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Transcript of Subjective refraction
SUBJECTIVE REFRACTION VERIFICATION BY SANJIVANI SINGH
SUBJECTIVE REFRACTION Meant for finding the most suitable lens to be prescribed and requires patients cooperation for proper estimation of the refractive error.
Can be carried out after objective refraction or even without that.
When cycloplegics used , subjective refraction (pmt) should be carried out after 3-4 days ( homatropine or cyclopentolate used) and 14 days when ( atropine used)
Steps of Subjective refraction:-
A. Monocular subjective refractionB. Binocular balancing and C. Correction for near vision
MONOCULAR SUBJECTIVE REFRACTION AIM-To find out cylindrical lens (exact power and axis) and best vision sphere for each eye separately.
PROCEDURE-Widely accepted protocol described includes:-
1. Selection and verification of baseline starting point. 2.Refinement and finalization of cylindrical lens axis and power. 3.Refinement and finalization of spherical lens.
SELECTION AND VERIFICATION OF BASELINE SARTING POINT:- Baseline starting point for objective can be obtained from: 1.Retinoscopy 2. Autorefractometer or 3. Evaluation of patients old glass
The subjective verification of sphere and cylindrical can be performed by Trial and error technique,( use of trial spherical BEST VISION SPHERE and cylindrical lens based on baseline starting point)
REFINEMENT AND FINALIZATION OF CYLINDRICAL LENS AXIS AND POWER
The cylindrical lens can be finalized by following techniques:-
1. Astigmatic clock dial and fogging technique 2. Jacksons cross-cylinder technique 3. Astigmatic fan and block technique
Astigmatic clock dial and fogging technique Steps :-1.Obtain best visual acuity in one eye occluding other.2.Fog the eye 3. ask patient to look at the dial and identify darkest and sharpest line 4.Add minus cylinder of progressively increasing power(90 degree to darkest and sharpest line) till line appear equal. 5.All lines appear in dial appear equally black.
ASTIGMATIC CLOCK DIAL
Jacksons cross cylinder technique:- Combination of two cylinder of equal strength with opposite sign and their axis at 90 degree.Used to refine cylinder axis and power of refraction already obtained
Steps:-1. Adjust sphere to the most plus or least minus.2.Discover the astigmatism.3. Refinement of the axis.4.Refinement of cylinder power.
JACKSONS CROSS CYLINDER
ASTIGMATIC FAN AND BLOCK TECNIQUE:- Also called Maddox V test
1. Obtain best vision sphere. 2. Add a positive sphere equal to half he amount of astigmatism estimated. 3. Refer the patient to the fan chart and ask which line and group of line appear clearest and darkest. 4. Direct the attention to the Maddox arrow and rotate it away from its blacker limbs appear equally blurred. 5. Directing attention now to blocks, add negative cylinder at the appropriate axis until the second clears as the first. 6. Now check by adding +o.50DS or critical observer then add + 0.25DS.
ASTIGMATIC FAN CHART
REFINEMENT AND FINALIZATION OF THE SPHERICAL LENS After the cylinder power and axis have been refined, the final step in monocular refraction is refining of the sphere which can be done by following techniques:-
1. Fogging technique using snellens visual acuity chart:-
A simple criteria is that to fog the eye and then unfog by reducing every time +0.25DS till the best snellens visual acuity is attained.
2. Duochrome test:
Based on the principle of chromatic aberration. In emmetropes yellow light (570mm) is focused on the retina, while red (620mm) and green light (535mm) are focused behind (0.24D) and in front (0.20D) of the retina. Easily and rapidly performed , has inherent weakness that is it doesnt relax accommodation, therefore patient is slightly fogged. Is not much useful with visual acuities worse than 6/9 (20/30), for the 0.5D difference between the two slides become difficult to distinguish.
3.Pinhole testing:-Pinhole test is helps in confirming whether the optical correction is correct or not.
BINOCULAR BALANCING The final step in the subjective refraction is binocular balancing- a process sometimes known as equalizing the accommodation effort or equalization of vision and allows both eye to have the retinal image simultaneously in focus .
A few commonly used methods are:-
1.FOGGING AND ALTERNATE OCULLUSION METHOD :- In this method , both eye (with best accepted lens) are fogged with >/< 1.0DS , reducing vision and then alternate cover test is performed and ask patient to whether the eye showing comparatively clear image. If balanced then equal blur in both eye and if not then add +025DS until both eyes r equally blurred.2
2. DUOCHROME TEST WITH FOGGING:- In this , each eye ( best correcting lenses) observes the vision chart while the fellow eye is fogged with +1.0DS. Sphere before observing eye is adjusted to give red or green preference.
3.PRISM DISSOCIATION METHOD:- This method is done by , fogging both eye with >/< 1.0DS and a vertical prism of 3or 4 placed base down in front of right eye and base up in front of left eye and then single line usually 6/12 is projected on the chart. If binocular balance present then patient se the same line with both eyes simultaneously, if not ( difference in clarity) then +0.25DS is placed before the eye with better vision , done until two lines are equally distinct for both eye. Considered as the most sensitive method and so practised more commonly.
4. TURVILLE INFINITY BALANCE TECHNIQUE
5. POLAROID FILTERS
CORRECTION FOR NEAR VISION Correction for near vision is indicated usually after 40 years of age.When the distance vision has been corrected , the visual acuity for near should be estimated using any of the near vision charts:-Jaegers chart, Snellens reading test type or number point.1.DETERMINATION PF AMPLITUDE OF ACCOMMODATION: Estimated for presbyopic correction. Average amplitude of accommodation in following table:- AGE ACCOMMODATION AMPLITUDE FROM 8YEARS OF AGE DECREASES BY 1D FOR EVERY 4 YEAR TILL 40 YEARSFROM 40 -48 YEARS OF AGE DECREASES BY 1.5D FOR EVERY 4 YEARS FROEM 48 YEARS OF AGE DECREASES BY 0.5 FOR EVERY 4YEARS
2. DETERMINATION FOR NEAR POINT OF CONVERGENCE 3. DYNAMIC RETINOSCOPY
4. DETERMINATION OF NEAR ADD