SUBJECTIVE REFRACTION

By

praveen

Subjective refraction technique rely on the patient’s response to obtain the refractive correction that gives the best visual acuity .

If all the refractive errors were simply spherical ,subjective refraction will be easy.

Determining the astigmatic portion of the correction is more complex .

Different subjective refraction techniques are employed .09323330039 paul

Cross-cylinder technique[jackson cross cylinder ] First step in cross cylinder refraction is

adjusting the sphere to yield best visual acuity without accommodation .

Place the prescription the patient is wearing ,if any ,into a trial frame or phoropter .

Fog the eye to be examined with plus sphere while the patient views a visual acuity chart.

Then decrease the fog until best visual acuity is obtained

If no cylindrical correction is present initially ,the cross cylinder may still be used ,placed arbitrarily at 90 0 and 180 0 .

To check the presence of astigmatism . If a preferred flip position is

found ,cylinder is added with axis parallel to the respective plus or minus axis of the cross cylinder until the two flip choices are equal .

If no preference is found with the cross- cylinder axis at 90 0 and 180 0 ,45 0 and 135 0 should always be checked before assuming that no astigmatism is present .

Once any cylinder power is found , axis and power are refined in the usual manner .

Cylinder axis is always refined first . This sequence is necessary because

the correct axis can be found in the presence of an in correct power .

But the full cylinder power will not be found in the presence of an incorrect axis .

To refine the axis place the cross cylinder with its principal meridians 45 0 away from the principal meridians of the correcting cylinder .

Present the patient with alternative flip choices and inquiring which is” blackest and sharpest “.

Rotate the axis of the correcting cylinder toward the corresponding plus or minus axis of the cross cylinder .

Low-power cylinder are rotated in large steps ,high powered cylinder are rotated in small steps ,repeat the procedure until the flip choices appear equal .

To refine cylinder power ,align the cross – cylinder axis with the principal meridians of the correcting lens .

As the examiner changes cylinder power according to the patient’s response .

The spherical equivalent of the refractive correction should remain constant .

That is for every 0.50 D of cylinder power change ,the sphere is changed 0.25 D in the opposite direction .

If the cylinder power is changed a large amount ,the sphere power should be readjusted for best visual acuity .

Continue to refine cylinder power until choices appear “about the same “to the patient ,that will be the correct endpoint .

Remember always to use the proper cross cylinder for the patient’s visual acuity level .

For eg :- +- 0.25 D cross cylinder is commonly used with visual acuity levels of 20/30 and better .

A high cross cylinder [+- 0.50 D or +- 1.00 D ] should be used with poorer vision .

Following steps are used in cross –cylinder refraction .

Adjust sphere to the most plus or least minus that gives best visual acuity .

Use test figure one or two lines larger than the patient’s BCVA , because the introduction of the JCC produce blur .

If cylinder correction is not already present ,look for astigmatism .

Refine axis first Refine cylinder power . Refine sphere ,cylinder axis ,and

cylinder power .

Tricking

Avoid confusion with previous choices by giving different number to subsequent choices

Which is better ,one or two ,three or four?

If the patient persists in always choosing either the first or the second number ,reverse the order .

Clock Dial

When the clock dial is presented to the patient under sufficient fog .

The examiner’s first job is to determine the axis of the correcting cylinder .

This is done by first asking the patient if he or she can see three lines in any or all of the spokes .

And then ask to report in which of the spokes the three lines are the sharpest or most distinct .

Most clock dial charts equipped with numbers similar to those on the face of a clock .

Expected response of the patient is that the spoke from 12 to 6 ,from 1 to 7 ,from 2 to 8 o’clock is most distinct .

To determine the axis of the correcting cylinder ,the smaller of the two number reported by the is multiplied by 30.

For example ,if the patient reports that the 12 t 6 o’clock spoke is most distinct the examiner would place the axis of the correcting cylinder at 180 degrees.

The examiner then begins adding minus cylinder power ,0.25 D at a time .

Questioning the patient each time as to the relative sharpness of the lines in the spokes representing the two principal meridians of the eye .

If sufficient minus cylinder power is added ,the horizontal and vertical focal lines will be located in the same plane .

Then the patient will report that the 12 to 6 and the 3-to -9 spokes are equally distinct .

This procedure is referred to as “collapsing the conoid of sturm “ the horizontal and vertical focal line are replaced by a point image .

Monocular subjective refraction

Monocular subjective refraction consists of the following procedure

Determining the cylindrical correction under fog.

Refining the cylindrical correction without fog .

Determining the spherical end point . Following these procedure ,one or more

binocular balancing tests are performed .

