|Year : 1983 | Volume
| Issue : 6 | Page : 735-739
Optical treatment of myopia including contact lenses
VK Dada, SK Angra, Madan Mohan, VK Kalra
Dr. Rajendra Prasad Centre for Ophthalmic Sciences, AIIM.S., New Delhi, India
V K Dada
Dr. Rajendra Prasad Centre for Ophthalmic Sciences, AIIM.S., New Delhi
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Dada V K, Angra S K, Mohan M, Kalra V K. Optical treatment of myopia including contact lenses. Indian J Ophthalmol 1983;31:735-9
Ideal treatment of myopia is aimed
1. To give optimal visual correction.
2. To halt the progression of myopia.
3. To cause regression in myopia
4. To treat associated muscle imbalance.
Myopic correction is a multifactorial major problem which responds fairly well to correction with minus lenses. Due to confused state of etiology where the blame swings from simple retractive status to a pathological entity and widely accepted view of environmental influence in the development has lead on to modifications in the conventional treatment with minus lenses. These factors may be accommodational, or extra ocular muscles producting a rise of intra-ocular pressure causing a stretch of the weakened ocular coats. Therefore treatment is accordingly modified to control accommodation & convergence.
General Guidelines regarding Optical Prescription
The Prescription of glasses includes the following parameters & their related significance is highlighted (This is called formula of prescription).
1. Spherical Power.
2. Cylindrical Power Visual Parameters.
3. Axis of cylinder
4. Prism Incorporation. Important in muscle imbalance.
5. Addition: Important in accommodative problems.
6. Tint of Glasses: Important in cutting down reflection & transmission.
7. Thickness: Weight of lens & strength of lens.
8. B.C.: Important in Optical efficiency & cosmetic, and magnification.
9. Special Lenses.- Plastic Lenses for children.
- High refractive index glass (Highlite) for High myopia.
- Aspheric Lens- in high myopia. - Toughened Lenses: for all cases.
- Occupational - Selective and hand tints cruxite-& infrabar.
Types of Frames:
They should be sturdy, bigger, deep grooved, plastic.
Placement of Lens in Frame: Anisekonic considerations
Edge:- Anisekonic consideration
1/ 3 & 2/ 3
- bevel 1/ 2 & 1/2
Surface.- Front should be at least +2.00 D.
Antideflection coating to decrease reflection.
Decentring:- A compromise for distant & near prescription.
Placement of Frame on the Face:
This includes pupillary distance, nose bridge, short & long term effects on the face. Pentoscopic tilt of 15°.
General Guidelines are:
Prescription of Glasses
(i) Infants & children should have the refraction done under cycloplegia. Acceptance is started with the retinoscopic findings without adding working distance factor. End point for prescription is minimum minus in trial frame which gives the visual acuity of 6/9 in each eye & 6/6 with both eyes open.
The subject is now discussed
Optical prescription for myopia.
(ii) Role of Contact Lenses in correction o f myopia
(a) Routine prescription of contact lens. (b) Retrospective studies on the behaviour of myopia under a well filled lens over the years.
(c) Special fitting techniques for causing reduction in myopia (Orthokeratology). Optical treatment for Myopia:
Basic aim is to provide a minus lens to provide the best possible visual acuity. The reason for full correction of myopia is that the accommodation-convergence relationship is permitted to develop along the normal lanes otherwise under correction causes higher degree of exophoria for near & symptoms pertaining to convergence fatigue.
In low myopia, a full correction may be safely given without too much difficulty in regard to acceptance. In very low myopia the flatter concave lenses create reflected virtual images which prove excessively annoying. In such cases a 15° pentoscopic tilt of lens to displace the image or to use non-reflective coating is indicated.
In medium myopia, in absence of previous minus correction, the sudden demands on accommodation may be more severe & intermittent or spastic esophoria may be manifested because of and upon accommodation. This can be treated by Bifocals, atropinisation & prism therapy. In cases of high myopia in which minus lenses have not been prescribed before, the effect on accommodation is even more severe. In addition diminishing effect of high minus correction upon the size of image & the restriction of the extent of peripheral visual field require even further adjustments of the pt. One might have to resort to Bifocals, weak cycloplegics or full correction should be given in stages.
Pathological myopia in addition needs low visual aids.
Bifocals & atropinisation:- These have a special role in treatment of
(ii) III sustatinerd accommodation with a sudden demand on accommodation upon full correction.
(iii) Prolonged accommodative fatigue after full correction. (bifocals).
(iv) Prepresbiopic myopes.
The purpose of these is to avoid accommodational effort & associated accommodational strain. Secondly preventing the rise of IOP & its deleterious effects on weak scleral coats. In bifocal, on addition of 0.75 D is usually adequate.
