is established on the retina and the dimensions of the objects. The enlargement of the images on the retina has instead importance for what it regards the visual acuity of the eye. Obviously short-sighted eye will draw less advantage from the correction in respect to the hypermetropic eye because reduced images are projected on the retina.
Related articles: The prescription of corrective lenses: Secondary effects of the optical correction of ametropy (1/2)
From Optometria e Oftalmologia of Prof. Sergio Villani (Volume Two).
Pseudoaphakia: The IOL.
Tuesday, March 18th, 2008
The IOL is generally constituted of two parts:
- optical zone
- support elements
The optical zone is by now universally composed of the material discovered by Ridley: Perspex C.Q. (= P.M.M.A. Clinical Quality). The lenses made with this material need to be refined by hand. Some IOL producers, however, use a different type of PMMA, that allows for shaping by injection; but, the lens manufactured this way turns out to have deforming elements of support and, with cutting margins, it is today preferred to use those molated or levigated by hand. The diameter of the optical zone varies for several of the types of lenses between 5mm and 6mm. Today it is possible to decide from a vast range of powers: from + 6,00 to + 27,00D. In the IOL in the posterior chamber the optical zone has 20 or 4 holes (of the diameter of 0,4 - 0,5 milimeter) that serve as points of attachment for the surgical instruments to position the lens in its seat. The support elements are defined in this way according to how much support is offered to the lens on the ocular structures. They have various denominations according to the IOL group in consideration.
They can be composed of the same material (e.g., IOL of Choyce and IOL of Kelman), or be constituted of other materials in order to obtain a better flexibility: es. Polypropylene (previously also metals like plastic tungsten, platinum etc and other materials like nylon, which turns out to be inferior) were used. The elements of support of the lenses for iridea fixation, for irido-capsular fixation and for the posterior chamber are called loops or handles.
CLASSIFICATION OF THE IOL
Based on the anatomical center of insertion we can distinguish 4 classes of IOL:
1) IOL for the front chamber
2) IOL for iridea fixation
3) IOL for irido-capsular fixation
4) IOL for the posterior chamber
The appraisal of the corrected power of the IOL system is based on two methods:
1) On the use of the Tooriche formulae, based on the principles of the optical geometry applied to the schematic eye;
2) On the use of emperical formulae, based on the retrospective analysis of the data.
In both cases, essential parameters are constituted by the measurement of the corneal refractional power (measured with a oftalmometer or cheratometer), and by the corrected measurement of the axial length of the eye (usually calculated with one of the many biometric ultrasonic To or B-Scan, in commerce).
Related article: Pseudoaphakia: Contraindications to IOL implantation
Da Optometria e Oftalmologia del Prof. Sergio Villani (Volume Uno).
The prescription of corrective lenses: Secondary effects of the optical correction of ametropy (1/2)
Saturday, March 15th, 2008
The eye-corrective lens system differs from the optical system of the uncorrected eye not only for what concerns the focusing of the image on the retina but also for other characteristics, some of which are very important for the vision.
The most important are:
- Variation of the viewing field
The viewing field of the lens system, i.e., the points that can be seen through the lens with an unmoving eye is more restricted than that of the eye and has smaller angular dimensions with a smaller dimension of the lens and the distance from the eye. With identical diameter, distance and power, the positive lenses reduce the viewfield more than negative lenses, due to different prismatic deviations.
- Variations of the entry of light in the pupil
The aperture of the rays used by the eye depends on the diameter of the pupil. If the eye is corrected with a negative lens, the aperture is reduced for the same pupil diameter, i.e., the pupil of the eye-lens system is smaller than the eye’s pupil.
- With positive lenses the effect is the contrary. The variation of the pupil procuced by the lens is greater the greater is the power and the greater the distance between the lens and the eye. This variation has an effect on the luminosity of the image on the retina, but this can be compensated by the variations of the pupil’s diameter.
There is however an anti-esthetic effect for those who see the pupil through the corrective lens: the pupil may seem small through negative lenses and large with positive lenses. To reduce this effect to the minimum the lenses should be mounted as close as possible to the eye, compatibly with the requirement that the eyelashes do not touch the lens and this is most important for powerful lenses.Related article: The prescription of corrective lenses: aphakia
From Optometria e Oftalmologia by Prof. Sergio Villani (Volume Two).
Pseudoaphakia: Contraindications to IOL implantation
Wednesday, March 12th, 2008
The most severe contraindications to the employment of IOL is for one-eyed patients as advised by authorative experts in the sector. Other contraindications are:
- Diabetic retinopatia whether or not associated with a rubeosis of the iris
- Distrophy or corneal degeneration, i.e., the risk that the surgical trauma causes corneal discompensations
- Chronic uveitis
- Microphtalmos
- Patients that were already subject to retinal dehiscence or separation
- Glaucoma, only that today it is not an absolute contraindication
- Aniridia or pre-existent sectorial iridectomia
- Irreversible damage of the pupilar sphyncster
- Ambilopia
- Acute ocular or peripheral infections
- Very low front chamber; flat cornea
- Heavy loss of vitreo during the intervention
- Ocular blood loss between operations
- Retinal or neurological damages
- Children of less than 12 years of age
Obviously subjects that are contrary to the implantation or result psichologically inadequate should not be considered.