Determining the cylinder under fog

Suitable fogging lenses will have to be put into place before beginning the monocular subjective refraction .

The left eye is occluded ,a block of letter is introduced at the 6-m distance .

Plus power in front of the right eye is reduced ,0.25 at a time until the patient can read all of the 20 /40 letters .

Ensures that the eye is fogged by approximately 1.00 D ,so the entire conoid of sturm is in front of the retina .

The astigmatic chart is presented ,and the axis of the correcting cylinder is determined .

If clock dial is used ,the patient is asked first whether three lines can be seen in any or all of the spokes .

If he can see then ask the patient to report in which of the spokes the three lines are the most distinct .

Minus cylinder is added to the indicated axis 0.25 D at a time until equality is obtained .

Additional minus cylinder power is then added 0.25 D at a time to obtain a reversal .

Then the examiner returning to the lowest-power cylinder that brings about equality .

The right is then occluded and the procedure are repeated for the left eye .

Refining the cylinder without fog On completion of the astigmatic chart test . The right eye is defogged ,reducing plus

lens power 0.25 D at a time ,until the best visual acuity line [20/20 or 20/15 ] is reads.

When the removal of additional plus power [or addition of minus power ]fails to improve visual acuity ,the spherical lens power is returned to the maximum plus or minimum minus power that resulted in the best visual acuity for that eye .

Beginning with the cylinder power and axis found with the astigmatic chart under fog .

The practitioner first refines cylinder axis and then cylinder power .

Crossed –cylinder test for Axis

Crossed –cylinder Test for power

Additional cylinder check tests

Additional check test for cylinder power . Many patient tended to reject cylinder

power in the crossed- cylinder test even though it has been evident both in retinoscopy and in the astigmatic chart test .

This occurs ,the patient is asked to watch the smallest readable row of letters .

An additional -0.25 D cylinder is placed in the refractor ,and the patient is asked to report which of the two views appears to be more distinct .

If the additional -0.25 D cylinder does not improve the clarity of the letters ,the original cylinder power is left in the refractor .

If the additional cylinder power causes the letters to be more distinct ,the examiner can choose to leave the additional cylindrical power in the refractor.

Additional check test of axis The additional cylinder check test for

axis is sometime referred to as bracketing .

It is useful mainly when the power of the correcting cylinder is 1.00 D or more .

The patient is asked to view a row of 20/20 or 20/15 letter

And ask to report when the cylinder lens in the refractor is slowly rotated.

When the blur is reported ,the change in the axis is noted mentally .

And the test is repeated with the same instructions .

For example : if the original correcting cylinder axis is located at 180 degrees .

The patient reports a blur at 15 degrees and blur at 165 degrees ,then the examiner can assume that the orginal axis was correct .

Determining the spherical end point Each eye is fogged to 0.75 or 1.00 D and

defogged to best acuity. Maximum plus power of best visual

acuity . Examiner should present a block of letters

extending from 20/40 to 20/15. We should make a mental note of the

patient’s acuity for each eye through the +0.75 or +1.00 D fogging lens prior to defogging .

It is important that accommodation be relaxed in arriving at the monocular end point .

For example : +1.00 D fog may blur the right eye to 20/30 ,but it may blur the left eye only to 20/20 .

This evidence that the left eye is underplussed [or over minused ] ,and the subjective end point for both eye should be determined again .

Patient instruction

To determine the monocular endpoint correct patient instruction are important .

The examiner should understand that the patient’s subjective evaluation of the clarity or distinctness of the letters is not the important consideration .

The important ,overriding consideration is the ability of the patient to resolve the letters .

As plus power is decreased 0.25 D at a time .

When the point has been reached where an additional decrease in plus power of 0.25 D does not make any more letter readable.

On the other hand ,if the examiner allows the patient to respond in terms of clarity of the letter or in terms of which lens is preferred .

Many young patient will continue to accommodate with each 0.25 D of reduction in plus ,with the result that the end point will be completely invalid.

When minus lens power is added to the point that accommodation is necessary to keep the letters in sharp focus on the retina

Many patient will notice that the letter appear to be smaller .

This is purely optical effect ,known as accommodative micropsia .

it is right to ask the patient if the letter looks smaller to avoid overminusing the patient .

If patient report that the letter are not smaller should not be taken as evidence .

Binocular balance The purpose of binocular balancing tests is

not to balance the visual acuity but to balance the state of accommodation of the two eyes .

If the corrected visual acuity is same in both eye ,the balancing procedure may consist of comparison of the visual acuity for the two eyes .