Prism base-in is frequently prescribed along with orthoptic treatment of myopia. Theoretically this causes a reduction in the action of extrinsic muscles during convergence. This will also avoid accommodation associated with convergence. The following parameters are aimed to effect the progression of myopia by authors at -
(a) Precise correction
(h) Under correction (Bifocals)
(ii) By affecting Convergence:
- Pirsm base-in
(III) Lens Power variation
- Full correction as such
- Full correction worn full time
- Worn only on vision demand.
Worn either way with prism base in
- Worn either way with accompanying orthoptic exercises.
- Worn only for distance.
Under correction of lenses
- Full for distance & near
- Full correction for distance & near
- Under correction for near (Bifocals).
Contact Lenses In Myopia:
Routube Fitting: It is advantageous to use contact lens rather than spectacles because of
- Wider field of vision.
- Lesser distortions
- Minimal prismatic effect.
- Higher image magnification.
- Negligible distortion of shape & Better adaptation in anisometropia
The choice of lens fitted depends upon:
(a) Fitters likes & dislikes.
(b) Patient's demands
(c) lens availability. Lenses are
a - PMMA
b - HEMA
c - G.P.L.
d - CAB lenses
e - Silicone lenses. f - Silicone acrylate g - Sterene Lenses.
Effect of Contact Lens on Myopia:
It is well established fact that contact lenses can alter the refraction & keratometric readings. The study was undertaken at RAPCOS on the behaviour of myopia in young subjects. All the patients were provided with a well fitted hard contact lenses.
| MATERIALS & METHODS|| |
130 eyes in the age group of 12- 20 years were studied. Keratometry & refraction was done at 3 stages.
Stage 1- 1. Before fitting the C.L
Stage II- After 3 years of regular C. L. wear
Stage III - After 3 weeks of total C.L. with drawal
It is clear from [Table - 1] that 76% showed a decrease in myopia which has gone to an extent of 2,25 D, in somehow it has been achieved with contact lens seems to be due to its effect on the accommodational status.
As the natural course of myopia in subjects is not known it is difficult to interpret the findings. However, following factors may contribute in contact lens causing a retardation in progress of myopia.
(1) Contact lens has a pushing back effect on the streched coats of eye ball.
(2) Contact lens causes a natural shift to wards the accomodation convergence relationship.
(3) Contact lens has retaining effect on the curvature.
(4) Hypoxia caused by contact lens may cause a curvatural alteration.
After studying the behaviour of myopia under a well fitted hard C. L. study was undertaken to know the effect of a contact lens in modifying the refractive status (Orthokeratology which is defined as the procedure designed to effect the reduction or elimination of refractive anomalies by programmed application of contact lens. The study was undertaken, RAPCOS,, New Delhi. 60 eyes of patients aged 12 to 28 years were fitted with hard contact lenses. The refractive errors ranged between - 0.5D to -8.ODD. All the cases had their visual acuity correctable to 6/6 with glasses. Baseline refraction was done under cycloplegia. Keratometry & naked eye visual acuity was noted at first visit.
Choice of base curve & overall diameter was seen to be consistent with the dgood physical & physiological outcome during the day's continuous wear. The patients were called for follow ups every week and they were off the lenses for at least 24 hrs. before the parameters were studied. At each subsequent visit, keratometry or refraction equal to or greater than 0.50D was considered significant only if it was found at 2 or more occasions. A new pair was given with a reduction in B.C. equal to keratometric flattening. Patients subjective symptoms were noted and no lenses were dispensed which produced irritation or inability to tolerate the lenses during all the watering hrs.
Patients who did not respond to this methodology for 3 months were tried with secondary methodology of going.
( i) More flat.
(ii) Increasing the overall diameter.
(iii) Increase in lens thickness.
(iv) Increase in wearing time.
Results are as under -
When keratometary was taken as a criteria 61.7% showed a positive response (Flattening) & 38.3% non responders. Change in keratometry noted.
It is obvious from preceding tables that 70% of the patients responded to the initial treatment & 28% did not respond. Duration of positive response started in 2 weeks and 14.3% of cases showed positive response in 3 months. Highest number of 47.6% responded within 1 month. Out of secondary methodology increase of O.D. and thickness were the only methods of success.
It is a preliminary report the response of cases is being highlighted with regard to changes in base curve O.D., thickness and duration of wear in compassion to conventional methodology.
It could be predicted that patients with keratometric readings below 45.00 D and spectacle correction in between -3.00 to -6.00 D & younger age responded best to the technique.
However at present the study is continuing to guide the variations in individual parameters which will make the authors to predict a positive or a negative response on the basis of known ocular parameters.
A few common problems which patients found during methodology include frequent lens displacement, frequent lens loss. More photophobia, induced astigmatism or spherical corneas which is partially recoverable after discontinuing the lens.
[Table - 1], [Table - 2], [Table - 3], [Table - 4], [Table - 5], [Table - 6], [Table - 7]