Subjective Refraction Techniques

Specimen Collection

Instruments required

• Manual refraction unit or phoropter
• Trial frame
• Trial lenses and confirmation set
• Snellen's chart
• Pinhole
• Occluder
• Jackson cross cylinder- They combine two cylindrical lenses with equal power but the opposite sign (+/-), having an axis perpendicular to each other - they are used to diagnose astigmatism
• Duochrome test- To check the spherical power[6] 

Procedures

Subjective Refraction Steps

• Subjective refraction monocularly
• Binocular balancing
• Near vision correlation[2]

Monocular Subjective Refraction

The primary goal of monocular refraction is to find the spherical power, cylindrical power, and axis.

Steps

• Baseline starting point selection and verification
• Cylindrical lens power and axis refinement and finalization
• Spherical lens refinement and finalization

Baseline Starting Point Selection and Verification

The subject is seated at 6 m. A trial frame is placed over the eye, and the visual acuity is tested for each eye separately. Although subjective refraction can be easily obtained directly, it is always recommended to get objective refraction done first.[7]  

The objective refraction is done using retinoscopy, old glass prescription, and autorefractometry. While starting to perform the subjective refraction, the uncorrected visual acuity will give an idea about the amount of ametropia.[8] The verification of cylinder and sphere is usually performed by trial and error.[9]  

Trial and Error Method

Spherical Lenses

This should be verified first. In the case of myopia, the weakest concave lens, and in hypermetropia, the strongest convex lens forms the best vision sphere.[10]  To find the best vision sphere, a series of low power spheres such as +0.25, +0.5, -0.25, and -0.5 are quickly interchanged over the front trial lenses. The myopic patients are asked which correction makes the letter appear clearer and darker.[11] 

Cylindrical Lenses

The cylindrical lenses need axis and power verification. Usually, the axis is checked first. 28030877

Axis Verification- This is done by rotating the cylinder's axis in increments of 5 and 10 degrees in either direction and confirming from the patient whether the visual acuity improved or not.[12] It is challenging to appreciate the change in smaller cylindrical powers at which axis the acuity is better. Under such cases, a more substantial cylindrical power is tried to verify the axis.[13] 

Power Verification- is done after axis verification by changing the cylindrical power.[14] 

Cylindrical Axis and Power Finalization and Refinement

Usually, the spherical component is refined and corrected after cylindrical astigmatism has been taken care of to obtain a sharper and clearer image. Hence, it is always mandatory to correct and refine the cylindrical astigmatic error before the spherical component. There are various methods of purifying the cylindrical correction.[15] 

 These include 

• Astigmatic fan and block technique 
• Jackson cross-cylinder technique
• Astigmatic clock dial and fogging technique

Astigmatic Fan and Block Technique

In some cases, the patient cannot respond well to cross-cylinder testing. In these cases, the Maddox V test comes in handy, an old fan and block technique method.[16]  This consists of a series of radiating lines spaced at an interval of 10 degrees and arranged in the form of sun rays having a V and two sets of mutually perpendicular lines called blocks. The V and block can be simultaneously rotated at 180 degrees.[16] 

Steps of Astigmatic Block and Fan Technique

Best Vision Sphere - The first step should be to obtain the visual acuity using a sphere only. It is thought that the best sphere gives the circle of least diffusion on the retina.[17] 

Positive Sphere Addition - The positive sphere, half the amount of estimated astigmatism, should be added to bring the eye to a state of simple myopic astigmatism.[18]

Fan Chart- The patient should be seated at the fan chart and asked which line or group appears dark and which appears clearer. This gives a fair idea of the direction of astigmatic error. After this, a simple check must be performed by adding a +0.5 D sphere to confirm that the eye is in a state of simple myopic astigmatism. The black line should appear blurred, but if not more positive sphere should be added till it seems to be confused.[12] 

In some cases, the clear line change through 90 degrees, indicating a state of simple hypermetropic astigmatism with an anterior focal line on the retina. Hence, furthermore, the sphere must be added till it appears blurred.[19] 

Maddox Arrow - The focus should be directed to the Maddox arrow, and rotation should be done from the black limb until both limbs appear blurred. This helps in knowing the axis of astigmatism.