If the correcting visual acuity is not same in both eyes ,then a method not based on visual acuity must be used .

Procedure used for balancing the state of accommodation for the two eyes are often referred to as equalization tests or as binocular balancing tests .

Balancing with little or no fog On completion of the monocular

subjective refraction ,the lenses are left in the refractor .

The occluders has to remove from both eyes .

The patient’s attention is called to a block of letters at 6 m.

Plus lens power is added in front of both eyes until the 20/20 letter are blurred but the 20/25 letter are easily resolved .

This normally requires an increase in plus or a decrease in minus of 0.25 to 0.50 D.

Then ask to compare the clarity of the 20/25 letters for the two eyes .

Using either prism dissociation or alternative occlusion .

Prism dissociation

The examiner place 3 of base –down prism in front of the right eye and 3 of base-up prism in front of the left eye .

The patient will see two charts separated –vertically .

The upper chart seen by right eye . Patient attention is called to the 20 /25

letters .

Then asked to report whether the letter are more distinct or easier to read in the upper chart or the lower chart .

If the equally distinct for the two eyes, the accommodative state of the two eyes is considered to be balanced ,and the test is over .

If the patient reports a difference in clarity of the letters for the two eyes.

0.25 D is added in front of the eye with the better vision and the test is repeated .

Often the patient fails to report equal clarity for any lens combination .

This problem can be solved by instructing the patient to report “which of the lens combination causes the upper and lower charts to be more nearly equal .

Another way of resolving this difficulty is to give better acuity to the dominant eye .

Once the patient’s acuity is balanced at 20/25 the patient is defogged binocularly.

Alternate occlusion

Patient is instructed to compare alternate views of the chart while the each eye is alternately occluded.

Occluder in the refractor can be used for occlusion .

Fogging to 20/25 . Adding 0.25 D to the better eye ,are

done in the same manner of prism dissociation test .

Check Test for the Binocular End point

Once the examiner is satisfied with the binocular balance ,binocular end point can be verified .

Patient’s attention is called to the 20 /20 letter .

Add +0.25 D sphere to both eye and ask for any difference in the clarity of the letters.

The same question is again asked after adding 0.25 D sphere .

Again third +0.25 D sphere is added ,ask of the difference in the clarity .

Expected response are that the 20/20 letters will be “slightly blurred “. With the first 0.25 D of plus .

“badly blurred “ with the second 0.25D. “blurred out “ with the third 0.25 D .

Bichrome test As a monocular End –point test

The bichrome test must be done in an almost completely darkened room .

Starting with the result of monocular subjective finding .

+0.50D or + 0.75 D of spherical power is placed in front of each eye.

The red and green filter is placed on the projector.

The patient is asked to report which of the letters ,those on the red background or those in the green background –are “sharper blacker ,or more distinct “.

The patient is expected to report that the letters on the red background are more distinct than those on the green background .

As the plus power is reduced 0.25 D at a time .

At some point the patient should report that the letter on red and green background are equally distinct .

As plus power is further reduced ,the patient should say those on the green side are more distinct than the red side .

If the original monocular subjective endpoint was correct ,the patient will typically report that the red letters are more distinct with +0.75,+0.50,+0.25 D fog .

When all the fog is removed the letter will appear to be equally distinct on both the red green sides .

Prescribing for children Prescribing visual correction for children often

has two goals : Providing a focused retinal image . Achieving the optimal balance between

accommodation and convergence .

In some cases subjective refraction is impossible because child’s inability to cooperate.

The optimal refraction in an infant requires the paralysis of accommodation with complete cycloplegia .

The presence of strabismus may modify normal prescribing guidelines .

Myopia

[general guidelines for correction of significant childhood myopia]

Cycloplegic refraction are mandatory . Full refractive error ,including

cylinder ,should be corrected .young children tolerate cylinder well.

Some will under correct myopia ,other may even use bifocals with or without atropine ,on the theory that prolonged accommodation hastens .

Intentional undercorrection of a myopica esotrope to decrease the angle of deviation is rarely tolerated .

Indentional overcorrection of a myopic error can be of some value in controlling an intermittent exodeviation .

Parents should be educated about the natural progression of myopia and the need for frequent refraction and possible prescription changes .

Contact lenses may be desirable in older children to avoid the problem of image minification found with high- minus lenses.

Hyperopia When hyperopia and esotropia

coexist ,initial management includes full correction of the cycloplegic refractive error .

In school age child ,the full refractive correction may cause blurring of distance vision because of the inability to relax accommodation fully .

The amount of correction may have to be reduced for the child to accept the glasses.