Related article: Pseuodoaphakia: Indications for IOL implantation
From Optometria e Oftalmologia by Prof. Sergio Villani (Volume One).
The prescription of corrective lenses: aphakia
Sunday, March 9th, 2008
Analogous considerations are valid in the case of aphakia, i.e., when the chrystal has been extracted with a surgical intervention. It can be that the corrective lens has to be modified frequently because of an corneal astigmatism caused by the surgical intervention. This is the reason why one has to wait 5-6 weeks before the definitive correction can be implanted while a partial correction cen be applied after a few days.
In many cases, the lens that allows for maximum sharpness is not well tolerated at first and usually a correction is prescribed that allows for a distinct vision at 1-2 meters, that is with 1 or .5D of undercorrection. Later the use of lenticulars, especially if bifocals allow to obtain the final correction; even if distorsions cannot be avoided, with this type one obtains the least weight and without effort one can reach a good close and far eyesight. Difficulties get worse when a patient has one aphakic eye and the other is in, almost, good conditions. The difference in size of the images that are projected on the retina and the prismatic effect on the two eyes make a binocular vision impossible and, if the aphakic eye was first dominant, the suppression of that image becomes difficult. In these cases till 20 years ago the good eye was ocluded in order to use the aphakic eye. From the 70’s contact lenses are used, with significant functional and esthetic advantages; with these one can obtain a single binocular vision without aniseiconian disturbances, something that cannot be achieved with spectacles.
Another method that is used in 90% of the cases consists in the introduction of an intraocular lens directly at the same same spot of the chrystal. In this way the functional result is surely acceptable.
Related article: The prescription of corrective lenses: severe myopia
From Optometria e Oftalmologia by Prof. Sergio Villani (Volume Two)
Vision of the hypermetrope
Thursday, March 6th, 2008
With a slight hypermetropia a young person can easily compensate it with accomodation; very often, a hypermetrope can see better than an emmetrope.
With a higher hypermetropia the accomodative action is such that, if we exclude children, it is difficult to obtain a compensation of the hypermetropia; there will instead be a discrete part of absolute hypermetropia and the visus in these conditions are scarce, especially for close-by vision. If the vision of an uncorrected hypermetropic eye is normal, it remains such with a positive lens because that lens only reduces the accomodation.
If there is a slight improvement, it is due to the magnifying action of the positive lens which stimulates a larger number of retinal receptors, something that is not evident because the corrective action is entrusted to the accomodation.
When the eyesight is subnormal the corrective lens can enhance it till it is normal. This is not always true though because the eyesight, if not corrected in the first years of life, cannot develop normally.
In the case of microftalmo instead, the eye is underdeveloped and the sensitive elements are less and the retinal image in a small eye will be smaller than normal. The lens therefore will magnify the image and the eyesight will return normal.
When the accomodation is such that it neutralizes the flaw, there is no relation between the eyesight and the hypermetropia.
If the accomodation is almost absent, the condition is like with absolute hypermetropia; the eyesight becomes like with a myopia of the same degree but an external examination of the eye will show a much smaller pupil (myotic) than in the case of hypermetropia.
From Optometria e Oftalmologia by Prof. Sergio Villani (Volume One)
Related article: The vision of the hypermetropic eye (1/2)
The prescription of corrective lenses: severe myopia
Monday, March 3rd, 2008
The correction of a severe myopia, that is a myopia superior to 10.0D, often presents problems because a complete correction is often not tolerated, even if it should from a technical point of view. The criterion though is always the patient’s wellbeing. Modification of the prescription is still empirical and varies from one individual to the other. The reason is, in part, the effect of high-power lenses to increase the image size projected on the retina and in part the alteration of the compensation system, especially for the convergence. While for a slight or medium myopia a complete correction is tolerated and also a hypercorrection to see in the distance, in the case of severe refraction the complete correction cannot be tolerated. Often it is necessary to reduce it by 2-4 diopters, even if the best reduction value will be found in the course the eye examinations.
It is also possible that the excessive weight is to blame, to eliminate this factor we can use synthetic resin lenses or titanium frames whenever necessary.
Much benefit can be obtained with contact lenses which, often, if negative, allow for a sharper vision than with ordinary lenses.
The use of synthetic resin (organic) lenses whose weight are reduced to acceptable levels have the inconvenience that they are thicker.
Related article: The prescription of corrective lenses: severe Astigmatism (2/2)
From Optometria e Oftalmologia by Prof. Sergio Villani (Volume Two)