Blocks - Then direct the focus towards blocks and add a negative cylinder at the required axis until the second one becomes apparent as the first one. Suppose this doesn't happen, first under-correct and then overcorrect the astigmatic error.[20]

Second Check - A double-check must be performed by adding a +0.5 D sphere or a +0.25 D sphere with keen observation. Both blocks must blur equally, but astigmatism has been corrected if the black lines interchange. If the darker block becomes black again, the probably initial sphere correction was wrong and must be reassessed.[8]

Jackson Cross Cylinder Technique

This is a combination of two cylinders of equal strength but having an opposite sign, first described by Edward Jackson. The cylinders are mounted on a handle, and their axis is at the right angle. The commonly employed ones are +/- 0.25 and +/- 0.50. These are more useful than any other lenses in clinical refraction. These are usually employed to refine the cylinder's axis but can be used to assess complete astigmatic refraction.[21]  

Steps

Sphere Adjustment

The sphere should be adjusted to most plus or least minus correction, giving the best-corrected visual acuity. This is achieved by performing fogging ( adding a positive sphere before the eye) while checking the visual acuity on the chart and then reducing the fog till a visual acuity is achieved. The primary aim is to place the circle of least diffusion on the retina, thus resulting in mixed astigmatism.[22]

Finding Astigmatism

If the cylindrical correction is not known initially, the Jackson cross-cylinder can still be used arbitrarily at 90 and 180 degrees to assess astigmatism. If the flip position is found, a cylindrical correction can be added with the axis parallel to the plus or minus axis of the cross-cylinder until the flip choices are equal. If the astigmatism is not discoverable at 90 and 180 degrees, then  45 and 135 degrees should be checked before assuming that no astigmatism is present.[12] 

Axis Refinement

This should be attempted first. This is because a correct axis can be determined in the presence of an incorrect power, but full cylindrical power cannot be determined in the presence of an incorrect axis. For axis refinement, a cross-cylinder of +/- 0.5 D can be placed in front of the eye with the handle parallel to the axis in the trial frame, and then the patient is asked to tell any changes in the visual acuity.

If the patient discerns no difference between the two positions, the cylinder's axis is correct. If the visual acuity improves in a particular position, a plus cylinder should be rotated in the direction of the plus cylindrical component of the cross-cylinder. The test must be repeated several times till a neutral point is achieved.[12]

Cylindrical Power Refinement

To assess the cylinder's power, a cross-cylinder of +/- 0.25 D is placed in front of the eye, with its axis parallel to the cylinder's axis, first with the same sign and then with the opposite sign. In the first position, the correction is improved by 0.25 D, and in the second, it is diminished by the same power. If there is no improvement in any positions, then the cylinder's power is correct. However, if there is an improvement, the power should be verified again and improved.[2] 

Astigmatic Clock Dial and Fogging

Steps

First, the visual acuity is obtained by placing a sphere with the other eye being occluded.[23]

Second, make the eye in compound myopic astigmatism but put a plus sphere before the eye to focus all the meridians before the eye. When the eye is fogged, the accommodation makes the image more blurred; hence the patient tends to relax accommodation and stabilize the refractive error of the eye.[24]

Third, the patient is asked to focus on the astigmatic dial and determine the darkest and sharpest lines.[25]

Fourth, minus cylindrical lenses of increasing power are used with an axis perpendicular to the black and sharp lines till the lines appear equal. A rotating cross dial is used and aligned with the principal meridian to compare the two meridians. The rule of 30 is used to calculate the axis of the minus cylinder. The lower number of the astigmatic dial is multiple with 30 to get the axis. For example, if the black line is seen at the 6 to 12 o'clock position, the lower number is multiplied by 30 to get the axis of the minus cylinder, i.e., 30 x 6= 180.[26]

Once all the lines in the astigmatic dial appear black equally, but still not incorrect focus because the eye is fogged. Now use the distance vision chart and reduce the plus sphere till the patient achieves a clear vision. [15]

Spherical Lens Refinement and Stabilization

After the cylindrical axis and power have been determined, the final step is sphere refinement by using fogging technique with Snellen's visual acuity chart, duo-chrome test, and pinhole visual acuity test.[27] 

Fogging

A perfect way to determine spherical power is to fog the eye and then start unfogging by reducing the sphere by a +0.25 D sphere until a clear image is obtained. While termination of the test, it would be difficult to ascertain the endpoint as small changes in spherical power may not further refine the axis, and the examiner may land in a state of confusion. Hence, the Duochrome test is used to get the endpoint correction at this stage.[28]

Duochrome Test

This test is based on the chromatic aberration principle. The yellow light is focused on the retina in a normal human eye. The red light is focused at 0.24 D behind the retina, and the green light is concentrated at 0.20 D in front of the retina. In this test, the patient is told to read letters graded from 6/18 to 6/5 with green and red backgrounds.

