Subjective Refraction

                Subjective Refraction

Objective Refraction-------

Objective determination of the refractive Status, Independent of patient response.

  -Objective tests are usually quicker than subjective, but their results usually need to be refined using subjective methods.

A retinoscopy is a form of objective in which the judgment of a human operator is required to determine the refractive error.

Subjective Refraction----------

Subjective Refraction is an attempt to determine, by trial and error using the patient's cooperation, the combination of lenses that will provide the best-corrected visual acuity (BCVA).

Subjective determination of the refractive status, dependent on patient response.

Subjective Techniques----

Subjective Refraction has two technique-----A. Monocular TechniqueB. The binocular technique (Binocular balancing)

A. Monocular Technique:-

Monocular distance subjective tests

1. Fogging

2. Duochrome test

3. Stenopaic Slit

4. Astigmatic Fan

5. Jackson’s Cross Cylinder

B. The binocular technique (Binocular balancing):-

  6. Binocular balancing

1. Fogging------

  Aim-

To relax the accommodation

To determine maximum plus (minimum minus) spherical power which provides the subject maximum visual acuity.

Principle- To make patient artificially myopic by plus lenses.

Prerequisites:-

Objective Refraction

Dark room

Procedure:-

•Performed Monocularly
•Start with gross retinoscopy value
•Defog the patient by +0.25DS or by +0.50DS, till the maximum visual acuity is obtained.

End Point:

           Maximum plus with best visual acuity

           Minimum minus with best visual acuity

2. Duochrome test:-

                            A duochrome test is a test commonly used to refine the final sphere in refraction, which makes use of the transverse chromatic aberration of the eye. Because of the chromatic aberration of the eye, the shorter wavelengths (green) are focused in front of the longer red wavelengths.

Principle: Longitudinal Chromatic Aberration

Aim: To evaluate the amount of correction

Procedure:

The room should be dark

Monocularly

The patient wears the correction

Ask the patient, letters in which

The background is clear.

3 Possibilities:

Letters in both backgrounds are equally clear or blur- NORMAL

Letters in the red background are clearer- ADD MORE +0.25D

Letters in green background are clearer- MINUS MORE -0.25D

End point: Equal sharpness of letters in 2 different color background
3. Stenopaic slit

It's an opaque disc perforated by a slit-like opening. It acts as an elongated pinhole.

Diameter: 25mm, width 1mm

Aim:

To find out the principal meridian of an astigmatism eye

To differentiate between cataractous and glaucomatous halos

Principle: Act as an elongated pinhole

Procedure:

Fog the patient

Compound myopic astigmatism is induced

The subject wears the spherical correction

Monocular testing

Place the slit on top of the correction.

Ask the patient to concentrate on 6/18 visual acuity line

Rotate the Slit in all meridians

Ask the subject to report if Sharpness and clearness of letters increases in any meridian

2 possibilities:

Vision is the same in all rotation 

In one meridian the vision improves

Correction:

Place the axis of minus cyl in same stenopaic slit position where the patient has reported of vision improvement.

Endpoint: In all rotation the vision the response should be the same

4. Astigmatic Fan

Aim: To evaluate Principle meridian from an astigmatism eye

Procedure:

Fog the patient

Done on top of spherical correction

Ask the patient to report which line is clear or blur

Endpoint: All the lines should be equally clear or blur

5. Jackson Cross Cylinder

Flip Cylinder

JCC is the type of sphero - cylindrical lens, having the combination power of both plus cylinder and minus cylinder of equal power

Principle:

Spherical equivalent is zero (eg.+0.25/-0.50X90)

Aim:

Is to refine the power and axis of the corrective lens

Procedure:

Start monocularly with the subject wearing the correction

Flipping of JCC is done and the response of the subject noted

Equal distinctness of the letters is the endpoint

For refining the power

The principal meridians of the JCC are located parallel to the principal meridian of corrective lens

Flipping is done

Endpoint: equal distinctness of letters

For refining the axis

The principal meridian of the JCC are located at 450to the principal meridian of corrective lens

Flipping is done

Endpoint: equal distinctness of letters

6. Binocular balancing

 The final important step of subjective refraction is to make certain that accommodation has been relaxed equally in the 2 eyes is known as binocular balancing.

PURPOSE: – To Equalize accommodation between the two eyes – Not to equalize the best-corrected visual acuity between the two eyes: CANNOT BE PERFORMED WHEN: – patient suppresses one eye – strabismus exists – px has only one eye

Why? – Pre-presbyopic patients can accommodate during the distance monocular refraction – Patients may actually accommodate different amounts in the left and right eyes

 Methods for binocular balance technique: 

There are various methods that may be employed to perform the binocular balancing of a prescription.

 These include: – 

1. Successive Alternate Occlusion

2. Vertical Prism Dissociation

3. Blurring or fogging (Humphriss Immediate Contrast) 

4.Polarized or Vector graphic

5 Turville Infinity Balance (Septum technique)

1. Successive Alternate Occlusion:-

Successive Alternate Occlusion:

Procedure: – ADD plus lenses binocularly till px barely sees 20 /50 line(6/18) over the Rx you just achieved from the monocular subjective refraction – examiner performs slow alternate occlusion – Patient compares of letters between 2 eyes – “I know this is blurry, I want you to tell me which image is clearer, or are they both equally blurred.”

ENDPOINT – the endpoint is equal acuity b/w 2 eyes – If one eye clearer than – Plus lenses added in + 0.25DS steps before the clearer eye to equalize the blur

 2. Vertical Prism Dissociation:

Procedure: 

The two eyes will be dissociated by using the prisms in the technique 

The same amount of fog as last technique +1.00 

The same isolated line used 20/50 

Keep the prisms of 3 or 4 Δ BD in OD and 3 or 4 Δ BU in OS

Equal and opposite 

BD prism will cause the image to be higher (top)

The image will be displaced towards the apex (point) of the prism Δ BU prism will cause the image to be lower (bottom) 

BD for OD and BU for OS, therefore; – EGNU5 seen by OD – EGNU5 seen by OS

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