In emmetropia, both colors give an equally sharp image. In myopia, the red appears clearer, and in hypermetropia, the green one. The lenses then should be adjusted to make the image appear clear. The only limitation with this test is that it doesn't relax the accommodation. Hence the test should be performed when the eye is slightly fogged. The letters on the red should appear clear, and minus power should be added till the red and green are equally clearer.[29]  

Pinhole Testing

Finally, a pinhole test should be done to know that the correction in the trial frame is correct. Any improvement in the pinhole acuity indicates that the correction is incorrect.[30] 

Binocular Balancing

This is the final step of subjective refraction, also known as vision equalization or equalization of accommodative effort. This process helps in the simultaneous focus of retinal images. A difference in appearance will lead to asthenopic symptoms and an imbalance in accommodation.[2] 

The various proposed methods of binocular balancing are

• Fogging and alternate occlusion method
• Duochrome test with fogging
• Prism dissociation method
• Polaroid filters
• Turville infinity balance technique

Near Vision Correction

Correction of near vision is often necessary after the age of 40 years. Once the distant vision is corrected, the near vision should be corrected using near-vision charts such as Jaeger's chart, Snellen's chart, or number test types standardized by the faculty of Ophthalmologists N5 to N48.[31]   

If the near vision is defective, further testing should be done as follows:

• Near the point of accommodation and amplitude of accommodation
• Determination of near point of convergence
• Dynamic retinoscopy
• Determination of near add[32]  

Methodology to Assess Subjective Refraction

• History
• Subjective Refraction
• Assessing the fixation target
• Instruction to patients
• Visual acuity assessment
• Starting point
• Localizing principal meridian
• Astigmatism procedure
• Near vision assessment + Near correction
• Measurement of interpupillary distance
• Advice and prescription[2]

Indications

• Glass prescription
• Contact lens prescription
• Refractive error
• Keratoconus
• Early cataract
• Visual acuity not correlating with clinical findings[33]

Potential Diagnosis

The final diagnosis is made as follows:

Documenting the visual acuity with Snellen's chart and pinhole acuity. For example

• Uncorrected distant visual acuity (UDCVA) or visual acuity without correction – 20/40
• Best-corrected distant visual acuity (BCVA) or corrected distant visual acuity (CDVA)- 20/20
• Spherical correction- -2D sphere
• Cylindrical correction- -1D cylinder[34]

The prescription is written as follows:

OD-  -2 D/ -1D x 60 – 20/20

OS-  -2 D/ -1D x 150 – 20/20

Duochrome test

R=G, where R= red and G= green

Interfering Factors

• Subjective refraction is dependent on patient cooperation
• It depends on the patient's ability to understand and respond to commands accurately
• Difficult in patients with the language barrier
• Difficult to perform in patients who are deaf and mute
• Pediatric patients
• Lack of skill of the optometrists/ophthalmologists

Patient Safety and Education

It is vital to educate the patient regarding the importance and technique of subjective refraction. The patient should receive education on the need for refractive error correction and the importance of preventing amblyopia in pediatric patients. The need for regular spectacle wear should be emphasized, and then if needed, patients should understand the need for laser refractive correction in the future.

The patient should also be educated regarding timely follow-up and subjective refraction evaluation to detect changes in refractive error. Moreover, they should also be told they need Schiempflug imaging in cases of high cylindrical power and retinal assessment in subjects with high myopia.[35]

Clinical Significance

Any patient presenting for routine ophthalmic examination may have an underlying refractive error. It is the duty of the examining ophthalmologist or the optometrist to determine the underlying refractive error and treat them correctly. The patient may be advised to use contact lenses or spectacles based on the need. Correction of refractive error in the pediatric age group is critical to prevent amblyopia.[36]